Professor Roger Davies

Abstract:

By detecting ionised-gas emission in 75% of the cases, the SAURON integral-field spectroscopic survey has further demonstrated that early-type galaxies often display nebular emission. Furthermore, the SAURON data have shown that such emission comes with an intriguing variety of morphologies, kinematic behaviours and line ratios. Perhaps most puzzling was the finding that round and slowly rotating objects generally display uncorrelated stellar and gaseous angular momenta, consistent with an external origin for the gas, whereas flatter and fast rotating galaxies host preferentially co-rotating gas and stars, suggesting internal production of gas. Alternatively, a bias against the internal production of ionised gas and against the acquisition of retrograde material may be present in these two kinds of objects, respectively. In light of the different content of hot gas in these systems, with slowly rotating objects being the only systems capable of hosting massive X-ray halos, we suggest that a varying importance of evaporation of warm gas in the hot interstellar medium can contribute to explain the difference in the relative behaviour of gas and stars in these two kinds of objects. Namely, whereas in X-ray bright and slowly rotating galaxies stellar-loss material would quickly evaporate in the hot medium, in X-ray faint and fast rotating objects such material would be allowed to lose angular momentum and settle in a disk, which could also obstruct the subsequent acquisition of retrograde gas. Evidence for a connection between warm and hot gas phases, presumably driven by heat conduction, is presented for four slowly rotating galaxies with Chandra observations. (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 a study of the internal kinematics and stellar populations of early-type galaxies in the Fornax cluster. 10 galaxies in a luminosity range of 21.8MB17.4 were observed with the integral field units (IFU) of Gemini South GMOS and VLT-VIMOS. Velocity maps and age-metallicity diagrams are presented for NGC 1404 and NGC 1419. © 2008 Copyright International Astronomical Union 2008.