by Zheng Zheng, Cheng Li, Shude Mao
The alpha-to-iron ratio ([alpha/Fe]) is an important indicator for star formation histories because alpha-elements (such as O, Ne, Mg, Si, Ca, Ti) are mostly produced in core-collapse supernovae, whose progenitors are high-mass stars, while irons (Fe) are mostly produced by Type Ia supernovae, whose progenitors are low-mass compact stars. High-mass stars generally have a very short lifetime (~ 1 Million year), whilst low-mass compact stars almost all have ages larger than 1 Giga year. So a galaxy formed in a single burst or experienced fast quenching will be enhanced in [alpha/Fe] in comparison with a galaxy formed with an extended star formation history.
Environmental effects, such as harassment, strangulation, or gas stripping, have been recognized to play important roles in shutting down star formation in galaxies. Previous studies have shown that the [alpha/Fe] and the spectral index ratio Mgb/<Fe>, which is generally recognized as an indicator for [alpha/Fe], are best correlated with galaxy stellar velocity dispersion, sigma_*, and might have dependence on environment at the low velocity dispersion end. However, most of these studies are only based on early type galaxies and focus on the central part of galaxies. When compared to the central part, the outskirt of galaxies is expected to be more affected by environment.
A collaboration group led by Dr. Zheng Zheng at the National Astronomical Observatories, Chinese Academy of Sciences and Profs. Cheng Li, Shude Mao at Tsinghua Center for Astrophysics recently studied the environmental dependence of Mgb/<Fe> distribution in galaxies using a large sample of galaxies from the SDSS-IV MaNGA survey. They investigated the Mgb/<Fe>-sigma_* relation in different environments using both young (mostly late type) and old (mostly early type) galaxies. Thanks to the information provided by the ELUCID project (lead by Prof. Huiyuan Wang at USTC), they can study the dependence on different types of environments, such as local density, large scale structure type, central/satellite type, and even formation time of dark matter halos of galaxies. Also, with the state of the art integral field spectroscopy from MaNGA, they are able to explore the Mgb/<Fe>-sigma_* relation for different regions within galaxies.
They found that (1) all galaxies show a tight correlation between Mgb/<Fe> and sigma_* (Fig. 1); (2) `old' (H_beta < 3) low-sigma_* galaxies in high local density environment and inner regions within galaxy groups are enhanced in Mgb/<Fe>, while `young' (H_beta>3) galaxies and high-mass galaxies show no or less environmental dependence (Fig. 2); (3) `old' galaxies with high-z_f show enhanced Mgb/<Fe> over low- and medium-z_f (Fig. 3); (4) Mgb/<Fe> gradients are close to zero and show dependence on sigma_* but no obvious dependence on the environment or z_f (Figs. 4 & 5). These results indicate that stellar velocity dispersion or galaxy mass is the main parameter driving the alpha enhancement, although environments appear to have modest effects, particularly for low- and medium-mass galaxies.