The Impact of the CMB on high-$z$ AGNs jets

Luca Ighina

Insubria University (Como, Italy) + INAF–Brera (Milano, Italy)

Radio-Loud (RL) Active Galactic Nuclei (AGNs) are among the brightest
astrophysical sources at all wavelengths. Their relativistic jets can affect
both their Supermassive Black Holes (SMBHs) growth and the surrounding
intergalactic medium. While in the radio band these jets can be observed at
all scales (from pc to Mpc scales), their X-ray and $\gamma$-ray emission is
expected to be concentrated on very small scales (<10 pc). However, after the
launch of the Chandra X-ray telescope, several kpc-scale jets were detected
[1] and the mechanism responsible for their high-energy radiation at these
scales is still under debate. Understanding its origin is crucial also to
derive the physical properties of these jets (e.g. the power) at large scales
and, as a consequence, their impact on the environment. In the following, we
explore the Inverse Compton interaction of the relativistic electrons within
relativistic jets with the Cosmic Microwave background photons (IC/CMB) as
possible interpretation. Moreover, we also estimate how this interpretation
could also affect the observed evolution across cosmic times of the SMBHs
hosted in jetted systems.

The most distant large-scale relativistic jet, at $z=6.1$

After the detection of several large-scale jets with the Chandra telescope,
one of the most popular interpretations proposed for their X-ray emission was
that it is produced by the IC/CMB interaction [2]. Despite requiring quite
extreme assumptions on the physical parameters (e.g. bulk Lorentz factor of
the kpc-scale jets $\Gamma\sim$10-20), this model was successful in
reproducing the high X-ray fluxes observed in the majority of these large-
scale jets. However, after the launch of the Fermi $\gamma$-ray telescope,
this model was challenged by the non-detection of the expected strong
$\gamma$-ray emission in a few objects in the low-$z$ Universe ($z<1$).
Therefore, another interpretation was proposed, with the X-ray radiation
produced by a second population of highly-energetic electrons (with Lorentz
factors $\gamma\sim 10^{8-9}$) through synchrotron (see [3]). Nevertheless, we
still expect the IC/CMB process to take place, especially at high redshift.
Indeed, given the strong redshift dependence of the CMB energy density,
$U_{\rm CMB}\propto(1+z)^{4}$, and that the IC/CMB is significantly boosted
for jets oriented close to our line of sight (i.e. for sources classified as
blazars), the best way to constrain it is to focus on high-$z$ blazars. To
this end we performed a Chandra X-ray observation of the most distant blazar
currently known, PSO J0309+27 at $z=6.1$ [4]. This observation revealed the
presence of a $\sim$20 kpc jet extending in the North-East direction (see
Figure 1), making it the highest-redshift large-scale jet currently known.
From the detailed study of its multiwavelength emission (see [5]), we found
that its X-ray flux is fully consistent with the IC/CMB process with
$\Gamma\sim 2$, therefore without requiring the extreme bulk velocities
velocities needed at low redshift. This is due to a double effect of the CMB
energy density increase at high redshift: (a) it boosts the IC/CMB process by
a factor $(1+z)^{4}$; (b) it decreases the cooling timescale of the electrons,
which, in turn, cannot be accelerated up to the very high energies needed to
emit in the X-rays through synchrotron. Therefore, from our work we concluded
that, while in the local Universe the synchrotron process likely dominates the
X-ray emission observed in large-scale relativistic jets, at high redshift
($z\gtrsim 2$) the IC/CMB process becomes the most important mechanism.

Figure 1: Chandra X-ray image of PSO J0309+27 with the (3, 3$\sqrt{2}$,
6)$\times$RMS radio contours at 3 GHz from the VLASS survey overlaid. The kpc-
scale jet is highlighted by the blue ellipse. Adapted from [5].

Space density evolution of blazars

If, as explained above, the IC/CMB process was important and significant at
high redshift, we would expect the overall X-ray emission (core plus extended
jet emission) of blazars to increase as a function of redshift. Interestingly,
such a trend was found from the comparison of statistically complete samples
of blazars at high and low redshift (e.g. [6]). In particular, these studies
found that $z\sim 4.5$ blazars have, on average, X-ray-to-radio luminosity
(X/R) ratios about two times larger than their low-$z$ counterparts. This
difference could also have important consequences on our understanding of the
evolution of blazars, and therefore of the jetted SMBHs, through cosmic times.
Indeed, recent studies have found that the cosmological evolution of the
blazar population significantly differs when observed in the X-ray or the
radio band. In particular, X-ray studies (e.g. [7]) find that the number of
blazars at high redshift is significantly larger than the one expected from
the radio band (e.g. [8]). As shown also in Figure 2, the X-ray space density
evolution of blazars, computed starting from the radio Luminosity Function
(LF) derived by [8] and assuming a constant X/R conversion factor, under-
predicts the number of blazars compared to the observed X-ray data (green
points). However, if we now consider that a fraction of the overall X-ray
emission is due to the IC/CMB process, we expect the typical X/R ratios of
blazars to evolve with redshift as follows:

LX/Lr ($z$) = LX/Lr ($z=0$) $\times$ ($1-A_{0}$) +
$A_{0}~{}\times~{}(1+z)^{4}$,

where $A_{0}$ is the relative contribution of the IC/CMB emission at $z=0$.
Based on the study of the largest complete samples of blazars selected in the
radio band, we found $A_{0}$ to be of the order of $\sim 10^{-3}$, that is,
the IC/CMB contribution at low redshift is negligible, as expected. By
including this type of X/R evolution in the computation of the expected X-ray
space density starting from the radio LF, we found that the increase of the
X-ray emission related to the IC/CMB process can reconcile the evolutions
observed in the radio and X-ray band, see the solid red curve in Figure 2.
In order to improve the current statistics at high redshift and therefore to
further test the IC/CMB mechanism, the e-ROSITA X-ray mission will be crucial,
thanks to its relatively deep sensitivity over the entire sky. Indeed, the
actual number and redshift distribution of the blazars detected by this
mission will depend on the importance of the IC/CMB process to the observed
X-ray emission. At the same time, detailed studies on these X-ray-selected
sources with the Chandra telescope, like the one presented above, will further
constrain the IC/CMB model.

Figure 2: Observed X-ray space density of blazars: green squares from [9],
blue points from [7] as reference. The two lines are the expected space
density evolution of blazars with (solid red) and without (dashed blue) the
redshift evolution given by the the IC/CMB process. Adapted from [9].

Hunting down RL AGNs at high redshift

If, on the one side, the e-ROSITA mission will uncover the X-ray-brightest end
of the blazar population, on the other side, on-going and future radio
surveys, like the ones performed by the Square Kilometre Array Observatory and
its precursors, will uncover the much larger RL population of AGNs at high
redshift. Indeed, soon after the first data release of the Rapid ASKAP
Continuum Survey (RACS) we were able to identify one of the most distant RL
AGNs currently known: VIK J2318$-$3113, at $z=6.44$ [10]. Given this first
successful identification, we are now using the RACS survey as starting point
for the discovery of new RL AGNs at high redshift. In particular, from its
combination with optical/NIR wide-area surveys currently available (namely
Pan-STARRS and DES) and the use of the dropout technique we are building the
first statistically complete sample of RL AGNs at $5<5<6.5$. In Figure 3 left
panel we report the sky coverage of the RACS and the optical/NIR surveys used
together with the sky distribution of the currently known $z>5$ RL AGNs and
our newly selected candidates. While the spectroscopic confirmation of the
majority of the candidates is still ongoing, we were already able to identify
two new $z>6$ RL AGNs through dedicated Gemini–South/GMOS observations
(GS-2021-DD-112, P.I. Ighina; see Figure 3 right panel). Chandra observations
of these sources will potentially allow us to detect and study more kpc-scale
jets at $z>6$ to further test the IC/CMB model.

Figure 3: Left panel: Sky coverage of the radio and optical/NIR surveys used
for our selection together with the currently known, the new selected
candidates and the newly identified RL AGNs at $z>5$. Right panel: zoom on the
optical/NIR dropout of the two newly confirmed RL AGNs at $z>6$.

References

## References

  * Schwartz [1999] Schwartz, D. A.,, et al., 2000, ApJ, 540, 69
  * Tavecchio [2000] Tavecchio, F., et al., 2000, ApJ, 544, L23
  * Georganopoulous [2016] Georganopoulos, M., et al., 2016, Galaxies, 4, 65
  * Belladitta [2020] Belladitta, S., et al. 2020, A&A, 635, L7
  * Ighina [2022] Ighina, L., et al. 2022, A&A, 659, A93
  * Ighina [2019] Ighina, L., et al. 2019, MNRAS, 489, 2732
  * Ajello [2009] Ajello, M., et al., 2009, ApJ, 699, 603
  * Mao [2017] Mao, P., et al., 2017, ApJ, 842, 87
  * Ighina [2021b] Ighina, L., et al. 2021, MNRAS, 505, 4120
  * Ighina [2021a] Ighina, L., et al. 2021, A&A, 647, L11

Short CV 2018–2020: MSc in Astrophysics, Milano–Bicocca University, Milano,
Italy 2020–present: PhD in Physics and Astrophysics, Insubria University,
Como, Italy 2024–on: Relaxing on a beach somewhere in the Caribbeans