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Register a new userThe Sami Galaxy Survey: stellar populations of passive spiral galaxies in different environment
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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.
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We present an analysis of the global stellar populations of galaxies in the SAMI Galaxy Survey. Our sample consists of 1319 galaxies spanning four orders of magnitude in stellar mass and includes all morphologies and environments. We derive luminosity-weighted, single stellar population equivalent stellar ages, metallicities and alpha enhancements from spectra integrated within one effective radius apertures. Variations in galaxy size explain the majority of the scatter in the age--mass and metallicity--mass relations. Stellar populations vary systematically in the plane of galaxy size and stellar mass, such that galaxies with high stellar surface mass density are older, more metal-rich and alpha-enhanced than less dense galaxies. Galaxies with high surface mass densities have a very narrow range of metallicities, however, at fixed mass, the spread in metallicity increases substantially with increasing galaxy size (decreasing density). We identify residual correlations with morphology and environment. At fixed mass and size, galaxies with late-type morphologies, small bulges and low Sersic n are younger than early-type, high n, high bulge-to-total galaxies. Age and metallicity both show small residual correlations with environment; at fixed mass and size, galaxies in denser environments or more massive halos are older and somewhat more metal rich than those in less dense environments. We connect these trends to evolutionary tracks within the size--mass plane.
The well-established correlations between the mass of a galaxy and the properties of its stars are considered evidence for mass driving the evolution of the stellar population. However, for early-type galaxies (ETGs), we find that $g-i$ color and stellar metallicity [Z/H] correlate more strongly with gravitational potential $Phi$ than with mass $M$, whereas stellar population age correlates best with surface density $Sigma$. Specifically, for our sample of 625 ETGs with integral-field spectroscopy from the SAMI Galaxy Survey, compared to correlations with mass, the color--$Phi$, [Z/H]--$Phi$, and age--$Sigma$ relations show both smaller scatter and less residual trend with galaxy size. For the star formation duration proxy [$alpha$/Fe], we find comparable results for trends with $Phi$ and $Sigma$, with both being significantly stronger than the [$alpha$/Fe]-$M$ relation. In determining the strength of a trend, we analyze both the overall scatter, and the observational uncertainty on the parameters, in order to compare the intrinsic scatter in each correlation. These results lead us to the following inferences and interpretations: (1) the color--$Phi$ diagram is a more precise tool for determining the developmental stage of the stellar population than the conventional color--mass diagram; and (2) gravitational potential is the primary regulator of global stellar metallicity, via its relation to the gas escape velocity. Furthermore, we propose the following two mechanisms for the age and [$alpha$/Fe] relations with $Sigma$: (a) the age--$Sigma$ and [$alpha$/Fe]--$Sigma$ correlations arise as results of compactness driven quenching mechanisms; and/or (b) as fossil records of the $Sigma_{SFR}proptoSigma_{gas}$ relation in their disk-dominated progenitors.
We study the internal radial gradients of the stellar populations in a sample comprising 522 early-type galaxies (ETGs) from the SAMI (Sydney- AAO Multi-object Integral field spectrograph) Galaxy Survey. We stack the spectra of individual spaxels in radial bins, and derive basic stellar population properties: total metallicity ([Z/H]), [Mg/Fe], [C/Fe] and age. The radial gradient ($ abla$) and central value of the fits (evaluated at R$_e$/4) are compared against a set of six possible drivers of the trends. We find that velocity dispersion ($sigma$) - or, equivalently gravitational potential - is the dominant driver of the chemical composition gradients. Surface mass density is also correlated with the trends, especially with stellar age. The decrease of $ abla$[Mg/Fe] with increasing $sigma$ is contrasted by a rather shallow dependence of $ abla$[Z/H] with $sigma$ (although this radial gradient is overall rather steep). This result, along with a shallow age slope at the massive end, imposes stringent constraints on the progenitors of the populations that contribute to the formation of the outer envelopes of ETGs. The SAMI sample is split between a field sample and a cluster sample. Only weak environment-related differences are found, most notably a stronger dependence of central total metallicity ([Z/H]$_{e4}$) with $sigma$, along with a marginal trend of $ abla$[Z/H] to steepen in cluster galaxies, a result that is not followed by [Mg/Fe]. The results presented here serve as constraints on numerical models of the formation and evolution of ETGs.
The Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey is an ongoing project to obtain integral field spectroscopic observations of ~3400 galaxies by mid-2016. Including the pilot survey, a total of ~1000 galaxies have been observed to date, making the SAMI Galaxy Survey the largest of its kind in existence. This unique dataset allows a wide range of investigations into different aspects of galaxy evolution. The first public data from the SAMI Galaxy Survey, consisting of 107 galaxies drawn from the full sample, has now been released. By giving early access to SAMI data for the entire research community, we aim to stimulate research across a broad range of topics in galaxy evolution. As the sample continues to grow, the survey will open up a new and unique parameter space for galaxy evolution studies.
We explore stellar population properties separately in the bulge and the disk of double-component cluster galaxies to shed light on the formation of lenticular galaxies in dense environments. We study eight low-redshift clusters from the Sydney-AAO Multi-object Integral field (SAMI) Galaxy Survey, using 2D photometric bulge-disk decomposition in the $g$, $r$ and $i$-bands to characterize galaxies. For 192 double-component galaxies with $M_{*}>10^{10~}M_{odot}$ we estimate the color, age and metallicity of the bulge and the disk. The analysis of the $g-i$ colors reveals that bulges are redder than their surrounding disks with a median offset of 0.12$pm$0.02 mag, consistent with previous results. To measure mass-weighted age and metallicity we investigate three methods: (i) one based on galaxy stellar mass weights for the two components, (ii) one based on flux weights and (iii) one based on radial separation. The three methods agree in finding 62% of galaxies having bulges that are 2-3 times more metal-rich than the disks. Of the remaining galaxies, 7% have bulges that are more metal-poor than the disks, while for 31% the bulge and disk metallicities are not significantly different. We observe 23% of galaxies being characterized by bulges older and 34% by bulges younger with respect to the disks. The remaining 43% of galaxies have bulges and disks with statistically indistinguishable ages. Redder bulges tend to be more metal-rich than the disks, suggesting that the redder color in bulges is due to their enhanced metallicity relative to the disks instead of differences in stellar population age.
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