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

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 ≲ z ≲ 4, combining ultradeep Hyper Suprime-Cam photometry and various (near-)infrared surveys within a total area of ∼3.1 deg 2 . We find that for LRDs with M 5500 < −22.5, their number density declines from ∼10 −4.5 cMpc −3 at z > 4 to ∼10 −5.3 cMpc −3 at 2.7 < z < 3.7 and ∼10 −5.7 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 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 FWHM ≈ 2400 km s −1 and with nearly symmetric narrow H α absorption. Additionally, DEEP23-z2LRD1 has extremely narrow [O iii ] lines with FWHM ≈ 140 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.