Professor Roger Davies

Abstract:

As a demonstration of the capabilities of the new Oxford SWIFT integral field spectrograph, we present first observations for a set of 14 early-type galaxies in the core of the Coma cluster. Our data consist of I- and z-band spatially resolved spectroscopy obtained with the Oxford SWIFT spectrograph, combined with r-band photometry from the SDSS archive for 14 early- type galaxies. We derive spatially resolved kinematics for all objects from observations of the calcium triplet absorption features at \sim 8500 {AA} . Using this kinematic information we classify galaxies as either Fast Rotators or Slow Rotators. We compare the fraction of fast and slow rotators in our sample, representing the densest environment in the nearby Universe, to results from the ATLAS3D survey, finding the slow rotator fraction is \sim 50 per cent larger in the core of the Coma cluster than in the Virgo cluster or field, a 1.2 {\sigma} increase given our selection criteria. Comparing our sample to the Virgo cluster core only (which is 24 times less dense than the Coma core) we find no evidence of an increase in the slow rotator fraction. Combining measurements of the effective velocity dispersion {\sigma_e} with the photometric data we determine the Fundamental Plane for our sample of galaxies. We find the use of the average velocity dispersion within 1 effective radius, {\sigma_e}, reduces the residuals by 13 per cent with respect to comparable studies using central velocity dispersions, consistent with other recent integral field Fundamental Plane determinations.

Abstract:

Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars, which depends on the present number of each type of star in the galaxy. The present number depends on the stellar initial mass function (IMF), which describes the distribution of stellar masses when the population formed, and knowledge of it is critical to almost every aspect of galaxy evolution. More than 50 years after the first IMF determination, no consensus has emerged on whether it is universal among different types of galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot both be universal, but they could not convincingly discriminate between the two possibilities. Only recently were indications found that massive elliptical galaxies may not have the same IMF as the Milky Way. Here we report a study of the two-dimensional stellar kinematics for the large representative ATLAS(3D) sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass, using detailed dynamical models. We find a strong systematic variation in IMF in early-type galaxies as a function of their stellar mass-to-light ratios, producing differences of a factor of up to three in galactic stellar mass. This implies that a galaxy's IMF depends intimately on the galaxy's formation history.

Abstract:

Surveying 18 12CO-bright galaxies from the ATLAS 3D early-type galaxy sample with the Institut de Radio Astronomie Millimétrique (IRAM) 30-m telescope, we detect 13CO(1-0) and 13CO(2-1) in all 18 galaxies, HCN(1-0) in 12/18 and HCO +(1-0) in 10/18. We find that the line ratios 12CO(1-0)/ 13CO(1-0) and 12CO(1-0)/HCN(1-0) are clearly correlated with several galaxy properties: total stellar mass, luminosity-weighted mean stellar age, molecular-to-atomic gas ratio, dust temperature and dust morphology. We suggest that these correlations are primarily governed by the optical depth in the 12CO lines; interacting, accreting and/or starbursting early-type galaxies have more optically thin molecular gas while those with settled dust and gas discs host optically thick molecular gas. The ranges of the integrated line intensity ratios generally overlap with those of spirals, although we note some outliers in the 12CO(1-0)/ 13CO(1-0), 12CO(2-1)/ 13CO(2-1) and HCN/HCO +(1-0) ratios. In particular, three galaxies are found to have very low 12CO(1-0)/ 13CO(1-0) and 12CO(2-1)/ 13CO(2-1) ratios. Such low ratios may signal particularly stable molecular gas which creates stars less efficiently than 'normal' (i.e. below Schmidt-Kennicutt prediction), consistent with the low dust temperatures seen in these galaxies. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Abstract:

We investigate the accuracy in the recovery of the stellar dynamics of barred galaxies when using axisymmetric dynamical models. We do this by trying to recover the mass-to-light ratio (M/L) and the anisotropy of realistic galaxy simulations using the Jeans Anisotropic Multi-Gaussian Expansion (JAM) modelling method. However, given that the biases we find are mostly due to an application of an axisymmetric modelling algorithm to a non-axisymmetric system and in particular to inaccuracies in the deprojected mass model, our results are relevant for general axisymmetric modelling methods. We run N-body collisionless simulations to build a library with various luminosity distribution, constructed to mimic real individual galaxies, with realistic anisotropy. The final result of our evolved library of simulations contains both barred and unbarred galaxies. The JAM method assumes an axisymmetric mass distribution, and we adopt a spatially constant M/L and anisotropy distributions. The models are fitted to two-dimensional maps of the second velocity moments of the simulations for various viewing angles [position angle (PA) of the bar and inclination of the galaxy]. We find that the inclination is generally well recovered by the JAM models, for both barred and unbarred simulations. For unbarred simulations the M/L is also accurately recovered, with negligible median bias and with a maximum one of just Δ(M/L) < 1.5 per cent when the galaxy is not too close to face on. At very low inclinations the M/L can be significantly overestimated (9 per cent in our tests, but errors can be larger for very face-on views). This is in agreement with previous studies. For barred simulations the M/L is on average (when PA = 45°) essentially unbiased, but we measure an over/underestimation of up to Δ(M/L) = 15 per cent in our tests. The sign of the M/L bias depends on the PA of the bar as expected: overestimation occurs when the bar is closer to end-on, due to the increased stellar motion along the line-of-sight, and underestimation otherwise. For unbarred simulations, the JAM models are able to recover the mean value of the anisotropy with bias, within the region constrained by the kinematics. However when a bar is present, or for nearly face-on models, the recovered anisotropy varies wildly, with biases up to Δβz≈ 0.3. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.