Counting Little Red Dots at $z<4$ with Ground-based Surveys and Spectroscopic Follow-up

Abstract

Little red dots (LRDs) are a population of red, compact objects discovered by JWST at $z>4$. At $4<z<8$, they are roughly 100 times more abundant than UV-selected quasars. However, their number density is uncertain at $z<4$ due to the small sky coverage and limited blue wavelength coverage of JWST. We present our ground-based search for LRDs at $2\lesssim z\lesssim 4$, combining ultra-deep Hyper Suprime-Cam photometry and various (near-)infrared surveys within a total area of $\sim 3.1\mathrm{de}{\mathrm{g}}^2$. We find that for LRDs with $M_{5500}<-22.5$, their number density declines from $\sim 10^{-4.5}\mathrm{cMp}{\mathrm{c}}^{-3}$ at $z>4$ to $\sim 10^{-5.3}\mathrm{cMp}{\mathrm{c}}^{-3}$ at $2.7<z<3.7$ and $\sim 10^{-5.7}\mathrm{cMp}{\mathrm{c}}^{-3}$ at $1.7<z<2.7$. We also present the Magellan/FIRE spectrum of our first followed-up candidate, DEEP23-z2LRD1 at $z_{\mathrm{spec}}=2.26$, as a proof of concept for our sample selection. Similar to high-redshift LRDs, the spectrum of DEEP23-z2LRD1 exhibits broad H$\hat{I}\pm$ emission with $\mathrm{FWHM}\approx 2400\mathrm{km}{\mathrm{s}}^{-1}$ and with nearly symmetric narrow H$\hat{I}\pm$ absorption. Additionally, DEEP23-z2LRD1 has extremely narrow [OIII] lines with $\mathrm{FWHM}\approx 140\mathrm{km}{\mathrm{s}}^{-1}$, suggesting the presence of an accreting black hole in a low-mass host galaxy. Limited by the angular resolution of ground-based surveys, we emphasize that spectroscopic follow-ups are required to characterize the contamination fraction of this sample and pin down LRD number density at $z<4$.

15 pages, 8 figures, submitted to ApJL. Comments are welcome