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 Ly α halos surrounding nine galaxy groups and clusters at 2 < z < 3.3, including five new detections and one upper limit. We find observational evidence for the cold-stream to hot-accretion transition predicted by theory by measuring a decrease in the ratio between the spatially extended Ly 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, 5 σ 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 2.6 σ and 1.3 σ , respectively. The higher scatter in these tracers suggests the Ly α 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 Ly α /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 Ly α 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.