environment indicators

I worked on galaxy envionment indicators today.

If you are measuring galaxy environments, you can choose from some options: You can choose a measure of local density that either (1) has a fixed physical length scale (radius) or else (2) is high in signal-to-noise. The reason for this dichotomy is that the former are done, basically, by counting the (necessarily small) number of galaxies in the relevant local volume, while the latter uses something like the Nth closest or a clustocentric distance or tesselation, where the scale is set by what gives good signal-to-noise (and is therefore different in different environments). We have shown that a fixed scale of 1 Mpc is about the best possible scale, but you can't measure it at high signal-to-noise.

After this, you can choose a measure that either (a) uses redshift information or else (b) works with imaging data alone. The latter allows you to go fainter into the luminosity function, and work with cheaper (and more readily available) data, but it costs you a lot in signal-to-noise, because you have (even with good photometric redshifts) a lot of projected galaxies (either foreground or background). Purely imaging indicators also have the problem that they almost never have systematics that are constant with redshift, since foreground/background and photometric redshifts all vary strongly with redshift for reasonable (read: the usual) choices that can be made.