Finding Young Stellar Populations in Elliptical Galaxies from Independent Components of Optical Spectra

Elliptical galaxies are believed to consist of a single population of old stars formed together at an early epoch in the Universe, yet recent analyses of galaxy spectra seem to indicate the presence of significant younger populations of stars in them. The detailed physical modelling of such populations is computationally expensive, inhibiting the detailed analysis of the several million galaxy spectra becoming available over the next few years. Here we present a data mining application aimed at decomposing the spectra of elliptical galaxies into several coeval stellar populations, without the use of detailed physical models. This is achieved by performing a linear independent basis transformation that essentially decouples the initial problem of joint processing of a set of correlated spectral measurements into that of the independent processing of a small set of prototypical spectra. Two methods are investigated: (1) A fast projection approach is derived by exploiting the correlation structure of neighboring wavelength bins within the spectral data. (2) A factorisation method that takes advantage of the positivity of the spectra is also investigated. The preliminary results show that typical features observed in stellar population spectra of different evolutionary histories can be convincingly disentangled by these methods, despite the absence of input physics. The success of this basis transformation analysis in recovering physically interpretable representations indicates that this technique is a potentially powerful tool for astronomical data mining.Comment: 12 Pages, 7 figures; accepted in SIAM 2005 International Conference on Data Mining, Newport Beach, CA, April 200

FLASH redshift survey - I. Observations and Catalogue

The FLAIR Shapley-Hydra (FLASH) redshift survey catalogue consists of 4613 galaxies brighter than \bJ = 16.7 (corrected for Galactic extinction) over a 605 sq. degree region of sky in the general direction of the Local Group motion. The survey region is an approximately 60\degr \times 10\degr strip spanning the sky from the Shapley Supercluster to the Hydra cluster, and contains 3141 galaxies with measured redshifts. Designed to explore the effect of the galaxy concentrations in this direction (in particular the Supergalactic plane and the Shapley Supercluster) upon the Local Group motion, the 68% completeness allows us to sample the large-scale structure better than similar sparsely-sampled surveys. The survey region does not overlap with the areas covered by ongoing wide-angle (Sloan or 2dF) complete redshift surveys. In this paper, the first in a series, we describe the observation and data reduction procedures, the analysis for the redshift errors and survey completeness, and present the survey data.Comment: 12 pages, 12 figures, mn.sty, submitted to MNRA

First results of the XI Groups Project: Studying an unbiased sample of galaxy groups

X-ray observations of hot, intergalactic gas in galaxy groups provide a useful means of characterizing the global properties of groups. However, X-ray studies of large group samples have typically involved very shallow X-ray exposures or have been based on rather heterogeneous samples. Here we present the first results of the XI (XMM/IMACS) Groups Project, a study targeting, for the first time, a redshift-selected, statistically unbiased sample of galaxy groups using deep X-ray data. Combining this with radio observations of cold gas and optical imaging and spectroscopy of the galaxy population, the project aims to advance the understanding of how the properties and dynamics of group galaxies relate to global group properties. Here, X-ray and optical data of the first four galaxy groups observed as part of the project are presented. In two of the groups we detect diffuse emission with a luminosity of L_X ~ 10^41 erg/s, among the lowest found for any X-ray detected group thus far, with a comparable upper limit for the other two. Compared to typical X-ray selected groups of similar velocity dispersion, these four systems are all surprisingly X-ray faint. We discuss possible explanations for the lack of significant X-ray emission in the groups, concluding that these systems are most likely collapsing for the first time. Our results strongly suggest that, unlike our current optically selected sample, previous X-ray selected group samples represented a biased picture of the group population. This underlines the necessity of a study of this kind, if one is to reach an unbiased census of the properties of galaxy groups and the distribution of baryons in the Universe.Comment: 14 pages, 8 figures, accepted for publication in MNRA

Cold gas in group-dominant elliptical galaxies

We present IRAM 30m telescope observations of the CO(1-0) and (2-1) lines in a sample of 11 group-dominant elliptical galaxies selected from the CLoGS nearby groups sample. Our observations confirm the presence of molecular gas in 4 of the 11 galaxies at >4 sigma significance, and combining these with data from the literature we find a detection rate of 43+-14%, comparable to the detection rate for nearby radio galaxies, suggesting that group-dominant ellipticals may be more likely to contain molecular gas than their non-central counterparts. Those group-dominant galaxies which are detected typically contain ~2x10^8 Msol of molecular gas, and although most have low star formation rates (<1 Msol/yr) they have short depletion times, indicating that the gas must be replenished on timescales ~100 Myr. Almost all of the galaxies contain active nuclei, and we note while the data suggest that CO may be more common in the most radio-loud galaxies, the mass of molecular gas required to power the active nuclei through accretion is small compared to the masses observed. We consider possible origin mechanisms for the gas, through cooling of stellar ejecta within the galaxies, group-scale cooling flows, and gas-rich mergers, and find probable examples of each type within our sample, confirming that a variety of processes act to drive the build up of molecular gas in group-dominant ellipticals.Comment: 9 pages, 5 postscript figures, 4 tables, accepted by A&A. Revised throughout in response to referee's comments, including updates to Table 1 and Figure 4, and addition of Figure

Young stellar populations in early-type galaxies in the Sloan Digital Sky Survey

We use a purely data-driven rectified factor analysis to identify early-type galaxies with recent star formation in DR4 of the SDSS Spectroscopic Catalogue. We compare the spectra and environment of these galaxies with `normal' early-types, and a sample of independently selected E+A galaxies. We calculate the projected local galaxy surface density (Sigma_5 and Sigma_10) for each galaxy in our sample, and find that the dependence, on projected local density, of the properties of E+As is not significantly different from that of early-types with young stellar populations, dropping off rapidly towards denser environments, and flattening off at densities < 0.1-0.3 Mpc^-2. The dearth of E+A galaxies in dense environments confirms that E+As are most likely the products of galaxy-galaxy merging or interactions, rather than star-forming galaxies whose star formation has been quenched by processes unique to dense environments. We see a tentative peak in the number of E+A galaxies at Sigma_10 \~ 0.1-0.3 Mpc^-2, which may represent the local galaxy density at which the rate of galaxy-galaxy merging or interaction rate peaks. Analysis of the spectra of our early-types with young stellar populations suggests that they have a stellar component dominated by F stars, ~ 1-4 Gyr old, together with a mature, metal-rich population characteristic of `typical' early-types. The young stars represent > 10% of the stellar mass in these galaxies. This, together with the similarity of the environments in which this `E+F' population and the E+A galaxy sample are found, suggests that E+F galaxies used to be E+A galaxies, but have evolved by a further ~ one to a few Gyr. Our factor analysis is sensitive enough to identify this hidden population. (Abridged)Comment: 7 pages, 5 figures, submitted to MNRAS, minor revisio

AGN feedback with the Square Kilometer Array (SKA) and implications for cluster physics and cosmology

AGN feedback is regarded as an important non-gravitational process in galaxy clusters, providing useful constraints on large-scale structure formation. It modifies the structure and energetics of the intra-cluster medium (ICM) and hence its understanding is crucially needed in order to use clusters as high precision cosmological probes. In this context, particularly keeping in mind the upcoming high quality radio data expected from radio surveys like SKA with its higher sensitivity, high spatial and spectral resolutions, we review our current understanding of AGN feedback, its cosmological implications and the impact that SKA can have in revolutionizing our understanding of AGN feedback in large-scale structures. Recent developments regarding the AGN outbursts and its possible contribution to excess entropy in the hot atmospheres of groups and clusters, its correlation with the feedback energy in ICM, quenching of cooling flows and the possible connection between cool core clusters and radio mini-halos, are discussed. We describe current major issues regarding modeling of AGN feedback and its impact on the surrounding medium. With regard to the future of AGN feedback studies, we examine the possible breakthroughs that can be expected from SKA observations. In the context of cluster cosmology, for example, we point out the importance of SKA observations for cluster mass calibration by noting that most of $z>1$ clusters discovered by eROSITA X-ray mission can be expected to be followed up through a 1000 hour SKA-1 mid programme. Moreover, approximately $1000$ radio mini halos and $\sim 2500$ radio halos at $z<0.6$ can be potentially detected by SKA1 and SKA2 and used as tracers of galaxy clusters and determination of cluster selection function.Comment: 14 pages, 10 figures, Review article accepted in Journal of Astrophysics and Astronomy (JOAA

Deep Chandra Observations of HCG 16 - II. The Development of the Intra-group Medium in a Spiral-Rich Group

We use a combination of deep Chandra X-ray observations and radio continuum imaging to investigate the origin and current state of the intra-group medium in the spiral-rich compact group HCG 16. We confirm the presence of a faint ($L_{X,{\rm bolo}}$=1.87$^{+1.03}_{-0.66}$$\times$10$^{41}$ erg/s), low temperature (0.30$^{+0.07}_{-0.05}$ keV) intra-group medium (IGM) extending throughout the ACIS-S3 field of view, with a ridge linking the four original group members and extending to the southeast, as suggested by previous Rosat and XMM-Newton observations. This ridge contains 6.6$^{+3.9}_{-3.3}$$\times$10$^9$ solar masses of hot gas and is at least partly coincident with a large-scale HI tidal filament, indicating that the IGM in the inner part of the group is highly multi-phase. We present evidence that the group is not yet virialised, and show that gas has probably been transported from the starburst winds of NGC 838 and NGC 839 into the surrounding IGM. Considering the possible origin of the IGM, we argue that material ejected by galactic winds may have played a significant role, contributing 20-40% of the observed hot gas in the system.Comment: 11 pages, 6 figures, 1 table, accepted for publication in ApJ; updated references and fixed typos identified at proof stag

Deep Chandra Observations of HCG 16 - I. Active Nuclei, Star formation and Galactic Winds

We present new, deep Chandra X-ray and Giant Metrewave Radio Telescope 610~MHz observations of the spiral-galaxy-rich compact group HCG 16, which we use to examine nuclear activity, star formation and the high luminosity X-ray binary populations in the major galaxies. We confirm the presence of obscured active nuclei in NGC 833 and NGC 835, and identify a previously unrecognized nuclear source in NGC 838. All three nuclei are variable on timescales of months to years, and for NGC 833 and NGC 835 this is most likely caused by changes in accretion rate. The deep Chandra observations allow us to detect for the first time an Fe-K$\alpha$ emission line in the spectrum of the Seyfert 2 nucleus of NGC 835. We find that NGC 838 and NGC 839 are both starburst-dominated systems, with only weak nuclear activity, in agreement with previous optical studies. We estimate the star formation rates in the two galaxies from their X-ray and radio emission, and compare these results with estimates from the infra-red and ultra-violet bands to confirm that star formation in both galaxies is probably declining after galaxy-wide starbursts were triggered ~400-500 Myr ago. We examine the physical properties of their galactic superwinds, and find that both have temperatures of ~0.8 keV. We also examine the X-ray and radio properties of NGC 848, the fifth largest galaxy in the group, and show that it is dominated by emission from its starburst.Comment: 18 pages, 11 figures, 11 tables, accepted for publication in ApJ; updated references and fixed typos identified at proof stag

The Structure of the Outer Halo of the Galaxy and its Relationship to Nearby Large-Scale Structure

We present evidence to support an earlier indication that the Galaxy is embedded in an extended, highly inclined, triaxial halo outlined by the spatial distribution of companion galaxies to the Milky Way. Signatures of this spatial distribution are seen in 1) the angular variation of the radial-velocity dispersion of the companion galaxies, 2) the spatial distribution of the M~31 sub-group of galaxies, 3) the spatial distribution of the isolated, mainly dwarf irregular, galaxies of the Local Group, 4) the velocity anisotropy quadrupole of a sub-group of high-velocity clouds, and 5) the spatial distribution of galaxies in the Coma-Sculptor cloud. Tidal effects of M~31 and surrounding galaxies on the Galaxy are not strong enough to have affected the observed structure. We conclude that this distribution is a reflection of initial conditions. A simple galaxy formation scenario is proposed which ties together the results found here with those of Holmberg (1969) and Zaritsky et al. (1997) on the peculiar distribution of satellites around a large sample of spiral galaxies.Comment: Accepted for publication in the Astron J., March 2000, 12 pages with 1 figur