COSMOS2015

COSMOS2015
Name	COSMOS2015
Type	Astronomical survey
Field	Cosmology
Coordinates	10h00m28.6s, +02°12′21.0″
Startdate	2015
Telescope	Subaru Telescope, Very Large Telescope, Hubble Space Telescope, Spitzer Space Telescope, Galex, Chandra X-ray Observatory
Collaborators	National Astronomical Observatory of Japan, Max Planck Society, European Southern Observatory, California Institute of Technology, Space Telescope Science Institute

COSMOS2015 is a multiwavelength astronomical catalog and survey product centered on the COSMOS field that provides deep photometric and derived physical properties for millions of extragalactic sources. It extends prior datasets by combining imaging from space-based observatories and large ground-based facilities to produce matched photometry, photometric redshifts, and stellar population parameters across ultraviolet to mid-infrared wavelengths. The release has been widely used by research teams studying galaxy evolution, large-scale structure, active galactic nuclei, and dark matter through gravitational lensing.

Overview

COSMOS2015 builds on the legacy of the COSMOS project that integrates observations from Hubble Space Telescope, Subaru Telescope, Very Large Telescope, Spitzer Space Telescope, Chandra X-ray Observatory, and Galex to produce deep multiwavelength coverage over a ~2 square degree field originally defined for the COSMOS survey. The catalog provides photometry, photometric redshifts, stellar masses, star-formation rates, and rest-frame colors for sources detected primarily in near-infrared bands from instruments such as VISTA and UltraVISTA, coupled with optical imaging from Suprime-Cam and spectroscopic training samples from programs using VIMOS and DEIMOS. Collaborations include teams from institutions such as National Astronomical Observatory of Japan, California Institute of Technology, Max Planck Society, European Southern Observatory, and Space Telescope Science Institute.

Survey Design and Data Collection

The COSMOS2015 dataset was assembled by coordinating observations across facilities: deep near-infrared imaging from VISTA/UltraVISTA and Subaru/MOIRCS, optical imaging from Subaru Telescope/Suprime-Cam, ultraviolet measures from Galex, mid-infrared from Spitzer Space Telescope/IRAC, and X-ray catalogs from Chandra X-ray Observatory and XMM-Newton. The design prioritized uniform coverage of the COSMOS field to enable large-scale structure studies comparable to surveys like Sloan Digital Sky Survey and to provide complements to space programs including Hubble Space Telescope imaging from the COSMOS Treasury. Spectroscopic samples for calibration and validation were drawn from campaigns with VIMOS on VLT, DEIMOS on Keck Observatory, and archival redshifts from zCOSMOS and VANDELS-like efforts. The coordination involved institutional partners such as Max Planck Institute for Astronomy, Institut d'Astrophysique de Paris, University of California, and National Astronomical Observatory of Japan.

Photometric and Spectroscopic Catalogs

Photometry in COSMOS2015 combines aperture-matched measurements across bands including GALEX/FUV, Subaru/BVRiz, VISTA/YJHK, and Spitzer/IRAC channels. The catalog reports multi-aperture magnitudes, photometric errors, and flags for blending or saturation informed by imaging from Hubble Space Telescope/ACS. Photometric redshifts were calibrated using spectroscopic redshifts from surveys such as zCOSMOS, VIMOS Ultra Deep Survey, DEEP2, and targeted programs with Keck Observatory and Very Large Telescope. Additional columns include rest-frame magnitudes, stellar masses derived using stellar population synthesis models anchored to libraries like those of Bruzual & Charlot and initial mass functions such as Chabrier IMF or Salpeter IMF for comparison. AGN identification leverages cross-matches with Chandra X-ray Observatory sources and mid-infrared color diagnostics from Spitzer/IRAC.

Data Reduction and Calibration

Image processing for COSMOS2015 employed pipelines tailored to each instrument: reduction steps referenced standards used in projects associated with Subaru Telescope, VISTA, and Spitzer Space Telescope. Astrometric calibration used reference catalogs anchored to 2MASS and Gaia to ensure consistent coordinate systems. Photometric zero-points were tied to standard star observations and cross-checked against catalogs from CFHT and SDSS Photometric Catalogs where overlap permitted. PSF homogenization across bands was performed to enable robust color measurements, with convolution kernels constructed following techniques applied in surveys like HST CANDELS and COSMOS-DASH. Noise models incorporated correlated pixel noise estimates and variance propagation similar to procedures used by HST and VLT deep-field reductions.

Key Scientific Results

COSMOS2015 enabled a range of studies: measurements of the stellar mass function to high redshift that built on frameworks used in SDSS and CANDELS analyses; investigations of galaxy quenching and color bimodality connecting to work by groups from Max Planck Institute for Astrophysics and Institute of Astronomy, Cambridge; mapping of large-scale structure and cosmic web reconstruction comparable to results from GAMA and VIPERS; lensing-based mass measurements of dark matter halos using shape catalogs from Hubble Space Telescope imaging and techniques pioneered in CFHTLenS. The catalog supported studies of active galactic nuclei demographics in the context of Chandra and Spitzer AGN selections and enabled cross-comparisons with theoretical predictions from simulations such as Illustris and EAGLE.

Data Release and Accessibility

COSMOS2015 was released to the community through project data portals hosted by institutions including NASA/IPAC, IPAC Infrared Science Archive, and the COSMOS collaboration servers, with accompanying documentation and data tables. The release format followed conventions established by surveys like SDSS and HST legacy programs, providing machine-readable catalogs, imaging mosaics, and ancillary products. Subsequent analyses and value-added catalogs have been produced by groups at Caltech, Princeton University, University of Tokyo, and INAF and are often cross-referenced in later releases and collaborative repositories.

Category:Astronomical surveys