------------------------------------------------------------ COSMOS GALAXY AND X-RAY GROUP MEMBERSHIP CATALOGS 2011 September 14, updated 2011 November 15 (v20110914a) Matt George and Alexie Leauthaud mgeorge@astro.berkeley.edu, asleauthaud@lbl.gov ------------------------------------------------------------ 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. arxiv:1109.6040) -Spectroscopic redshifts (observed_targets_20110513.dat, M. Salvato, used for "ZBEST" membership; redshifts not included in this release) We consider 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. We expect that this is the version of the catalog that will be made publicly available. Please let us know if you find problems or have suggested changes to be made. ----------READING FILES---------- The catalogs are in the FITS binary table format with data stored in the first extension. I/O packages for reading this file format are available in a variety of languages include IDL, C, and Python. As an example, to read the catalogs in IDL: group_file = "/path/to/catalogs/cosmos_xgroups_20110914a_public.fits" group = mrdfits(group_file, 1) gal_file = "/path/to/catalogs/cosmos_xgroup_galaxies_20110914a_public.fits" gal = mrdfits(gal_file, 1) To check the contents of these data structures in IDL once they have been loaded: help, group, /str help, gal, /str ----------EXAMPLES FOR SELECTING OBJECTS---------- To get started, here are a few common examples for how to select certain lists of objects using IDL. The parameters and flags used are described in the README files for each catalog. To select member galaxies above a certain probability threshold: sel=where(gal.p_mem GT 0.5) ...with a quality cut on groups: sel=where(gal.p_mem GT 0.5 AND gal.group_flag EQ 1) ...or to select all candidate members of the first group in the catalog: sel=where(gal.group_id EQ group[0].group_id) To select "central" galaxies (as defined below) selected by photometric redshifts only: sel=where(gal.mmgg_scale EQ 1) ...or to select member galaxies replacing photometric redshifts with spectroscopic redshifts when available, with a quality cut on groups: sel=where(gal.p_mem_zbest EQ 1 AND gal.group_flag_zbest EQ 1) To select low mass groups passing quality cuts: sel=where(group.m200c LT 13. AND group.flag_include EQ 1) ----------GROUP MEMBERSHIP AND CENTERING DETAILS---------- Galaxies are associated with groups using their redshifts and proximity to X-ray group centers. A Bayesian membership probability is assigned by comparing the galaxy redshift PDF with the expected group redshift distribution and the field density at that redshift. Tests on mock catalogs and comparing galaxies selected with spectroscopic redshifts suggest that a membership probability cut of P>0.5 maximizes the purity and completeness of the member sample. We define various centers based on the member galaxies and use weak gravitational lensing to test which center most closely traces the center of mass on average. Preliminary results show that the Most Massive Group Galaxy within the scale radius of the X-ray position (MMGGs) is the favored center. Specifically, MMGGs is defined as the galaxy with the highest stellar mass near the X-ray position of the group (i.e., with a projected distance smaller than the sum of the NFW scale radius and the X-ray positional uncertainty) and with P>0.5. We assign group membership probabilities a first time around the X-ray positions in order to identify MMGGs, and a second time to identify group members with MMGGs as the new center of the group. Galaxies within R200 of the new center are assigned membership probabilities from zero to one. ----------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 ----------VERSION HISTORY---------- 20101006 - initial release (beta version) 20110209 - fixed bug in field density estimate, shifting membership threshold from P>0.7 to 0.5. Minor fix replacing a small number of missing photozs for bright sources with speczs. Replaced X-ray catalog Xmass_cha54xmm_mie_ext5_zupdate0510_zerr.cat with Xmass_chaxmm10_mie_ext6_id_update1010_zupdate.cat 20110914 - modified field names and null values. updated spectroscopic catalog to include zCOSMOS 20k and exclude IMACS spectra. Trimmed catalogs to groups with XFLAG<=3, and galaxies with a stellar mass. Removed some fields for streamlining. 20110914a - modified field names again to follow IRSA guidelines. Added MMGGS_ZBEST column to galaxy catalog. The data should be unchanged from 20110914.