6
Environmental effects on H i and star
formation properties of massive galaxies
that disks in very dense environments exhibit a deficiency of their Hi content (Giovanelli
& Haynes 1985), which decreases strongly toward the cluster cores (Gavazzi 1989). The trend in Hi depletion matched the predicted effect ofram pressure stripping, a mechanism first introduced by Gunn & Gott (1972). The cold gas stripping rate in galaxies moving through a dense intracluster medium (ICM) is proportional to the density of the ICM and the velocity with which the galaxy moves through it, i.e. ∝ρICMv2glx. Stripping is efficient in removing the gas when the ram pressure exceeds the gravitational force binding the ISM to the galaxy. This implies that a strong effect is expected in the dense, central regions of clusters. Interestingly, observations of Hi deficient galaxies that were still star forming showed normal molecular gas in their central regions (Kenney & Young 1989). A second mechanism explaining why cluster galaxies are gas-deficient that has been proposed is star-vation. Larson et al. (1980) suggested that the diffuse hot gas component which surrounds a galaxy is easily stripped by the ICM, thus preventing new material from cooling onto the disk and forming new stars. After a few Gyrs, the galaxy will then convert all its Hi into molecular gas, consume its reservoir, “starve” and stop forming stars.
Hydrodynamic mechanisms such ram-pressure stripping could not explain an eventual morphological transformation of spirals into early-type objects; more dramatic events like mergers between galaxies are required for such changes. Investigations were carried out of the dependencies of star formation on environment at fixed morphological type. Most early studies were restricted to a field/cluster comparison and results were controversial.
Some analyses found that galaxies in clusters exhibit reduced star formation (e.g. Ken-nicutt 1983; Dressler et al. 1985), while others found enhanced activity (e.g. KenKen-nicutt et al. 1984; Gavazzi & Jaffe 1985). It was still not possible to understand whether the observed changes were internally driven or caused by the external environment. In the late 90s it became clear that only with a careful parametrization of environmental den-sity and morphology the question could be properly addressed. Hashimoto et al. (1998) used a sample of 15,749 galaxies from the Las Campanas Redshift Survey to study the effects of environment on star formation using a three dimensional local density estima-tor. After removing the effect of the morphology - density relation using the concentration index (as objective morphological tracer), they proved that star formation rate of galax-ies with a given morphology is a continuous, decreasing function of increasing local density.
From 2003 till now, large optical surveys have provided complete samples of galaxies large enough to allow galaxies to be binned according to several properties at once. The first strong result of these investigations has been that galaxy evolution is to first order set
6.1 Introduction 93 by “nature” because the main properties of galaxies are determined by their stellar mass, and only weakly depend on environment (Kauffmann et al. 2003b; Tanaka et al. 2004).
Once stellar mass is fixed, an environmental “nurture” effect does modify their properties.
A second important result is that the correlation between colour/star formation and en-vironment is actually stronger than the one between morphology and enen-vironment (Ball et al. 2008; Bamford et al. 2009; Skibba et al. 2009). Dense environments are dominated by an increasing fraction of red and quiescent objects (Bower & Balogh 2004; Balogh et al.
2004, 2009).
Disentangling different physical processes, instead, has been more difficult. Pure op-tical data are not sufficient to investigate the physics behind the observations, because they do not provide complete information about different structural and physical param-eters. Models of galaxy evolution are currently the most successful tool we have to try to improve our understanding of the data. Semianalytical models (SAM) have been signifi-cantly improved with the aid of SDSS data, which have provided better and more detailed constraints on physics in the models. One of the most recent semi-analytic models (e.g.
Guo et al. 2011) includes only a starvation-like environmental effect in their recipes, which acts on galaxies once they enter the virial radius of a bigger halo and became “satellites”.
Strictly speaking, starvation is not a physical process, but the global effect of a variety of mechanisms that remove the hot gas in galaxies. In Guo’s SAMs, starvation is produced through both tidal and ram-pressure stripping of the hot component. To avoid confusion, here and throughout the chapter ram-pressure refers only to the stripping of the cold gas, and starvation to any mechanism removing the hot one. Models predict that, at fixed stellar mass, satellites are redder than the corresponding centrals because their star forma-tion is not sustained by fresh material. Once a similar division in satellites and centrals is applied to redshift surveys, observations and models are qualitatively consistent (van den Bosch et al. 2008; Weinmann et al. 2009; Kimm et al. 2009). Since the optical observations of the last decade pointed toward weak environmental mechanisms that were not strongly dependent on group mass and that extended out to large distances from the center of the halo (Bower & Balogh 2004; Balogh et al. 2004; Weinmann et al. 2006), and because these effects were reproduced by the models, starvation has come to be considered as the main driver of environmental evolution in galaxies. Ram-pressure stripping of the cold gas is currently considered unlikely to play a major role in shaping galaxies properties, at least in lower density environments. According to simulations, to observe a significant effect environments richer than Virgo are necessary (Quilis et al. 2000).
Although the Hi is a sensitive tracer of interactions with the ICM, current data are
still limited compared to what available in the optical. Samples are small because Hi observations are time consuming, and targeted environmental studies mainly focused on the field/cluster dichotomy. All observations have actually supported the stripping sce-nario, finding that the fraction of Hi-deficient galaxies increases in very-rich environments (Solanes et al. 2001; Boselli & Gavazzi 2009), along with the frequent presence of signa-tures of disturbed morphologies (Bravo-Alfaro et al. 2000; Chung et al. 2009). Comparison between dynamical models or SPH simulations and data (Abadi et al. 1999; Vollmer 2009) has shown that ram pressure can in fact be responsible for the observed distortion of the Hidisk in cluster galaxies. Observations of statistically significant samples are still lacking, however, so it has been difficult to quantify at what density or halo mass the effect sets in.
Fortunately, the astronomical community is currently putting a lot of effort in making a step forward in our picture of the Hicontent of galaxies, similar to what SDSS has done for the optical observations. The planned, next generation radio telescopes and instruments will make large, complete and deep Hi surveys feasible, so that the analysis of the cold gas depletion as a continuous function of the local density, and in bins of several properties, will be possible.
Current blind Hi surveys like ALFALFA do map the whole density range from void to rich clusters, but because they are shallow, the galaxies with Hi detections are biased against dense environments where objects are known to be gas poor. Nevertheless, with the large amount of data available from ALFALFA, we can use the stacking technique to perform the first systematic study of the cold gas content as a function of local density for a statistically significant sample. If we constrain the (relative) effect of local density on different galaxy properties, including Hi and star formation, we can gain insight into the processes at work, and try to address the still open issue of the dominant mechanisms acting at different environmental densities.
In this chapter, we study the relation between Hi and SF as a function of the envi-ronment. We start by presenting the local density tracer we use for our analysis (§6.2).
To compare star formation and cold gas dependence on environment, we measure average scaling relations for both parameters as a function of M?, because it is crucial to disentangle the effect of stellar mass and environment. In section 6.3 we present star formation rate scaling relations, and in section 6.4 the Hi ones. Finally, we constrain therelative effect of environment on galaxy properties. To gain insight into the processes at work, we compare our results with mock catalogues from semi-analytic models applied to the Millennium Simulation. (§6.5). Summary and discussion on future work are presented in Section 6.6.