The role of massive halos in the star formation history of the Universe

Abstract

Context. The most striking feature of the cosmic star formation history (CSFH) of the Universe is a dramatic drop in the star formation (SF) activity after z ~ 1. <BR /> Aims: In this work we investigate whether the very same process of assembly and growth of structures is one of the major drivers of the observed decline in the Universe's SF activity. <BR /> Methods: We study the contribution to the CSFH of galaxies in halos of different masses. This is done by studying the total SF rate-halo mass-redshift plane from redshift 0 to redshift ~1.6 in a sample of 57 groups and clusters by using the deepest available mid- and far-infrared surveys conducted with Spitzer MIPS and Herschel PACS and SPIRE, on blank (ECDFS, CDFN, and the COSMOS) and cluster fields. <BR /> Results: Our results show that low mass groups (M<SUB>halo</SUB> ~ 6 × 10<SUP>12</SUP>-6 × 10<SUP>13</SUP> M<SUB>☉</SUB>) provide a 60-80% contribution to the CSFH at z ~ 1. This contribution has declined faster than the CSFH in the past 8 billion years to less than 10% at z < 0.3, where the overall SF activity is sustained by lower mass halos. More massive systems (M<SUB>halo</SUB> > 6 × 10<SUP>13</SUP> M<SUB>☉</SUB>) provide only a marginal contribution (<10%) at any epoch. A simplified abundance-matching method shows that the large contribution of low mass groups at z ~ 1 is due to a large fraction (>50%) of very massive, highly star-forming main sequence galaxies. Below z ~ 1 a quenching process must take place in massive halos to cause the observed faster suppression of their SF activity. Such a process must be a slow one, though, since most of the models implementing a rapid quenching of the SF activity in accreting satellites significantly underpredict the observed SF level in massive halos at any redshift. This would rule out short time-scale mechanisms such as ram pressure stripping. Instead, starvation or the satellite's transition from cold to hot accretion would provide a quenching timescale of 1 to few Gyr that is more consistent with the observations. <BR /> Conclusions: Our results suggest a scenario in which, owing to the structure formation process, more and more galaxies experience the group environment and the associated quenching process in the past 8 billion years. This leads to the progressive suppression of their SF activity so that it shapes the CSFH below z ~ 1. <P />Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.