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

Yilun Ma, Jenny E. Greene, David J. Setton, Andy D. Goulding, Marianna Annunziatella, Xiaohui Fan, Vasily Kokorev, Ivo Labbe, Jiaxuan Li, Xiaojing Lin, Danilo Marchesini, Jorryt Matthee, Luke Robbins, Anna Sajina, Marcin Sawicki, O. Grace Telford

First listed 2025-04-10 | Last updated 2025-04-10

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 $2\lesssim z\lesssim4$, combining ultra-deep Hyper Suprime-Cam photometry and various (near-)infrared surveys within a total area of $\sim3.1\,\mathrm{deg^{2}}$. We find that for LRDs with $M_{5500}<-22.5$, their number density declines from $\sim10^{-4.5}\,\mathrm{cMpc^{-3}}$ at $z>4$ to $\sim10^{-5.3}\,\mathrm{cMpc^{-3}}$ at $2.7<z<3.7$ and $\sim10^{-5.7}\,\mathrm{cMpc^{-3}}$ 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$α$ emission with $\mathrm{FWHM}\approx2400\,\mathrm{km\,s^{-1}}$ and with nearly symmetric narrow H$α$ absorption. Additionally, DEEP23-z2LRD1 has extremely narrow [OIII] lines with $\mathrm{FWHM}\approx140\,\mathrm{km\,s^{-1}}$, 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 $z<4$.

Short digest

Using ~3.1 deg^2 of ultra-deep HSC plus (near-)IR imaging, the authors assemble a ground-based sample of Little Red Dots at 2≲z≲4 and measure their abundance for M5500<-22.5. They find a sharp decline in number density from ~10^-4.5 cMpc^-3 at z>4 to ~10^-5.3 at 2.7<z<3.7 and ~10^-5.7 at 1.7<z<2.7, extending LRD demographics below z~4. A Magellan/FIRE spectrum of DEEP23-z2LRD1 (z_spec=2.26) shows broad Hα (FWHM≈2400 km s^-1) plus nearly symmetric narrow Hα absorption and extremely narrow [OIII] (FWHM≈140 km s^-1), consistent with an accreting BH in a low-mass host. Because ground-based resolution limits can admit contaminants, the paper stresses that spectroscopy is required to firm up the z<4 LRD number densities.

Key figures to inspect

• Number-density vs. redshift for M5500<-22.5: read off the drop from ~10^-4.5 (z>4) to ~10^-5.3 (2.7<z<3.7) and ~10^-5.7 (1.7<z<2.7), and compare to UV-selected quasar counts.
• Survey footprint/depth map (~3.1 deg^2 across HSC + NIR surveys): check area coverage and limiting depths that set the LRD luminosity threshold.
• Color–color/SED selection panel: see how the ground-based blue coverage separates compact red candidates from dusty/low-z interlopers and sets the M5500 cut.
• Magellan/FIRE spectrum around Hα for DEEP23-z2LRD1: inspect the broad Hα (FWHM≈2400 km s^-1) with nearly symmetric narrow absorption and continuum shape.
• [OIII] λ5007 line profile for DEEP23-z2LRD1: verify the extremely narrow FWHM≈140 km s^-1 and lack of strong outflow wings, informing the low-mass host/BH interpretation.