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Register a new userThe SAURON project - XI. Stellar Populations from Absorption Line Strength Maps of 24 Early-Type Spirals
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Reynier F. Peletier
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We present absorption line strength maps of a sample of 24 representative early-type spiral galaxies, mostly of type Sa, obtained as part of the SAURON survey of nearby galaxies using our custom-built integral-field spectrograph. Using high-quality spectra, spatially binned to a constant signal-to-noise, we measure several key age, metallicity and abundance ratio sensitive indices from the Lick/IDS system over a contiguous two-dimensional field including bulge and inner disc. We present maps of H beta, Fe 5015 and Mg b, for each galaxy The absorption line maps show that many galaxies contain some younger populations (<= 1 Gyr), distributed in small or large inner discs, or in circumnuclear star forming rings. In many cases these young stars are formed in circumnuclear mini-starbursts, which are dominating the light in the centres of some of the early-type spirals. These mini-starburst cause a considerable scatter in index-index diagrams such as Mg b- H beta and Mg b -Fe 5015, more than is measured for early-type galaxies. We find that the central regions of Sa galaxies display a wide range in ages, even within the galaxies. 50% of the sample show velocity dispersion drops in their centres. All of the galaxies of our sample lie on or below the Mg b- $sigma$ relation for elliptical galaxies in the Coma cluster, and above the H beta absorption line - $sigma$ relation for elliptical galaxies. If those relations are considered to be relations for the oldest local galaxies we see that our sample of spirals has a considerable scatter in age, with the largest scatter at the lowest $sigma$. This is in disagreement with highly inclined samples, in which generally only old stellar populations are found in the central regions. All this can be understood if ... (see paper for rest of abstract)
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(Abridged) We present a stellar population analysis of the absorption line strength maps for 48 early-type galaxies from the SAURON sample. Using the line strength index maps of Hbeta, Fe5015, and Mgb, measured in the Lick/IDS system and spatially binned to a constant signal-to-noise, together with predictions from up-to-date stellar population models, we estimate the simple stellar population-equivalent (SSP-equivalent) age, metallicity and abundance ratio [alpha/Fe] over a two-dimensional field extending up to approximately one effective radius. We find a large range of SSP-equivalent ages in our sample, of which ~40% of the galaxies show signs of a contribution from a young stellar population. The most extreme cases of post-starburst galaxies, with SSP-equivalent ages of <=3 Gyr observed over the full field-of-view, and sometimes even showing signs of residual star-formation, are restricted to low mass systems(sigma_e <= 100 k/ms or ~2x10^10 M_sol). Spatially restricted cases of young stellar populations in circumnuclear regions can almost exclusively be linked to the presence of star-formation in a thin, dusty disk/ring, also seen in the near-UV or mid-IR. The flattened components with disk-like kinematics previously identified in all fast rotators (Krajnovic et al.) are shown to be connected to regions of distinct stellar populations. These range from the young, still star-forming circumnuclear disks and rings with increased metallicity preferentially found in intermediate-mass fast rotators, to apparently old structures with extended disk-like kinematics, which are observed to have an increased metallicity and mildly depressed [alpha/Fe] ratio compared to the main body of the galaxy. The slow rotators generally show no stellar population signatures over and above the well known metallicity gradients and are largely consistent with old (>=10 Gyr) stellar populations.
The unexpected rising flux of early-type galaxies at decreasing ultraviolet (UV) wavelengths is a long-standing mystery. One important observational constraint is the correlation between UV-optical colours and Mg2 line strengths found by Burstein et al. (1988). The simplest interpretation of this phenomenon is that the UV strength is related to the Mg line strength. Under this assumption, we expect galaxies with larger Mg gradients to have larger UV colour gradients. By combining UV imaging from GALEX, optical imaging from MDM and SAURON integral-field spectroscopy, we investigate the spatially-resolved relationships between UV colours and stellar population properties of 34 early-type galaxies from the SAURON survey sample. We find that galaxies with old stellar populations show tight correlations between the FUV colours (FUV-V and FUV-NUV) and the Mgb index, H{beta} index and metallicity [Z/H]. The equivalent correlations for the Fe5015 index, {alpha}-enhancement [{alpha}/Fe] and age are present but weaker. We have also derived logarithmic internal radial colour, measured line strength and derived stellar population gradients for each galaxy and again found a strong dependence of the FUV-V and FUV-NUV colour gradients on both the Mg b line strength and the metallicity gradients for galaxies with old stellar populations. In particular, global gradients of Mg b and [Z/H] with respect to the UV colour across galaxies are consistent with their local gradients within galaxies, suggesting that the global correlations also hold locally. From a simple model based on multi-band colour fits of UV upturn and UV-weak galaxies, we have identified a plausible range of parameters that reproduces the observed radial colour profiles. In these models, the centers of elliptical galaxies, where the UV flux is strong, are enhanced in metals by roughly 60% compared to UV-weak regions.
We present absorption line strength maps of 48 representative elliptical and lenticular galaxies obtained as part of the SAURON survey of nearby galaxies. Using high-quality spectra, spatially binned to a constant signal-to-noise, we measure four key age, metallicity and abundance ratio sensitive indices from the Lick/IDS system over a two-dimensional field extending up to approximately one effective radius. We modify the classical Fe5270 index to define a new index, Fe5270S, which maximizes the usable spatial coverage of SAURON. Maps of Hbeta, Fe5015, Mgb and Fe5270S are presented for each galaxy. We use the maps to compute average line strengths integrated over circular apertures of one-eighth effective radius, and compare the resulting relations of index versus velocity dispersion with previous long-slit work. The metal line strength maps show generally negative gradients with increasing radius roughly consistent with the morphology of the light profiles. Remarkable deviations from this general trend exist, particularly the Mgb isoindex contours appear to be flatter than the isophotes of the surface brightness for about 40% of our galaxies without significant dust features. Generally these galaxies exhibit significant rotation. We infer from this that the fast-rotating component features a higher metallicity and/or an increased Mg/Fe ratio as compared to the galaxy as a whole. The Hbeta maps are typically flat or show a mild positive outwards radial gradient, while a few galaxies show strong central peaks and/or elevated overall Hbeta-strength likely connected to recent star-formation activity. For the most prominent post-starburst galaxies even the metal line strength maps show a reversed gradient. (abridged)
The SAURON project will deliver two-dimensional spectroscopic data of a sample of nearby early-type galaxies with unprecedented quality. In this paper, we focus on the mapping of their stellar populations using the SAURON data, and present some preliminary results on a few prototypical cases.
We present line-strength gradients for 22 spectral indices measured in a sample of 82 early-type galaxies in different environments,including the high-density core of the Coma cluster, the Virgo cluster,poor groups,and field galaxies. We derive age and metallicity gradients, and compare the mean values with the predictions of different galaxy formation models. We explore the behaviour of individual chemical species by deriving the metallicity gradient with different indicators.We find that the strength of the metallicity gradient inferred from stellar population models depends on the specific Lick index employed. In particular, metallicity gradients obtained with CN2 and C4668 combined with Hb are steeper than when measured using Ca4227 or Fe4383. The correlation of the metallicity gradients with other parameters also depends on the specific index employed. If the metallicity gradient is obtained using CN2 and Mgb then it correlates with the central age of the galaxies. On the contrary, if Fe4383 or Ca4227 are used, the metallicity gradient correlates with the velocity dispersion gradient.This may suggests that several mechanism have helped to set the age and metallicity gradients in early-type galaxies. While we do not find any correlation between the metallicity gradient and the central velocity dispersion for galaxies in low-density environments, we find a marginal correlation between the metallicity gradient and the mass for galaxies in the centre of the Coma cluster. We also find a trend for which galaxies in denser environments show a steeper metallicity gradient than galaxies in less dense environments.We interpret these results in light of the different models to explain the differences between galaxies as a function of environment.
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