On what scales does environment matter?
From NYU CCPP Wiki
This page is a description of Blanton et al. (2004d). We have here a version with high resolution figures.
Blanton et al. (2004) show that the relationship between the star formation histories of galaxies and their environments is fundamental, whereas the relationship between the structural properties of galaxies and environment is secondary. Here (Blanton et al. 2004) we investigate on what scales the environment of galaxies affect star-formation histories --- whether only the very local environment (say, within 1 Mpc/h) matters or whether the larger scale environment is important as well. We find that the larger scale environment is unimportant once one has specified the local environment. This result is encouraging for descriptions of galaxy formation which depend only on the host halo mass, since they assume that the larger scale environment is unimportant.
Blanton et al. (2004) describe the sample we use and its fundamental properties. Basically, it is a sample of low redshift (less than 0.05) galaxies from the SDSS. The advantage to using this sample is that it is volume limited for absolute magnitudes less than -18.5 in the r band. A volume limited sample which goes to such low luminosities provides a density of tracer galaxies much higher than those normally used for studies of this sort.
Our basic results are in the following figures. First, below is the average overdensity on scales of 1 Mpc/h as a function of luminosity and color:
Here we see the facts that color is important at low luminosity, and luminosity is important at high luminosity. In addition, the population of low luminosity red galaxies is in dense regions; these are primarily satellites in large groups (Berlind et al., in preparation).
Another way of looking at similar results is to consider the fraction of blue galaxies (relative to all galaxies) as a function of density. If we look at the scales 0.5, 1.0 and 6 Mpc/h scales separately, we naturally find that in denser regions the fraction of blue galaxies is smaller:
But these effects are not independent, necessarily: the fraction of blue galaxies may decline in large-scale overdensities simply because those regions contain preferentially more small-scale overdensities.
To test whether there is an independent relationship between the large-scale environment and galaxy color, we show as contours the blue fraction as a function of a small-scale overdensity (1 Mpc/h) and a large-scale overdensity (6 Mpc/h):
Here you can see that at a given density on small scales, there is no dependence of the blue fraction on large scale overdensity; that is, the contours are nearly perfectly vertical. This result indicates that the color of a galaxy depends only the number of nearby galaxies --- a number which traces well the mass of the host halo --- and not on the larger scale environment, just as assumed by many semi-analytic descriptions of galaxy formation.
This result contradicts some previous results in the literature. We show in the paper that the previous results suffer from low signal-to-noise ratio in their density estimates due to the low mean number density of their environment tracers. This finding emphasizes the importance (and difficulty) of having high signal-to-noise estimates of the density field.
