The Atlas3D Project - XXX. Star formation histories and stellar population scaling relations of early-type galaxies

Abstract: We present the stellar population content of early-type galaxies from the Atlas3D survey. Using spectra integrated within apertures covering up to one effective radius, we apply two methods: one based on measuring line-strength indices and applying single stellar population (SSP) models to derive SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and one based on spectral fitting to derive non-parametric star-formation histories, mass-weighted average values of age, metallicity, and half-mass formation timescales. Using homogeneously derived effective radii and dynamically-determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane (M_JAM, Sigma_e, R_maj), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alpha-enhanced than their larger counterparts. From non-parametric star-formation histories, we find that the duration of star formation is systematically more extended in lower mass objects. Assuming that our sample represents most of the stellar content of today's local Universe, approximately 50% of all stars formed within the first 2 Gyr following the big bang. Most of these stars reside today in the most massive galaxies (>10^10.5 M_sun), which themselves formed 90% of their stars by z~2. The lower-mass objects, in contrast, have formed barely half their stars in this time interval. Stellar population properties are independent of environment over two orders of magnitude in local density, varying only with galaxy mass. In the highest-density regions of our volume (dominated by the Virgo cluster), galaxies are older, alpha-enhanced and have shorter star-formation histories with respect to lower density regions.

• The paper reveals that compact early-type galaxies are older, more metal-rich, and alpha-enhanced, indicating rapid formation histories.
• It employs dual methodologies—line-strength indices with SSP models and spectral fitting—to derive stellar population parameters across effective galaxy radii.
• The study finds that galaxy mass, rather than environment, chiefly determines star formation duration and stellar population properties.

Stellar Population Scaling Relations in Early-Type Galaxies: An Analysis of the ATLAS$^{3D}$ Survey

The paper in discussion presents a comprehensive analysis of the stellar population content in early-type galaxies, utilizing the extensive data set from the ATLAS$^{3D}$ survey. The study emphasizes the investigation of star formation histories and stellar population scaling relations through the lens of dynamical and spectroscopic measurements.

Key Methodologies

The research employs two principal methodologies to derive stellar population parameters. Firstly, the analysis utilizes line-strength indices combined with Single Stellar Population (SSP) models to calculate SSP-equivalent values of age, metallicity, and alpha enhancement. Secondly, a spectral fitting approach is utilized to extract non-parametric star formation histories, alongside mass-weighted average age and metallicity. These approaches enable the authors to assess the stellar populations across one effective radius in the sample galaxies.

Principal Findings

1. Distribution on the Mass Plane: The study reveals that, at fixed mass, compact early-type galaxies are systematically older, more metal-rich, and exhibit greater alpha enhancement compared to their larger counterparts. This suggests that such compact galaxies formed earlier and within shorter timeframes.
2. Star Formation Duration: Through non-parametric star formation histories, the research delineates that lower mass galaxies have more protracted star formation durations. The study posits that approximately half of all stars formed within the first 2 billion years after the Big Bang, predominantly in the most massive galaxies which had completed 90% of their star formation by redshift $z \sim 2$.
3. Environmental Independence: A notable finding is that stellar population properties appear independent of environmental density variations over two orders of magnitude, implicating galaxy mass as the dominant factor influencing these properties.
4. Influence of High-Density Regions: In contrast, galaxies within the highest-density environments, specifically those in the Virgo cluster, are identified as older, more alpha-enhanced, and exhibiting shorter star-formation histories compared to those from lower density regions.

Implications and Future Directions

The paper has significant theoretical implications, primarily in understanding the processes governing the evolution of early-type galaxies. The findings reinforce the concept that galaxy mass is a more critical determinant of stellar population features than immediate environmental influences over a broad range of densities. Practically, the results emphasize the need for varied data collection methods — including both spectroscopic and imaging data — to accurately capture the evolutionary narratives of galaxies.

Looking towards future work, further exploration into the dynamical processes affecting galaxy evolution is suggested, particularly in understanding the divergent evolutionary paths prompted by variations in galaxy compactness and mass. The study also hints at the importance of refining spectral models to better address discrepancies in age estimations seen in comparisons with the age of the universe itself.

In conclusion, the paper provides a thorough analysis enriched with robust data, positing galaxy mass as the central pillar in the architecture of early-type galaxies' stellar histories and characteristics, while offering insights into the nuanced relationship between cosmic structures and their environments.