LoCuSS: THE STEADY DECLINE AND SLOW QUENCHING OF STAR FORMATION IN CLUSTER GALAXIES OVER THE LAST FOUR BILLION YEARS

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/MIPS 24µm data with extensive spectroscopy of cluster members. The specific-SFRs of massive (M 10 10 M ⊙ ) star-forming cluster galaxies within r 200 are found to be systematically ∼28% lower than their counterparts in the field at fixed stellar mass and redshift, a difference significant at the 8.7σ 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 star formation rates (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 r 200 with SFRs>3 M ⊙ yr −1 , of the form f SF ∝(1 + z) 7.6±1.1 . We dissect the origins of the Butcher-Oemler effect, revealing it to be due to the combination of a ∼3× decline in the mean specific-SFRs of star-forming cluster galaxies since z∼0.3 with a ∼1.5× 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. The observed sharp decline in star formation activity among cluster galaxies since z∼0.4 likely reflects the increased susceptability of low-redshift spiral galaxies to gas removal mechanisms as their gas surface densities decrease with time. We find no evidence for the build-up of cluster S0 bulges via major nuclear star-burst episodes.

The evolution of galaxy star formation activity in massive haloes

“…Figure 6 shows the comparison of the Guo et al (2014) IR LF in groups at z < 0.1 with the IR LF of the Shapley supercluster studied in Haines et al (2010), which as shown in Haines et al (2013), is consistent with the IR LF of the Coma cluster and A3266 studied by Bai et al (2006) and Bai et al (2009), respectively. Our IR LF in groups at 0.1 < z < 0.4 is compared with the stacked IR LF of 30 clusters observed at 0.15 < z < 0.30 in the LoCuSS survey (Haines et al 2013). Our IR LF in groups at 0.4 < z < 0.8 is compared with the stacked IR LF of six rich clusters at 0.6 < z < 0.8 studied in Finn et al (2010).…”

“…The group galaxy IR LF shows a smoother decline at high luminosity which is more consistent with a Schechter or a modified Schechter function than with a double power law. The double power law predicts too many bright galaxies at the bright end, especially in the two lowest redshift bins, where we observe a rather fast decline of the LF Saunders et al (1990) for the composite IR LF of our group sample, the local mean IR LF derived from the work of Guo et al (2014), and for the cluster IR LF of Haines et al (2013), respectively.…”

“…The main difference Fig. 4: blue for the nearby groups of Guo et al (2014) and the Shapley superclusters (Haines et al 2010) at z < 0.1, black for 0.1 < z < 0.4 groups and 0.15 < z < 0.3 LoCuSS clusters (Haines et al 2013), and green for 0.4 < z < 0.8 groups and for the 0.6 < z < 0.8 rich cluster LF of Finn et al (2010). For clarity we only show here the best fit derived with the modified Schechter function of Saunders et al (1990). between the group and the cluster LF is obviously the normalization, since the clusters are much richer, so they contain many more star-forming and IR-emitting galaxies.…”

Spectacular tails of ionized gas in the Virgo cluster galaxy NGC 4569

“…It is also supported by the observations of several clusters at intermediate redshift (e.g. Haines et al 2013Haines et al , 2015. In its current form, however, this scenario over-predicts the number of red dwarf galaxies compared to what is observed in nearby clusters (Kang & van den Bosch 2008;Font et al 2008;Kimm et al 2009;De Lucia 2011;Weinmann et al 2011;Taranu et al 2014).…”

SATELLITE QUENCHING AND GALACTIC CONFORMITY AT 0.3 < z < 2.5*

“…At low redshift, it has been argued that environmental processes shut down star formation in satellites over a long timescale of ∼2-7 Gyr in order to explain the distribution of satellite quiescent fractions (e.g., Balogh et al 2004;Finn et al 2008;Weinmann et al 2009;McGee et al 2011;De Lucia et al 2012;Haines et al 2013;Wetzel et al 2013). The reduced quenching of satellites at higher redshifts (Figure 7) is therefore expected, since satellites will not have had time to quench; however, these timescales are still too long to explain the existence of quenched satellites at these higher redshifts.…”