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Register a new userOn the Nature of X-ray Sources in Early-Type Galaxies
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Adrienne M. Juett
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We show that the observed relationship between the fraction of low-mass X-ray binaries (LMXBs) found in globular clusters (GCs) and the GC-specific frequency for early-type galaxies is consistent with an LMXB formation model in which the field population of LMXBs is formed in situ via primordial binary formation. The suggestion that a significant fraction of the field LMXB population in early-type galaxies was formed in GCs is not required by the data. Finally, we discuss observational studies that will test this model more thoroughly.
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The advanced capabilities of the Chandra and XMM-Newton observatories mean that, for the first time, the detailed study of the brightest point-like X-ray sources in nearby galaxies outside of the local group is a realistic aim. Here, we present the results of a Chandra ACIS-S study of two of the nearest and brightest sources in the rare ultraluminous (L(X) > 10^39 erg s^-1) X-ray source (ULX) class, NGC 5204 X-1 and NGC 4559 X-1. When considered with new optical integral field spectroscopy data this provides powerful diagnostics as to the nature of these sources, in particular suggesting that NGC 5204 X-1 is a high-mass X-ray binary, and showing new evidence linking it to the Galactic microquasar phenomenon. We also find that both ULX appear to be located in cavities in emission-line gas nebulae that surround the sources. In addition, we present the results of a Chandra observation of the interacting galaxies NGC 4485/NGC 4490, a pair of late-type spiral galaxies that, remarkably, contain a total of six ULX. We identify one as a supernovae, and the remainder as probable black hole X-ray binaries. All six are located in star formation regions, underlining the emerging link between ULX and active star formation activity.
We use deep Chandra observations to measure the emissivity of the unresolved X-ray emission in the elliptical galaxy NGC 3379. After elimination of bright, low-mass X-ray binaries with luminosities >10^{36 erg/sec, we find that the remaining unresolved X-ray emission is characterized by an emissivity per unit stellar mass L_x/M_stars ~8.2x10^{27} erg/s/M_sun in the 0.5-2 keV energy band. This value is in good agreement with those previousely determined for the dwarf elliptical galaxy M32, the bulge of the spiral galaxy M31 and the Milky Way, as well as with the integrated X-ray emissivity of cataclysmic variables and coronally active binaries in the Solar neighborhood. This strongly suggests that i) the bulk of the unresolved X-ray emission in NGC 3379 is produced by its old stellar population and ii) the old stellar populations in all galaxies can be characterized by a universal value of X-ray emissivity per unit stellar mass or per unit K band luminosity.
The past decade has seen a large progress in the X-ray investigation of early-type galaxies of the local universe, and first attempts have been made to reach redshifts z>0 for these objects, thanks to the high angular resolution and sensitivity of the satellites Chandra and XMM-Newton. Major advances have been obtained in our knowledge of the three separate contributors to the X-ray emission, that are the stellar sources, the hot gas and the galactic nucleus. Here a brief outline of the main results is presented, pointing out the questions that remain open, and finally discussing the prospects to solve them with a wide area X-ray survey mission such as WFXT.
X-ray luminosity, temperature, gas mass, total mass, and their scaling relations are derived for 94 early-type galaxies using archival $Chandra$ X-ray Observatory observations. Consistent with earlier studies, the scaling relations, $L_X propto T^{4.5pm0.2}$, $M propto T^{2.4pm0.2}$, and $L_X propto M^{2.8pm0.3}$, are significantly steeper than expected from self similarity. This steepening indicates that their atmospheres are heated above the level expected from gravitational infall alone. Energetic feedback from nuclear black holes and supernova explosions are likely heating agents. The tight $L_X - T$ correlation for low-luminosities systems (i.e., below 10$^{40}$ erg/s) are at variance with hydrodynamical simulations which generally predict higher temperatures for low luminosity galaxies. We also investigate the relationship between total mass and pressure, $Y_X = M_g times T$, finding $M propto Y_{X}^{0.45pm0.04}$. We explore the gas mass to total mass fraction in early-type galaxies and find a range of $0.1-1.0%$. We find no correlation between the gas-to-total mass fraction with temperature or total mass. Higher stellar velocity dispersions and higher metallicities are found in hotter, brighter, and more massive atmospheres. X-ray core radii derived from $beta$-model fitting are used to characterize the degree of core and cuspiness of hot atmospheres.
We present a study of the ionized gas in a sample of 65 nearby early-type galaxies, for which we have acquired optical intermediate-resolution spectra. Emission lines are detected in ~89 % of the sample. The incidence of emission appears independent from the E or S0 morphological classes. According to classical diagnostic diagrams, the majority of the galaxies are LINERs. However, the galaxies tend to move toward the Composites region (at lower [NII]/Halpha values) as the emission lines are measured at larger galacto-centric distances. This suggests that different ionization mechanisms may be at work in LINERs.
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