What turns galaxies off? The different morphologies of star-forming and quiescent galaxies since z~2 from CANDELS (1110.3786v2)

Abstract: We use HST/WFC3 imaging from the CANDELS Multicycle Treasury Survey, in conjunction with the Sloan Digital Sky Survey, to explore the evolution of galactic structure for galaxies with stellar masses >3e10M_sun from z=2.2 to the present epoch, a time span of 10Gyr. We explore the relationship between rest-frame optical color, stellar mass, star formation activity and galaxy structure. We confirm the dramatic increase from z=2.2 to the present day in the number density of non-star-forming galaxies above 3e10M_sun reported by others. We further find that the vast majority of these quiescent systems have concentrated light profiles, as parametrized by the Sersic index, and the population of concentrated galaxies grows similarly rapidly. We examine the joint distribution of star formation activity, Sersic index, stellar mass, inferred velocity dispersion, and stellar surface density. Quiescence correlates poorly with stellar mass at all z<2.2. Quiescence correlates well with Sersic index at all redshifts. Quiescence correlates well with `velocity dispersion' and stellar surface density at z>1.3, and somewhat less well at lower redshifts. Yet, there is significant scatter between quiescence and galaxy structure: while the vast majority of quiescent galaxies have prominent bulges, many of them have significant disks, and a number of bulge-dominated galaxies have significant star formation. Noting the rarity of quiescent galaxies without prominent bulges, we argue that a prominent bulge (and perhaps, by association, a supermassive black hole) is an important condition for quenching star formation on galactic scales over the last 10Gyr, in qualitative agreement with the AGN feedback paradigm.

• The paper confirms that nearly all massive quiescent galaxies exhibit high Sérsic indices, indicating a transformation to bulge-dominated structures.
• The paper demonstrates that star formation quenching correlates more strongly with galaxy structure than with stellar mass, highlighting the role of morphology.
• The paper shows that a prominent bulge, likely linked to AGN feedback, is essential for ceasing star formation over the past 10 billion years.

The Role of Galaxy Morphology in the Evolution of Star Formation from Redshift 2 to the Present

This paper investigates the factors contributing to the cessation of star formation in galaxies over the past ten billion years using data from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). The paper particularly examines the changing morphologies of galaxies and their relationship with star formation activity, considering a sample of galaxies with stellar masses greater than $3 \times 10^{10} M_{\sun}$ from redshift $z=2.2$ to the current epoch.

Key Findings

1. Increase in Quiescent Galaxies: The paper confirms a significant increase in the number density of non-star-forming (quiescent) galaxies with masses higher than $3 \times 10^{10} M_{\sun}$ from $z=2.2$ to today, corroborating previous research in the field. The paper finds that nearly all quiescent galaxies exhibit concentrated light profiles as characterized by high Sérsic indices, suggesting a transformation towards more bulge-dominated structures.
2. Galactic Structure and Star Formation: The paper highlights the correlation between a galaxy's structural properties, such as its Sérsic index, stellar mass, velocity dispersion, and surface density, with its star formation activity. While quiescence does not strongly correlate with stellar mass, it shows a robust relationship with the Sérsic index across all redshifts considered in the paper. This implies that a galaxy's morphology plays a crucial role in its ability to suppress star formation.
3. Prominent Bulge as a Quenching Factor: A standout finding is the necessity for a prominent bulge, potentially signifying the presence of a supermassive black hole, as a condition for quenching star formation at galactic scales over the past 10 billion years. The research supports the AGN feedback paradigm, suggesting that feedback mechanisms from central black holes could be instrumental in the cessation of star formation.

Implications and Future Directions

The implications of this paper extend our understanding of galaxy evolution, particularly the morphological transformations that accompany or perhaps lead to the suppression of star formation. The prevalence of high Sérsic index morphologies among quiescent galaxies highlights the pivotal role of structural evolution in the broader context of galaxy formation and evolution theories.

These findings underscore the need for further exploration into the exact physical mechanisms enabling or driving these structural changes, particularly in the context of feedback processes from star formation or supermassive black holes. Future observational campaigns and theoretical models should aim to disentangle the causal relationships between galactic structure, bulge prominence, and star formation suppression, with potential insights from enhanced imaging and spectroscopy techniques.

Continued investigation in this field could benefit from larger sample sizes and advanced redshift surveys, which would enhance the statistical robustness of trends observed across varying environmental contexts and deeper epochs in cosmic history. Such studies would contribute significantly to refining models of galaxy formation and the role of AGN feedback in quenching star formation.