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Register a new userLine-of-sight velocity distribution corrections for Lick/IDS indices of early-type galaxies
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Authors
Harald Kuntschner
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We investigate line-of-sight velocity distribution (LOSVD) corrections for absorption line-strength indices of early-type galaxies in the Lick/IDS system. This system is often used to estimate basic stellar population parameters such as luminosity weighted ages and metallicities. Using single stellar population model spectral energy distributions by Vazdekis (1999) we find that the LOSVD corrections are largely insensitive to changes in the stellar populations for old galaxies (age >3 Gyr). Only the Lick/IDS Balmer series indices show an appreciable effect, which is on the order of the correction itself. Furthermore, we investigate the sensitivity of the LOSVD corrections to non-Gaussian LOSVDs. In this case the LOSVD can be described by a Gauss-Hermite series and it is shown that typical values of h_3 and h_4 observed in early-type galaxies can lead to significant modifications of the LOSVD corrections and thus to changes in the derived luminosity weighted ages and metallicities. A new, simple parameterisation for the LOSVD corrections, taking into account the h_3 and h_4 terms, is proposed and calibrations given for a subset of the Lick/IDS indices and two additional indices applicable to old (>3 Gyr) stellar populations.
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We have extended our evolutionary synthesis code, GALEV, to include Lick/IDS absorption-line indices for both simple and composite stellar population models (star clusters and galaxies), using the polynomial fitting functions of Worthey et al. (1994) and Worthey & Ottaviani (1997). We present a mathematically advanced Lick Index Analysis Tool (LINO) for the determination of ages and metallicities of globular clusters (CGs): An extensive grid of GALEV models for the evolution of star clusters at various metallicities over a Hubble time is compared to observed sets of Lick indices of varying completeness and precision. A dedicated chi^2 - minimisation procedure selects the best model including 1-sigma uncertainties on age and metallicity. We discuss the age and metallicity sensitivities of individual indices and show that these sensitivities themselves depend on age and metallicity; thus, we extend Wortheys (1994) concept of a metallicity sensitivity parameter for an old stellar population at solar metallicity to younger clusters of different metallicities. We find that indices at low metallicity are generally more age sensitive than at high metallicity. Our aim is to provide a robust and reliable tool for the interpretation of star cluster spectra becoming available from 10m class telescopes in a large variety of galaxies: metal-rich & metal-poor, starburst, post-burst, and dynamically young. We test our analysis tool using observations from various authors for Galactic and M31 GCs, for which reliable age and metallicity determinations are available in the literature, and discuss in how far the observational availability of various subsets of Lick indices affects the results. For M31 GCs, we discuss the influence of non-solar abundance ratios on our results.
We introduce BAYES-LOSVD, a novel implementation of the non-parametric extraction of line-of-sight velocity distributions (LOSVDs) in galaxies. We employ bayesian inference to obtain robust LOSVDs and associated uncertainties. Our method relies on principal component analysis to reduce the dimensionality of the base of templates required for the extraction and thus increase the performance of the code. In addition, we implement several options to regularise the output solutions. Our tests, conducted on mock spectra, confirm the ability of our approach to model a wide range of LOSVD shapes, overcoming limitations of the most widely used parametric methods (e.g. Gauss-Hermite expansion). We present examples of LOSVD extractions for real galaxies with known peculiar LOSVD shapes, i.e. NGC4371, IC0719 and NGC4550, using MUSE and SAURON integral-field unit (IFU) data. Our implementation can also handle data from other popular IFU surveys (e.g. ATLAS3D, CALIFA, MaNGA, SAMI). Details of the code and relevant documentation are freely available to the community in the dedicated repositories.
We present an improved study of the expected shape of the line-of-sight velocity distribution in shell galaxies. We found a simple analytical expression connecting prominent and in principle observable characteristics of the line profile and mass-distribution of the galaxy. The prediction was compared with the results from a test-particle simulation of a radial merger.
We investigate the Mg-sigma and <Fe>-sigma relations in a sample of 72 early-type galaxies drawn mostly from cluster and group environments using a homogeneous data-set which is well-calibrated onto the Lick/IDS system. The small intrinsic scatter in Mg at a given sigma gives upper limits on the spread in age and metallicity of 49% and 32% respectively, if the spread is attributed to one quantity only and if the variations in age and metallicity are uncorrelated. The age/metallicity distribution as inferred from the Hbeta vs <Fe> diagnostic diagram reinforces this conclusion, as we find mostly galaxies with large luminosity weighted ages spanning a range in metallicity. In our sample we do not find significant evidence for an anti-correlation of ages and metallicities which would keep the index-sigma relations tight while hiding a large spread in age and metallicity. As a result of correlated errors in the age-metallicity plane, a mild age-metallicity anti-correlation cannot be completely ruled out given the current data. Correcting the line-strengths indices for non-solar abundance ratios following the recent paper by Trager et al., leads to higher mean metallicity and slightly younger age estimates while preserving the metallicity sequence. The [Mg/Fe] ratio is mildly correlated with the central velocity dispersion and ranges from [Mg/Fe]=0.05 to 0.3 for galaxies with sigma > 100 km/s. Under the assumption that there is no age gradient along the index-sigma relations, the abundance-ratio corrected Mg-sigma, <Fe>-sigma and Hbeta-sigma relations give consistent estimates of Delta [M/H]/ Delta log sigma = 0.9 (+- 0.1). The slope of the Hbeta-sigma relation limits a potential age trend as a function of sigma to 2-3 Gyrs along the sequence.(abriged)
In this paper we present measurements of velocity dispersions and Lick indices for 509 galaxies in the local Universe, based on high signal-to-noise, long slit spectra obtained with the 1.52 m ESO telescope at La Silla. The conversion of our measurements into the Lick/IDS system was carried out following the general prescription of Worthey and Ottaviani 1997. Comparisons of our measurements with those of other authors show, in general, good agreement. We also examine the dependence between these indices (e.g., Hbeta, Mg_2, Fe5270 and NaD) and the central velocity dispersion (sigma), and we find that they are consistent with those previously reported in the literature. Benefiting from the relatively large size of the sample, we are able to investigate the dependence of these relations on morphology and environment, here represented by the local galaxy density. We find that for metallic lines these relations show no significant dependence on environment or morphology, except in the case of NaD, which shows distinct behavior for E and S0. On the other hand, the Hbeta-logsigma shows a significant difference as a function of the local density of galaxies, which we interpret as being caused by the truncation of star formation in high density environments. Comparing our results with those obtained by other authors we find a few discrepancies, adding to the ongoing debate about the nature of these relations. Finally, we report that the scatter of the Mg indices versus sigma relations correlate with Hbeta, suggesting that age may contribute to the scatter. Furthermore, this scatter shows no significant dependence on morphology or environment. Our results are consistent with the current downsizing model, where low mass galaxies have an extended star formation history (abridged).
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