A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields

TL;DR

This study presents a census of photometrically selected little red dots (LRDs) at 4 < z < 9 in JWST blank fields to characterize obscured AGN demographics in the early universe. It combines color-color selection, compactness criteria, and quasar template fitting to identify 260 reddened AGN candidates over ~640 arcmin^2, enabling a wide dynamic range in luminosity. The inferred properties indicate moderate dust extinction with A_V ~ 1.6, bolometric luminosities spanning L_bol ~ 10^44–10^47 erg s^-1, and rest-frame UV magnitudes M_UV ~ -17 to -22, revealing a dusty AGN population that outnumbers UV-selected quasars by about two orders of magnitude at similar magnitudes. The results imply a substantial contribution of dusty AGN to the high-z SMBH mass function, with a number density ~100× higher than UV-selected quasars, a rising fraction among UV-selected galaxies at z ~ 7, and a lower limit on the SMBH mass function at z ~ 5 consistent with theory, while leveraging blank-field data to mitigate cosmic variance.

Abstract

Observations with the James Webb Space Telescope (JWST) have uncovered numerous faint active galactic nuclei (AGN) at $z\sim5$ and beyond. These objects are key to our understanding of the formation of supermassive black holes (SMBHs), their co-evolution with host galaxies, as well as the role of AGN in cosmic reionization. Using photometric colors and size measurements, we perform a search for compact red objects in an array of blank deep JWST/NIRCam fields totaling $\sim640$ arcmin$^{2}$. Our careful selection yields 260 reddened AGN candidates at $4<z_{\rm phot}<9$, dominated by a point-source like central component ($\langle r_{\rm eff} \rangle <130$ pc) and displaying a dichotomy in their rest-frame colors (blue UV and red optical slopes). Quasar model fitting reveals our objects to be moderately dust extincted ($A_{\rm V}\sim1.6$), which is reflected in their inferred bolometric luminosities of $L_{\rm bol}$ = 10$^{44-47}$ erg/s, and fainter UV magnitudes $M_{\rm UV} \simeq$ $-17$ to $-22$. Thanks to the large areas explored, we extend the existing dusty AGN luminosity functions to both fainter and brighter magnitudes, estimating their number densities to be $\times100$ higher than for UV-selected quasars of similar magnitudes. At the same time they constitute only a small fraction of all UV-selected galaxies at similar redshifts, but this percentage rises to $\sim$10\% for $M_{UV}\sim -22$ at $z\sim7$. Finally, assuming a conservative case of accretion at the Eddington rate, we place a lower limit on the SMBH mass function at $z\sim5$, finding it to be consistent with both theory and previous JWST observations.

Figures (1)

• Figure 1: Selection and analysis of LRD candidates. Top: Sample selection criteria. The left and central panels show modified "red 1" ($z\lesssim 6$) and "red 2" ($z\gtrsim 6$) color-color cuts from labbe23. The right panel shows the compactness cut of our sample. Selected objects are highlighted as maroon circles, while grayscale hexbins show the full catalog. The compact red sources are clear outliers in color-color-compactness space. Colorbar is shared between all plots. Bottom: An example of best-fit SEDs to the photometry of LRD candidates with the dust-free (blue) and dusty (red) AGN templates vandenberk01glikman06 at representative redshifts of $z\sim6$ and $z\sim8$. The combined model is shown in black. Detections ($>3\sigma$) are shown as red circles, while upper limits (primarily from HST) are shown as downward arrows. On the right of each SED we show 1$.\!\!^{\prime\prime}$5 color composite cutouts in the short (F115W/F150W/F200W) and long (F277W/F356W/F444W) NIRCam filters.