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Abstract:

We present the results of deep Westerbork Synthesis Radio Telescope observations of the neutral hydrogen in 12 nearby elliptical and lenticular galaxies selected from a representative sample of nearby galaxies that were studied earlier at optical wavelengths with the integral-field spectrograph sauron. The observed objects are field galaxies, or (in two cases) are located in poor-group environments. We detect Hi in 70% of the galaxies. This detection rate is much higher than in previous, shallower single-dish surveys, and is similar to that for the ionised gas. The results suggest that at faint detection levels the presence of Hi is a relatively common characteristic of field early-type galaxies. The presence of regular disc-like structures is as common as Hi in offset clouds and tails. All galaxies where Hi is detected also contain ionised gas, whereas no Hi is found around galaxies without ionised gas. Galaxies with regular HI discs tend to have strong emission from ionised gas. In these cases, the similar kinematics of the neutral hydrogen and ionised gas suggest that they form one structure. We do not find a trend between the presence of Hi and the global age of the stellar population or with the global dynamical characteristics of the galaxies. If fast and slow rotators represent the relics of different formation paths, this does not appear in the presence and characteristics of the Hi. The links observed between the large-scale gas and the characteristics on the nuclear scale (e.g. the presence of kinematically decoupled cores and radio continuum emission), suggest that for the majority of the cases the gas is acquired through merging, but the lack of correlation with the stellar-population age suggests that smooth, cold accretion could be an alternative scenario, at least in some galaxies. In either case, the data suggest that early-type galaxies continue to build-up their mass to the present. (c) 2006 Elsevier B.V. All rights reserved.

Abstract:

ABSTRACT We present absorption line strength maps for a sample of 18 Sb-Sd galaxies observed using the integral-field spectrograph SAURON operating at the William Herschel Telescope on La Palma, as part of a project devoted to the investigation of the kinematics and stellar populations of late-type spirals, a relatively unexplored field. The SAURON spectral range allows the measurement of the Lick/IDS indices Hβ, Fe5015 and Mgb, which can be used to estimate the stellar population parameters. We present here the two-dimensional line strength maps for each galaxy. From the maps, we learn that late-type spiral galaxies tend to have high Hβ and low Fe5015 and Mgb values, and that the Hβ index has often a positive gradient over the field, while the metal indices peak in the central region. We investigate the relations between the central line strength indices and their correlations with morphological type and central velocity dispersion, and compare the observed behaviour with that for ellipticals, lenticulars and early-type spirals from the SAURON survey. We find that our galaxies lie below the Mg-σ relation determined for elliptical galaxies and that the indices show a clear trend with morphological type. From the line strength maps we calculate age, metallicity and abundance ratio maps via a comparison with model predictions; we discuss the results from a one-SSP (single stellar population) approach and from a two-SSP approach, considering the galaxy as a superposition of an old (≈13 Gyr) and a younger (age ≤5 Gyr) population. We confirm that late-type galaxies are generally younger and more metal-poor than ellipticals and have abundance ratios closer to solar values. We also explore a continuous star formation scenario, and try to recover the star formation history using the evolutionary models of Bruzual & Charlot, assuming constant or exponentially declining star formation rate. In this last case, fixing the galaxy age to 10 Gyr, we find a correlation between the e-folding time-scale τ of the starburst and the central velocity dispersion, in the sense that more massive galaxies tend to have shorter τ, suggesting that the star formation happened long ago and has now basically ended, while for smaller objects with larger values of τ it is still active now. © 2007 The Authors. Journal compilation © 2007 RAS.

Abstract:

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 (Spectrographic Areal Unit for Research on Optical Nebulae) survey of nearby galaxies using our custom-built integral-field spectrograph. Using high-quality spectra, spatially binned to a constant signal-to-noise ratio, we measure several key age, metallicity and abundance ratio sensitive indices from the Lick Observatory Image Dissector Scanner (Lick/IDS) system over a contiguous two-dimensional field including bulge and inner disc. We present maps of Hβ, Fe 5015 and Mg b for each galaxy. We find that Sa galaxies on the average have slightly smaller Mg b and Fe 5015 line strengths than ellipticals and S0s, and higher Hβ values, but with a much larger scatter. 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 ministarbursts, which are dominating the light in the centres of some of the early-type spirals. These ministarburst cause a considerable scatter in index-index diagrams such as Mg b-Hβ 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. We find that the central regions of early-type spirals are often dusty, with a good correlation between the presence of young central stellar populations and a significant amount of dust extinction. 50 per cent of the sample show velocity dispersion drops in their centres. All of the galaxies of our sample lie on or below the Mg b -σ relation for elliptical galaxies in the Coma cluster, and above the Hβ absorption line-σ 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 σ. This is in disagreement with highly inclined samples, in which generally only old stellar populations are found in the central regions. The discrepancy between our sample and highly inclined samples, and the presence of so many stellar velocity dispersion dips, i.e. so-called σ drops, in these spiral galaxies with large bulges (type Sa) can be understood if the central regions of Sa galaxies contain at least two components: a thin, disc-like component, often containing recent star formation, and another, elliptical-like component, consisting of old stars and rotating more slowly, dominating the light above the plane. These components together form the photometrically defined bulge, in the same way as the thin and the thick disc co-exist in the solar neighbourhood. In this picture, consistent with the current literature, part of the bulge, the thicker component, formed a very long time ago. Later, stars continued to form in the central regions of the disc, rejuvenating in this way the bulge through dynamical processes. This picture is able to explain in a natural way the heterogeneous stellar populations and star formation characteristics that we are seeing in detailed observations of early-type spiral galaxies. © 2007 RAS.