Environmental dependence of alpha abundance for nearby galaxies

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Environmental dependence of alpha abundance for nearby galaxies

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.


Relevant publication:

Zheng Zheng, Cheng Li, Shude Mao et al., 2019, ApJ, 873, 63


Figure 1. Upper panel: Mgb/<Fe> versus velocity dispersion for the central regions; the solid line is a linear fit to all the data points. The slope of of the fitted line and the Spearman's rank correlation coefficient (rS) of the data are shown on the plot. Lower panel: the residuals, i.e. deviations of individual data points of the upper panel from the fitted straight line. Symbols are color-coded by H_beta, which is an age indicator (larger H_beta value means younger stellar age). The plot shows that Mgb/<Fe> is well correlated with sigma_* for both old and young galaxies.
Figure 2. Weighted mean values of the residuals (\delta log(Mgb/<Fe>)) for different galaxy regions and different galaxy environment indicators. The columns are ordered in galaxy regions: central (left column), intermediate (middle column), and outer (right column). The rows are ordered in environment indicators: local density (top row), large scale structure (second row), isolated/central/satellite (third row) and radii to group centers (bottom row, satellite galaxies only). The upward triangles, downward triangles and diamonds for each row are denoted in the legends of the plot. Red symbols are for galaxies with H_beta<3 (old) and blue symbols are for galaxies with H_beta > 3 (young). This plot shows that old low-sigma_* galaxies in high local density environment and inner regions within galaxy groups are enhanced in Mgb/<Fe>, while young galaxies and high-mass galaxies have no or less environmental dependence.
Figure 3. The plot is the same as the Fig. 2 but only shows dependence on the formation redshift: low-z (z_f<0.9, upward triangles), medium-z (0.91.5, diamonds). This plot shows that old galaxies with high-z_f have enhanced Mgb/<Fe> over low- and medium-z_f
Figure 4. Distributions of the inner-to-outer ratio of Mgb/<Fe> in different sigma_* and local density bins. The upper, middle and lower panels show galaxies in high local density, medium local density and low local density environments respectively. The histograms are color-coded by sigma_*. This plot shows that Mgb/<Fe> gradients are close to zero and have dependence on sigma_* but no obvious dependence on local density environment. A more obvious of Mgb/<Fe> gradients on sigma_* is shown in Fig. 5.
Figure 5. Inner-to-outer ratio of Mgb/<Fe> versus sigma_*. Each dot shows one galaxy and the dots are color-coded by local density rho/rho_0. The straight line is a linear fit to all the dots.