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

Abstract

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.