LoCuSS: The steady decline and slow quenching of star formation in cluster galaxies over the last four billion years (1307.1135v1)

Abstract: We present an analysis of the levels and evolution of star formation activity in a representative sample of 30 massive galaxy clusters at 0.15<z\<0.30 from the Local Cluster Substructure Survey (LoCuSS), combining wide-field Spitzer 24um data with extensive spectroscopy of cluster members. The specific-SFRs of massive (M\>10^10 M_sun) star-forming cluster galaxies within r200 are found to be systematically 28% lower than their counterparts in the field at fixed stellar mass and redshift, a difference significant at the 8.7-sigma level. This is the unambiguous signature of star formation in most (and possibly all) massive star-forming galaxies being slowly quenched upon accretion into massive clusters, their SFRs declining exponentially on quenching time-scales in the range 0.7-2.0 Gyr. We measure the mid-infrared Butcher-Oemler effect over the redshift range 0.0-0.4, finding rapid evolution in the fraction (f_SF) of massive (M_K<-23.1) cluster galaxies within r200 with SFRs>3M_sun/yr, of the form f_SF (1+z)^7.6. We dissect the origins of the Butcher-Oemler effect, revealing it to be due to the combination of a ~3x decline in the mean specific-SFRs of star-forming cluster galaxies since z~0.3 with a ~1.5x decrease in number density. Two-thirds of this reduction in the specific-SFRs of star-forming cluster galaxies is due to the steady cosmic decline in the specific-SFRs among those field galaxies accreted into the clusters. The remaining one-third reflects an accelerated decline in the star formation activity of galaxies within clusters. The slow quenching of star-formation in cluster galaxies is consistent with a gradual shut down of star formation in infalling spiral galaxies as they interact with the intra-cluster medium via ram-pressure stripping or starvation mechanisms. We find no evidence for the build-up of cluster S0 bulges via major nuclear star-burst episodes.

The Slow Quenching of Star Formation in Cluster Galaxies

Haines et al. investigate the decline in star formation within galaxies in massive clusters over the past four billion years, utilizing a sample of 30 galaxy clusters from the Local Cluster Substructure Survey (LoCuSS). The research combines mid-infrared data from Spitzer/MIPS at 24μm with extensive spectroscopic data to analyze star-forming activity among cluster galaxies, comparing this with field galaxies over the same period.

A notable finding of the paper is the specific star formation rates (sSFR) of massive cluster galaxies (M ≥ 10^10 M_⊙) which are found to be approximately 28% lower compared to their field counterparts, with this difference significant at an 8.7σ level. This suggests that star formation in these cluster galaxies is quenching upon their accretion into the cluster environment. The decline in sSFR is estimated to occur exponentially over quenching time scales of approximately 0.7 to 2.0 Gyr, indicative of a slow quenching process rather than an abrupt cessation.

The authors also measure the mid-infrared Butcher-Oemler effect from redshifts 0.0 to 0.4, noting rapid evolution in the fraction of star-forming massive cluster galaxies within the virial radius. This is characterized by a decline in both their sSFR and number density, with two-thirds of the reduction attributed to the cosmic decline in sSFRs among field galaxies accreting into clusters, and one-third due to an accelerated decline within the clusters themselves, suggesting enhanced environmental quenching in clusters as opposed to the field.

The slow quenching observed could be attributed to mechanisms like ram-pressure stripping or starvation which gradually deplete the star-forming gas within spiral galaxies as they encounter the intra-cluster medium. The observed sharper decline in star formation activity since z~0.4 may reflect an increased susceptibility of low-redshift spiral galaxies to such gas removal mechanisms due to decreased gas surface densities over time.

While the research observes a systematic suppression of star formation in cluster galaxies, it finds no evidence for a significant population of starburst galaxies indicative of triggered star formation events or mergers. Therefore, the transformation of galaxies from star-forming spirals to S0s in clusters likely lacks the pronounced starburst phases, particularly in the low-redshift universe.

In future work, the implications for galaxy evolution models include refining the environmental quenching processes and examining the interaction of galaxies with the cluster medium in even greater detail. This underscores the complex interplay between galaxy accretion, cluster dynamics, and star formation processes which shape galaxy evolution in dense environments.