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Published March 10, 2008 | Published
Journal Article Open

Evolution of the bar fraction in COSMOS: quantifying the assembly of the Hubble sequence


We have analyzed the redshift-dependent fraction of galactic bars over 0.2 < z < 0.84 in 2157 luminous face-on spiral galaxies from the COSMOS 2 deg^2 field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z ~ 0.84 this fraction drops to about 20%. Over this redshift range the fraction of strong bars (SBs) drops from about 30% to under 10%. It is clear that when the universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z ~ 0.84, whereas for the low-mass, blue spirals it declines significantly with redshift beyond z = 0.3. There is also a slight preference for bars in bulge-dominated systems at high redshifts that may be an important clue toward the coevolution of bars, bulges, and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate.

Additional Information

© 2008 The American Astronomical Society. Received 2007 July 27, accepted for publication 2007 October 23. We are indebted to the anonymous referee for many helpful comments and suggestions that have greatly improved this paper. We are also thankful for the insightful and helpful discussions we have had with Karin Menendez-Delmestre, Neal Evans, Lee Armus, Isaac Shlosman, Wyn Evans, Donald Lynden-Bell, Mark Dickinson, David Elbaz, Francois Schweizer, Tomasso Treu, Jason Melbourne, Dan Kelson, and Luis Ho. The HST COSMOS 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. It is a pleasure the 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. We also wish to acknowledge support for the COSMOS Science meeting in May 2005 which was supported in part by NSF grant OISE-0456439.

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