EVIDENCE FOR THE COLD-STREAM TO HOT-ACCRETION TRANSITION AS TRACED BY Lyα EMISSION FROM GROUPS AND CLUSTERS AT 2<z<3.3

We present Keck Cosmic Web Imager (KCWI) observations of giant Lyα 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 Lyα 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, 5σ 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 2.6σ 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 photo-ionization from SFR and AGNs; this is also supported by energetics considerations. Below Mstream (cold-stream regime) we measure LLyα/BAR = 1040.51±0.16 erg s−1 M−1 yr, consistent with predictions, and SFR/BAR= 10−0.54±0.23: on average 30+20 −10% of the cold streams go into stars. Above Mstream (hot-accretion regime), LLyα 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. Subject headings: galaxies: evolution — galaxies: formation — galaxies: clusters: general — galaxies: highredshift