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Published September 2007 | Published
Journal Article Open

Deep GALEX Imaging of the COSMOS HST Field: A First Look at the Morphology of z ~ 0.7 Star-forming Galaxies


We present a study of the morphological nature of redshift z ~ 0.7 star-forming galaxies using a combination of HST ACS, GALEX, and ground-based images of the COSMOS field. Our sample consists of 8146 galaxies, 5777 of which are detected in the GALEX near-ultraviolet band (2310 Å or ~1360 Å rest frame) down to a limiting magnitude of 25.5 (AB), and all of which have a brightness of F814W(HST) < 23 mag and photometric redshifts in the range 0.55 < z < 0.8. We make use of the UV to estimate star formation rates, correcting for the effect of dust using the UV slope, and of the ground-based multiband data to calculate masses. For all galaxies in our sample, we compute, from the ACS F814W images, their concentration (C), asymmetry (A), and clumpiness (S), as well as their Gini coefficient (G) and the second moment of the brightest 20% of their light (M_20). We observe a bimodality in the galaxy population in asymmetry and in clumpiness, although the separation is most evident when either of those parameters is combined with a concentration-like parameter (C, G, or M_20). We further show that this morphological bimodality has a strong correspondence with the FUV-g color bimodality and conclude that UV-optical color predominantly evolves concurrently with morphology. We observe many of the most star-forming galaxies to have morphologies approaching that of early-type galaxies, and we interpret this as evidence that strong starburst events are linked to bulge growth and constitute a process through which galaxies can be brought from the blue to the red sequence while simultaneously modifying their morphology accordingly. We conclude that the red sequence has continued growing at z ≲ 0.7. We also observe z ~ 0.7 galaxies to have physical properties similar to that of local galaxies, except for higher star formation rates. Whence we infer that the dimming of star-forming galaxies is responsible for most of the evolution in star formation rate density since that redshift, although our data are also consistent with a mild number evolution.

Additional Information

© 2009 American Astronomical Society. Received 2006 July 28; accepted 2007 January 9. The COSMOS HST Treasury program was supported through NASA grant HST-GO-09822. We wish to thank Tony Roman, Denise Taylor, and David Soderblom for their assistance in planning and scheduling of the extensive COSMOS observations. We gratefully acknowledge the contributions of the entire COSMOS collaboration consisting of more than 70 scientists. More information on the COSMOS survey is available at http://www.astro.caltech.edu/~cosmos. It is a pleasure to acknowledge the excellent services provided by the NASA IPAC/IRSA staff (Anastasia Laity, Anastasia Alexov, Bruce Berriman, and John Good) in providing online archive and server capabilities for the COSMOS data sets. The COSMOS Science meeting in 2005 May was supported in part by the NSF through grant OISE-0456439. GALEX (Galaxy Evolution Explorer) is a NASA Small Explorer, launched in 2003 April. We gratefully acknowledge NASA's support for construction, operation, and science analysis for the GALEX mission, developed in cooperation with the Centre National d'Etudes Spatiales of France and the Korean Ministry of Science and Technology. Facilities: HST(ACS), GALEX, Subaru, CFHT, KPNO:2.1m, CTIO:1.5m Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555; and with the NASA Galaxy Evolution Explorer (GALEX); also based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation; the Canada-France-Hawaii Telescope with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the National Research Council of Canada, the Canadian Astronomy Data Centre, the Centre National de la Recherche Scientifique de France, TERAPIX, and the University of Hawaii.

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