Evidence for Cold-stream to Hot-accretion Transition as Traced by Ly{\alpha} Emission from Groups and Clusters at 2 < z < 3.3

TL;DR

This study provides observational evidence for the transition from cold to hot gas accretion in galaxy groups and clusters at high redshift, using Lyα emission to trace the process and its impact on star formation and AGN activity.

Contribution

It offers the first direct observational evidence of the cold-stream to hot-accretion transition at high redshift through Lyα emission measurements, confirming theoretical predictions.

Findings

Decreased Lyα luminosity to baryonic accretion rate ratio with increasing halo mass.

Approximately 30% of cold streams convert into stars.

Lyα emission correlates with cold accretion rather than outflows or photo-ionization.

Abstract

We present Keck Cosmic Web Imager (KCWI) observations of giant Lya halos surrounding 9 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 Lya luminosity and the expected baryonic accretion rate (BAR), with increasing elongation above the transition mass Mstream). This implies a modulation of the share of BAR that remains cold diminishing quasi-linearly (logarithmic slope of 0.97+-0.19, 5 sigma significance) with the halo to Mstream mass ratio. The integrated star-formation rates (SFRs) and AGN bolometric luminosities display a potentially consistent decrease, albeit significant only at 2.6 sigma and 1.3 sigma, respectively. The higher scatter in these tracers suggests the Lya emission might be mostly a direct product of cold accretion in these structures rather than indirect, mediated by outflows and photo-ionization from SFR and AGNs; this is also supported by energetics considerations. Below Mstream (cold-stream regime) we measure LLya/BAR=10^{40.51+-0.16}~erg/s/Msun*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 Mstream (hot-accretion regime), LLya is set by Mstream (within 0.2~dex scatter in our sample), independent of the halo mass but rising tenfold from z=2 to 3.