COSMOS-Web: The over-abundance and physical nature of "little red dots"--Implications for early galaxy and SMBH assembly (2406.10341v1)

Abstract: JWST has revealed a population of compact and extremely red galaxies at $z>4$, which likely host active galactic nuclei (AGN). We present a sample of 434 ``little red dots'' (LRDs), selected from the 0.54 deg$^2$ COSMOS-Web survey. We fit galaxy and AGN SED models to derive redshifts and physical properties; the sample spans $z\sim5$-$9$ after removing brown dwarf contaminants. We consider two extreme physical scenarios: either LRDs are all AGN, and their continuum emission is dominated by the accretion disk, or they are all compact star-forming galaxies, and their continuum is dominated by stars. If LRDs are AGN-dominated, our sample exhibits bolometric luminosities $\sim10^{45-47}$ erg\,s$^{-1}$, spanning the gap between JWST AGN in the literature and bright, rare quasars. We derive a bolometric luminosity function (LF) $\sim100$ times the (UV-selected) quasar LF, implying a non-evolving black hole accretion density of $\sim10^{-4}$ M$\odot$ yr$^{-1}$ Mpc$^{-3}$ from $z\sim2$-$9$. By contrast, if LRDs are dominated by star formation, we derive stellar masses $\sim10^{8.5-10}\,M\odot$. MIRI/F770W is key to deriving accurate stellar masses; without it, we derive a mass function inconsistent with $\Lambda$CDM. The median stellar mass profile is broadly consistent with the maximal stellar mass surface densities seen in the nearby universe, though the most massive $\sim50$\% of objects exceed this limit, requiring substantial AGN contribution to the continuum. Nevertheless, stacking all available X-ray, mid-IR, far-IR/sub-mm, and radio data yields non-detections. Whether dominated by dusty AGN, compact star-formation, or both, the high masses/luminosities and remarkable abundance of LRDs implies a dominant mode of early galaxy/SMBH growth.

• The paper reveals that 'little red dots' may be either AGN-driven or dusty star-forming systems, challenging existing models of SMBH growth.
• It employs robust SED fitting on 434 high-redshift sources to determine key properties like bolometric luminosity (~10^45-47 erg/s) and stellar mass.
• The analysis bridges the gap between JWST AGN data and bright quasar studies, prompting a reexamination of early galaxy formation theories.

Early Galaxy and SMBH Assembly: Insights from COSMOS-Web and the Study of "Little Red Dots" (LRDs)

The paper discussed, titled "COSMOS-Web: The over-abundance and physical nature of `little red dots'—Implications for early galaxy and SMBH assembly," addresses a compelling astrophysical anomaly using observational data from the COSMOS-Web survey. The investigation revolves around the enigmatic "little red dots" (LRDs), a population of compact and extremely red galaxies observed at high redshifts ($z \gtrsim 4$) with the James Webb Space Telescope (JWST). These LRDs might significantly advance our understanding of the early universe's cosmic structures, particularly regarding galaxies and supermassive black holes (SMBHs).

Exploring LRDs: Two Interpretations

The authors, led by Hollis B. Akins, use meticulous criteria to identify 434 LRDs from the COSMOS-Web survey, postulating two extreme physical scenarios about their nature. In one scenario, these LRDs are presumed to be active galactic nuclei (AGNs) whose continuum emission is primarily dominated by the accretion disk; in the other, they are viewed as compact, dusty star-forming galaxies where starlight chiefly dominates the continuum. This dual interpretation sets the stage for understanding the physical and evolutionary aspects of these high-redshift entities.

Analytical Techniques and Findings

Utilizing models to fit galaxy and AGN spectral energy distributions (SEDs), the researchers derive insightful physical properties and redshifts for the sample, emphasizing specific redshift ranges, thereby excluding contaminants like brown dwarfs. These observations reveal the LRDs' considerable bolometric luminosities ($\sim 10^{45-47}$ erg/s), bridging the luminosity gap between existing JWST AGN data and bright quasars known from prior studies. This constitutes a notable inference as it implies a bolometric luminosity function that is significantly denser than previously noted UV-selected quasar functions, suggesting a constant black hole accretion density from $z \sim 2$ to $z \sim 9$. Such findings hint at an unexpectedly populous mode of substantial black hole growth in the early universe, with potential impact on models of cosmic structure formation.

If instead the LRDs are assumed to be dominated by star formation, they exhibit substantial stellar masses ($\sim 10^{8.5-10}\,M_\odot$). However, such a mass function raises inconsistencies with $\Lambda$CDM cosmology unless contributions from AGN are also considered. Despite maximum stellar mass surface density constraints observed in the contemporary universe, many LRDs exceed these limits, compelling a reconsideration of their physical modeling.

Implications and Future Directions

This paper's analysis of LRDs leads to impactful theoretical implications. If dominated by star-forming galaxies, the LRDs not only challenge aspects of current cosmological models but also inspire a reevaluation of the processes underlying massive compact galaxy formations. Conversely, if primarily AGN-dominated, the derived overabundance of luminous AGNs at these epochs necessitates reconsiderations in AGN activity frameworks and SMBH growth paradigms.

Practical implications include guiding subsequent research to simulate and interpret SMBH and massive galaxy formation processes more precisely. Additionally, the paper underlines the importance of multi-wavelength observations that offer vertical insights into photometric redshift estimation, line emission, and dust content.

The authors advocate for future studies that integrate deeper spectroscopic surveys and advanced modeling to further disentangle the AGN and stellar contributions to LRD spectra. Such efforts should bolster understanding of the balance between host galaxy evolution and black hole growth, essential to constructing more comprehensive cosmological theories.

In conclusion, this paper effectively situates LRDs within both the contexts of SMBH assembly and galaxy evolution at cosmic dawn, emphasizing their pivotal role in enriching our narrative of the universe's early epochs—a pursuit that not only enhances astrophysical knowledge but also informs future investigative orientations in the field of observational cosmology.