------------------------------------------------------------ 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 ------------------------------------------------------------ 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