Evidence for Cold-stream to Hot-accretion Transition as Traced by Lyα Emission from Groups and Clusters at 2 < z < 3.3

Abstract

We present Keck Cosmic Web Imager observations of giant Lyα halos surrounding nine galaxy groups and clusters at 2 < z < α luminosity and the expected baryonic accretion rate (BAR), with increasing elongation above the transition mass (M_(stream)). This implies a modulation of the share of BAR that remains cold, diminishing quasi-linearly (logarithmic slope of 0.97 ± 0.19, 5σ significance) with the halo to M_(stream) mass ratio. The integrated star formation rates (SFRs) and active galactic nucleus (AGN) bolometric luminosities display a potentially consistent decrease, albeit significant only at 2.6σ and 1.3σ, respectively. The higher scatter in these tracers suggests the Lyα emission might be mostly a direct product of cold accretion in these structures rather than indirect, mediated by outflows and photoionization from SFR and AGNs; this is also supported by energetics considerations. Below M_(stream) (cold-stream regime), we measure L_(Lyα)/BAR = 10^(40.51 ± 0.16) erg s⁻1 M_⊙⁻¹ yr, consistent with predictions, and SFR/BAR = 10^(−0.54 ± 0.23): on average, 30₋₁₀⁺²⁰% of the cold streams go into stars. Above M_(stream) (hot-accretion regime), L_(Lyα) is set by M_(stream) (within 0.2 dex scatter in our sample), independent of the halo mass but rising 10-fold from z = 2 to 3.