Elliptical Galaxy Formation
I developed an image-processing method to select high-quality elliptical galaxies based on their smooth surface brightness profile. Form the SDSS survey, I selected a sample of about 2000 elliptical galaxies at z<0.05. This is by far the largest high-fidelity elliptical sample that I know of. The figure below shows 20 ellipticals randomly selected from the sample. The data is available on my old data website:
This sample enables systematic studies of the stellar populations of elliptical galaxies. In Zhu et al. (2010), I presented a comprehensive study on the dependence of the star-formation history on mass and environment. The two figures on the first row below show the mass dependence of the SED. The SED is redder with stronger stellar absorption for higher-sigma (higher-mass) ellipticals, showing they are older, more metal rich, and have higher alpha-abundance ratio. It means more massive ellipticals formed earlier and in a shorter period of time as well.
The two figures on the second row below show the environmental dependence of the SED. The SED is slightly redder with slightly stronger stellar absorption for ellipticals in richer groups. The dependence is weak but significant. It means ellipticals (at a given mass) host slightly older and more metal rich stellar populations.
You can learn more in Zhu et al. (2010).
And here is a bonus dual-core galaxy I found while analyzing hundreds of thousands of images.
