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COSMOS GALAXY AND X-RAY GROUP MEMBERSHIP CATALOGS
NASA/IPAC IRSA Release January 2012
Matt George and Alexie Leauthaud
mgeorge@astro.berkeley.edu, alexie.leauthaud@me.com
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The reference for this release is George et al. 2011 ApJ, 742, 125. 
There are two catalogs in this release, one for group properties and
one for galaxy properties. Information is drawn from a number of
sources:

-ACS imaging catalog (Leauthaud et al. 2007, updated)
-Photometric redshifts (Ilbert et al. 2009, 
     photoz_vers1.7_010809, pdzBay_v1.7_010809.out.gz) 
-X-ray group properties (Finoguenov et al. in prep. [also 2007],
     Xmass_chaxmm10_mie_ext6_id_update1010_zupdate.cat) 
-X-ray group galaxy members (George et al. 2011, ApJ, 742, 125)
-Spectroscopic redshifts (observed_targets_20110513.dat, M. Salvato, 
     used for "ZBEST" membership; redshifts not included in this release)

We include galaxies with MAG_AUTO<24.2 (ACS F814W band) and groups
with z<1. The galaxy magnitude cut is set by the K-band completeness
limit of ~24 since a detection is required for stellar mass
estimates. Photoz uncertainties also rise steeply beyond this
magnitude.

The galaxy catalog contains ACS photometric parameters, stellar
masses, photoz information, and group membership associations. The
group catalog contains X-ray properties, lensing-calibrated masses,
quality flags, aggregate member galaxy statistics, and coordinates for
the center of each group.


----------GALAXY CATALOG INFO----------

This is a group membership catalog drawn from the COSMOS ACS 
galaxy catalog, similar to the one presented in
Leauthaud et al 2007. The main difference between this catalog and the
Leauthaud et al 2007 one is that the raw ACS images have now been
corrected for the effects of charge transfer inefficiency (CTI, see
Massey et al. 2010 for further details). Since the CTI correction
scheme slightly changes the noise properties of the raw images, the
detections have also changed. For this reason, the GAL_ID field in this
catalog can not be used to match to objects in the 2007 catalog. The
pixel scale for this catalog is 0.03". To reference this ACS catalog
please reference Leauthaud 2007 with updates presented in Leauthaud et
al (in prep). This catalog is truncated at F814W (MAG_AUTO) < 24.2 due 
to the K-band completeness limit for stellar masses and because photoz 
uncertainties rise near this limit. Objects within ACS masks have also 
been removed (these are the same masks as in Leauthaud et al. 2007) 
and a variety of bad detections have been removed ("clean"=1 and 
"mu_class"=1) as well as galaxies without stellar masses.


----------SEXTRACTOR FIELDS----------

GAL_ID                   - unique number that identifies object for the catalog
MAG_AUTO            	- F814W total magnitude from Sextractor
RA			- from Sextractor
DEC			- from Sextractor


----------STELLAR MASS----------

Stellar mass calculated by Kevin Bundy (details can be found in
section 2.3 of Leauthaud et al. 2010,
http://adsabs.harvard.edu/abs/2010ApJ...709...97L). A Chabrier IMF is
assumed.

SM			- log of stellar mass
SM_ERR     		- error on stellar mass


----------GROUP PARAMETERS----------

Group parameters are described in George et al. 2011. 
Galaxies are associated with groups based on their
redshift and proximity to X-ray group positions, and assigned
probabilities for membership. Mock catalogs and spectroscopic tests
suggest that a membership cut of P>0.5 maximizes the purity and
completeness of the sample. One can also weight measured properties by
the probability that a galaxy belongs to a group. See group catalog
file for a description of group quality flags (group_flag = 1 is good).

GROUP_ID		- ID of group to which galaxy has maximum membership probability
GROUP_FLAG	     	- FLAG_INCLUDE value for GROUP_ID from group catalog
P_MEM			- probability that this galaxy is a member of GROUP_ID
MMGGs			- flag for "BCGs" (1 if galaxy is MMGGs of GROUP_ID)
DIST_MMGGs		- projected distance of object from MMGGs for GROUP_ID as a fraction of R200c

See group catalog for description of group center. From the lensing
signals stacked around different centers, MMGGs appears to be the
best tracer of the center of mass. The above fields are for member
galaxies selected by photozs only. We also select members using the
best available redshift (replacing photozs with speczs when
available). Each of the above group membership fields has a
corresponding entry for this photoz+specz selection, just append _ZBEST, e.g. 
GROUP_ID_ZBEST
GROUP_FLAG_ZBEST
...


----------PHOTOZ CATALOG FROM OLIVIER----------

Fields from Olivier Ilbert's photoz catalog v1.7
(photoz_vers1.7_010809). A matching radius of 1 arcsecond has been
used to match the catalogs.

ZPHOT			- photo-z measured using the galaxy templates. Median of the maximum likelihood distribution ("zPDF" in oliviers catalog)
ZPHOT_ERRLO         	- lower limit, 68% confidence level for zMinChi2
ZPHOT_ERRHI        	- upper limit, 68% confidence level for zMinChi2
MNUV_MR              	- Dust corrected color, M(NUV)-M(R) corrected from dust-extinction. Classification used in mass function paper: <1.2 for highly star-forming and >3.5 for quiescent



----------EXCEPTIONAL CASES AND FAQs----------

- MMGGs is not necessarily the most massive member of a group,
since a more massive galaxy can live in the outskirts. It is also not
necessarily the brightest member in any particular band.

- Selection based on spectroscopic redshifts and photometric redshifts
do not always agree so we use both approaches. If you want a uniformly
selected sample, use the photozs. If you favor purity over uniformity,
use the photoz+specz selection ("ZBEST") which replaces photozs with
speczs when available. 

- When a galaxy can belong to multiple groups, the highest membership
probability is saved to P_MEM and the corresponding group ID to
GROUP_ID. Central galaxies are exceptions. Each group has the most
massive galaxy near its X-ray position with membership probability
P>0.5 (MMGGs) locked into the group. P_MEM and GROUP_ID are not
altered for this galaxy, even if the redshift weighting produces a
higher membership probability for a different group. We see no
instances where a galaxy qualifies as MMGGs for multiple groups.

- Not all groups have a member galaxy near the X-ray position. These
groups are flagged and the algorithm currently selects no members.

- A small number of objects do not have photozs because of saturation
or deblending issues. When a spectroscopic redshift is available, we
replace the photoz PDF with the specz convolved with a gaussian of
width equal to the typical photoz uncertainty for the galaxy's
magnitude. In cases where a photoz does not exist because an object
falls in a ground-based imaging mask (e.g. near a bright foreground
star), we do not replace the redshift. Groups with significantly
masked areas are flagged. 

- To first order, the positional dependence of the membership
probability is a step function at R200. However, because the
probability depends on the relative group and field densities, there
is some positional dependence (a higher group density results in
higher membership probabilities for candidate members). If a group has
different centers chosen by the photoz and photoz+specz selections,
the group density estimate can change. Thus, there can be differences
between P_MEM and P_MEM_ZBEST even if a galaxy does not have
a specz. 


----------COSMOLOGICAL PARAMETERS (WMAP5)----------

Omega b		          0.0438000
Omega m 	       	  0.258000
Omega l  	       	  0.742000
H0    		       	  72.0000
Sigma_8  	       	  0.796000


----------EXTERNAL FILES USED----------

X-ray catalog				  - Xmass_chaxmm10_mie_ext6_id_update1010_zupdate.cat
X-ray SExtractor coords			  - cosmos_m46finalmie_ext5.txt
Photoz catalog				  - photoz_vers1.7_010809
Photoz PDFs				  - pdzBay_v1.7_010809.out.gz
Image mask (FLAG_MASK)			  - COSMOS_borders_F814W_mask.fits
Spectroscopic catalog                     - observed_targets_20110513.dat