Professor Roger Davies

Abstract:

We report a detailed spectral analysis of the population of low-massX-ray binaries (LMXBs) detected in the elliptical galaxy NGC 4278 with Chandra. Seven luminous sources were studied individually, four in globular clusters (GCs) and three in the stellar field. The range of (0.3-8 keV) LX for these sources is ∼(3-8) × 1038 erg s-1, suggesting that they may be black hole binaries (BHBs). Fitting the data with either single thermal accretion disk or power-law (PO) models results in best-fit temperatures of ∼0.7-1.7 keV and Λ ∼ 1.2-2.0, consistent with those measured in Galactic BHBs. Comparison of our results with simulations allows us to discriminate between disk and power-lawdominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the LX ≥ T in4 relation of Galactic BHBs are in the 5-15M⊙ range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges (LX ≥ 1.5 × 1038 erg s-1, 6 × 1037 erg s-1 ≤ LX < 1.5 × 1038 erg s-1, and LX < 6 × 1037 erg s-1) suggests that while the high-luminosity sources have prominent thermal disk emission components, power-law components are likely to be important in the mid- and low-luminosity spectra. Comparing low-luminosity average spectra, we find a relatively larger NH in the GC spectrum; we speculate that this may point to either a metallicity effect or to intrinsic physical differences between field and GC accreting binaries. Analysis of average sample properties uncovers a previously unreported L X-RG correlation (where RG is the galactocentric radius) in the GC-LMXB sample, implying richer LMXB populations in more central GCs. No such trend is seen in the field LMXB sample. We can exclude that the GC LX-RG correlation is the by-product of a luminosity effect and suggest that it may be related to the presence of more compact GCs at smaller galactocentric radii, fostering more efficient binary formation. © 2010. The American Astronomical Society. All rights reserved.

Abstract:

From a deep multi-epoch Chandra observation of the elliptical galaxy NGC 3379 we report the spectral properties of eight luminous LMXBs (LX < 1.2 × 1038 erg s-1). We also present a set of spectral simulations, produced to aid the interpretation of low-count single-component spectral modeling. These simulations demonstrate that it is possible to infer the spectral states of X-ray binaries from these simple models and thereby constrain the properties of the source. Of the eight LMXBs studied, three reside within globular clusters (GCs) and one is a confirmed field source. Due to the nature of the luminosity cut, all sources are either neutron star (NS) binaries emitting at or above the Eddington luminosity or black hole (BH) binaries. The spectra from these sources are well described by single-component models, with parameters consistent with Galactic LMXB observations, where hard-state sources have a range in photon index of 1.5-1.9 and thermally dominant (TD) sources have inner-disk temperatures between ∼0.7 and 1.55 keV. The large variability observed in the brightest GC source (LX< 4×1038 erg s-1) suggests the presence of a BH binary. At its most luminous this source is observed in a TD state with kTin = 1.5 keV, consistent with a BH mass of ∼4M ⊙. This observation provides further evidence that GCs are able to retain such massive binaries. We also observed a source transitioning from a bright state (LX ∼ 1 × 1039 erg s-1), with prominent thermal and non-thermal components, to a less luminous hard state (LX = 3.8 × 1038 erg s-1, Λ = 1.85). In its high flux emission, this source exhibits a cool-disk component of ∼0.14 keV, similar to spectra observed in some ultraluminous X-ray sources (ULXs). Such a similarity indicates a possible link between "normal" stellar-mass BHs in a high accretion state and ULXs. © 2010. The American Astronomical Society. All rights reserved.

Abstract:

The mass assembly of galaxies leaves various imprints on their surroundings, such as shells, streams and tidal tails. The frequency and properties of these fine structures depend on the mechanism driving the mass assembly: e.g. a monolithic collapse, rapid cold-gas accretion followed by violent disk instabilities, minor mergers or major dry/wet mergers. Therefore, by studying the outskirts of galaxies, one can learn about their main formation mechanism. I present here our on-going work to characterize the outskirts of Early-Type Galaxies (ETGs), which are powerful probes at low redshift of the hierarchical mass assembly of galaxies. This work relies on ultra-deep optical images obtained at CFHT with the wide-field of view MegaCam camera of field and cluster ETGs obtained as part of the ATLAS3D and NGVS projects. State of the art numerical simulations are used to interpret the data. The images reveal a wealth of unknown faint structures at levels as faint as 29 mag arcsec-2 in the g-band. Initial results for two galaxies are presented here. © Copyright International Astronomical Union 2011.

Abstract:

The molecular gas content of local early-type galaxies is constrained and discussed in relation to their evolution. First, as part of the ATLAS 3D survey, we present the first complete, large (260 objects), volume-limited single-dish survey of CO in normal local early-type galaxies. We find a surprisingly high detection rate of 22%, independent of luminosity and at best weakly dependent on environment. Second, the extent of the molecular gas is constrained with CO synthesis imaging, and a variety of morphologies is revealed. The kinematics of the molecular gas and stars are often misaligned, implying an external gas origin in over a third of the systems, although this behaviour is drastically diffferent between field and cluster environments. Third, many objects appear to be in the process of forming regular kpc-size decoupled disks, and a star formation sequence can be sketched by piecing together multi-wavelength information on the molecular gas, current star formation, and young stars. Last, early-type galaxies do not seem to systematically obey all our usual prejudices regarding star formation, following the standard Schmidt-Kennicutt law but not the far infrared-radio correlation. This may suggest a greater diversity in star formation processes than observed in disk galaxies. Using multiple molecular tracers, we are thus starting to probe the physical conditions of the cold gas in early-types. © Copyright International Astronomical Union 2011.

Abstract:

HARMONI has been conceived as a workhorse visible and near-infrared (0.47-2.45 microns) integral field spectrograph for the European Extremely Large Telescope (E-ELT). It provides both seeing and diffraction limited observations at several spectral resolutions (R= 4000, 10000, 20000). HARMONI can operate with almost any flavor of AO (e.g. GLAO, LTAO, SCAO), and it is equipped with four spaxel scales (4, 10, 20 and 40 mas) thanks to which it can be optimally configured for a wide variety of science programs, from ultra-sensitive observations of point sources to highangular resolution spatially resolved studies of extended objects. In this paper we describe the expected performance of the instrument as well as its scientific potential. We show some simulated observations for a selected science program, and compare HARMONI with other ground and space based facilities, like VLT, ALMA, and JWST, commenting on their synergies and complementarities. © 2010 Copyright SPIE - The International Society for Optical Engineering.