Two new catalogs of blazar candidates in the WISE infrared sky
Raffaele D'Abrusco Nuria Alvarez Crespo Francesco Massaro Riccardo Campana Vahram Chavushyan Marco Landoni Fabio La Franca Nicola Masetti Dan Milisavljevic Alessandro Paggi Federica Ricci Howard A. Smith



















































Raffaele D'Abrusco
rdabrusc@cfa.harvard.edu

0000-0003-3073-0605]Raffaele D'Abrusco
Center for Astrophysics  Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 20138, USA


Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, I-10125 Torino, 
Italy
European Space Agency (ESA), European Space Astronomy Centre (ESAC), E-28691 Villanueva de la 
Cañada, Madrid, Spain


Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, I-10125 Torino, 
Italy
Istituto Nazionale di Fisica Nucleare, Sezione di Torino, via Pietro Giuria 1, I-10125 Torino, Italy
INAF - Osservatorio Astrofisico di Torino, via Osservatorio 20, I-10025 Pino Torinese, Italy


INAF - Osservatorio di Astrofisica e Scienza dello Spazio, via Gobetti 93/3, I-40129, Bologna, Italy


Instituto Nacional de Astrofísica, Óptica y Electrónica, Apartado Postal 51 y 216, 72000
Puebla, México


INAF - Istituto Nazionale di Astrofisica, via Emilio Bianchi 46, I-23807 Merate (LC), Italy


Dipartimento di Matematica e Fisica, Università Roma Tre, via della Vasca Navale 84,
I-00146, Roma, Italy


INAF - Osservatorio di Astrofisica e Scienza dello Spazio, via Gobetti 93/3, I-40129, Bologna, Italy
Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, 
Santiago, Chile


Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, 
IN, 47907, USA


Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, I-10125 Torino, Italy
Istituto Nazionale di Fisica Nucleare, Sezione di Torino, via Pietro Giuria 1, I-10125 Torino, Italy
INAF - Osservatorio Astrofisico di Torino, via Osservatorio 20, I-10025 Pino Torinese, Italy



Instituto de Astrofísica and Centro de Astroingeniería, Facultad de 
Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile


Center for Astrophysics  Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 20138, USA
















We present two catalogs of radio-loud candidate blazars whose  mid-infrared 
colors are selected to be consistent with the colors of confirmed γ-ray emitting 
blazars. The first catalog is the improved and expanded release of the WIBRaLS catalog presented 
by <cit.>: it includes sources detected in all four  
filters, spatially cross-matched with radio source in one of three radio surveys
and radio-loud based on their q_22 spectral parameter. WIBRaLS2 includes 9541 
sources classified as BL Lacs, FSRQs or mixed candidates based on their  colors. 
The second catalog, called KDEBLLACS, 
based on a new selection technique, contains 5579 candidate BL Lacs extracted 
from the population of  sources detected in the first three  passbands 
([3.4], [4.6] and [12]) only, whose mid-infrared colors are similar to those of 
confirmed, γ-ray BL Lacs. 
KDBLLACS members area also required to have a radio counterpart and be 
radio-loud based on the parameter q_12, defined similarly to q_22 used 
for the WIBRaLS2. We describe the properties 
of these catalogs and compare them with the largest samples of confirmed 
and candidate blazars in the literature. We crossmatch the two 
new catalogs with the most recent catalogs of γ-ray sources 
detected by  LAT instrument. Since spectroscopic observations of candidate 
blazars from the first WIBRaLS catalog within the uncertainty
regions of γ-ray unassociated sources confirmed that ∼90% of 
these candidates are blazars, we anticipate that these new catalogs will play again 
an important role in the identification of the γ-ray sky.

























§ INTRODUCTION



Blazars represent one of the most extreme classes of active galactic nuclei (AGNs). These radio-loud sources 
are characterized by flat radio spectra even at low radio frequencies (i.e., 
below ∼1 GHz) <cit.>, superluminal 
motions <cit.>, 
peculiar infrared colors <cit.>, high optical 
polarization <cit.> and, not least, 
rapid and irregular variability <cit.> at all frequencies with 
uncorrelated  
amplitudes and 
different time scales ranging between minutes to weeks <cit.>. Blazars can 
reach bolometric luminosities up 
to 10^49 erg s^-1 during γ-ray flaring states <cit.>. 

The emission from 
blazars, according to the unification scenario of radio-loud AGNs, arises from a relativistic
jet, closely aligned along the line of sight, that, in some cases, can outshine the host 
galaxy and the other AGN emission components <cit.>.

A close connection between the γ-ray and the infrared (IR) properties of blazars has 
recently emerged from the investigation of the all-sky surveys carried out with Wide-Field Infrared Survey 
Explorer () <cit.> and  satellites <cit.>. The observed
correlations across more than 10 
decades of frequency has been proven to be an extremely powerful tool to identify new γ-ray emitting 
blazars among  sources with unknown or uncertain lower-energy 
counterparts <cit.>.

With a sky density of ∼ 0.1 sources per square degree, blazars dominate the γ-ray 
sky in the MeV-TeV energy range and represent ∼40% of the sources in the Third  -Large Area 
Telescope (LAT) Source Catalog <cit.>. Blazars are mainly divided into two 
sub-classes based on their optical spectra: (i) BL Lac objects characterized by featureless optical 
spectra or 
showing emission and/or absorption lines of equivalent widths EW< 5Å  <cit.>, and (ii)
Flat Spectrum Radio Quasars (FSRQs) with a typical quasar-like optical spectrum. Hereinafter, we adopt the 
nomenclature used in the  catalog <cit.>, where BL Lacs and FSRQs are 
referred to as BZBs and BZQs, respectively.

The Spectral Energy Distributions (SEDs) of blazars are characterized by two broad components, a  
low-energy one peaking between the IR and X-ray bands and a high energy one whose emission ranges between
the X-rays and the γ-rays. The former component is interpreted as synchrotron 
radiation from relativistic particles accelerated in a jet while the component at higher energies, according to leptonic models, is due to inverse Compton emission with seed photons that can 
have different origins <cit.>, while hadronic 
models invoke the synchrotron emission by protons or secondary particles produced in proton-photon 
interactions <cit.>.


The BL Lac 
population is also divided, according to their SEDs, in “Low-frequency peaked BL Lac objects” 
(LBLs) when the peak of the first 
component lie in the IR-to-optical energy range and “High-frequency peaked BL Lac objects” (HBLs) when 
the synchrotron peak falls in the UV-to-X-rays energy range <cit.>. Another, more recent 
classification distinguishes blazars as low-synchrotron peaked (LSP), intermediate-synchrotron peaked (ISP) 
or high-synchrotron peaked (HSP) based on the peak frequency ν_S,peak of the synchrotron 
component of their SEDs <cit.>. Here we will adopt the LBLs/HBLs sub-classification for 
the BZBs as it does not strictly depend on the exact location of the peak frequency.

The discovery of the peculiar  IR colors of blazars has been used to search for blazar-like sources within 
the positional uncertainty regions of the unidentified/unassociated γ-ray sources (UGSs) that could 
be their potential counterparts <cit.>. Several procedures based on  data 
have been developed 
to investigate the nature of the UGSs listed in all  source catalogs as well as to verify the nature of blazars
candidates of uncertain type <cit.>. However, all these methods 
require spectroscopic confirmation of the natures of the blazar candidates selected.

One of the largest sources of candidate blazars used to identify unassociated γ-ray sources 
observed by the  
LAT has been the catalog of  Blazar-Like Radio-Loud Sources (WIBRaLS; Paper I). 
This catalog contains  sources detected in all four  bands, whose 
mid-IR colors are similar to those of confirmed  blazars. WIBRaLS sources were also required to a) have a radio 
counterpart from one of three major surveys, namely the National Radio Astronomy Observatories Very Large Array VLA Sky 
Survey <cit.> the VLA Faint Images of the Radio Sky at 
Twenty-cm Survey <cit.>
and the Sydney University Molonglo Sky Survey Source Catalog <cit.>, and b) to be radio-loud, 
that is to have an observed ratio between radio and 22 μm mid-infrared flux densities >3 (Paper I).

Since the publication of the WIBRaLS catalog, extensive optical spectroscopic campaigns whose goal 
is to verify the
nature of WIBRaLS1 blazar candidates that can be spatially associated to γ-ray sources observed 
by  have been carried out <cit.>.
The total number of WIBRaLS candidate blazars which have been spectroscopically followed up and reported 
upon
in either one of the papers listed above is, to date, 159 split in 126 candidate BZBs, 16 candidate BZQs and 17 
candidates with no spectral classification available (Mixed). The analysis of the optical spectra 
confirmed that 
∼93% of candidate BZBs have featureless optical spectra typical of BL Lacs and ∼52% of the 
candidate BZQs show FSRQ spectra. Only ∼3%, ∼12% and ∼11% of the spectra of observed 
candidate BZBs, BZQs
or Mixed blazars cannot be classified as belonging to blazars, yielding a weighted average efficiency
of the selection ∼95%. 
Checks of spectra already published in the literature for 28 additional candidate blazars
from WIBRaLS1 confirmed a BL Lacs or FSRQs nature for 27 of them.

Since the publication of the first release of the WIBRaLS catalog (Paper I), new versions of 
some of the main datasets used to 
define its selection method have been released:


    
  * The ROMA BZCat, which contains the list of bona fide, spectroscopically confirmed blazars 
    used to define the locus occupied by γ-ray emitting blazars in the  color space, 
    has reached its 5th release <cit.>. This release contains ∼ 3600 sources, vs ∼ 
    3050 sources in the version used by <cit.>. 
    
  * The LAT 3-year Point Source Catalog (3FGL) <cit.> of 
    γ-ray sources detected by   containing ∼ 3000 sources, has also become available. 
    This recent update to the catalog of  LAT sources is more than twice as large as the 2-year 
    2FGL <cit.> catalog (∼ 2250 members) used to extract the first version of the 
    WIBRaLS catalog.


These two new datasets yield jointly a larger sample of confirmed 
γ-ray blazars that can be used to characterize more accurately their  mid-IR properties and
hence to identify more effectively blazars that may be detected in the γ-ray 
energy range. In this paper we describe a 
new release of the WIBRaLS catalog that, by taking advantage 
of these most recent data available, maximizes the legacy value of  observations for the investigation 
of blazars.

The completeness of the WIBRaLS catalog is a function of the blazar spectral class, and decreases significantly 
for BL Lacs, which often are not detected in the [22] μm  band. BZBs, in particular 
HBLs, have lower detection rate in the [22]  band (Paper I), since their emission 
in the mid-infrared at 22 μm may be lower than the limiting sensitivity in the fourth  band. 

For this reason, in this paper we will also present a new, complementary 
catalog of candidate BZBs selected with a novel technique that employs the  colors obtained from 
the first three  bands only and has been applied to All sources not detected in W4.
We focus only on the BZB spectral class for two main reasons: i) the region of the IR color-color space occupied 
by BZB is less contaminated by spurious IR sources than that of BZQs, ii) during our campaign of spectroscopic 
follow-up of candidate BL Lacs selected with  colors, we found that a large fraction of UGSs and BCUs are 
classified as candidate BZBs, providing an indication that they are the most elusive counterparts of  
sources <cit.>.

The paper is organized as follows: in Section <ref> we provide a brief introduction to the  and 
radio catalogs used to extract both samples of candidate blazars. Sections <ref> 
and <ref> describe the selection methods of the new WIBRaLS catalog and the catalog of BL Lac 
candidates selected using two  colors only, respectively. The comparison between the two 
catalogs presented in this paper and the literature is described in 
Section <ref>. Finally, our conclusions are summarized in Section <ref>. 

We use cgs units unless otherwise stated and spectral indices, α, are defined by flux density 
S_ν∝ν^-α indicating as flat spectra those with α<0.5.  magnitudes used 
here are in the Vega system and are not corrected for the Galactic extinction. As shown in our previous 
analyses <cit.>, such correction affects only the magnitude at 3.4 μm for sources 
lying at low Galactic latitudes, and it ranges between 2% and 5% of the magnitude, thus not affecting 
significantly our results. 



§ DATA





 §.§ Infrared data



All the candidate blazars discussed in this paper were sources extracted from images produced by the  space 
telescope <cit.>.  has observed the whole sky from 2009 to 2011 in the four bands W1, W2, W3 
and W4 centered on 3.4, 4.6, 12 and 22 μm, respectively. 

The All source catalog has superseded the previously available  All-Sky catalog; it was 
produced by combining  single exposures images from the first two years of the mission with the 
post-cryogenic phase data <cit.>, that observed the sky in three (W1, W2 and W3) and then only 
two (W1 and W2) bands. The final result is a two-fold improvement in the depth-of-coverage in 
the first two bands thanks to the additional observations that have increased the sensitivity of the stacked 
images, and improved photometric accuracy in all filters thanks to updated background measurements. The Allsensitivities are 54, 71, 730 and 5000 μJy in the W1, W2, W3 and W4 passbands respectively, with 
angular resolutions of 6.1, 6.4, 6.5 and 12. Due to the  survey strategy, 
the limiting sensitivity of the catalog of sources extracted from the  images is not uniform on the sky 
(compare with Figure 8 at the Explanatory Supplement to the All Data Release 
Products[<http://wise2.ipac.caltech.edu/docs/release/allwise/expsup/sec4_2.html>]). 

The All Source Catalog contains astrometry and photometry in the IR for 747,634,026 objects; 
only 25,882,082 of these sources (∼3.5%) were seen in all the four bands, increasing up to 
99,118,890 sources (∼13.3%) 
detected in the first three bands W1, W2 and W3 only. These two catalogs represent the parent samples for the 
updated WIBRaLS (Section <ref>) and the new catalog of BL Lacs candidates (Section <ref>), 
respectively. 



 §.§ Radio data



We have crossmatched the catalog of All sources according to the procedure described in 
Section <ref> with the NVSS, the FIRST and the SUMSS radio surveys. A brief description
for each of these three radio surveys is given below.


    
  * NVSS <cit.> is a 1.4 GHz continuum survey performed using the Very Large Array (VLA) 
    and covering the entire sky north of -40 deg declination, i.e. 82% of the celestial sphere with 
    a beam size of 45 FWHM. The result is a catalog of over 1.8 million discrete sources brighter 
    than S ∼ 2.5 mJy in the entire survey. 
    
  * FIRST <cit.> is a project designed to produce the radio equivalent of the 
    Palomar Observatory Sky Survey over 
    10,000 square degrees of the North and South Galactic Caps using the NRAO Very Large Array (VLA). 
    The beam size varies between 5.4 FWHM for circular beam and 6.8 along the major axis
    for elliptical shape, as a function of the declination of the observation. The 
    survey area has been chosen to coincide with that of the Sloan Digital Sky 
    Survey <cit.> and at the m_v ∼23 limit of SDSS, ∼40 % of the 
    optical counterparts to FIRST sources are detected. At the 1 mJy source detection threshold, there 
    are ∼90 sources per square degree.
    
  * SUMSS <cit.> is a radio imaging survey of the southern sky carried out with the Molonglo 
    Observatory Synthesis Telescope (MOST) operating at 843 MHz, with a beam size ≈ 43 FWHM. 
    The catalog covers approximately 3500 deg^2 with declination δ < 
    -30^∘, about 43% of the total survey area. The survey has a limiting peak brightness of 6 mJy/beam at 
    declinations δ≤ -50^∘, and 10 mJy/beam at δ > -50^∘. The SUMSS is therefore 
    similar in sensitivity and resolution to the NVSS, with ∼7000 sources found in the overlap region.






§ THE SECOND RELEASE OF THE WIBRALS CATALOG



In this paper, a new release of the catalog of  Blazar-like RAdio-Loud Sources (WIBRaLS) has 
been built up by following an improved version of the procedure described by Paper I and including
new samples of confirmed blazars associated with  γ-ray sources. Schematically, the steps we followed 
to extract the WIBRaLS catalog are the following: 


	
  * We select  sources detected in all four  bands, with IR colors similar to those of associated 
	 blazars (Section <ref>). We call these sources “ blazar-like sources”. 
	
  * The  blazar-like sources are positionally cross-matched with sources extracted from either one
	of the radio surveys NVSS, SUMSS and FIRST (Section <ref>) and only those with a radio
	counterpart are retained.
	
  * Among the WISE blazar-like sources with a radio counterpart, only radio-loud sources 
	are selected as members of the WIBRaLS catalog (Section <ref>).
	

In the following, we provide descriptions of each step above.



 §.§  color selection of blazar-like sources



We define the blazar-like locus using the sample of confirmed Fermi blazars, based on the 5^th version of 
the ROMA-BZCat catalog <cit.> and the 3FGL catalog <cit.>, associated with All 
counterparts detected in all four  filters. This locus thus includes newly identified 4-band
sources and extends the locus presented in Paper I.

The BZCat v.5.0 contains 3561 bona fide blazars with spectroscopic confirmation. Optical spectra are 
also used to classify members of the BZCat as BZBs or BZQs, according to the total equivalent width of 
all emission and absorption features, while blazars whose properties are intermediate 
between BZB and BZQ are tagged as Uncertain (BZU) and other blazar-like objects whose emission is
mostly contaminated by light from the host galaxy are labeled as BZGs. 

All BZCat sources classified as BZU and BZG were discarded, leaving 1151 BZBs and 1909 BZQs. In order to 
spatially crossmatch the positions of these BZCat sources with All sources detected in the four 
filters, we adopted the maximum radial distance of 3.3 obtained by conservatively combining a nominal
uncertainty of 1on the radio positions of BZCat sources with positional uncertainty in the  W4
passband <cit.>. This sample was then spatially crossmatched with γ-ray 
sources included in the 
3FGL catalog <cit.>, the latest release of sources detected by the LAT instrument 
on board NASA's  spacecraft, based on the first 48 months of survey data. The crossmatch has been performed 
by taking into account the position angles and the lengths of the semi-major and semi-minor axes of the 95% 
confidence region of each  source, available in 3FGL, and the positional uncertainty of the All 
counterparts of the BZCat sources. The final sample used to define the  three-dimensional locus 
consists of 901 confirmed γ-ray emitting blazars, split in 497 BZBs and 404 BZQs, and is more than twice
as large as the analogous list in Paper I, which contained 447 sources.

Following Paper I, we define a model of the locus in the three-dimensional Principal Components 
(PCs) space generated by the three independent  colors W1-W2 ([3.4]-[4.6]), W2-W3 ([4.6]-[12]) and 
W3-W4 ([12]-[22]). 
The locus is modeled as a set of three coaxial cylinders whose axes lie on the direction of the first 
PC (PC1). 
The two extremal cylinders are populated by blazars of similar spectral classes, namely BZB 
and BZQ respectively, while the intermediate cylinder contains significant (≥25%) fractions of 
both BZBs and BZQs. The length along the PC1 axis of the two extremal cylinders is defined so that 
they contain at least 75% of blazars classified as candidate BZBs and BZQs, respectively, while 
excluding the locus sources with PC1 value smaller than the 1%-st percentile and larger than 
the 99%-th percentile of the distribution of locus PC1 coordinates.
The mixed cylinder contains fractions of BZBs and BZQs each smaller than 75%. The radii of the three 
cylinders, which 
lie in the plane generated by PC2 and PC3 and orthogonal to PC1, are defined to contain 95% of the 
sources whose PC1 coordinates fall in each of the three cylinders. Sources located within the 
locus are selected on the base of the value of their “score”, a quantitative measure of the distance 
of a generic source to the locus model, defined as follows.

The  colors of a  source and the associated uncertainties are 
projected into the PC three-dimensional space, where they define an “uncertainty ellipsoid”. The position and
orientation of this ellipsoid - relative to the model of each cylinder separately - are used to calculate a numeric 
value defined between 0 and 1. This normalized distance is then weighted by the volume of the error ellipsoid, 
so that two  sources placed in the same position relative to the locus model but with different 
uncertainties are assigned different scores. A more detailed description of the score and its properties 
can be found in Paper I. 

The score is also used to classify candidate blazars compatible with the model of the locus 
according to their reliability. All  sources with non-zero score for either cylinder 
are ranked according to the decreasing compatibility with the locus model in the classes A, B, C and D, 
defined by the 90%-th, 60%-th, 20%-th and 5%-th percentiles of the score distribution for the sources 
of the locus sample. These classes are defined to facilitate quick prioritization of 
candidate blazars as targets of follow-up observation across sources of different spectral types (BZB-like,
BZQ-like or Mixed) and regardless of the specific score distributions for each spectral type.
Nonetheless, classes do not replace scores as a quantitative indicator of the degree of compatibility 
of each candidate blazar with the locus model in the  color space; all scientific 
analysis on the final list of WIBRaLS should be performed using scores.
The lower 5% threshold corresponds to the fraction of locus sources that are 
located outside the locus model by definition. The values of the score thresholds used to define 
the three classes are reported in Table <ref>. 

To all sources with score larger than the 5-th percentile threshold, for any one of the three cylinders, are assigned 
the corresponding type (BZB, BZQ or Mixed) and selected as  blazar-like sources. The Mixed type does 
indicate a specific spectral class, since the Mixed cylinder contains comparable fractions of both BZBs and BZQs.

The total number of  blazar-like sources selected is 526,681, split in 156,506 BZB candidates 
(∼30%), 348,805 (∼60%) BZQ candidates and the remaining 21,370 (∼4%) sources located 
in the Mixed region. The  blazar-like sources can also be split in 7,807 Class A sources (∼1%), 27,986 
Class B sources (∼5%), 149,052 Class C sources (∼28%) and the remaining 341,836 
(∼65%) belonging to the class D.

cccc
	Values of the score thresholds s_5%, s_20%, s_60% and s_90%, 
	used for the extraction of the  blazar-like sources and to define the classes
	as described in Section <ref>. These values are determined as the 5%-th,
	20%-th, 60%-th and 90%-th percentiles of the distribution of scores of the locus 
	sample, separately for BZB, Mixed and BZB cylinders in the locus model.
    
	        BZB-like 	    Mixed	    BZQ-like  
	    				
    s_5% (D)	    0.24	    0.32	    0.22	

	s_20% (C)	    0.51	    0.54	    0.48	

	s_60% (B)	    0.74	    0.85	    0.77	

	s_90% (A)	    0.91	    0.99	    0.91 	
	
	



 §.§ Radio counterparts



Following Paper I, we determined the optimal radii for the association of the  blazar-like sources 
with their potential radio counterpart in the NVSS, SUMSS and FIRST surveys. 

We adopted a modified version of the procedure illustrated by <cit.> and <cit.>. They 
computed the optimal radius for the spatial crossmatch of NVSS and FIRST radio detections with optical sources 
in SDSS by setting a threshold on the fraction of spurious associations (i.e. the contamination) obtained for different 
values of maximum crossmatch radius. In this paper, similarly at what done in Paper I, the optimal association 
radius is indeed set as 
the radial distance that provides a given fixed efficiency of the selection e_thr=99%, corresponding 
to a contamination c_thr=1%, where c(ϑ)=100%-e(ϑ). The efficiency or purity 
of the selection is defined as the number of sources around real radio positions n_real(ϑ), reduced 
by the number of sources around mock radio positions n_mock(ϑ) and divided by the number of 
“real” matches.

We estimated n_real(ϑ) by counting the number of  sources detected in all four bands within 
circular regions of radius ϑ between 0 and 60  centered on a sample of 5·10^4 sources 
randomly extracted from each of the three radio surveys. The value of n_mock(ϑ) was calculated by 
averaging over one hundred mock realizations of the coordinates of each real radio source, generated by moving the 
real position in a random direction and by a random radial distance in the [60, 120] range.

The maximum cross-match radii evaluated for the NVSS and SUMSS surveys are ϑ_NVSS=10.4, 
ϑ_SUMSS=7.4 respectively, identical to those determined for the 1^st release of the 
WIBRaLS catalog. This approach does not work for the FIRST survey because of the very high density 
of FIRST sources. For this reason, we assume as optimal search radius for FIRST the value previously determined 
ϑ_FIRST=3.4 (Paper I), that was obtained by combining conservative estimates 
of the positional uncertainties of FIRST and All sources. 

The number of  blazar-like sources associated with at least one radio counterpart in either of the three surveys 
within the maximum radial distances discussed above is 32630, split in 18903 with a NVSS counterpart, 1040 with 
a FIRST counterpart and 3323 with a SUMSS counterpart. In order to exclude  sources associated to distinct 
radio sources associated to the emission of lobes of the same radio galaxy, we searched and removed all duplicate 
radio sources whose positions would fall within 6from each other. This radius matches the typical positional 
uncertainty of γ-ray sources detected by LAT in the 3FGL <cit.>. In these cases, the All source 
with the largest score among all the duplicates was retained. After this step, the number of  blazar-like sources
with a radio counterpart in the NVSS, FIRST and SUMSS catalogs is 18693, 10227 and 3287 respectively, for a total of 
32207 sources. This sample includes 5547 sources with a counterpart 
from both the NVSS and FIRST surveys and 709 sources with one counterpart from both NVSS and one SUMSS. 
The number of unique  blazar-like sources associated to a radio counterpart (i.e., the previous sample 
after removing sources with radio counterpart listed in two catalogs) is 25951, with ∼72% (18693) has a NVSS 
counterpart, ∼18% (4687) has a FIRST counterpart and the remaining ∼10% (2578) are associated 
to a FIRST source. Moreover, 7101 sources (∼27%) of this list are classified as candidate BZBs, 2186 (∼8%) 
as Mixed candidates and the remaining 1664 (∼64%) as candidate BZQs [The selection 
of candidate BZQs using solely 
their  photometry can be contaminated by normal quasars as they share the same region of the  colors
space <cit.>. Also, the existence of radio counterparts to  -selected BZQ candidates alone, 
lacking radio spectral characterization, does not remove the degeneracy <cit.>], 
with 1366 class A (∼5%), 3302 class B (∼13%), 9265 class C (∼36%) and the remaining 
12018 (∼47%) as class D sources.




 §.§ Radio-loudness selection



Blazars are radio-loud AGNs but not every radio-loud AGN is a blazar. Blazars are generally hosted in 
elliptical galaxies, whose emission at radio frequencies 
is dominated by synchrotron emission from particles accelerated in 
the AGN relativistic jet pointed along the line of sight. In order to distinguish blazars from other radio 
sources, when lacking radio spectral information, we adopted the approach described in what 
follows. <cit.> as well as <cit.>
suggested that AGN-powered radio-sources can be 
identified using q_24, a modified definition of the so-called q parameter <cit.>, i.e. the 
logarithm of the ratio of far-IR to radio flux density, to overcome the dearth of accurate flux density measurements 
at far-IR frequency. The q_24 parameter is defined as:


    q_24=log(S_24μ m/S_1.4GHz)
 

where S_24μ m is the observed flux density at 24 μm and S_1.4GHz is the 
flux density measured at 1.4 GHz. Following <cit.>, we adopted a similar criterion to 
select “radio-loud sources” among 
the  blazar-like sources with a radio-counterpart by using the parameter q_22 defined as:


    q_22=log(S_22μ m/S_radio)
 

where the flux density in the 24 μm band of the Multi-band Imaging Photometer used on 
Spitzer (MIPS) is replaced by the  W4 ([22] μm) band. This approach is possible 
thanks to the similarities of the two wavebands (see Paper I for additional details). 
For the S_radio in Equation <ref> we used the radio flux density at 1.4 GHz for sources with a 
NVSS or FIRST radio counterparts. Given the lack of flux measurement at 1.4 GHz for SUMSS sources, for 
those sources we used flux densities at 843 MHz. 

Due to the remarkable flatness of radio spectra of blazars <cit.>, 
replacing the flux density at 1.4 GHz with the same quantity at 843 MHz produces a
small effect of the value of the q_22 parameter, whose size was
estimated using the sample of  blazar-like 
sources with a radio 
counterpart detected in both NVSS and SUMSS (508 sources). The distribution of the difference between 
the values of q_22(1.4 GHz) and q_22(843 MHz) is fairly constant across the 
interval of q_22(1.4 GHz) covered by our sample, with Δ q_22≈-0.07,
confirming a fairly flat radio spectrum for the sources in this sample. Based on 
this finding, for the  blazar-like sources associated with SUMSS counterparts only, 
we have used the corrected q_22 
value defined as q^'_22(843 MHz)=q_22(843 MHz)+Δ q_22. 

 

Both the q_24 and q_22 parameters show a dependence on the redshift of the source, as their values 
decrease for larger redshifts <cit.>. Figure <ref> shows the distribution 
of q_22 values for the confirmed γ-ray emitting blazars in the locus sample with reliable 
redshift measurements, as a function of the redshift and color-coded according to their spectral classification 
from the BZCat. Locus sources (mostly BZBs) with uncertain or unknown redshifts were not used, reducing the 
number of locus members to 563 (split in 159 BZBs and 404 BZQs). In Figure <ref>, the same 
trend reported by <cit.> and Paper I is observed. If redshift estimates were available for 
all  blazar-like sources associated to a radio counterpart, we could have determined different q_22 
thresholds in different redshift bins. Since redshifts are not available, we used fixed values of q_22. 

In this paper, we improve over the approach used in Paper I by determining different q_22 thresholds 
for  blazar-like sources with radio counterpart classified as candidate BZQs, BZBs and Mixed. The thresholds 
for candidate 
BZBs and BZQs, calculated as the 95% of the q_22 distribution of all blazars in the locus sample classified 
as BZB and BZQ, are q_22^BZB≤-0.61 and q_22^BZB≤-0.97, 
respectively. The threshold for Mixed sources is set to the mean value of the thresholds for candidate BZQs and BZBs,
q_22^(Mixed)≤-0.79. These thresholds are shown as horizontal lines in Figure <ref>.

We evaluated the effect of the unknown underlying redshift distribution of the  blazar-like sources 
with radio counterpart on our selection based on fixed q_22 thresholds. We followed the same strategy used 
in Paper I. We computed the q_22 for all sources in the locus sample after varying their 
observed redshifts over an equally spaced grid covering the [0, 4] range with bins of 0.05 width, which includes 
the interval [0, ∼1.25] covered by the observed redshifts. We assumed power-law spectral energy distribution 
with slope constrained by the observed flux densities at 22 μm and at 1.4 GHz. The fractions of candidate 
BZQs and BZBs that satisfy the q_22 conditions based on the observed redshift distribution are 
∼91% and ∼93%, respectively. These fractions are slightly lower than the ∼94% fraction of 
sources recovered with the q_22≤-0.5 threshold used in Paper I.

lcccc
	Members of the WIBRaLS catalog, split according to their 
	 spectral types and classes, and provenance of the radio counterpart.
	
		    	BZB-like	    Mixed    BZQ-like		    	 Total	

	    
	Class A	   	55	        5		   61			    	121	    

	Class B	   	273	       124	   317		    	714	    

	Class C	   	1086	   579	   1746		    	3411	

	Class D	   	2330	   0		   2965		   	5295	

			   		       		   			   		    

	NVSS	   	3024	    557	   4093 		    	7664	

	FIRST	    	36	       7		   21	 		    	64	    

	SUMSS	    	694	        144	   975 		    	1813	
		
	Total 	    	3744	   	708	    5089		    	9541	

	 
 
The total number of  blazar-like sources with a radio counterpart that satisfies the radio-loudness 
criteria based on the q_22 parameter and, thus, belong to the second release of the WIBRaLS catalog 
is 9541. The break-down of the catalog according to  spectral type, class and provenance of the 
radio counterpart is given in Table <ref>, while the basic parameters for a subset of sources 
in the catalog are displayed in Table <ref>. The number of sources of the locus
sample of confirmed γ-ray emitting blazars used to determine the WIBRaLS selection, that are found 
in the final WIBRaLS2 is 666, ∼ 26% less than the original size of the locus
sample (901 sources). The exclusion of these 235 locus sources is the result of a) the definition of the 
 model (Section <ref>), that excludes 2% of the sources based on their location along the PC1 axis, 
and 5% of each spectral class because the radii of the three cylinders are defined to contain the 95% of associated
sources, and b) the definition of the q_22 thresholds (Section <ref>), which remove 5% of the 
remaining locus sources for each spectral class, by definition. 

lrrrrrrrrrrlrr
   
 	Sample of rows of the catalog of WIBRaLS sources.
    
	   All namea     R.A.b     
 	    Dec.c     
 	    W1-W2d     W2-W3e     
 	    W3-W4f     s_BZBg      
 	    s_MIXh     
 	    s_BZQi     Classj      
 	    Typek     Radio counterpartl      
 	    S_radiom     q_22n
	   
	J000011.09-433316.4     0.0462203     -43.5545611     1.31     2.85     2.67     0.0     0.0     0.39     D     BZQ     SUMSSJ000011.2-433317     70.9     -1.41

   	J000020.40-322101.2     0.0850076     -32.3503443     1.35     3.26     2.47     0.0     0.0     0.52     C     BZQ     NVSSJ000020-322059     520.9     -1.94

  	J000029.07-163620.2     0.1211645     -16.6056221     0.39     2.01     2.45     0.53     0.0     0.0     C     BZB     NVSSJ000029-163621     89.1     -1.24

   	J000047.05+312028.2     0.1960452     31.3411703     0.82     2.48     2.4     0.32     0.0     0.0     D     BZB     NVSSJ000047+312027     46.4     -1.43

   	J000056.54-402206.4     0.235613     -40.368453     0.71     2.69     2.6     0.28     0.0     0.0     D     BZB     SUMSSJ000056.7-402208     76.0     -1.36

   	J000101.04+240842.5     0.2543718     24.1451458     1.23     3.06     2.13     0.0     0.0     0.34     D     BZQ     NVSSJ000101+240842     46.6     -1.26

   	J000105.29-155107.2     0.2720486     -15.8520035     1.18     3.42     2.14     0.0     0.0     0.33     D     BZQ     NVSSJ000105-155106     347.5     -2.01

   	J000108.11-373857.1     0.2838199     -37.6492076     1.32     2.72     2.29     0.0     0.0     0.39     D     BZQ     SUMSSJ000108.0-373901     23.1     -1.12

   	J000118.01-074626.9     0.3250683     -7.7741395     0.95     2.58     2.19     0.64     0.13     0.0     C     BZB     NVSSJ000118-074626     208.4     -1.23

   	J000131.63+165413.8     0.3818138     16.9038342     1.21     2.92     2.45     0.0     0.0     0.59     C     BZQ     NVSSJ000131+165416     63.1     -1.08

   	J000132.22+135258.4     0.3842501     13.8829081     1.07     3.0     2.48     0.0     0.0     0.23     D     BZQ     NVSSJ000132+135258     74.2     -1.59

   	J000132.34+240230.3     0.3847546     24.0417769     0.85     2.14     1.84     0.81     0.0     0.0     B     BZB     NVSSJ000132+240231     359.2     -1.28

   	J000132.74-415525.2     0.3864515     -41.9236926     0.68     2.17     2.09     0.48     0.0     0.0     D     BZB     SUMSSJ000133.1-415526     11.6     -0.84

   	J000132.83+145607.9     0.3868025     14.9355453     0.92     2.85     2.77     0.0     0.1     0.31     D     BZQ     NVSSJ000132+145609     314.6     -1.69

   	J000137.07+431543.9     0.4044828     43.2622002     0.72     2.61     2.32     0.76     0.0     0.0     B     BZB     NVSSJ000137+431544     61.8     -0.9

    (a):  name; (b): Right Ascension (J2000); (c): Declination (J2000); (d): W1-W2  color; 
    (e): W2-W3  color; (f): W3-W4  color; (g): Score for the BZB region of the locus; (h): Score for the Mixed region of the locus; (i):
    Score for the BZQ region of the locus; (j): Class (see Section <ref>);
    (k): Spectral type (see Section <ref>); (l): Name of the radio counterpart; 
    (m): Radio flux density [mJy]; (n): Radio-loudness parameter q_22
 



§ CATALOG OF  Γ-RAY BL LAC CANDIDATES



As mentioned briefly in Section <ref>, only ≈ 2.5% of the All sources are 
detected at [22] μm. 
For this reason, the WIBRaLS catalog, that requires its members to be detected in all four  bands, 
will certainly be incomplete for BZBs whose SEDs peak in the X-rays and are not bright 
enough to be detected in the W4  band. This occurrence is schematically displayed 
in Figure <ref>, where the typical shapes of the SEDs of HBLs and LBLs
are plotted together with the  sensitivity limits for the four  filters. 



We designed a novel method that relies only on the colors obtained with the W1, W2 and W3  bands to 
produce a list of candidate BL Lacs, extracted from the All survey. The steps adopted for 
our procedure can be summarized as follows:


	
  * We select all All sources detected only in the first three  bands, having IR colors 
	similar to 
	those of known  BZBs belonging to the ROMA-BZCat v5.0. 
	
  * From this sample, we further select only those sources with a radio counterpart in either one 
	of the NVSS, SUMSS and FIRST radio surveys.
	
  * We identify as candidate BZBs sources whose values of the radio-to-infrared 
	flux density ratio are compatible with those of confirmed  BL Lacs. 


Details on these steps are given in the following sections.



 §.§  color selection of BZB-like sources



The training set used to identify the  mid-IR colors of BL Lac objects was built selecting all the  
sources belonging to the 3FGL and associated with BZBs in the latest release of the Roma-BZCAT 
catalog <cit.>. We only considered sources whose All counterparts are not detected in the
W4 band. The total number of unique sources in the training set is 93. Among these sources, 34 are associated
with a NVSS radio counterpart only, 3 are associated with a FIRST source only and the remaining 56 have both a 
NVSS and FIRST counterpart.

We selected  sources with IR colors similar to those of our training set by adopting the same procedure 
used in previous analyses <cit.>, based on the Kernel Density Estimation 
(KDE). KDE is a non-parametric procedure that estimates the Probability Density Function (PDF) of a multivariate 
distribution with no assumption on the properties of the parent population. The KDE depends on only one 
parameter, i.e., the bandwidth of the kernel of the density estimator, which is qualitatively analogous to the 
window size for one-dimensional running average.

We applied the KDE to the 2-dimensional distribution of training set sources in the  W2-W3 vs W1-W2 
color-color plane to determine its PDF. Then, we selected infrared sources in the All catalog not detected in 
the W4 passband and detected 
in the other three filters, whose colors are located within the isodensity contour enclosing 90% of the BL Lac 
training set. Only sources whose colors uncertainties ellipses are entirely contained within the 90% isodensity
contour were retained. Figure <ref> shows the distribution of the training set in the  color 
plane, together
with the iso-density contours determined by the KDE method, with the 90% contour displayed as a thick black line.  
The projections on this color-color diagram of the WIBRaLS2 catalog is also shown for reference.

The total number of  sources extracted applying this method is ∼14406, corresponding to ∼0.01% 
of the parent sample of All sources not detected only in W4.





 §.§ Radio counterparts



We further select possible BL Lacs candidates by searching for radio counterparts of the sample of sources selected
based on their  colors. This procedure obviously misses “radio weak BL Lacs” but to date they are  
extremely rare and their associations with  sources has not yet been verified <cit.>. 

We crossmatched the color-selected  sources with the radio surveys NVSS, SUMSS and FIRST using 
the same maximum association radii established as described in Section <ref>. We 
found a total of 17826 sources with at least one radio counterpart: 5532 are associated with a FIRST 
source within 3.4, 
10166 with a NVSS source within 10and the remaining 2128 with a SUMSS source with 7.4  
(see Table <ref>). After removing from our list sources with SUMSS or FIRST counterparts 
that are also associated with a NVSS source, there remain 2099  sources with a unique FIRST 
counterpart, 1680  sources with a unique SUMSS counterpart and the 10166 associated with a NVSS radio 
source, for a total of 13945 sources. Similarly at what was done for the WIBRaLS catalog 
in Section <ref>, 
in order to avoid possible contamination from radio lobes originating from radio-galaxies, 
we also checked for sources that had from another radio source
closer than 6, and, for FIRST radio counterparts only, sources with side lobe 
probability ≥ 0.05. These constraints reduced the number of sources to 2049 sources 
with a unique FIRST counterpart, 1671 sources associated to a unique SUMSS source and 10084 w
ith a radio counterpart in NVSS, for a total of 13804 candidates. 




 §.§ Infrared-to-radio ratio selection



The last step of the procedure to select  BL Lac candidates is based on the characterization of the 
distribution of their infrared-to-radio ratios q_12, similar to the q_22 parameter used to select 
WIBRaLS sources <ref>. We define the parameter q_12 as:


    q_12=log(S_12μm/S_radio)
 

i.e., the logarithm of the ratio between the  flux density measured in the W3 passband and 
the radio flux density. Flux density at 20 cm has been used to calculate q_12 of sources with a unique 
counterpart in the NVSS and FIRST surveys. 
Given the flatness of the radio spectrum of BL Lacs <cit.> and our estimate of the 
Δ q_22≈-0.07 for the  -selected 
sources with both SUMSS and NVSS radio counterparts (see Section <ref>), 
we assume a flat spectrum (α_radio=0) and use the flux density at 36 cm to
calculate the q_12 parameter for sources with SUMSS counterparts. [We estimate that for 
radio spectra with 
α_radio=±0.1, the effect on the values of 
q_12(843 MHz) calculated at 36 cm for SUMSS counterparts 
would be a factor ranging from ≈0.95 to ≈1.05. In the case of a 
much larger spectral index α_radio=0.5, the corrective factor would still be 
≈0.77.]

We consider BL Lac candidates those sources whose q_12 values are consistent with the observed q_12 
distribution of the training set sources. The q_12 values for sources with a FIRST or NVSS radio 
counterpart (59 and 90 respectively) have been used 
separately to determine the intervals of acceptable q_12 values for both surveys, as in both surveys flux 
densities are measured at the same wavelength (20 cm). 



Figure <ref> shows the distributions of the q_12 values of the BL Lacs
training set sources (light blue histograms), control samples including random  
sources with radio counterparts (solid gray histograms) and of the candidate BL Lacs 
selected according to their  colors as described in Section <ref> (dashed gray 
histograms). FIRST (upper panel), NVSS (mid panel) and 
SUMSS (lower panel) radio counterparts are shown separately; the light blue histogram is missing from the
SUMSS panel as no source in the training set has a radio counterpart from the SUMSS survey. It is
also interesting to notice that the peaks of the distributions of q_12 values for the control sample and 
the  -selected candidates with SUMSS radio associations (lower panel in Figure <ref>)
are significantly shifted towards smaller values of q_12 relative to the NVSS and FIRST distributions. 
This difference is due to relative shallowness of the SUMSS survey, whose sensitivity allows the detection of
sources with minimum density flux at 36 cm S_radio^SUMSS(36 cm)=5 mJy, larger than the 
∼2 mJy and ∼0.2 mJy for NVSS and FIRST, respectively. 

We defined the lower and upper limits of q_12 values used to select the candidate BL Lacs as
the 10-th and 90-th percentiles of the q_12 distribution of the training set (red lines in 
Figure <ref>). The q_12 intervals used to select the candidates are between -1.85 and -1 for 
NVSS training set sources and between -1.64 and -1.01 for sources with FIRST counterparts.
Given the lack  of training set BL Lacs with SUMSS counterpart, we have conservatively defined 
the upper and lower thresholds for the q_12 selection of SUMSS BL Lac candidates 
as the highest and lowest values of the lower and upper thresholds 
on the q_12 distribution of training set sources with a FIRST or 
NVSS counterpart, respectively. As a result, the interval of allowed q_12 values for SUMSS counterparts 
ranges between -1.64 and -1.
After applying the q_12 selections to the sample of 13804 -radio selected sources, we obtain 
5941 candidates, split in 327 associated to 
a FIRST source, 5310 with a NVSS counterpart and the remaining 305 associated with a 
SUMSS source. 

Then, as final step, we discarded all sources in the catalog located at Galactic latitudes |b|<10,
since γ-ray sources at low galactic latitudes suffer from a higher detection threshold due 
to a higher Galactic diffuse emission background <cit.>. This constraint reduces the total number
of sources in the catalog of KDE-selected BL Lac candidates to 5579, due to the exclusion of 363 sources 
with a NVSS radio counterpart. In what follows, the sources in the catalog 
of KDE-selected candidate BL Lacs will be called KDEBLLACS. The break-down of the number of BL Lacs candidates 
selected at different stages of the procedure as a function of the different radio surveys is displayed 
in Table <ref>, while the properties available for a sample of sources
in the final KDEBLLACS catalog is shown in Table <ref>


lcccc
	Break-down of the number of  sources identified as BL Lacs candidate at each 
	step of the selection described in Section <ref>, split by provenance of the radio 
	counterparts.
	
	        NVSS	    FIRST       SUMSS       	 Total	

	    
     selection	           	10166   	    5532	   2128		    	17826	

	duplicates removal		   	10166	        2099	    1680		   	13945	

	close sources removal	   	100084		    2049	    1671		   	13804	
 	
	q_12 selection	       	5310		    327	    305		   	5942	

	b selection   	   	4947		    327	    305		   	5579	

	 

lrrrrlrr
	Sample of rows of the catalog of KDEBLLACS.
	
	    All namea     R.A.b     Dec.c      
	    W1-W2d     W2-W3e     Radio namef     
	    S_20cmg     
	    q_12i
	        
	J000007.63+420725.5     0.0318093     42.1237527     0.35     1.92     NVSSJ000007+420722     18.2     -1.5

	  J000010.29-363405.2     0.042887     -36.5681267     0.53      2.33     NVSSJ000010-363407     6.2     -1.3

	  J000056.22-082742.0     0.2342813     -8.4616809     0.43      1.94     NVSSJ000056-082747     14.4     -1.6

	  J000116.37+293534.5     0.3182368     29.5929424     0.61      2.46     NVSSJ000116+293534     3.5     -1.02

	  J000126.44+733042.6     0.3601711     73.5118347     0.77      2.22     NVSSJ000126+733042     23.6     -1.63

	  J000137.86-103727.3     0.4077672     -10.6242584     0.40     2.07     NVSSJ000137-103727     10.2     -1.41

	  J000147.28+455015.2     0.4470018     45.8375759     0.78      1.91     NVSSJ000147+455016     4.2     -1.22

	  J000236.06-081532.4     0.6502775     -8.2590058     0.71      2.11     NVSSJ000236-081533     28.3     -1.61

	  J000302.99-105638.1     0.7624893     -10.9439389     0.67     2.36     NVSSJ000302-105637     18.2     -1.61

	  J000311.94-070144.3     0.7997588     -7.0289838     0.61     1.98     FIRST J000311.9-070144     4.17     -1.01

	(a):  name; (b): Right Ascension (J2000); (c): Declination (J2000); (d): W1-W2  color; 
    (e): W2-W3  color; (f): Name of the radio counterpart; 
    (h): Radio flux density [Jy]; (n): Radio-loudness parameter q_12
 



§ DISCUSSION



The full characterization of the two catalogs presented in this paper would require
optical, spectroscopic follow-up observations to confirm the nature of the candidates, and will be 
discussed in a future paper. In this Section, we examine the global spatial and 
 photometric properties of the WIBRaLS2 and KDEBLLACS catalogs, and discuss how they compare with the 
most recent catalogs of  γ-ray sources. 

The Aitoff projections of the sky positions in galactic coordinates of the WIBRaLS2 and KDEBLLACS catalogs 
are shown in Figure <ref>. The coverage of the WIBRaLS2 catalog is mostly uniform 
across the sky (left panel in Figure <ref>), with the exception of a region along
the galactic plane where radio sources are not available from any of the three surveys used. Two regions 
of higher density can be observed where the FIRST and SUMSS surveys
overlap with the NVSS coverage, respectively north and south of the Galactic plane. In the right 
panel, the sky distribution of the KDEBLLACS catalog features prominently a lower density of sources
associated to SUMSS radio counterparts and the lack of sources due to the Galactic latitude selection 
described in Section <ref>. 



It is interesting to compare the regions of the  color space occupied by the WIBRaLS2 and 
KDEBLLACS catalogs. The location of the sources belonging to the above catalogs in the  
color space is displayed in Figure <ref>.
The two samples occupy partially overlapping but distinct regions of the three
dimensional  color space. Since KDEBLLACS are not detected in the W4 passband by definition, 
their positions along the W3-W4 axis cannot be established, but the three-dimensional volume of the 
space potentially occupied by these sources can be determined by using the upper limits on their W4 
brightness (i.e., lower limits on their W4 magnitude) available in the All catalog.



The two catalogs of blazar candidates described in this paper are complementary because their members
can differ in spectral properties (see discussion in Section <ref>) and in brightness. 
Figure <ref>, that displays the histograms of the magnitude values for the three filters W1, W2 and W3 for the WIBRaLS2 and KDEBLLACS samples together, confirms that the KDE-based method 
selects sources that are increasingly fainter than those in the WIBRaLS catalog as the wavelength increase 
going from filter W1 to W3. 






 §.§ Comparison with literature



One of the main goals of the production of catalogs of candidate blazars, like the two discussed
in this paper, is the discovery of multi-wavelength counterparts of unassociated γ-ray 
sources (UGSs) observed by Fermi. While the thorough analysis that is necessary to reliably 
associate low energy candidate blazars to UGS <cit.>
is beyond the goals of this paper, as a consistency check and in order to assess the potential of the 
two catalogs here discussed to improve the characterization of the currently known γ-ray sources, we 
compared the WIBRaLS2 and KDEBLLACS catalogs with the most recent catalogs of sources detected by  LAT. 

We spatially crossmatched the WIBRaLS2 catalog with the 3FGL catalog <cit.> using the 98% 
elliptical uncertainty regions for 3FGL sources and a fixed positional uncertainty of 1on the 
WIBRaLS2 radio coordinates. 
We found a total of 1049 matches, including the 666 sources in the locus sample selected 
as WIBRaLS2 members (see Section <ref>). Among the remaining
373 sources, 49 are unassociated; the 320 sources that are associated or identified with a known 
multi-wavelength counterpart in 3FGL are all classified, according to the CLASS1 parameter in 
3FGL <cit.>, as BL Lacs, FSRQs or BZU (Blazar candidate of Uncertain type), except for 
3 rdg (radio-galaxies) and 3 ssrq (soft-spectrum radio quasars). The crossmatch between the 
KDEBLLACS catalog and 3FGL, using the same positional uncertainties, returned 186 matches (all 
distinct from the WIBRaLS2 crossmatches), with 57 unassociated sources. ∼95% of the remaining 131
3FGL sources crossmatched with a KDEBLLACS source that are associated or identified, are classified as 
BZU or BL Lacs according to the 3FGL CLASS parameter. The total number of unique crossmatched sources from 
either the WIBRaLS2 or KDEBLLACS catalogs increases to 1757 (1404 and 353, respectively) when 
the preliminary LAT 8-year Point Source List 
(FL8Y)[https://fermi.gsfc.nasa.gov/ssc/data/access/lat/fl8y/] (which contains a total of 5523 
γ-ray sources) is used, for a total of 152 unassociated sources. Out of the 1605
associated or identified sources, ∼ 99% are classified as either BL Lacs, FSRQs or BZU.  

Following the same approach described above, we also crossmatched the WIBRaLS2 and KDEBLLACS 
catalogs with the Third Catalog of Hard -LAT Sources <cit.>. The spatial
crossmatch returns a total of 807 distinct matches, split in 647 from WIBRaLS2 and 160 from 
KDEBLLACS, with 33 unassociated 3FHL sources and the remaining 774 composed by BL Lacs 
(520, ∼ 68% of the total), 
and blazars of uncertain type (BZU, ∼ 16% of the total). The 83% of the WIBRaLS2 members 
crossmatched with 3FHL sources are classified as candidate BZBs or MIXED candidates, based 
on their  colors. 

It is also useful to compare the two catalogs of candidate blazars presented in this paper 
with the largest catalog of candidate HSPs available in the literature to date, namely the second 
WISE High Synchrotron Peaked blazar (2WHSP) catalog <cit.>, with 1691 entries. The 2WHSP is an 
expansion of the 1WHSP catalog <cit.>, 
and contains HSPs candidates drawn from the All catalog that can be associated to radio and X-ray counterparts. 
The 2WSHPs candidates are further selected by requiring that their radio-to-IR and IR-to-X-ray broad-band 
spectral slopes are consistent with those of known, confirmed HSPs sources <cit.>, and that the peak frequency 
of the synchrotron emission component of their SEDs ν_peak is >10^15 Hz <cit.>. The 
2WHSP contains 1691 unique candidates or confirmed HSPs: 460 of these sources are associated to All sources
detected in four bands and 717 sources are associated to sources detected in the first three (W1, W2 and W3)
filters. The remaining sources cannot be compared to the catalogs discussed in this paper because not detected 
in both the W3 and W4 passbands. We determined that 248 of the 460 2WSHPs sources detected in all All filters are
also in the new WIBRaLS catalog, while 267 of the 717 sources not detected in W3 have been selected as 
KDEBLLACS. The main cause of difference between our catalogs and the 2WHSP is 
the maximum spatial radius used to crossmatch All sources with radio 
counterparts from one of the three radio surveys FIRST, NVSS and SUMSS. <cit.> report that 
radio counterparts were selected within 0.3 and 0.1 for NVSS/SUMSS and FIRST catalogs, 
respectively. These radii are significantly larger than the radii used in this catalog, discussed
in Sections <ref> and <ref> (10.4, 3.4 and 7.4 for NVSS, 
FIRST and SUMSS respectively). As a consequence, only 311 All sources with a unique radio counterpart 
contained in the sample used to extract the KDEBLLACS can also be found in the 
2WSHP catalog. Another possible source of difference between the KDEBLLACS and the 2WHSP catalogs is 
the extent of the region in the  color-color diagram where 2WHSP sources are located, which is 
significantly larger than the area where the BL Lacs candidates are selected from 
(see Figure <ref>).




 §.§ Comparison with the WIBRaLS1 catalog


The second release of the WIBRaLS catalog, WIBRaLS2, described in this paper, contains 5025 candidate blazars also 
found in the first WIBRaLS (WIBRALS1 hereinafter) catalog (Paper I). Of the total 7885 members of 
WIBRALS1, 2830 sources are not included in WIBRaLS2 because their q_22 values (for ∼ 95% of them) 
are larger than the new, more stringent thresholds adopted for BZQs in WIBRaLS2 (see Section <ref>). 
The increase in the size of the training set of confirmed γ-ray blazars used to define the WIBRaLS 
locus has led to an increase of the volume in  color space that has produced 4711 sources in 
the WIBRaLS2 not included in WIBRaLS1.




§ SUMMARY AND CONCLUSIONS



In this paper, we present two new catalogs of blazar candidates, selected on 
the basis of their  mid-IR colors, their association to a radio counterpart and their 
radio-loudness. These new catalogs contain a combined total of 15196 candidate blazars, 
including both candidate FSRQs and BL Lacs, distributed on ∼ 90% of the sky. 

The second, enhanced release of the WIBRaLS catalog supersedes the original WIBRaLS1 (Paper I) catalog, 
which has been extensively employed to associate  unidentified sources with their low energy 
counterparts. WIBRaLS2 contains candidate blazars drawn from All sources detected
in all four  passbands with colors similar to those of spectroscopically confirmed, γ-ray 
emitting blazars, that are associated to radio counterparts and identified as radio-loud. 
Spectral classification as candidate BZBs, BZQs or Mixed candidate blazars, following the ROMA-BZCat <cit.> terminology, 
and derived on the  color properties is also provided. WIBRaLS2 contains 9541 candidate blazars, 
a ∼25% increase over the first version of WIBRaLS.

The KDEBLLACS catalog complements the WIBRaLS2 by identifying BL Lacs candidates 
that, because of their typical low infrared-to-radio ratios and/or  brightness (cp. HBLs),  
have gone undetected in the 
W4  filter and, as a consequence, cannot be considered for selection in WIBRaLS2. The KDEBLLACS members 
are required to be detected in the three  filters W1, W2 and W3 and are selected based on their positions
in the W2-W3 vs W1-W2  color-color diagrams using the KDE technique. They are also associated to a radio 
counterpart and identified as radio-loud according to the q_12 radio-to-MIR spectral parameter. We select
5579 sources in the KDEBLLACS. 

Previous samples of candidate blazars selected on the basis of the IR colors, eventually combined with 
radio and/or multifrequency observations <cit.> have been 
used by the -LAT collaboration for 
the preparation of the 3FGL, the Third Catalog of Active Galactic Nuclei <cit.> and the 
Second and the Third Catalog of Hard -LAT Sources <cit.>.

This work will contribute to a more comprehensive understanding of the diverse and fascinating γ-ray 
sky as observed by Fermi. In particular, the community will benefit from the WIBRaLS2 and KDEBLLACS 
catalogs of candidate blazars presented 
in this paper and the subsequent programs of follow-up spectroscopic observations needed to confirm their nature and, 
possibly, determine their redshifts by using them to: (i) improve our knowledge of the luminosity function of BL 
Lacs <cit.>; (ii) select potential targets for the Cherenkov Telescope Array (CTA) 
as shown by <cit.> ; (iii) obtain more stringent 
limits on the dark matter annihilation in sub-halos <cit.>; (iv) search for 
counterparts of new flaring γ-ray sources <cit.> and of high energy 
neutrino emission <cit.>; (v) test new γ-ray 
detection algorithms <cit.>; (vi) and, finally, perform 
population studies of the remaining UGSs <cit.>.



[Abdo et al.(2010)]abdo10 Abdo, A. A., Ackermann, M., Agudo, I., et al. 2010, , 716, 30.
[Acero et al.(2013)]acero13 Acero, F., Donato, D., Ojha, R., et al. 2013, , 779, 133.
[Acero et al.(2015)]acero15 Acero, F., Ackermann, M., Ajello, M., et al. 2015, The Astrophysical Journal Supplement Series, 218, 23.
[Ackermann et al.(2015)]ackermann15 Ackermann, M., Ajello, M., Atwood, W. B., et al. 2015, , 810, 14.
[Ackermann et al.(2016)]ackermann16 Ackermann, M., Ajello, M., Atwood, W. B., et al. 2016, The Astrophysical Journal Supplement Series, 222, 5.
[Agudo et al.(2014)]agudo14 Agudo, I., Thum, C., Gómez, J. L., et al. 2014, , 566, A59.
[Ajello et al.(2014)]ajello14 Ajello, M., Romani, R. W., Gasparrini, D., et al. 2014, , 780, 73.
[Ajello et al.(2017a)]ajello17a Ajello, M., Atwood, W. B., Baldini, L., et al. 2017, 
The Astrophysical Journal Supplement Series, 232, 18.
[Ajello et al.(2017a)]ajello2017b Ajello, M., Atwood, W. B., Baldini, L., et al. 2017, The Astrophysical Journal Supplement Series, 232, 18.
[Álvarez Crespo et al.(2016a)]alvarez16a Álvarez Crespo, N., Massaro, F., D'Abrusco, R., et al. 2016, , 361, 316.
[Álvarez Crespo et al.(2016b)]alvarez16b Álvarez Crespo, N., Masetti, N., Ricci, F., et al. 2016, , 151, 32.
[Álvarez Crespo et al.(2016c)]alvarez16c Álvarez Crespo, N., Massaro, F., Milisavljevic, D., et al. 2016, , 151, 95.
[Angelakis et al.(2016)]angelakis2016 Angelakis, E., Hovatta, T., Blinov, D., et al. 2016, , 463, 3365.
[Arsioli et al.(2015)]arsioli15 Arsioli, B., Fraga, B., Giommi, P., et al. 2015, , 579, A34.
[Becker et al.(1995)]becker95 Becker, R. H., White, R. L., & Helfand, D. J. 1995, , 450, 559.
[Berlin et al.(2014)]berlin14 Berlin, A., Hooper, D., & McDermott, S. D. 2014, , 89, 115022.
[Bernieri et al.(2013)]bernieri13 Bernieri, E., Campana, R., Massaro, E., et al. 2013, , 551, L5.
[Best et al.(2005)]best05 Best, P. N., Kauffmann, G., Heckman, T. M., et al. 2005, , 362, 9.
[Blandford, & Rees(1978)]blandford78 Blandford, R. D., & Rees, M. J. 1978, BL Lac Objects, 328.
[Bonzini et al.(2013)]bonzini13 Bonzini, M., Padovani, P., Mainieri, V., et al. 2013, , 436, 3759.
[Böttcher(2007)]bottcher07 Böttcher, M. 2007, , 309, 95.
[Böttcher(2012)]bottcher12 Böttcher, M. 2012, Relativistic Jets from Active Galactic Nuclei, 17.
[Bruni et al.(2018)]bruni18 Bruni, G., Panessa, F., Ghisellini, G., et al. 2018, , 854, L23.
[Campana et al.(2015)]campana15 Campana, R., Massaro, E., Bernieri, E., et al. 2015, , 360, 19.
[Campana et al.(2016)]campana16 Campana, R., Massaro, E., & Bernieri, E. 2016, , 361, 185.
[Campana et al.(2017)]campana17 Campana, R., Maselli, A., Bernieri, E., et al. 
2017, , 465, 2784.
[Chang et al.(2017)]chang2017 Chang, Y.-L., Arsioli, B., Giommi, P., et al. 2017, , 598, A17.
[Chiaro et al.(2016)]chiaro16 Chiaro, G., Salvetti, D., La Mura, G., et al. 2016, , 462, 3180.
[Condon et al.(1998)]condon98 Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, , 115, 1693.
[Cowperthwaite et al.(2013)]cowperthwaite13 Cowperthwaite, P. S., Massaro, F., D'Abrusco, R., et al. 2013, , 146, 110.
[D'Abrusco et al.(2012)]dabrusco12 D'Abrusco, R., Massaro, F., Ajello, M., et al. 2012, , 748, 68.
[D'Abrusco et al.(2013)]dabrusco13 D'Abrusco, R., Massaro, F., Paggi, A., et al. 2013, The Astrophysical Journal Supplement Series, 206, 12.
[D'Abrusco et al.(2014)]dabrusco14 D'Abrusco, R., Massaro, F., Paggi, A., et al. 2014, The Astrophysical Journal Supplement Series, 215, 14.
[Dermer, & Schlickeiser(1993)]dermer1993 Dermer, C. D., & Schlickeiser, R. 1993, , 416, 458.
[Donoso et al.(2009)]donoso09 Donoso, E., Best, P. N., & Kauffmann, G. 2009, , 392, 617.
[Falomo et al.(2014)]falomo14 Falomo, R., Pian, E., & Treves, A. 2014, Astronomy and Astrophysics Review, 22, 73.
[Giroletti et al.(2016)]giroletti16 Giroletti, M., Massaro, F., D'Abrusco, R., et al. 2016, , 588, A141.
[Gunn et al.(1998)]gunn1998 Gunn, J. E., Carr, M., Rockosi, C., et al. 1998, , 116, 3040.
[Healey et al.(2007)]healey07 Healey, S. E., Romani, R. W., Taylor, G. B., et al. 2007, The Astrophysical Journal Supplement Series, 171, 61.
[Helfand et al.(2015)]helfand15 Helfand, D. J., White, R. L., & Becker, R. H. 2015, , 801, 26.
[Helou et al.(1985)]helou85 Helou, G., Soifer, B. T., & Rowan-Robinson, M. 1985, , 298, L7.
[Homan et al.(2002)]homan02 Homan, D. C., Ojha, R., Wardle, J. F. C., et al. 2002, , 568, 99.
[Hovatta et al.(2016)]hovatta2016 Hovatta, T., Lindfors, E., Blinov, D., et al. 2016, , 596, A78.
[IceCube Collaboration et al.(2018)]icecube2018 IceCube Collaboration, Aartsen, M. G., Ackermann, M., et al. 2018, Science, 361, eaat1378.
[La Mura et al.(2015)]lamura2015 La Mura, G., Chiaro, G., Ciroi, S., et al. 2015, Journal of Astrophysics and Astronomy, 36, 447.
[Landoni et al.(2015)]landoni15 Landoni, M., Massaro, F., Paggi, A., et al. 2015, , 149, 163.
[Landoni et al.(2018)]landoni2018 Landoni, M., Paiano, S., Falomo, R., et al. 2018, , 861, 130.
[Lister, & Homan(2005)]lister2005 Lister, M. L., & Homan, D. C. 2005, , 130, 1389.
[Lister et al.(2009)]lister2009 Lister, M. L., Aller, H. D., Aller, M. F., et al. 2009, , 137, 3718.
[Mainzer et al.(2011)]mainzer11 Mainzer, A., Grav, T., Bauer, J., et al. 2011, , 743, 156.
[Marchesi et al.(2018)]marchesi2018 Marchesi, S., Kaur, A., & Ajello, M. 2018, , 156, 212.
[Marchesini et al.(2019)]marchesini2019 Marchesini, E. J., Peña-Herazo, H. A., Álvarez Crespo, 
N., et al. 2019, , 364, 5.
[Maselli et al.(2013)]maselli13 Maselli, A., Massaro, F., Cusumano, G., et al. 2013, The Astrophysical Journal Supplement Series, 206, 17.
[Maselli et al.(2015)]maselli15 Maselli, A., Massaro, F., D'Abrusco, R., et al. 2015, , 357, 141.
[Massaro et al.(2008a)]massaro2008 Massaro, F., Giommi, P., Tosti, G., et al. 2008, , 489, 1047.
[Massaro et al.(2008b)]massaro2008f Massaro, F., Tramacere, A., Cavaliere, A., et al. 2008, , 478, 395.
[Massaro et al.(2009)]massaro09 Massaro, E., Giommi, P., Leto, C., et al. 2009, , 495, 691.
[Massaro et al.(2011)]massaro11a Massaro, E., Giommi, P., Leto, C., et al. 2011, Multifrequency Catalogue of Blazars (3rd Edition.
[Massaro et al.(2011)]massaro11 Massaro, F., D'Abrusco, R., Ajello, M., et al. 2011, , 740, L48.
[Massaro et al.(2012a)]massaro12a Massaro, F., D'Abrusco, R., Tosti, G., et al. 2012, , 752, 61.
[Massaro et al.(2012b)]massaro12b Massaro, F., D'Abrusco, R., Tosti, G., et al. 2012, , 750, 138.
[Massaro et al.(2013a)]massaro13a Massaro, F., D'Abrusco, R., Paggi, A., et al. 2013, The Astrophysical Journal Supplement Series, 206, 13.
[Massaro et al.(2013b)]massaro13b Massaro, F., Giroletti, M., Paggi, A., et al. 2013, The Astrophysical Journal Supplement Series, 208, 15.
[Massaro et al.(2013c)]massaro13e Massaro, F., Paggi, A., Errando, M., et al. 2013, The Astrophysical Journal Supplement Series, 207, 16.
[Massaro et al.(2013d)]massaro13f Massaro, F., D'Abrusco, R., Giroletti, M., et al. 2013, The Astrophysical Journal Supplement Series, 207, 4.
[Massaro et al.(2013e)]massaro13d Massaro, F., D'Abrusco, R., Paggi, A., et al. 2013, The Astrophysical Journal Supplement Series, 209, 10.
[Massaro et al.(2014)]massaro14 Massaro, F., Masetti, N., D'Abrusco, R., et al. 2014, , 148, 66.
[Massaro et al.(2015)]massaro15spectra Massaro, F., Landoni, M., D'Abrusco, R., et al. 2015, , 575, A124.
[Massaro et al.(2015)]massaro15a Massaro, E., Maselli, A., Leto, C., et al. 2015, , 357, 75.
[Massaro et al.(2015)]massaro15b Massaro, F., D'Abrusco, R., Landoni, M., et al. 2015, The Astrophysical Journal Supplement Series, 217, 2.
[Massaro et al.(2015)]massaro15c Massaro, F., Thompson, D. J., & Ferrara, E. C. 2015, Astronomy and Astrophysics Review, 24, 2.
[Massaro et al.(2016)]massaro16 Massaro, F., Álvarez Crespo, N., D'Abrusco, R., et al. 2016, , 361, 337.
[Massaro, & D'Abrusco(2016)]massaro16b Massaro, F. & D'Abrusco, R. 2016, , 827, 67.
[Massaro et al.(2017)]massaro17 Massaro, F., Marchesini, E. J., D'Abrusco, R., et al. 2017, , 834, 113.
[Mauch et al.(2003)]mauch03 Mauch, T., Murphy, T., Buttery, H. J., et al. 2003, , 342, 1117.
[Mücke, & Protheroe(2001)]mucke2001 Mücke, A., & Protheroe, R. J. 2001, Astroparticle Physics, 15, 121.
[Murase et al.(2012)]murase2012 Murase, K., Dermer, C. D., Takami, H., et al. 2012, , 749, 63.
[Nolan et al.(2012)]nolan12 Nolan, P. L., Abdo, A. A., Ackermann, M., et al. 2012, The Astrophysical Journal Supplement Series, 199, 31.
[Nori et al.(2014)]nori14 Nori, M., Giroletti, M., Massaro, F., et al. 2014, The Astrophysical Journal Supplement Series, 212, 3.
[Orienti et al.(2014)]orienti14 Orienti, M., D'Ammando, F., Giroletti, M., et al. 2014, , 444, 3040.
[Padovani, & Giommi(1996)]padovani96 Padovani, P., & Giommi, P. 1996, , 279, 526.
[Padovani et al.(2011)]padovani11 Padovani, P., Miller, N., Kellermann, K. I., et al. 2011, , 740, 20.
[Paggi et al.(2013)]paggi13 Paggi, A., Massaro, F., D'Abrusco, R., et al. 2013, The Astrophysical Journal Supplement Series, 209, 9.
[Paggi et al.(2014)]paggi14 Paggi, A., Milisavljevic, D., Masetti, N., et al. 2014, , 147, 112.
[Paiano et al.(2017a)]paiano2017a Paiano, S., Falomo, R., Franceschini, A., et al. 2017, , 851, 135.
[Paiano et al.(2017b)]paiano2017b Paiano, S., Landoni, M., Falomo, R., et al. 2017, , 844, 120.
[Paiano et al.(2019)]paiano2019 Paiano, S., Falomo, R., Treves, A., et al. 2019, , 871, 162.
[Pavlidou et al.(2014)]pavlidou2014 Pavlidou, V., Angelakis, E., Myserlis, I., et al. 2014, , 442, 1693.
[Peña-Herazo et al.(2017)]pena17 Peña-Herazo, H. A., Marchesini, E. J., Álvarez Crespo, N., et al. 2017, , 362, 228.
[Ricci et al.(2015)]ricci15 Ricci, F., Massaro, F., Landoni, M., et al. 2015, , 149, 160.
[Schinzel et al.(2015)]schinzel15 Schinzel, F. K., Petrov, L., Taylor, G. B., et al. 2015, The Astrophysical Journal Supplement Series, 217, 4.
[Stern et al.(2005)]stern2005 Stern, D., Eisenhardt, P., Gorjian, V., et al. 2005, , 631, 163.
[Stern et al.(2012)]stern2012 Stern, D., Assef, R. J., Benford, D. J., et al. 2012, , 753, 30.
[Stickel et al.(1991)]stickel91 Stickel, M., Padovani, P., Urry, C. M., et al. 1991, , 374, 431.
[Taylor(2005)]taylor05 Taylor, M. B. 2005, Astronomical Data Analysis Software and Systems XIV, 29.
[Taylor et al.(2007)]taylor07 Taylor, G. B., Healey, S. E., Helmboldt, J. F., et al. 2007, , 671, 1355.
[Takeuchi et al.(2013)]takeuchi13 Takeuchi, Y., Kataoka, J., Maeda, K., et al. 2013, The Astrophysical Journal Supplement Series, 208, 25.
[Urry, & Padovani(1995)]urry95 Urry, C. M., & Padovani, P. 1995, Publications of the Astronomical Society of the Pacific, 107, 803.
[Vermeulen, & Cohen(1994)]vermeulen94 Vermeulen, R. C., & Cohen, M. H. 1994, , 430, 467.
[White et al.(1997)]white97 White, R. L., Becker, R. H., Helfand, D. J., et al. 1997, , 475, 479.
[Wright et al.(2010)]wright10 Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, , 140, 1868.
[Zechlin, & Horns(2012)]zechlin12 Zechlin, H.-S., & Horns, D. 2012, Journal of Cosmology and Astro-Particle Physics, 2012, 50.



R.D'A. is supported by NASA contract NAS8-03060 (Chandra X-ray Center).

The work of F.M. and A.P. is partially supported by the “Departments of 
Excellence 2018-2022” Grant awarded by the Italian Ministry of Education, 
University and Research (MIUR) (L. 232/2016) and made use of resources
provided by the Compagnia di San Paolo for the grant awarded on the BLENV
project (S1618_L1_MASF_01) and by the Ministry of Education, 
Universities and Research for the grant MASF_FFABR_17_01. F.M. 
also acknowledges financial contribution from the agreement ASI-INAF 
n.2017-14-H.0 while A.P. the financial support from the Consorzio 
Interuniversitario per la fisica Spaziale (CIS) under the agreement 
related to the grant MASF_CONTR_FIN_18_02.

F.R. acknowledges support from FONDECYT Postdoctorado 3180506 and CONICYT project 
Basal AFB-170002.

V.C. is partially supported by the CONACyT research grant 280789






This research has made use of data obtained from the high-energy Astrophysics Science Archive Research 
Center (HEASARC) provided by NASA's Goddard Space Flight Center.

The SIMBAD database operated at CDS, Strasbourg, France; the NASA/IPAC Extragalactic Database
(NED) operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract 
with the National Aeronautics and Space Administration.

Part of this work is based on the NVSS (NRAO VLA Sky Survey): The National Radio Astronomy Observatory is 
operated by Associated Universities, Inc., under contract with the National Science Foundation and on the VLA 
low-frequency Sky Survey (VLSS).

The Molonglo Observatory site manager, Duncan Campbell-Wilson, and the staff, Jeff Webb, Michael White and 
John Barry, are responsible for the smooth operation of Molonglo Observatory Synthesis Telescope (MOST) and the 
day-to-day observing programme of SUMSS. The SUMSS survey is dedicated to Michael Large whose expertise and 
vision made the project possible. The MOST is operated by the School of Physics with the support of the Australian 
Research Council and the Science Foundation for Physics within the University of Sydney.

This publication makes use of data products from the Wide-field Infrared Survey Explorer, 
which is a joint project of the University of California, Los Angeles, and 
the Jet Propulsion Laboratory/California Institute of Technology, 
funded by the National Aeronautics and Space Administration.

This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University 
of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the 
National Aeronautics and Space Administration and the National Science Foundation.

TOPCAT[http://www.star.bris.ac.uk/∼mbt/topcat/] 
<cit.> for the preparation and manipulation of the tabular data and the images.

