The Little Red Dots LRDs are red and compact galaxies detected in JWST deep fields mainly in the redshift range z  48. Given their compactness and the inferred stellar masses in the hypothesis that LRDs are starburst galaxies the implied stellar densities are immense. This Research Note uses an extensive catalog of LRDs from the PRIMER and the COSMOSWeb surveys to investigate these densities. We find a median upper limit on the effective radius of 80 pc which leads to median lower limit values of the core density of 10SUP4SUP M SUBSUB pcSUP3SUP and individual densities as high as 10SUP8SUP M SUBSUB pcSUP3SUP which is 10 times higher than the density necessary for runaway collisions to take place. For 35 of the LRDs investigated the lower limits are higher than the highest stellar densities observed in any system in any redshift range.
['Galaxy evolution', 'Early universe', 'Surveys', 'Galaxies', '594', '435', '1671', '573', 'Astrophysics - Astrophysics of Galaxies', 'Astrophysics - Cosmology and Nongalactic Astrophysics', 'Astrophysics - High Energy Astrophysical Phenomena']
Sizes and Stellar Masses of the Little Red Dots Imply Immense Stellar Densities
{'Sizes and Stellar Masses of the Little Red Dots Imply Immense Stellar Densities': '<!-- image -->  \n1 Harvard College, Cambridge, MA 02138, USA  \n2 Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA 3 Black Hole Initiative, Harvard University, 20 Garden St, Cambridge, MA 02138, USA 4 Department of Physics and Astronomy, Colby College, Waterville, ME 04901, USA  \n(Accepted August 22, 2024)  \nSubmitted to Research Notes of the AAS', 'ABSTRACT': "The 'Little Red Dots' (LRDs) are red and compact galaxies detected in JWST deep fields, mainly in the redshift range z = 4 -8. Given their compactness and the inferred stellar masses in the hypothesis that LRDs are starburst galaxies, the implied stellar densities are immense. This Research Note uses an extensive catalog of LRDs from the PRIMER and the COSMOS-Web surveys to investigate these densities. We find a median (upper limit) on the effective radius of 80 pc, which leads to median (lower limit) values of the core density of ∼ 10 4 M ⊙ pc -3 , and individual densities as high as ∼ 10 8 M ⊙ pc -3 , which is ∼ 10 times higher than the density necessary for runaway collisions to take place. For ∼ 35% of the LRDs investigated, the lower limits are higher than the highest stellar densities observed in any system in any redshift range.  \nKeywords: Galaxy evolution (594) - Galaxies (573) - Surveys (1671) - Early universe (435)", '1. INTRODUCTION': "The 'Little Red Dots' (LRDs) are highz sources detected in several JWST deep fields (e.g., Kocevski et al. 2023; Harikane et al. 2023; Maiolino et al. 2023; Labb'e et al. 2023; Matthee et al. 2024), at a median redshift of z ∼ 6 and with a distribution spread over ∼ 1 Gyr of cosmic history (Kocevski et al. 2024; Akins et al. 2024; Kokorev et al. 2024). These sources are strikingly red (Matthee et al. 2024) and extremely compact. While the median effective radius is r e ∼ 150 pc (Baggen et al. 2023), some of them are significantly smaller, with r e < 35 pc (Furtak et al. 2023).  \nLRDs are either highly dense and compact starburst galaxies (Labb'e et al. 2023; P'erez-Gonz'alez et al. 2024), or they host supermassive black holes (SMBHs) of 10 6 -10 8 M ⊙ (Harikane et al. 2023; Maiolino et al. 2023), which are overmassive with respect to their host's stellar mass (Pacucci et al. 2023; Durodola et al. 2024); remarkably, despite showing broad emission lines typical of Type-1 AGN (Greene et al. 2024), they are undetected in the X-rays (Maiolino et al. 2024; Yue et al. 2024), possibly because they are accreting at super-Eddington rates (Pacucci & Narayan 2024).  \nThe stellar-only interpretation warrants further discussion. Stellar masses derived from SED fitting are large, in some cases ∼ 10 11 M ⊙ (Labb'e et al. 2023). At highz , the fact that the Balmer break redshifts out of the NIRCam bands may lead to overestimating such masses, alleviating the tension. Measurements or, in most cases, upper limits on the effective radius are much firmer than any estimates on the black hole or stellar masses, and they are typically of the order of ∼ 100 pc, which is ∼ 2% of the effective radius of the Milky Way (Lian et al. 2024). Hence, the implied stellar densities are immense (as recently shown in Baggen et al. 2024 for three galaxies), and whether stable stellar systems can even exist in these conditions is unclear.  \nfabio.pacucci@cfa.harvard.edu  \nFigure 1. Statistical descriptors of the population of LRDs presented in Akins et al. (2024) and Kocevski et al. (2024). Left: distribution of the inferred core stellar densities. Right: combined distribution of effective radii and densities.  \n<!-- image -->  \nIn this Research Note, we consider the stellar mass and effective radius estimates from the Cosmos-Web and PRIMER surveys derived in Akins et al. (2024) and Kocevski et al. (2024) to infer the implied stellar densities.", '2. METHODS': 'The catalog of LRDs provided by Akins et al. (2024) contains 434 galaxies at 5 < z < 9 from the COSMOS-Web survey. This dataset was augmented with 64 PRIMER sources from Kocevski et al. (2024), removing a few duplicates based on their coordinates, thus leaving 475 sources for analysis. We consider the inferred stellar masses in the star-only model and their (model-independent) effective radii to estimate the implied average stellar and core densities.  \nWe compute an upper limit on the physical effective radius from the redshift of each source and its angular effective radius (measured in the F444W filter). Most of the LRDs in Kocevski et al. (2024) and in Akins et al. (2024) are unresolved. The Plummer model (Plummer 1911) is adopted to construct stellar density profiles and infer the core density of the LRDs; the associated potential is Φ( r ) = -GM ⋆ / √ r 2 + R 2 p , where M ⋆ is the total stellar mass, and R p is the Plummer scale radius. As is customary both in local and highz studies (see, e.g., Baggen et al. 2024), we assume that the effective radius traces the half-mass radius and that R eff = 1 . 3 R p . The core density for each stellar system is calculated in the innermost 1 pc radius. This choice is motivated by comparing our stellar densities with the maximum surface density reached by dense stellar systems, which is ∼ 10 11 M ⊙ kpc -2 (Hopkins et al. 2010; Akins et al. 2024), or ∼ 5 × 10 4 M ⊙ pc -3 .', '3. RESULTS': 'We display in Fig. 1 (left) the distribution of the core density, computed with the mass within 1 pc. All LRDs are characterized by implied core stellar densities that are significantly higher than the typical stellar density in the solar neighborhood (i.e., ≈ 0 . 06 M ⊙ pc -3 , Xiang et al. 2018). Remarkably, most LRDs are significantly denser than typical Galactic globular clusters, characterized by core densities of ∼ 10 3 M ⊙ pc -3 (Djorgovski & Meylan 1994). In fact, the median value for the core density of the LRDs is ∼ 10 4 M ⊙ pc -3 . Interestingly, the maximum value reached in the core of any system is ∼ 10 8 M ⊙ pc -3 , which is 10 times higher than the density necessary for runaway collisions to take place (Ardi et al. 2008; Fujii et al. 2024).  \nTo conclude, Fig. 1 (right) displays the combined distribution of radii and core stellar densities for the LRDs. The most common LRD, expected to comprise around 24% of the population, would be characterized by a core density of 10 4 . 2 M ⊙ pc -3 with a radius of only ∼ 100 pc. The distribution also demonstrates that LRDs as small as 10 -20 pc with core densities as high as ∼ 10 8 M ⊙ pc -3 can be observed in around 5% of the dataset. The lower limits on the core stellar densities are, for ∼ 35% of the LRDs, higher than the highest stellar densities observed in various stellar systems in any redshift range.  \nSince most effective radii measurements are upper limits, our density estimates are lower limits. Hence, it is crucial to investigate whether these extremely dense stellar systems are gravitationally stable.  \nThis project is funded by the Harvard College Research Program for Summer 2024. The Authors thank Chris Lintott for the helpful suggestions for improving the manuscript.'}