Properties of galaxy groups in the Sloan Digital Sky Survey — I. The dependence of colour, star formation and morphology on halo mass

Simone M. Weinmann 1 2 Frank C. van den Bosch 0 2 Xiaohu Yang 3 H. J. Mo 3 0 Max-Planck-Institute for Astronomy , Konigstuhl 17, D-69117 Heidelberg, Germany 1 Institute for Theoretical Physics, University of Zurich , CH-8057 Zurich, Switzerland 2 Department of Physics, Swiss Federal Institute of Technology , ETH Honggerberg, CH-8093 Zurich, Switzerland 3 Department of Astronomy, University of Massachussets , 710 North Pleasant Street, Amherst, MA 01003-9305, USA A B S T R A C T Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey Data Release 2, we investigate the correlation between various galaxy properties and halo mass. We split the population of galaxies in early-types, late-types and intermediate-types, based on their colour and specific star formation rate. At fixed luminosity, the late- (early-)type fraction of galaxies increases (decreases) with decreasing halo mass. Most importantly, this mass dependence is smooth and persists over the entire mass range probed, without any break or feature at any mass-scale. We argue that the previous claim of a characteristic feature on galaxy group scales is an artefact of the environment estimators used. At fixed halo mass, the luminosity dependence of the type fractions is surprisingly weak, especially over the range 0.25 L /L 2.5: galaxy type depends more strongly on halo mass than on luminosity. In agreement with previous studies, the late- (early-)type fraction increases (decreases) with increasing halocentric radius. However, we find that this radial dependence is present in haloes of all masses probed (down to 1012 h1 M ), while previous studies did not find any radial dependence in haloes with M 1013.5 h1 M . We argue that this discrepancy owes to the fact that we have excluded central galaxies from our analysis. We also find that the properties of satellite galaxies are strongly correlated with those of their central galaxy. In particular, the early-type fraction of satellites is significantly higher in a halo with an early-type central galaxy than in a halo of the same mass but with a late-type central galaxy. This phenomenon, which we call 'galactic conformity', is present in haloes of all masses and for satellites of all luminosities. Finally, the fraction of intermediate-type galaxies is always 20 per cent, independent of luminosity, independent of halo mass, independent of halocentric radius, and independent of whether the galaxy is a central galaxy or a satellite galaxy. We discuss the implications of all these findings for galaxy formation and evolution. - The local population of galaxies consists roughly of two types: red galaxies, which reveal an early-type morphology and which have little or no ongoing star formation, and blue galaxies with active star formation and a late-type morphology. The case for two distinct classes of galaxies has recently been strengthened as the use of large galaxy redshift surveys has shown that the distributions of colour and star formation rate (SFR) of the galaxy population are bimodal (e.g. Strateva et al. 2001; Blanton et al. 2003b; Kauffmann et al. 2003; Baldry et al. 2004; Balogh et al. 2004a,b; Brinchmann et al. 2004; Kauffmann et al. 2004). In addition, studies at intermediate redshifts have shown that this bimodality exists at least out to z 1 (e.g. Bell et al. 2004; Tanaka et al. 2005; Weiner et al. 2005), but with different fractions of galaxies on both sides of the bimodality scale compared to z = 0 (Bell et al. 2004; Faber et al. 2005). An important, and largely open question in galaxy formation regards the origin of this bimodality. In particular, does this bimodality arise early on (the nature scenario), or is it a consequence of various physical processes that operate over a Hubble time (the nurture scenario)? In particular, are there two distinct formation channels, or are galaxies being transformed from one type to the other? In the latter case we need to know where, how and when these transformations occur. Important hints come from the observed correlations between galaxy properties and environment: galaxies in dense environments (i.e. clusters) have predominantly early-type morphologies (e.g. Oemler 1974; Dressler 1980; Whitmore, Gilmore & Jones 1993) and low SFRs (e.g. Balogh et al. 1997, 1999; Poggianti et al. 1999). At first sight this seems to suggest that cluster-specific processes, such as galaxy harassment (Moore et al. 1996), ram-pressure stripping (Gunn & Gott 1972) and/or interactions with the cluster potential (Byrd & Valtonen 1990) play a dominant role in transforming galaxy morphologies from late- to early-types, and in truncating their SFRs. However, starting with the work of Postman & Geller (1984), it has become clear that the environmental dependence of galaxy properties is not restricted to clusters, but smoothly extends to the scale of galaxy groups (see also Zabludoff & Mulchaey 1998; Tran et al. 2001). Consequently, it has been suggested that group-specific processes are of paramount importance for transforming galaxies. In particular, the relatively low velocity dispersion of groups implies that galaxygalaxy merging, which can transform disc galaxies into ellipticals (e.g. Toomre & Toomre 1972), is effective. In addition, as soon as a galaxy becomes a group member, i.e. becomes a satellite of a bigger system, it is deprived of its reservoir of hot gas. Consequently, it is expected that, after a delay time in which the galaxy consumes (part of) its cold gas, star formation in the galaxy comes to a halt (Larson, Tinsley & Caldwell 1980; Balogh, Navarro & Morris 2000). This process, often called strangulation, provides a natural explanation for the increasing fraction of red galaxies towards denser environments. Much of the earlier work on the relation between galaxy properties and environment was based on incomplete samples of clusters and groups. With the advent of large, homogeneous galaxy surveys, it has become possible to investigate this relation in far more detail, and over a much wider range of environments. In particular, using the Las Campanas Redshift Survey (LCRS; Shectman et al. 1996), the Two-degree Field Galaxy Redshift Survey (2dFGRS; Colless et al. 2001) and the Sloan Digital Sky Survey (SDSS; York et al. 2000; Stoughton et al. 2002) various authors have investigated the relation between environment and morphology (e.g. Hashimoto & Oemler 1999; Goto et al. 2003; Kuehn & Ryden 2005), between environment and SFR (e.g. Hashimoto et al. 1998; Domnguez et al. 2002; Lewis et al. 2002; G omez et al. 2003; Balogh et al. 2004a; Tanaka et al. 2004; Kelm, Focardi & Sorrentino 2005), and between environment and colour (e.g. Balogh et al. 2004b; Hogg et al. 2004; Tanaka et al. 2004). One of the numerous results that have emerged from these studies is that galaxy properties only seem to correlate (significantly) with environment above a characteristic surface density, which is roughly consistent w (...truncated)