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Register a new userThe stellar populations in the low luminosity, early-type galaxy NGC59
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Low luminosity galaxies may be the building blocks of more luminous systems. Southern African Large Telescope (SALT) observations of the low luminosity, early-type galaxy NGC59 are obtained and analysed. These data are used to measure the stellar population parameters in the centre and off-centre regions of this galaxy, in order to uncover its likely star formation history. We find evidence of older stars, in addition to young stars in the emission line regions. The metallicity of the stellar population is constrained to be [Z/H] ~ -1.1 to -1.6, which is extremely low, even for this low luminosity galaxy, since it is not classed as a dwarf spheroidal galaxy. The measured [alpha/Fe] ratio is sub-solar, which indicates an extended star formation history in NGC59. If such objects formed the building blocks of more massive, early-type galaxies, then they must have been gaseous mergers, rather than dry mergers, in order to increase the metals to observed levels in luminous, early-type galaxies.
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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 present the first in a series of papers in T$h$e role of $E$nvironment in shaping $L$ow-mass $E$arly-type $N$earby g$a$laxies (hELENa) project. In this paper we combine our sample of 20 low-mass early types (dEs) with 258 massive early types (ETGs) from the ATLAS$^{mathrm{3D}}$ survey - all observed with the SAURON integral field unit (IFU) - to investigate early-type galaxies stellar population scaling relations and the dependence of the population properties on local environment, extended to the low-{sigma} regime of dEs. The ages in our sample show more scatter at lower {sigma} values, indicative of less massive galaxies being affected by the environment to a higher degree. The shape of the age-{sigma} relation for cluster vs. non-cluster galaxies suggests that cluster environment speeds up the placing of galaxies on the red sequence. While the scaling relations are tighter for cluster than for the field/group objects, we find no evidence for a difference in average population characteristics of the two samples. We investigate the properties of our sample in the Virgo cluster as a function of number density (rather than simple clustrocentric distance) and find that dE ages negatively correlate with the local density, likely because galaxies in regions of lower density are later arrivals to the cluster or have experienced less pre-processing in groups, and consequently used up their gas reservoir more recently. Overall, dE properties correlate more strongly with density than those of massive ETGs, which was expected as less massive galaxies are more susceptible to external influences.
To understand the history and formation mechanisms of galaxies it is crucial to determine their current multidimensional structure. Here we focus on stellar population properties, such as metallicity and [$alpha$/Fe] enhancement. We devise a new technique to recover the distribution of these parameters using spatially resolved, line-of-sight averaged data. Our chemodynamical method is based on the made-to-measure (M2M) framework and results in an $N$-body model for the abundance distribution. We test our method on a mock data set and find that the radial and azimuthal profiles are well-recovered, however only the overall shape of the vertical profile matches the true profile. We apply our procedure to spatially resolved maps of mean [Z/H] and [$alpha$/Fe] for the Andromeda Galaxy, using an earlier barred dynamical model of M31. We find that the metallicity is enhanced along the bar, with possible maxima at the ansae. In the edge-on view the [Z/H] distribution has an X shape due to the boxy/peanut bulge; the average vertical metallicity gradient is equal to $-0.133pm0.006$ dex/kpc. We identify a metallicity-enhanced ring around the bar, which also has relatively lower [$alpha$/Fe]. The highest [$alpha$/Fe] is found in the centre, due to the classical bulge. Away from the centre, the $alpha$-overabundance in the bar region increases with height, which could be an indication of a thick disc. We argue that the galaxy assembly resulted in a sharp peak of metallicity in the central few hundred parsecs and a more gentle negative gradient in the remaining disc, but no [$alpha$/Fe] gradient. The formation of the bar lead to the re-arrangement of the [Z/H] distribution, causing a flat gradient along the bar. Subsequent star formation close to the bar ends may have produced the metallicity enhancements at the ansae and the [Z/H] enhanced lower-$alpha$ ring.
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.
We study the stellar population far into the halo of one of the two brightest galaxies in the Coma cluster, NGC 4889, based on deep medium resolution spectroscopy with FOCAS at the Subaru 8.2m telescope. We fit single stellar population models to the measured line-strength (Lick) indices (Hbeta, Mgb, [MgFe] and <Fe>). Combining with literature data, we construct radial profiles of metallicity, [alpha/Fe] element abundance ratio and age for NGC 4889, from the center out to ~60 kpc (~4Re). We find evidence for different chemical and star formation histories for stars inside and outside 1.2Re = 18 kpc radius. The inner regions are characterized by a steep [Z/H] gradient and high [alpha/Fe] at ~2.5 times solar value. In the halo, between 18 and 60 kpc, the [Z/H] is near-solar with a shallow gradient, while [alpha/Fe] shows a strong negative gradient, reaching solar values at ~60 kpc. We interpret these data in terms of different formation histories for both components. The data for the inner galaxy are consistent with a rapid, quasi-monolithic, dissipative merger origin at early redshifts, followed by one or at most a few dry mergers. Those for the halo argue for later accretion of stars from old systems with more extended star formation histories. The half-light radius of the inner component alone is estimated as ~6 kpc, suggesting a significantly smaller size of this galaxy in the past. This may be the local stellar population signature of the size evolution found for early-type galaxies from high-redshift observations.
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