The X-ray and radio activity of typical and luminous Ly α emitters from z ∼ 2 to z ∼ 6: evidence for a diverse, evolving population

Abstract

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 42.4 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 10 44.07±0.01 erg s −1 and an average black hole accretion rate (BHAR) of 0.42 ± 0.01 M 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 L * LyαLAEs. 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< 0.009 M 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 +6.6 −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.

J-PLUS: Unveiling the brightest end of the Lyαluminosity function at 2.0 <z< 3.3 over 1000 deg2

“…Further support to this picture is provided by the tomographic analysis of the high-z Lyα LF in the COSMOS field performed by Sobral et al (2018b) by using a combination of optical, infrared and X-Ray data. In their work, the major contribution to the LF at L Lyα 10 43 is provided by sources showing X-Ray loud counterparts, thus likely to be AGN (see also e.g., Matthee et al 2017b;Calhau et al 2020). Their work shows how this contribution completely vanishes at z 3.5, thus paralleling the peak of AGN activity usually observed at z ∼ 2 − 3 (e.g., Hasinger et al 2005;Miyaji et al 2015).…”

“…Recent compelling hints point towards identifying the majority of high-z Lyα-emitting sources at L Lyα > 2 × 10 43 erg s −1 as AGN/QSOs (see e.g., Nilsson et al 2011;Konno et al 2016;Matthee et al 2017b;Sobral et al 2018a,b;Calhau et al 2020). The work of Sobral et al (2018a), in particular, pointed out the co-existence of two different classes of luminous z ∼ 2 − 3 LAEs at roughly 3 L * , namely dust-free, highly star-forming galaxies and AGN.…”

“…On the other hand, the bright-end of the Lyα LF is populated by AGN/QSOs (Calhau et al 2020) and rare, bright and SFbursty Lyα-emitting systems (e.g., Matsuda et al 2011;Bridge et al 2013;Cai et al 2017bCai et al , 2018. Current constraints at high Lyα luminosity are somewhat poor, given the relatively small cosmological volumes probed by past works focused specifically on detecting high-z Lyα-emitting sources (e.g., Fujita et al 2003;Blanc et al 2011;Herenz et al 2019).…”

Distinguishing AGN from starbursts as the origin of double-peaked Lyman-alpha emitters in the reionization era

“…We can estimate the extent of this obscuration by comparing the derived properties of the AGN to similar quasars at these redshifts. Calhau et al (2020) studied the radio and X-ray properties of ∼4000 LAEs in the COSMOS SC4K field over 2 < z < 6 and found a correlation between the hardness ratio, which determines the level of obscuration, and the X-ray luminosity of X-ray detected LAEs, with the more luminous X-ray LAEs being observed to have smaller hardness ratios. Our estimate for the intrinsic X-ray luminosity of COLA-1 places it above the hardness 0 category when compared to the Calhau et al (2020) sample, with properties similar to those of obscured AGN discovered in the COSMOS field by Marchesi et al (2016).…”

“…Calhau et al (2020) studied the radio and X-ray properties of ∼4000 LAEs in the COSMOS SC4K field over 2 < z < 6 and found a correlation between the hardness ratio, which determines the level of obscuration, and the X-ray luminosity of X-ray detected LAEs, with the more luminous X-ray LAEs being observed to have smaller hardness ratios. Our estimate for the intrinsic X-ray luminosity of COLA-1 places it above the hardness 0 category when compared to the Calhau et al (2020) sample, with properties similar to those of obscured AGN discovered in the COSMOS field by Marchesi et al (2016). It was found that for AGN having log L X /ergs s −1 > 44.1 in the 2-10 keV range, obscured sources are two to three times as abundant as unobscured ones at z ∼ 6−6.5 (Marchesi et al 2016).…”