Louisa A. Nolan

F stars, metallicity and the ages of red galaxies at z > 1

We explore whether the rest-frame near-ultraviolet spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by first testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-ultraviolet spectra of F stars of known (subsolar and supersolar) metallicity. F stars are of particular interest because the rest-frame near-ultraviolet spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turn-off. We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥ 3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein-de Sitter universe, and favour a A-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3-5.

Premature dismissal of high‐redshift elliptical galaxies

It has recently been argued that single-collapse high-redshift models for elliptical galaxy formation can be rejected because they predict large numbers of very red galaxies at intermediate redshifts, which are not seen in deep optical--infrared surveys. We argue, however, that this conclusion is premature since, while much effort has been invested in refining the predictions of hierarchical CDM models, only very simplistic models have been used to study the evolution of galaxies in other cosmogonies (e.g. isocurvature models). We demonstrate that the use of a more realistic multizone chemo-dynamical single-collapse model yields colours at intermediate redshifts that are much bluer than those inferred from the one-zone model, and indeed are comparable to those predicted by hierarchical merging, despite still allowing > 90 per cent of the final stellar mass of elliptical galaxies to be formed in the first Gyr of their evolution. We, therefore, conclude that the one-zone model should be avoided for prediction of the colours of high-redshift galaxies and that the use of realistic multizone models allows the existence of ellipticals at high redshift, their dismissal being premature.

A data-driven Bayesian approach for finding young stellar populations in early-type galaxies from their ultraviolet-optical spectra

Efficient predictive models and data analysis techniques for the analysis of photometric and spectroscopic observations of galaxies are not only desirable, but also required, in view of the overwhelming quantities of data becoming available. We present the results of a novel application of Bayesian latent variable modelling techniques, where we have formulated a data-driven algorithm that allows one to explore the stellar populations of a large sample of galaxies from their spectra, without the application of detailed physical models. Our only assumption is that the galaxy spectrum can be expressed as a linear superposition of a small number of independent factors, each a spectrum of a stellar subpopulation that cannot be individually observed. A probabilistic latent variable architecture that explicitly encodes this assumption is then formulated, and a rigorous Bayesian methodology is employed for solving the inverse modelling problem from the available data. A powerful aspect of this method is that it formulates a density model of the spectra, based on which we can handle observational errors. Further, we can recover missing data both from the original set of spectra which might have incomplete spectral coverage of each galaxy, or from previously unseen spectra of the same kind. We apply this method to a sample of 21 ultraviolet–optical spectra of well-studied early-type galaxies, for which we also derive detailed physical models of star formation history (i.e. age, metallicity and relative mass fraction of the component stellar populations). We also apply it to synthetic spectra made up of two stellar populations, spanning a large range of parameters. We apply four different data models, starting from a formulation of principal component analysis (PCA), which has been widely used. We explore alternative factor models, relaxing the physically unrealistic assumption of Gaussian factors, as well as constraining the possibility of negative flux values that are allowed in PCA, and show that other models perform equally well or better, while yielding more physically acceptable results. In particular, the more physically motivated assumptions of our rectified factor analysis enable it to perform better than PCA, and to recover physically meaningful results. We find that our data-driven Bayesian modelling allows us to identify those early-type galaxies that contain a significant stellar population that is ≲1-Gyr old. This experiment also concludes that our sample of early-type spectra showed no evidence of more than two major stellar populations differing significantly in age and metallicity. This method will help us to search for such young populations in a large ensemble of spectra of early-type galaxies, without fitting detailed models, and thereby to study the underlying physical processes governing the formation and evolution of early-type galaxies, particularly those leading to the suppression of star formation in dense environments. In particular, this method would be a very useful tool for automatically discovering various interesting subclasses of galaxies, for example, post-starburst or E+A galaxies.

A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where Are the X-Ray Binaries?

We present results from a 34 ks Chandra ACIS-S observation of the low-mass X-ray binary (LMXB) population and the hot interstellar medium (ISM) in the nearby (d = 3.1 Mpc) lenticular galaxy NGC 5102, previously shown to have an unusually low X-ray luminosity. We detect 11 X-ray point sources within the D25 optical boundary of the galaxy (93% of the light), one-third to one-half of which are likely to be background active galactic nuclei (AGNs). One of the X-ray sources is coincident with the optical nucleus and may be a low-luminosity AGN. Only two sources with an X-ray luminosity greater than 1037 ergs s-1 in the 0.5-5.0 keV band were detected, one of which is statistically likely to be a background AGN. We expected to detect seven or five such luminous sources if the X-ray binary (XRB) population scales linearly with the B-band or J-band magnitudes, respectively, of the host galaxy. By this measure, NGC 5102 has an unusually low number of XRBs. The deficit of LMXBs is even more striking, because some of these sources may in fact be high-mass X-ray binaries (HMXBs). NGC 5102 is unusually blue for its morphological type and has undergone at least two recent bursts of star formation only ~1.5 × 107 and ~3 × 108 yr ago. We present the results of optical/UV spectral synthesis analysis and demonstrate that a significant fraction (>50%) of the stars in this galaxy are comparatively young (<3 × 109 yr old). We discuss the relationship between the XRB population, the globular cluster (GC) population, and the relative youth of the majority of stars in this galaxy. If the lack of X-ray binaries is related to the relative youth of most of the stars, this would support models of LMXB formation and evolution that require wide binaries to shed angular momentum on a timescale of Gyr. We have also analyzed archival Hubble Space Telescope (HST) images of NGC 5102 and find that it has an unusually low specific frequency of GCs (SN ~ 0.4). The lack of LMXBs could also be explained by the small number of GCs. We have also detected diffuse X-ray emission in the central ~1 kpc of the galaxy with an X-ray luminosity of 4.1 × 1037 ergs s-1 in the 0.1-2.0 keV band. This hot gas is most likely a superbubble created by multiple supernovae of massive stars born during the most recent star burst and is driving the shock into the ISM, which was inferred from previous [O III] λ5007 and Hα observations.

The star formation histories of elliptical galaxies across the Fundamental Plane

We present the first results from a study designed to test whet her, given high-quality spectrophotometry spanning the mid-ultraviolet‐optical wavelength regime, it is possible to distinguish the metal content and star-formation history of in dividual elliptical galaxies with sufficient accuracy to establish whether their formation hi story is linked to their detailed morphology and position on the Fundamental Plane. From a detailed analysis of ultraviolet-optical spectroph otometry of the ‘cuspy’ elliptical galaxy NGC 3605 and the giant elliptical NGC 5018 we find that: 1) optical spectra with λ > 3500 ˚ A may not contain sufficient information to robustly uncover all the stellar populations present in individual galaxies, even in such re latively passive objects as elliptical galaxies, 2) the addition of the ultraviolet data approachi ng λ = 2500 ˚ A holds the key to establishing well-constrained star-formation histories for th ese galaxies, from which we can infer a formation and evolution history which is consistent with their photometric properties, 3) despite the superficial similarity of their spectra, the two galaxies have very different ‘recent’ star-formation histories ‐ the smaller, cuspy elliptical N GC 3605 contains a high-metallicity population of age ≃ 1 Gyr, and has a position on the fundamental plane typical of the product of a low-redshift gas-rich merger (most likely at z ∼ 0.08), while the giant elliptical NGC 5018, with a sub-solar secondary population, appears to have gained its more recent stars via mass transfer / accretion of gas from its spiral companion, 4) despite these differences in detailed history, more than 85% of the stellar mass of both galaxies is associated with an old (9-12 Gyr) stellar population of near-solar metallicity. This pilot study provides strong motivation for the construction and analysis of highquality ultraviolet-optical spectra for a substantial sam ple of ellipticals spanning the fundamental plane.

On class visualisation for high dimensional data: exploring scientific data sets

Parametric Embedding (PE) has recently been proposed as a general-purpose algorithm for class visualisation. It takes class posteriors produced by a mixture-based clustering algorithm and projects them in 2D for visualisation. However, although this fully modularised combination of objectives (clustering and projection) is attractive for its conceptual simplicity, in the case of high dimensional data, we show that a more optimal combination of these objectives can be achieved by integrating them both into a consistent probabilistic model. In this way, the projection step will fulfil a role of regularisation, guarding against the curse of dimensionality. As a result, the tradeoff between clustering and visualisation turns out to enhance the predictive abilities of the overall model. We present results on both synthetic data and two real-world high-dimensional data sets: observed spectra of early-type galaxies and gene expression arrays.

Star formation history and isophotal shapes of fossil central galaxies

We study the structure and stellar populations of the luminous elliptical galaxies dominating fossil groups and compare them with the brightest galaxies in ordinary groups. Despite being over-luminous, the fossil central galaxies do not show boxy stellar isophotes which are usually associated with luminous elliptical galaxies. Boxy isophotes, according to the numerical simulations, are produced in gas poor mergers. The isophotal shapes of the fossil central galaxies, therefore, suggest a gas rich merger for fossil central galaxies. Using a two-component spectral fitting, we show that the dominant stellar population of the fossil and non-fossil galaxies is old and the second population is either old or intermediate age. However, the second stellar component (recently-formed stars) in fossil central galaxies is significantly more metal poor than that in the brightest galaxies of non-fossil groups.