COSMOS ACS Catalog Definitions
COSMOS Overview
The COSMOS Archive serves data taken for the Cosmic Evolution Survey (COSMOS) project. COSMOS is an astronomical survey designed to probe the formation and evolution of galaxies as a function of cosmic time (redshift) and large scale structural environment. The survey covers a 2 square degree equatorial field with imaging by most of the major space-based telescopes (Hubble, Spitzer, GALEX, XMM, Chandra) and a number of large ground based telescopes (Subaru, VLA, ESO-VLT, UKIRT, NOAO, CFHT, and others).
IRSA serves the COSMOS archive using the following search methods:
- Spatial search on all images and small catalogs, using Atlas
- Advanced search on only the larger catalogs, using Gator
COSMOS ACS catalog
This is the ACS catalog for the COSMOS survey that has been constructed from 575 ACS pointings. Please see Leauthaud et al. 2006, ApJ, for project and data details.
Detection was performed on the images subsampled at 0.03 arcseconds. For this reason, the noise is correlated within the images which can affect the detection at the very faint magnitudes. Because of the correlated noise, the error estimates produced by SExtractor are offset. The error estimates in the catalog have been corrected for the correlated noise, for more details on the derivation, see Leauthaud et al 2006.
This catalog has been produced using photometry detection software, SExtractor (version 2.4.3). SExtractor was run on the ACS FW814 Cycle 12 and Cycle 13 images. In order to correctly detect and deblend a wide range of objects, Sextractor has been run twice, once with a configuration optimized for the detection of only the brightest objects, once with a configuration optimized for the faint objects. The two samples are then merged together to form the master catalog.
The following cleaning tasks have been performed on the final catalog:
- A narrow band is removed around each image because of low S/N on the edges. No data is removed from the final catalog because the overlap between adjacent image is larger than this band.
- Masking is performed around brights stars
- Asteroid trails and other defects have been masked out.
- Duplicate detections have been identified and the object with a poorer detection has been discarded.
We furthermore recommend that users remove objects with mu_class equal to 3 (see mu_type_small.eps. This cut will remove many faint spurious detections from the catalog.
The mu_class flag is a Star/Galaxy classifier that we recommend over the class_star flag from SExtractor. This flag is defined by the locus of objects within a mag_auto/mu_max plane (see mu_type.ps).
The classification flag mu_class is defined as:
- mu_class = 1: Galaxies
- mu_class = 2: Stars
- mu_class = 3: Objects with mu_max higher than stars (fake objects and residual cosmic rays)
See also:
- mag_auto_counts.eps: Number counts of stars and galaxies
- mu_type_small.eps: Classification of objects in the mu_max mag_auto plane. The dotted region contains fake detections (mu_class=3). The full line region contains stars (mu_class=2) and other objects are galaxies (mu_class=1). For clarity, this plot only contains a small random selection of objects.
COSMOS ACS Catalog Column Definitions
Gator serves the COSMOS September 2006 ACS Catalog (60 columns [55 delivered + 5 IRSA-created], 1,117,274 data rows). The column descriptions can be found below.
| Column Name | Data Type | Units | Description |
| number | int | Running object number from SExtractor; do not use this number as a unique identification number | |
| mag_iso | float | mag | Isophotal magnitude |
| magerr_iso | float | mag | RMS error for isophotal magnitude |
| mag_isocor | float | mag | Corrected isophotal magnitude |
| magerr_isocor | float | mag | RMS error for corrected isophotal magnitude |
| mag_petro | float | mag | Petrosian magnitude. Kron_fact = 2.5 |
| magerr_petro | float | mag | RMS error for petrosian magnitude. |
| petro_radius | float | pixel | Petrosian radius. Pixel size is 0.03 arcseconds. |
| mag_aper | float | mag | Fixed aperture magnitude. Aperture = 0.6 arcseconds. |
| magerr_aper | float | mag | RMS error for fixed aperture mag. Aperture = 0.6 arcseconds. |
| mag_auto | float | mag | Kron-like elliptical aperture magnitude |
| magerr_auto | float | mag | RMS error for AUTO magnitude |
| mag_best | float | mag | Best of MAG_AUTO and MAG_ISOCOR |
| magerr_best | float | mag | RMS error for MAG_BEST |
| flux_auto | float | count | Flux within a Kron-like elliptical aperture |
| fluxerr_auto | float | count | RMS error for AUTO flux |
| kron_radius | float | pixel | Kron radius for AUTO magnitude. Pixel size is 0.03 arcseconds. |
| background | float | counts | Background at centroid position |
| threshhold | float | counts | Detection threshold above background |
| flux_max | float | counts | Peak flux above backgound |
| flux_radius | float | pixel | Half-light radius |
| isoarea_image | int | pixel2 | Isophotal area |
| x_image | float | pixel | Object position along x. Pixel size is 0.03 arcseconds. |
| y_image | float | pixel | Object position along y. Pixel size is 0.03 arcseconds. |
| xmin_image | int | pixel | Minimum x-coordinate among detected pixels. Pixel size is 0.03 arcseconds. |
| ymin_image | int | pixel | Minimum y-coordinate among detected pixels. Pixel size is 0.03 arcseconds. |
| xmax_image | int | pixel | Maximum x-coordinate among detected pixels. Pixel size is 0.03 arcseconds. |
| ymax_image | int | pixel | Maximum y-coordinate among detected pixels. Pixel size is 0.03 arcseconds. |
| xpeak_image | int | pixel | x-coordinate of the brightest pixel |
| ypeak_image | int | pixel | y-coordinate of the brightest pixel |
| alphapeak_j2000 | float | deg | Right ascension of the brightest pixel (J2000) |
| deltapeak_j2000 | float | deg | Declination of the brightest pixel (J2000) |
| a_image | float | pixel | Profile RMS along major axis. Pixel size is 0.03 arcseconds. |
| b_image | float | pixel | Profile RMS along minor axis. Pixel size is 0.03 arcseconds. |
| ra | float | deg | Right ascension of the barycenter (J2000) |
| dec | float | deg | Declination of the barycenter (J2000) |
| theta_image | float | deg | Position angle (CCW/x). Defined counterclockwise. |
| mu_threshhold | float | max*arcsec-2 | Detection threshold above background |
| mu_max | float | max*arcsec-2 | Peak surface brightness abouve background |
| isoarea_world | float | deg2 | Isophotal area above analysis threshold |
| x_world | float | deg | Object position along world x axis |
| y_world | float | deg | Object position along world y axis |
| a_world | float | deg | Profile RMS along major axis (world units) |
| b_world | float | deg | Profile RMS along minor axis (world units) |
| theta_world | float | deg | Position angle (CCW/world-x) |
| flags | int | SExtractor quality flag | |
| fwhm_image | float | pixel | FWHM assuming a gaussian core. Pixel size is 0.03 arcseconds. |
| fwhm_world | float | deg | FWHM assuming a gaussian core |
| cxx_image | float | pixel-2 | Cxx object ellipse parameter |
| cyy_image | float | pixel-2 | Cyy object ellipse parameter |
| cxy_image | float | pixel-2 | Cxy object ellipse parameter |
| elongation | float | A_IMAGE/B_IMAGE | |
| class_star | float | SExtractor star/galaxy classifier. Prefer the MU_CLASS classifier. | |
| field | char | Name of ACS pointing in which object is detected | |
| mu_class | int | Star/galaxy classifier. Type 1=galaxy, type 2=star, type 3=fake detection. Use this classifier rather than CLASS_STAR. |