Counting Little Red Dots at <i>z</i> < 4 with Ground-based Surveys and Spectroscopic Follow-up
Yilun 逸伦 Ma 马, Jenny E. Greene, David J. Setton, Andy D. Goulding, Marianna Annunziatella, Xiaohui Fan, Vasily Kokorev, Ivo Labbe, Jiaxuan 嘉轩 Li 李, Xiaojing Lin, Danilo Marchesini, Jorryt Matthee, Lucy Robbins, Anna Sajina, Marcin Sawicki, O. Grace Telford
Abstract
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 ≲ 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.