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Register a new userThe SAMI Galaxy Survey: Trends in [alpha/Fe] as a Function of Morphology and Environment
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We present a new set of index-based measurements of [$alpha$/Fe] for a sample of 2093 galaxies in the SAMI Galaxy Survey. Following earlier work, we fit a global relation between [$alpha$/Fe] and the galaxy velocity dispersion $sigma$ for red sequence galaxies, [$alpha$/Fe]=(0.378$pm$0.009)log($sigma$/100)+(0.155$pm$0.003). We observe a correlation between the residuals and the local environmental surface density, whereas no such relation exists for blue cloud galaxies. In the full sample, we find that elliptical galaxies in high-density environments are $alpha$-enhanced by up to 0.057$pm$0.014 dex at velocity dispersions $sigma$<100 km/s, compared with those in low-density environments. This $alpha$-enhancement is morphology-dependent, with the offset decreasing along the Hubble sequence towards spirals, which have an offset of 0.019$pm$0.014 dex. At low velocity dispersion and controlling for morphology, we estimate that star formation in high-density environments is truncated $sim1$ Gyr earlier than in low-density environments. For elliptical galaxies only, we find support for a parabolic relationship between [$alpha$/Fe] and $sigma$, with an environmental $alpha$-enhancement of at least 0.03 dex. This suggests strong contributions from both environment and mass-based quenching mechanisms. However, there is no evidence for this behaviour in later morphological types.
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The kinematic morphology-density relation of galaxies is normally attributed to a changing distribution of galaxy stellar masses with the local environment. However, earlier studies were largely focused on slow rotators; the dynamical properties of the overall population in relation to environment have received less attention. We use the SAMI Galaxy Survey to investigate the dynamical properties of $sim$1800 early and late-type galaxies with $log(M_*/M_{odot})>9.5$ as a function of mean environmental overdensity ($Sigma_{5}$) and their rank within a group or cluster. By classifying galaxies into fast and slow rotators, at fixed stellar mass above $log(M_*/M_{odot})>10.5$, we detect a higher fraction ($sim3.4sigma$) of slow rotators for group and cluster centrals and satellites as compared to isolated-central galaxies. Focusing on the fast-rotator population, we also detect a significant correlation between galaxy kinematics and their stellar mass as well as the environment they are in. Specifically, by using inclination-corrected or intrinsic $lambda_{R_e}$ values, we find that, at fixed mass, satellite galaxies on average have the lowest $lambda_{,R_e,intr}$, isolated-central galaxies have the highest $lambda_{,R_e,intr}$, and group and cluster centrals lie in between. Similarly, galaxies in high-density environments have lower mean $lambda_{,R_e,intr}$ values as compared to galaxies at low environmental density. However, at fixed $Sigma_{5}$, the mean $lambda_{,R_e,intr}$ differences for low and high-mass galaxies are of similar magnitude as when varying $Sigma_{5}$ {($Delta lambda_{,R_e,intr} sim 0.05$. Our results demonstrate that after stellar mass, environment plays a significant role in the creation of slow rotators, while for fast rotators we also detect an independent, albeit smaller, impact of mass and environment on their kinematic properties.
We study the Fundamental Plane (FP) for a volume- and luminosity-limited sample of 560 early-type galaxies from the SAMI survey. Using r-band sizes and luminosities from new Multi-Gaussian Expansion (MGE) photometric measurements, and treating luminosity as the dependent variable, the FP has coefficients a=1.294$pm$0.039, b= 0.912$pm$0.025, and zero-point c= 7.067$pm$0.078. We leverage the high signal-to-noise of SAMI integral field spectroscopy, to determine how structural and stellar-population observables affect the scatter about the FP. The FP residuals correlate most strongly (8$sigma$ significance) with luminosity-weighted simple-stellar-population (SSP) age. In contrast, the structural observables surface mass density, rotation-to-dispersion ratio, Sersic index and projected shape all show little or no significant correlation. We connect the FP residuals to the empirical relation between age (or stellar mass-to-light ratio $Upsilon_star$) and surface mass density, the best predictor of SSP age amongst parameters based on FP observables. We show that the FP residuals (anti-)correlate with the residuals of the relation between surface density and $Upsilon_star$. This correlation implies that part of the FP scatter is due to the broad age and $Upsilon_star$ distribution at any given surface mass density. Using virial mass and $Upsilon_star$ we construct a simulated FP and compare it to the observed FP. We find that, while the empirical relations between observed stellar population relations and FP observables are responsible for most (75%) of the FP scatter, on their own they do not explain the observed tilt of the FP away from the virial plane.
We investigate the relationship between stellar and gas specific angular momentum $j$, stellar mass $M_{*}$ and optical morphology for a sample of 488 galaxies extracted from the SAMI Galaxy Survey. We find that $j$, measured within one effective radius, monotonically increases with $M_{*}$ and that, for $M_{*}>$10$^{9.5}$ M$_{odot}$, the scatter in this relation strongly correlates with optical morphology (i.e., visual classification and Sersic index). These findings confirm that massive galaxies of all types lie on a plane relating mass, angular momentum and stellar light distribution, and suggest that the large-scale morphology of a galaxy is regulated by its mass and dynamical state. We show that the significant scatter in the $M_{*}-j$ relation is accounted for by the fact that, at fixed stellar mass, the contribution of ordered motions to the dynamical support of galaxies varies by at least a factor of three. Indeed, the stellar spin parameter (quantified via $lambda_R$) correlates strongly with Sersic and concentration indices. This correlation is particularly strong once slow-rotators are removed from the sample, showing that late-type galaxies and early-type fast rotators form a continuous class of objects in terms of their kinematic properties.
We investigate the stellar populations of passive spiral galaxies as a function of mass and environment, using integral field spectroscopy data from the Sydney-AAO Multi-object Integral field spectrograph Galaxy Survey. Our sample consists of $52$ cluster passive spirals and $18$ group/field passive spirals, as well as a set of S0s used as a control sample. The age and [Z/H] estimated by measuring Lick absorption line strength indices both at the center and within $1R_{rm e}$ do not show a significant difference between the cluster and the field/group passive spirals. However, the field/group passive spirals with log(M$_star$/M$_odot)gtrsim10.5$ show decreasing [$alpha$/Fe] along with stellar mass, which is $sim0.1$ dex smaller than that of the cluster passive spirals. We also compare the stellar populations of passive spirals with S0s. In the clusters, we find that passive spirals show slightly younger age and lower [$alpha$/Fe] than the S0s over the whole mass range. In the field/group, stellar populations show a similar trend between passive spirals and S0s. In particular, [$alpha$/Fe] of the field/group S0s tend to be flattening with increasing mass above log(M$_star$/M$_odot)gtrsim10.5$, similar to the field/group passive spirals. We relate the age and [$alpha$/Fe] of passive spirals to their mean infall time in phase-space; we find a positive correlation, in agreement with the prediction of numerical simulations. We discuss the environmental processes that can explain the observed trends. The results lead us to conclude that the formation of the passive spirals and their transformation into S0s may significantly depend on their environments.
We study the balance in dynamical support of 384 galaxies with stellar kinematics out to >1.5R_e in the Sydney AAO Multi-object Integral Field (SAMI) Galaxy Survey. We present radial dynamical profiles of the local rotation dominance parameter, V/sigma, and local spin, lambda_loc. Although there is a broad range in amplitude, most kinematic profiles monotonically increase across the probed radial range. We do not find many galaxies with kinematic transitions such as those expected between the inner in-situ and outer accreted stars within the radial range probed. We compare the V/sigma gradient and maximum values to the visual morphologies of the galaxies to better understand the link between visual and kinematic morphologies. We find that the radial distribution of dynamical support in galaxies is linked to their visual morphology. Late-type systems have higher rotational support at all radii and steeper V/sigma gradients compared to early-type galaxies. We perform a search for embedded discs, which are rotationally supported discy structures embedded within large scale slowly or non-rotating structures. Visual inspection of the kinematics reveals at most three galaxies (out of 384) harbouring embedded discs. This is more than an order of magnitude fewer than the observed fraction in some local studies. Our tests suggest that this tension can be attributed to differences in the sample selection, spatial sampling and beam smearing due to seeing.
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