stacked images on galaxy clusters, photo-z determination

June 1 2022

Single-frame 5 sigma point source depth for ATLAS is around 19.5, sky-limited. A stack of 100 images has 10X SNR so 5 sigma depth is 10X fainter, or 2.5 mag which gets us to 22nd. 

Two-band images give us a magnitude and a single color. Balmer break is at 364.5 nm. ATLAS band edges are:
blue edge of c: 420 nm, transition at z of 0.15
red edge of c and blue edge of c: 550 nm, transition at z of 0.51
red edge of c: 650 nm, transition at z of 0.81.

LRG's have absolute magnitudes of -22 to -23 (Tal et al MASS GROWTH AND MERGERS: DIRECT OBSERVATIONS OF THE LUMINOSITY FUNCTION OF LRG SATELLITE GALAXIES OUT TO z = 0.7 FROM SDSS AND BOSS IMAGES)

We can see m=M+mu=22=22 so threshold mu should be 44, corresponding to z of around 0.8. Wow. That's a detection threshold. For getting a decent color we need better than 10% photometry in each band so let's go out to 0.5 for now. 

Object	RA (J2000)	DEC (J2000)	z	notes
Abell 2218	16:35:52.4	+66:12:52	0.176
Abell 370	02:39:50	-01:35:08	0.375
PSZSPT J0117-5053	019.29320144653	-50.88529205322	unknown	Planck + SPT cluster but no SPT counterpart so likely low-z
PSZSPT J0109-4552	017.36057472229	-45.87871932983	0.02	really low-z, likely hard to pick out cluster members?

Typical cluster sizes are a few arcmin so a stacked image that is 1 deg x 1 deg would be plenty to get comparison regions as well. 

tasks: 

1) find ATLAS magnitudes of isolated SDSS LRG galaxies and construct photo-z estimator (like redmapper) for these objects.

2) Make star-galaxy discriminator perhaps by including WISE data? 

3) extract photometry and morphology from stacked images, perhaps add WISE or GAIA data to get another color?   

4) make color-magnitude diagram of plausible cluster members and look for LRG finger. 

5) run on entire e-Rosita Xray cluster catalog, for example https://www.aanda.org/articles/aa/full_html/2022/05/aa41120-21/aa41120-21.html