Evolution of the Bar Fraction in COSMOS: Quantifying the Assembly of the Hubble Sequence

TL;DR

This study investigates how the fraction of barred spiral galaxies changes with redshift in the COSMOS field, revealing a significant decline over time and its dependence on galaxy properties, shedding light on galaxy evolution.

Contribution

It provides the largest and deepest analysis of bar fractions over redshift, highlighting the evolution of the Hubble sequence and the role of galaxy mass and morphology.

Findings

Bar fraction declines from 65% locally to 20% at z~0.84.

Strong bars decrease from 30% to under 10% over the same redshift range.

Massive, luminous spirals maintain a constant bar fraction up to z~0.84.

Abstract

We have analyzed the redshift-dependent fraction of galactic bars over 0.2<z<0.84 in 2,157 luminous face-on spiral galaxies from the COSMOS 2-square degree 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 (SB) bars 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 which may be an important clue towards the co-evolution 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.