UltraVISTA K-selected Catalog v4.1 README ==================================== This catalog contains aperture fluxes with an AB magnitude zeropoint of 25. Magnitudes can be determined with m_filter = -2.5*log10(flux_filter) + 25. The catalog has been put through many quality tests (zphot vs. zspec, number counts, looking for unusual objects, comparison of some SEDs with NMBS) and is a science-grade catalog; however, be aware that there are many many objects and error checking is not completely fool proof. The catalog is just entering larger consumption so it is likely this will reveal issues that were not discovered in the initial error checking. If you are using the catalog and find issues, please contact me (muzzin@strw.leidenuniv.nl) so that I can address these in future catalogs. How the catalog was made, and what it contains ===================================== This catalog is made from PSF matching the imaging in 23 different filters, YJHK (UltraVISTA), u (CFHT), BVgriz (Subaru), and the 12 intermediate optical bands (Subaru). The images are PSF-matched to the worst-seeing image, which is a tie between the Subaru g-band and Subaru IA484 bands (~1.1"). Objects are detected in the UltraVISTA K-band and photometry is done in 2.1" diameter apertures. The growth curves from stars show that the PSF matching is good to better than 1% in this aperture. IRAC, MIPS and GALEX photometry were added by M. Stefanon using a source-fitting code developed by Labbe (see Wuyts et al. 2007, Labbe et al. 2010 for more description. These bands bring the total number of bands in the catalog to 30. The PSF matching was done by dividing the UltraVISTA into 9 separate pointings which roughly match the layout of the COSMOS SuprimeCam pointings. PSF matching is done separately in each of the 9 fields to try and optimize any field-to-field PSF variations, and then those individual catalogs are combined into a "master" catalog. Post facto the PSF variations turn out to be fairly small between the fields, but there are differences. zCOSMOS ======== The catalog is matched to the zCOSMOS spectroscopic catalogs. The matching is done within an 0.5" radius, but the vast majority of matches have separations < 0.05" suggesting very few spurious matches. Only the highest-confidence zCOSMOS redshifts were kept in the catalog because they are used for adjusting the zeropoints. If you wish to use the entire 10k sample you will have to perform more matching. Zeropoint Offsets ============= Zeropoint offsets have been calculated for all bands using code developed by Gabe Brammer for the NMBS. The code iteratively adjusts the zeropoints of objects so that the scatter between zphot and zspec is minimized. Zeropoint offsets in the standard ugrizBV bands were generally < 5%; however offsets in the Subaru medium bands can be substantial, from as small as a few percent and as high as 18%. Offsets in the UltraVISTA bands were between 8 - 20%. All offsets are calculated RELATIVE to the K-band. Also, be aware that zeropoint offsetting is done to maximize the quality of the photometric redshifts. It depends on which templates are used. The templates have substantial freedom so most of the offset should be real, but the templates could introduce biases, so be aware that rest-frame colors are not completely "pure". Bright Stars ========= COSMOS covers a wide field and hence has more than a few very bright stars. These cause significant problems with the photometry of nearby objects. Several different flags are in the catalog (see below) which are designed to identify objects whose photometry is likely to be badly contaminated by bright stars. You trust the photometry of objects that have contamination flags < 1. Best Practices for Use ================= Here are a few best practices for using the catalog - It is important to remember that the magnitudes list for all bands ARE NOT total magnitudes. They are aperture magnitudes inside the 2.1" aperture after PSF matching. The only total magnitude available is for the K-band, where the total is from SExtractor's flux_auto, plus a correction for flux outside the flux_auto radius estimated using the growth curve of bright stars. Total magnitudes in any band can be calculated via flux_x(total) = flux_x(aperture) * (flux_Ks_total/flux_Ks), where x is the band of interest. -Be aware that there are regions with very bright stars. It is very important to set the data quality flags (contamination = 0, nan_contam = 0, star = 0), otherwise there will be objects with nonsensical photometry and population parameters. - The catalog contains sources down to a S/N(Ks) = 3 in the 2.1" aperture. The catalog is highly incomplete at these magnitudes and the photo-z's and stellar masses are also uncertain. We recommend only using objects bright than the Ks_tot < 23.4 limit, which is the 90% completeness limit for science with "complete samples". Mass-complete samples can be defined by applying both the Ks_tot < 23.4 limit as well as using the mass-completeness limits available on the website in the data products section. - Be wary of "extreme" objects (very high-redshift, very massive). With such a wide field and ground-based photometry, blends of galaxies can occur, or the photometry could be from contaminated from an unknown source. Please make sure to verify that extreme objects are real using the images before using them for science. Catalog Column Definitions ===================== 1. ID - Galaxy identifier in the master catalog 2. RA/DEC - Right Ascension and Declination in J2000 3. xpix/ypix - Location of the galaxy in pixels on the master image. 4. Ks_tot - Total Ks flux and error determined using Sextractor's flux_auto. An additional aperture correction has also been applied to correct for flux outside 2.5 times the Kron radius 5. K through IB827 - 2.1" PSF-matched aperture flux and errors for the 23 filters in this catalog 6. eK through eIB827 - Errors in the aperture fluxes. These are calculated from the rms of 5000 empty apertures in each filter. Also contains an additional term for shot noise (only important for bright objects). 7. K_flag - Sextractor flags for objects in the K-band. One should be cautious using objects with K_flag > 3 as these are probably contaminated by a nearby object or are near saturated pixels. 8. K_star - Sextractor parameter for determining if the light profile of the object is stellar. Values > 0.9 are likely to be point sources. This should not be used to identify stars, use the "star" parameter. 9. K_Kron - The Kron radius as determined by Sextractor. This should contain 90% of the flux. Total magnitudes were calculated within 2 Kron radii which should be 96% of flux 10. apcor - Aperture correction THAT HAS ALREADY BEEN applied to the flux_auto to get Ks_tot. This is determined from the growth curve of stars in the field. DO NOT APPLY AGAIN 11. z_spec - Spectroscopic redshift from zCOSMOS, value = -1 if no spectroscopic redshift is available. 12. z_spec_cc - Spectroscopic redshift quality classification. For highest quality redshifts use 3 < z_spec_cc < 4. See Lilly et al. (2006) for full definition of these flags. 13. z_spec_id - ID of the spectroscopic redshift in the zCOSMOS catalog 14. star - Classification of the object as a star/galaxy. Value = 1 is a star, Value = 0 is a galaxy. Classification is done based on location of stars in the U-J vs. J-K color-color plot. 15. contamination - Value = 1 if this object is near a bright star. Object should be excluded. Bright stars are identified from the USNO and 2MASS catalogs, and adaptive radii based on magnitude are used to define this parameter. 16. nan_contam - If this object is near an object that is saturated and the flux is contaminated by PSF convolution of the saturated object this value = 1. The value is the TOTAL number of filters that have this type of contamination. The fluxes of these filters have been set to -99 so are ignored by EAZY and FAST. Objects with 0 < value < 5 are probably acceptable as are only missing a few filters of data. Objects with value > 3 should probably be excluded as many filters of data are contaminated and missing. 17. orig_cat_id - ID number in the original catalogs generated from the 9 COSMOS subfields 18. orig_cat_field - ID of which of the 9 subfields this object was photometered in. 19. USE - A simple switch for choosing galaxies with good photometry. Objects with USE = 1 have, K(ap) < 24.44 (i.e., 3-sigma detection), star = 0, contamination = 0, and nan_contam < 3. Be aware that this includes objects with Ks_tot > 23.4 so for truly complete sample that selection should also be done.