A similar dataset to that used for the bivariate luminosity functions was studied to examine the relationship between the u-r color, galaxy morphology and galaxy density environment, including residual relations to ascertain which is more fundamental.
A paper detailing the results is available at MNRAS 383 907 and astro-ph/0610171.
Here the color is the restframe model u-r color from the Fourth Data Release of the Sloan Digital Sky Survey. The morphologies are those assigned from artificial neural networks , the Petrosian inverse concentration index and the Sersic index of the light profile. The environmental density is that 5th nearest neighbor within a redshift slice of +/- 1000 km/s, the surface density sigma5. The magnitude limits are r < 17.77, -22.5 < M_r - 5logh < -19.5 for nearest-neighbor galaxies and -22.5 < M_r - 5logh < -17.5 for galaxies for which sigma5 is measured. The brighter M_r corresponds to an apparent r < 17.77 at the limiting redshift of z < 0.0889. This prevents densities biased low by Malmquist bias at higher redshifts in the sample.
In agreement with previous results, we find that the color is a more predicitive indicator of environment, and that there is no evidence for a residual relation in morphology once the relation due to color is removed.
We also find that the robust bimodality displayed in color is less clear in morphology, indicating either insensitivity to the process that causes the bimodality in colour (an numerous other quantities), a larger overlap of the populations, or a third (or more) population in transition between the other two.
Our results are consistent with other studies in which the bimodality is hypothesized to be caused by bulges and disks, formed by merger and accretion. The third population may then be S0 galaxies.
Future work may include adding the star formation rate to the analysis and the use of marked correlation functions as a more general and less arbitrary description of envionment, e.g. sigma5 can correspond to intra- or inter-halo galaxy separations. One could also analyze the galaxy population in terms of bulges and disks instead of galaxy 'types'.
Last modified: Mar 27th 2008