Hintze Centre for Astrophysical Surveys

Abstract:

We present the first low-luminosity [LX > (5-10) × 1036 ergs s-1] X-ray luminosity functions (XLFs) of low-mass X-ray binaries (LMXBs) determined for two typical old elliptical galaxies, NGC 3379 and NGC 4278. Because both galaxies contain little diffuse emission from hot ISM and no recent significant star formation (hence no high-mass X-ray binary contamination), they provide two of the best homogeneous sample of LMXBs. With 110 and 140 ks Chandra ACIS S3 exposures, we detect 59 and 112 LMXBs within the D25 ellipses of NGC 3379 and NGC 4278, respectively. The resulting XLFs are well represented by a single power law with a slope (in a differential form) of 1.9 ± 0.1. In NGC 4278, we can exclude the break at LX ∼ 5 × 1037 ergs s -1 that was recently suggested as being a general feature of LMXB XLFs. In NGC 3379, on the other hand, we find a localized excess over the power-law XLF at ∼4 × 1037 ergs s-1, but with a marginal significance of ∼1.6 σ. Because of the small number of luminous sources, we cannot constrain the high-luminosity break (at 5 × 1038 ergs s-1) found in a large sample of early-type galaxies. For our two galaxies, the ratios of the integrated LMXB X-ray luminosities to the optical luminosities differ by a factor of 4, but are consistent with the general trend of a positive correlation between the X-ray-to-optical luminosity ratio and the globular cluster specific frequency. © 2006. The American Astronomical Society. All rights reserved.

Abstract:

We summarize results from McDermid et al. (2006), who present a set of follow-up observations of the sauron representative survey of early-type galaxies. We used the oasis integral-field spectrograph (while at the Canada-France-Hawaii Telescope) to obtain high spatial resolution spectra of 28 elliptical and lenticular galaxies. These seeing-limited data have on average twice the spatial resolution of the sauron data, albeit over a smaller field. These new data reveal previously unresolved features in these objects' stellar kinematics, stellar populations, and ionized gas properties. In this contribution, we focus on the discovery of a population of compact kinematically decoupled cores in a number of our sample galaxies. These compact cores are related to regions of young stars, and counter-rotate around the host galaxy's minor axis. We compare these objects to previously known decoupled components, which in contrast are composed of old stars, and which rotate around axes unrelated to the host galaxy's kinematics or shape. A key difference between these two kinds of decoupled cores are their physical size and relative mass. The compact decoupled cores are smaller than a few hundred parsec, and constitute less than a few percent of the total galaxy mass. The classical decoupled cores exist on kiloparsec scales, and comprise around a factor 10 more mass. We suggest that the small components are only found with young ages because of their low mass-to-light ratio. We show that after a few Gyrs, these components fade into the background galaxy, making them more difficult to detect. We draw the following conclusions: 1) young stars found in early-type galaxies are very often associated with centrally-concentrated counter-rotating components; 2) the small mass fraction and kinematic decoupling of these cores suggests that the star formation is associated to minor accretion events, which effectively drive the spread in luminosity-weighted ages found in early-type galaxies; and 3) such decoupled components may be common in all early-type galaxies, but not directly observed due to their small contribution to the total galaxy light at older ages. © 2007 International Astronomical Union.

Abstract:

We discuss SAURON absorption line strength maps of a sample of 24 early-type spirals, mostly Sa. From the Lick indices H, Mgb and Fe 5015 we derive SSP-ages and metallicities. By comparing the scaling relations of Mg b and H and central velocity dispersion with the same relation for the edge-on sample of Falcn-Barroso et al. (2002) we derive a picture in which the central regions of Sa galaxies contain at least 2 components: one (or more) thin, disc-like component, often containing recent star formation, and another, elliptical-like component, consisting of old stars and rotating more slowly, dominating the light above the plane. If one defines a bulge to be the component responsible for the light in excess of the outer exponential disc, then many Sa-bulges are dominated by a thin, disc-like component containing recent star formation. © 2007 International Astronomical Union.

Abstract:

A long-standing question is whether active galactic nuclei (AGN) vary like Galactic black hole systems when appropriately scaled up by mass. If so, we can then determine how AGN should behave on cosmological timescales by studying the brighter and much faster varying Galactic systems. As X-ray emission is produced very close to the black holes, it provides one of the best diagnostics of their behaviour. A characteristic timescale--which potentially could tell us about the mass of the black hole--is found in the X-ray variations from both AGN and Galactic black holes, but whether it is physically meaningful to compare the two has been questioned. Here we report that, after correcting for variations in the accretion rate, the timescales can be physically linked, revealing that the accretion process is exactly the same for small and large black holes. Strong support for this linkage comes, perhaps surprisingly, from the permitted optical emission lines in AGN whose widths (in both broad-line AGN and narrow-emission-line Seyfert 1 galaxies) correlate strongly with the characteristic X-ray timescale, exactly as expected from the AGN black hole masses and accretion rates. So AGN really are just scaled-up Galactic black holes.