New age-metallicity diagnostic diagram for the Washington photometric system

The age calibration of the Washington deltaT1 index is mainly used to estimate ages of star clusters older than 1 Gyr, no age-metallicity degeneracy effect is considered. We have profusely exploited synthetic T1 versus C-T1 colour magnitude diagrams aiming at exploring the intrinsic behaviour of the deltaT1 index. The analysis shows that deltaT1 varies with age and metal content as well. In general, the dependence on age weakens for ages greater than ~ 6 Gyr, and results even less sensitive to age as the metallicity decreases. For ages younger than ~ 5 Gyr deltaT1 shows a strong correlation with both age and metallicity. The deltaC index -defined as deltaT1 for the C passband- is also a combined measurement of age and metallicity. We introduce a new age-metallicity diagnostic diagram, deltaT1 versus deltaC - deltaT1, which has shown the ability of unambiguously providing age and metallicity estimates, simultaneously. The new procedure allows to derive ages from 1 up to 13 Gyr and metallicities [Fe/H] from -2.0 up to +0.5 dex, and is independent of the cluster reddening and distance modulus. It does solve the constraints found in the deltaT1 index and surpasses the performance of the standard giant branch metallicity method. All these features make the diagnostic diagram a powerful tool for estimating accurate ages as well as metallicities.Comment: 14 pages, 7 figures, accepted for publication in MNRA

The usage of Stromgren photometry in studies of Local Group Dwarf Spheroidal Galaxies - Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients

In this paper we demonstrate how Stromgren uvby photometry can be efficiently used to: 1. Identify red giant branch stars that are members in a dwarf spheroidal galaxy. 2. Derive age-independent metallicities for the same stars and quantify the associated errors. Stromgren uvby photometry in a 11 x 22 arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using the Isaac Newton Telescope on La Palma. Members of the Draco dSph galaxy were identified using the surface gravity sensitive c_1 index which discriminates between red giant and dwarf stars. Thus enabling us to distinguish the (red giant branch) members of the dwarf spheroidal galaxy from the foreground dwarf stars in our galaxy. The method is evaluated through a comparison of our membership list with membership classifications in the literature based on radial velocities and proper motions. The metallicity sensitive m_1 index was used to derive individual and age-independent metallicities for the members of the Draco dSph galaxy. The derived metallicities are compared to studies based on high resolution spectroscopy and the agreement is found to be very good. We present metallicities for 169 members of the red giant branch in the Draco dwarf spheroidal galaxy (the largest sample to date). The metallicity distribution function for the Draco dSph galaxy shows a mean [Fe/H] = -1.74 dex with a spread of 0.24 dex. The correlation between metallicity and colour for the stars on the red giant branch is consistent with a dominant old, and coeval population. There is a possible spatial population gradient over the field with the most metal-rich stars being more centrally concentrated than the metal-poor stars.Comment: Accepted for publication in A&A. 26 pages, 27 figures (some at reduced resolution). High resolution version available at http://www.astro.lu.se/~daniel/draco/faria.p

Evidence for intermediate-age stellar populations in early-type galaxies from K-band spectroscopy

The study of stellar populations in early-type galaxies in different environments is a powerful tool for constraining their star formation histories. This study has been traditionally restricted to the optical range, where dwarfs around the turn-off and stars at the base of the RGB dominate the integrated light at all ages. The near-infrared spectral range is especially interesting since in the presence of an intermediate-age population, AGB stars are the main contributors. In this letter, we measure the near-infrared indices NaI and D$_{\rm CO}$ for a sample of 12 early-type galaxies in low density environments and compare them with the Fornax galaxy sample presented by Silva et al. (2008). The analysis of these indices in combination with Lick/IDS indices in the optical range reveals i) the NaI index is a metallicity indicator as good as C4668 in the optical range, and ii) D$_{\rm CO}$ is a tracer of intermediate-age stellar populations. We find that low-mass galaxies in low density environments show higher NaI and D$_{\rm CO}$ than those located in Fornax cluster, which points towards a late stage of star formation for the galaxies in less dense environments, in agreement with results from other studies using independent methods.Comment: 7 pages, 3 figures, accepted for publication in ApJ

On the Interpretation of the Age Distribution of Star Clusters in the Small Magellanic Cloud

We re-analyze the age distribution (dN/dt) of star clusters in the Small Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud Photometric Survey. For ages younger than 3x10^9 yr the dN/dt distribution can be approximated by a power-law distribution, dN/dt propto t^-beta, with -beta=-0.70+/-0.05 or -beta=-0.84+/-0.04, depending on the model used to derive the ages. Predictions for a cluster population without dissolution limited by a V-band detection result in a power-law dN/dt distribution with an index of ~-0.7. This is because the limiting cluster mass increases with age, due to evolutionary fading of clusters, reducing the number of observed clusters at old ages. When a mass cut well above the limiting cluster mass is applied, the dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is of small importance in shaping the dN/dt distribution and incompleteness causes dN/dt to decline. The reason that no (mass independent) infant mortality of star clusters in the first ~10-20 Myr is found is explained by a detection bias towards clusters without nebular emission, i.e. cluster that have survived the infant mortality phase. The reason we find no evidence for tidal (mass dependent) cluster dissolution in the first Gyr is explained by the weak tidal field of the SMC. Our results are in sharp contrast to the interpretation of Chandar et al. (2006), who interpret the declining dN/dt distribution as rapid cluster dissolution. This is due to their erroneous assumption that the sample is limited by cluster mass, rather than luminosity.Comment: 8 pages, 4 figures, accepted for publication in Ap

The Age, Metallicity and Alpha-Element Abundance of Galactic Globular Clusters from Single Stellar Population Models

Establishing the reliability with which stellar population parameters can be measured is vital to extragalactic astronomy. Galactic GCs provide an excellent medium in which to test the consistency of Single Stellar Population (SSP) models as they should be our best analogue to a homogeneous (single) stellar population. Here we present age, metallicity and $\alpha$-element abundance measurements for 48 Galactic globular clusters (GCs) as determined from integrated spectra using Lick indices and SSP models from Thomas, Maraston & Korn, Lee & Worthey and Vazdekis et al. By comparing our new measurements to independent determinations we are able to assess the ability of these SSPs to derive consistent results -- a key requirement before application to heterogeneous stellar populations like galaxies. We find that metallicity determinations are extremely robust, showing good agreement for all models examined here, including a range of enhancement methods. Ages and $\alpha$-element abundances are accurate for a subset of our models, with the caveat that the range of these parameters in Galactic GCs is limited. We are able to show that the application of published Lick index response functions to models with fixed abundance ratios allows us to measure reasonable $\alpha$-element abundances from a variety of models. We also examine the age-metallicity and [$\alpha$/Fe]-metallicity relations predicted by SSP models, and characterise the possible effects of varied model horizontal branch morphology on our overall results.Comment: 22 pages, 19 figures, accepted for publication in MNRA

The puzzlingly large Ca II triplet absorption in dwarf elliptical galaxies

We present central CaT, PaT, and CaT* indices for a sample of fifteen dwarf elliptical galaxies (dEs). Twelve of these have CaT* ~ 7 A and extend the negative correlation between the CaT* index and central velocity dispersion sigma, which was derived for bright ellipticals (Es), down to 20 < sigma < 55 km/s. For five dEs we have independent age and metallicity estimates. Four of these have CaT* ~ 7 A, much higher than expected from their low metallicities (-1.5 < [Z/H] < -0.5). The observed anti-correlation of CaT* as a function of sigma or Z is in flagrant disagreement with theory. We discuss some of the amendments that have been proposed to bring the theoretical predictions into agreement with the observed CaT*-values of bright Es and how they can be extended to incorporate also the observed CaT*-values of dEs. Moreover, 3 dEs in our sample have CaT* ~ 5 A, as would be expected for metal-poor stellar systems. Any theory for dE evolution will have to be able to explain the co-existence of low-CaT* and high-CaT* dEs at a given mean metallicity. This could be the first direct evidence that the dE population is not homogeneous, and that different evolutionary paths led to morphologically and kinematically similar but chemically distinct objects.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

The continuous rise of bulges out of galactic disks

(abridged) This study revolves around dmB, a new distance- and extinction-independent measure of the contribution by stellar populations older than 9 Gyr to the mean r-band surface brightness of the bulge component in 135 late-type galaxies (LTGs) from the CALIFA survey, spanning a range of 2.6 dex and 3 dex in total and bulge stellar mass (M*T~10^(8.9-11.5) M_solar and M*B~10^(8.3-11.3) M_solar, respectively). The main insight from this study is that LTG bulges form a continuous sequence of increasing dmB with increasing M*T, M*B, stellar mass surface density S* and mass-weighted age and metallicity: high-dmB bulges are the oldest, densest and most massive ones, and vice versa. Furthermore, we find that the bulge-to-disk age and metallicity contrast, as well as the bulge-to-disk mass ratio increase with M*T, raising from, respectively, ~0 Gyr, 0 dex and 0.25 to ~3 Gyr, ~0.3 dex and 0.67 across the mass range covered by our sample. Whereas gas excitation in lower-mass bulges is invariably dominated by star formation (SF), LINER- and Seyfert-specific emission-line ratios were exclusively documented in high-mass, high-S* bulges. The continuity both in the properties of LTG bulges themselves and in their age and metallicity contrast to their parent disks suggests that these components evolve alongside in a concurrent process that leads to a continuum of physical and evolutionary characteristics. Our results are consistent with a picture where bulge growth in LTGs is driven by a superposition of quick-early and slow-secular processes, the relative importance of which increases with M*T. These processes, which presumably combine in situ SF in the bulge and inward migration of material from the disk, are expected to lead to a non-homologous radial growth of S* and a trend for an increasing Sersic index with increasing galaxy mass.Comment: 24 pages, accepted for publication in A&

Galaxy And Mass Assembly (GAMA): Stellar-to-Dynamical Mass Relation I. Constraining the Precision of Stellar Mass Estimates

In this empirical work, we aim to quantify the systematic uncertainties in stellar mass $(M_\star)$ estimates made from spectral energy distribution (SED) fitting through stellar population synthesis (SPS), for galaxies in the local Universe, by using the dynamical mass $(M_\text{dyn})$ estimator as an SED-independent check on stellar mass. We first construct a statistical model of the high dimensional space of galaxy properties; size $(R_e)$, velocity dispersion $(\sigma_e)$, surface brightness $(I_e)$, mass-to-light ratio $(M_\star/L)$, rest-frame colour, S\'ersic index $(n)$ and dynamical mass $(M_\text{dyn})$; accounting for selection effects and covariant errors. We disentangle the correlations among galaxy properties and find that the variation in $M_\star/M_\text{dyn}$ is driven by $\sigma_e$, S\'ersic index and colour. We use these parameters to calibrate an SED-independent $M_\star$ estimator, $\hat{M}_\star$. We find the random scatter of the relation $M_\star-\hat{M}_\star$ to be $0.108\text{dex}$ and $0.147\text{dex}$ for quiescent and star-forming galaxies respectively. Finally, we inspect the residuals as a function of SPS parameters (dust, age, metallicity, star formation rate) and spectral indices (H$\alpha$, H$\delta$, $D_n4000)$. For quiescent galaxies, $\sim65\%$ of the scatter can be explained by the uncertainty in SPS parameters, with dust and age being the largest sources of uncertainty. For star-forming galaxies, while age and metallicity are the leading factors, SPS parameters account for only $\sim13\%$ of the scatter. These results leave us with remaining unmodelled scatters of $0.055\text{dex}$ and $0.122\text{dex}$ for quiescent and star-forming galaxies respectively. This can be interpreted as a conservative limit on the precision in $M_\star$ that can be achieved via simple SPS-modelling.Comment: Accepted for publication in the Astrophysical Journal on 14 June 202