The Rise of Faint, Red AGN at $z>4$: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields (2404.03576v3)

Abstract: We present a sample of 341 "little red dots" (LRDs) spanning the redshift range $z\sim2-11$ using data from the CEERS, PRIMER, JADES, UNCOVER and NGDEEP surveys. Unlike past use of color indices to identify LRDs, we employ continuum slope fitting using shifting bandpasses to sample the same rest-frame emission blueward and redward of the Balmer break. This enables the detection of LRDs over a wider redshift range and with less contamination from galaxies with strong breaks that otherwise lack a rising red continuum. The redshift distribution of our sample increases at $z<8$ and then undergoes a rapid decline at $z\sim4.5$, which may tie the emergence of these sources to the inside-out growth that galaxies experience during this epoch. We find that LRDs are $\sim1$ dex more numerous than X-ray and UV selected AGN at z~5-7. Within our sample, we have identified the first two X-ray detected LRDs. An X-ray spectral analysis confirms that these AGN are moderately obscured with $\log\,(N_{\rm H}/{\rm cm}^{2}$) of $23.3^{+0.4}_{-1.3}$ and $22.72^{+0.13}_{-0.16}$. Our analysis reveals that reddened AGN emission dominates their rest-optical light, while the rest-UV originates from their host galaxies. We also present NIRSpec observations from the RUBIES survey of 17 LRDs that show broad emission lines consistent with AGN activity. The confirmed AGN fraction of our sample is 71\% for sources with F444W<26.5. In addition, we find three LRDs with blue-shifted Balmer absorption features in their spectra, suggesting an outflow of high-density, low-ionization gas from near the central engine of these faint, red AGN.

• The paper identifies a novel population of faint, red AGN, termed 'Little Red Dots', that signal key phases of early black hole growth.
• It employs a continuum slope fitting method across multiple JWST surveys to precisely select obscured AGN candidates with minimal contamination.
• The results reveal a rapid rise in LRD density at z<8 and confirm AGN nature via X-ray and broad-line detections, guiding future follow-up studies.

The Rise of Faint, Red AGN at $z>4$: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields

The paper by Kocevski et al. investigates the emergence of faint, red active galactic nuclei (AGN) at high redshifts ($z>4$) using data from various JWST extragalactic legacy fields. The primary aim of the research is to identify and characterize a population of these previously hidden, dust-obscured AGN, colloquially termed "Little Red Dots" (LRDs), which are thought to represent a significant phase of black hole (BH) growth in the early Universe.

Key Contributions and Methodology

The paper presents a novel sample of 341 LRDs spanning the redshift range $z \sim 2-11$, detected over an area covering 587.8 arcmin$^2$. This paper makes use of data from multiple surveys, including CEERS, PRIMER, JADES, UNCOVER, and NGDEEP, leveraging the JWST's capabilities in the near-infrared to expand our understanding of AGN in the high-redshift Universe.

The authors employ a continuum slope fitting technique to select LRDs, an approach that utilizes shifting bandpasses to consistently sample rest-frame emissions indicative of obscured AGN across a broad range of redshifts. This method allows for a more precise identification of LRD candidates compared to previous fixed-band color index techniques, lessening the contamination from galaxies with strong breaks that lack a rising red continuum.

Findings and Redshift Distribution

The results reveal a sharp increase in the number density of LRDs at $z < 8$, followed by a rapid decline around $z \sim 4.5$. This trend suggests a close relationship between the emergence of these AGNs and the inside-out growth of galaxies during their formative epochs, where early galactic structures see intense, dust-enshrouded star formation and black hole accretion.

X-ray Detected LRDs

A significant advancement in this paper is the identification of X-ray detected LRDs (PRIMER-COS 3866 and JADES 21925), offering concrete examples of dust-obscured AGNs at lower redshifts. An X-ray spectral analysis confirms moderate obscuration, reinforcing the idea that high-density, dusty environments contribute significantly to the red colors observed. The paper estimates black hole masses of these sources and suggests they exhibit a higher-than-expected BH mass given their host galaxy mass, a property seen in other high-redshift galaxies detected by JWST.

Broad-line Detections

The paper identifies broad-line emissions in a substantial fraction (71% for F444W $< 26.5$) of the observed LRDs, confirming the AGN nature of these sources. This high detection fraction supports the notion that many of these red sources are indeed obscured quasars, not just heavily reddened galaxies or stellar contamination.

Implications and Future Prospects

The findings imply that LRDs, as a significant presence at high redshifts, play a crucial role in understanding the early growth phases of supermassive black holes, often obscured by dust. This potentially resolves discrepancies between observed luminous AGN and theoretical models of their formation and growth. The paper's methodology and results could guide future spectroscopic follow-ups to further elucidate the properties of AGNs in the early Universe, offering insights into the interstellar medium during periods of intense star formation and AGN activity.

In conclusion, the work adds a critical piece to the puzzle of early black hole and galaxy co-evolution, particularly in the obscured regimes where much of the growth occurs unaccounted for in earlier surveys. As JWST continues its observations, the techniques and findings presented here will be invaluable in refining our understanding of the cosmic history of supermassive black holes.