Bulge Growth and Quenching since z = 2.5 in CANDELS/3D-HST (1402.0866v2)

Abstract: Exploiting the deep high-resolution imaging of all 5 CANDELS fields, and accurate redshift information provided by 3D-HST, we investigate the relation between structure and stellar populations for a mass-selected sample of 6764 galaxies above 10^10 Msun, spanning the redshift range 0.5 < z < 2.5. For the first time, we fit 2-dimensional models comprising a single Sersic fit and two-component (i.e., bulge + disk) decompositions not only to the H-band light distributions, but also to the stellar mass maps reconstructed from resolved stellar population modeling. We confirm that the increased bulge prominence among quiescent galaxies, as reported previously based on rest-optical observations, remains in place when considering the distributions of stellar mass. Moreover, we observe an increase of the typical Sersic index and bulge-to-total ratio (with median B/T reaching 40-50%) among star-forming galaxies above 10^11 Msun. Given that quenching for these most massive systems is likely to be imminent, our findings suggest that significant bulge growth precedes a departure from the star-forming main sequence. We demonstrate that the bulge mass (and ideally knowledge of the bulge and total mass) is a more reliable predictor of the star-forming versus quiescent state of a galaxy than the total stellar mass. The same trends are predicted by the state-of-the-art semi-analytic model by Somerville et al. In the latter, bulges and black holes grow hand in hand through merging and/or disk instabilities, and AGN-feedback shuts off star formation. Further observations will be required to pin down star formation quenching mechanisms, but our results imply they must be internal to the galaxies and closely associated with bulge growth.

Analyzing Bulge Growth and Quenching in CANDELS/3D-HST from z = 2.5 to Present

The paper by Lang et al. focuses on the relationship between galaxy structure and stellar populations across redshifts 0.5 < z < 2.5, utilizing data from the CANDELS/3D-HST survey. This research explores the connection between the prominence of galaxy bulges and the quenching of star formation, offering insights that link morphological characteristics with evolutionary processes.

Core Objectives and Methodology

The paper examines a significant mass-selected sample of 6,764 galaxies, each with a stellar mass exceeding $10^{10} M_\odot$. This paper uniquely employs both single-component profiles and two-component (bulge + disk) decompositions on reconstructed stellar mass maps, in addition to H-band light distributions, to assess galaxy structures. Importantly, it extends previous studies by incorporating stellar mass maps derived from spatially resolved stellar population modeling, which allow for a more direct assessment of the mass distribution within galaxies rather than just optical light distribution.

Key Results and Numerical Findings

1. Bulge Prominence Among Quiescent Galaxies:
   • The data confirms that quiescent galaxies exhibit a notable increase in bulge prominence, characterized by elevated Sérsic indices (n) and bulge-to-total (B/T) light ratios. The paper finds that star-forming galaxies above $10^{11} M_\odot$ also show increased typical e\ indices and B/T ratios.
2. Predictive Power of Bulge Mass Over Total Stellar Mass:
   • The authors demonstrate that bulge mass is a more robust predictor for distinguishing between star-forming and quiescent galaxies than total stellar mass. This trend aligns with predictions from semi-analytic models such as those by Somerville et al., where bulge growth is closely associated with mechanisms driving star formation quenching.
3. Role of AGN and Quenching Mechanisms:
   • The paper outlines that observed trends support theoretical models where AGN feedback tied to bulge growth potentially drives quenching. Observational evidence also suggests internal processes linked to bulge formation are critical, rather than mass accretion or external influences.

Implications and Future Directions

The findings suggest a substantial role for bulge growth in the evolutionary track of galaxies, potentially preceding and even instigating the quenching of star formation in massive systems. This underscores the significance of internal galaxy processes, possibly influenced by AGN activity, in shaping galaxy evolution.

Future research directions could focus on refining semi-analytic models to improve alignment with observational data, particularly at high redshifts, where discrepancies still exist, notably an underprediction of quenched galaxy fractions. Enhanced observational techniques and facilities could further explore the dynamics within galaxies that drive bulge and black hole co-evolution, aiming to better understand the causal relationships governing galaxy quenching.

In conclusion, this paper significantly contributes to our understanding of the link between galaxy morphology, bulge growth, and star formation quenching, offering a nuanced perspective on the evolutionary pathways of galaxies across cosmic time.