A new sample of Little Red Dots at $z<0.45$ in DESI DR1: Broad Balmer lines, low ionization spectrum and no variability

TL;DR

This study reports the discovery of eight low-redshift Little Red Dots (LRDs) with broad Balmer lines and low variability in DESI DR1, providing insights into supermassive black hole evolution at recent cosmic times.

Contribution

First ground-based spectroscopic identification of low-redshift LRDs, revealing their spectral features, low variability, and low number density compared to high redshift counterparts.

Findings

Eight low-redshift LRDs discovered with spectral features similar to high-redshift LRDs.

Most sources show weak or no variability over 4-17 years in the rest-frame.

Number density of low-redshift LRDs is about 10,000 times lower than at high redshift.

Abstract

JWST has unveiled an abundant population of compact broad-line emitters largely at $z\gtrsim4$, the Little Red Dots (LRDs), which might represent a previously unprobed supermassive black hole evolution channel predominant at high redshift. However, the LRDs have remained mostly elusive at lower redshift ($z\lesssim2$) where detailed studies are possible from ground-based observatories. We searched for low-redshift LRDs in the Dark Energy Spectroscopic Instrument (DESI) survey. Our search is primarily based on emission line properties, as opposed to earlier approaches that searched for compact sources with specific photometric spectral energy distributions. We report the discovery of eight LRDs at $z=0.2-0.45$, which show spectral features akin to the high-redshift LRDs in the rest-frame optical. The sources are characterized by broad Balmer lines, steep Balmer decrements, compact morphologies, Balmer absorption features and/or strong He I emission, but weak or absent He II, [Ne V] or other high excitation lines typical of Type I AGN. For 7 out of 8 sources, we retrieve dense-cadence light curves from time-domain surveys and for most sources we find weak to no intrinsic variability ($0.0-0.1$ mag) over $4-17$ years in the rest-frame. We also highlight the identification of a quasar with similar Balmer line profiles as LRDs, but shows differences in Balmer decrement, significant variability, and high-ionisation lines. Given the effective volume $4.9{\rm Gpc^3}$ covered by DESI DR1 at $z<0.45$, our sample corresponds to a number density of $1.6\times10^{-9}$Mpc$^{-3}$, indicating a number density $\sim$10,000 times lower than in the first billion years of cosmic time. We find a dearth of luminous and red LRDs at $z<1$ compared to higher-redshift, which could suggest lower gas feeding rates of LRD activity due to higher metallicities at later cosmic epochs.