The X-ray and radio activity of typical and luminous Lyα emitters

Despite recent progress in understanding Lyα emitters (LAEs), relatively little is known regarding their typical black hole activity across cosmic time. Here, we study the X-ray and radio properties of∼4000 LAEs at 2.2 < z < 6 from the SC4K survey in the COSMOS field. By exploring deep Chandra Legacy data, we reach an X-ray luminosity of ∼ 10 erg s−1 for the deepest, full sample stack of ∼480 Ms. We detect 254 (6.8% ± 0.4%) of the LAEs individually in the X-rays (S/N>3) and find an average luminosity of 1044.07±0.01 erg s−1 and an average black hole accretion rate (BHAR) of 0.42 ± 0.01M yr−1, consistent with moderate to high accreting AGN. We detect 120 sources in deep radio data, implying a LAE radio AGN fraction of 3.2% ± 0.3%. Approximately half of the LAEs detected in the radio are also detected in the X-rays, resulting in a total AGN fraction of 8.6%± 0.4% for &LLyα LAEs. The total AGN fraction rises with Lyα luminosity and declines with increasing redshift. For AGN, we find that Lyα luminosities correlate with the BHARs, suggesting that Lyα luminosity becomes an accretion rate indicator. Most LAEs (93.1% ± 0.6%) at 2 < z < 6 have no detectable X-ray emission (BHARs< 0.009M yr−1). We estimate the median star formation rate (SFR) of star-forming LAEs from Lyα and radio luminosities and find a SFR of 5.6 −2.8 M yr −1. The black hole to galaxy growth ratio (BHAR/SFR) for LAEs is < 0.0016, consistent with typical star forming galaxies and the local BHAR/SFR relation. We conclude that LAEs at 2 < z < 6 include two different populations: an AGN population, where Lyα luminosity traces BHAR, making them bright in Lyα, and another with low SFRs which remain undetected in even the deepest X-ray stacks but is detected in the radio stacks.