------------------------------------------------------------ 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. ----------GROUP CATALOG INFO---------- This is a COSMOS X-ray group membership catalog, combining X-ray group properties from Finoguenov et al. in prep. (also 2007) (catalog Xmass_chaxmm10_mie_ext6_id_update1010_zupdate.cat) with estimates for masses and radii calibrated from weak lensing (Leauthaud et al. 2010), and member galaxy information (George et al. 2011). Group redshifts have been determined by searching for red sequence overdensities within 500 kpc of the X-ray centers and are refined by using spectroscopic redshifts when available. We use groups with z<1 to ensure good optical identifications and small photoz uncertainties. ----------BASIC PROPERTIES AND X-RAY MEASUREMENTS---------- GROUP_ID - Group ID from X-ray catalog REDSHIFT - Group redshift (specz when available, red sequence otherwise) REDSHIFT_ERR - Group redshift uncertainty LX - log10[X-ray luminosity 0.1-2.4 keV], from Alexis' catalog LX_ERR - log10[uncertainty in LX], from Alexis' catalog LX_SCALE - log10[X-ray luminosity/E(z)], from Alexis' catalog XRAY_FLUX - log10[extrapolated flux (0.5-2 keV band) within R500 in ergs/s/cm^2], from Alexis' catalog XRAY_FLUX_ERR - log10[error on the flux], from Alexis' catalog RA - X-ray RA from Sextractor ellipse (cosmos_m46finalmie_ext5.txt) DEC - X-ray Dec from Sextractor ellipse (cosmos_m46finalmie_ext5.txt) XRAY_POS_ERR - X-ray position uncertainty (32" for XFLAG>1, 32"/significance for XFLAG=1) NAME - IAU formatted name COSMOS CL JHHMMSS.S+DDMMSS using coordinates from Alexis' catalog ----------GROUP QUALITY FLAGS---------- Alexis' catalog includes a quality flag based on the confidence in the optical identification. We include a few additional flags to identify cases where two groups are overlapping and membership identification is unclear (FLAG_MERGER), where we do not find a significant concentration of photoz members (FLAG_POOR), and where member selection may be affected by masks (FLAG_MASK). FLAG_INCLUDE is meant to combine these quality flags into a single selection. XFLAG - quality flag from X-ray catalog 1 - can use X-ray center, all good 2 - good ID, but cannot use X-ray center 3 - ID needs to be checked spectroscopically (Higher flags not included in this catalog) 4 - projection effects, no unique ID 5 - ID is potentially wrong 6 - ID is ok, but this is not a group 7 - no ID possible 8 - no photometry is available FLAG_MERGER - merger flag 1 - LENSING_R200 overlaps by more than 25% in radius and dz<0.01 with another group FLAG_POOR - poorness flag 1 if N_MEM <= 3 FLAG_MASK - mask flag 1 - >10% of area within LENSING_R200 or area within Rscale masked by stars, image defects, or edge of ACS coverage (ACS + Subaru B, V, I, z masks) FLAG_INCLUDE - combined quality flag 1 - good, FLAG1<3, FLAG_MERGER=0, FLAG_POOR=0, FLAG_PROJECTION=0, FLAG_MASK=0 ----------LENSING-CALIBRATED PROPERTIES---------- M200c - log10[M200] from WL-calibrated LX-M scaling relation (mass within radius where mean density is 200 times critical density) R200C_MPC - R200 in Mpc R200C_AS - R200 in arcsec, at group redshift CONC200c - C200 concentration from Zhao 2009 M-c (C=R200/Rscale) ----------MEMBER STATISTICS---------- NMEM - Number of objects in galaxy catalog with P_MEM > 0.5 for this group NMEM_W - Sum of P_MEM for objects in galaxy catalog for this group ----------GROUP CENTERS---------- Multiple "centers" are defined, and corresponding positions, galaxy IDs, stellar masses, and positional uncertainties are saved. From the lensing signals stacked on these centers, MMGGs appears to be the best tracer of the center of mass. These results do not imply that MMGGs is at the center of mass of every group, however. To study central galaxies, one may wish to confirm that a galaxy is both the most massive within the scale radius as well as the most massive in the group. MMGGs - most massive group galaxy within Rscale + X-ray positional error of X-ray center, using photoz selection only MMGGs_ZBEST - most massive group galaxy within Rscale + X-ray positional error of X-ray center, using speczs when available, else photozs Each center has a position associated with it, e.g ALPHA_MMGGs - RA (J2000) of MMGGs DELTA_MMGGs - Dec (J2000) of MMGGs ... We save the object ID in the galaxy catalog and the galaxy stellar mass, e.g. ID_MMGGs - ID for MMGGs galaxy from galaxy catalog SM_MMGGs - stellar mass for MMGGs galaxy ... ----------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