S. Zepf

Modeling the Dynamical Evolution of the M87 Globular Cluster System

We study the dynamical evolution of the M87 globular cluster system (GCS) with a number of numerical simulations. We explore a range of different initial conditions for the GCS mass function (GCMF), for the GCS spatial distribution, and for the GCS velocity distribution. Our simulations include the effects of two-body relaxation, dynamical friction, and mass loss due to stellar evolution. We first confirm that an initial power-law GCMF such as that observed in young cluster systems can be readily transformed through dynamical processes into a bell-shaped GCMF. However, only models with initial velocity distributions characterized by a strong radial anisotropy increasing with the galactocentric distance are able to reproduce the observed constancy of the GCMF at all radii. We show that such strongly radial orbital distributions are inconsistent with the observed kinematics of the M87 GCS. The evolution of models with a bell-shaped GCMF with a turnover similar to that currently observed in old GCSs is also investigated. We show that models with this initial GCMF can satisfy all the observational constraints currently available on the GCS spatial distribution, the GCS velocity distribution, and on the GCMF properties. In particular, these models successfully reproduce both the lack of a radial gradient of the GCS mean mass recently found in an analysis of Hubble Space Telescope images of M87 at multiple locations and the observed kinematics of the M87 GCS. Our simulations also show that evolutionary processes significantly affect the initial GCS properties by leading to the disruption of many clusters and changing the masses of those that survive. The preferential disruption of inner clusters flattens the initial GCS number density profile, and it can explain the rising specific frequency with radius; we show that the inner flattening observed in the M87 GCS spatial distribution can be the result of the effects of dynamical evolution on an initially steep density profile.

MODELS FOR LOW-MASS X-RAY BINARIES IN THE ELLIPTICAL GALAXIES NGC 3379 AND NGC 4278 : COMPARISON WITH OBSERVATIONS

We present theoretical models for the formation and evolution of populations of low-mass X-ray binaries (LMXB) in the two elliptical galaxies NGC 3379 and NGC 4278. The models are calculated with the recently updated StarTrack code (Belczynski et al. 2007), assuming only a primordial galactic field LMXB population. StarTrack is an advanced population synthesis code that has been tested

Effects of the dissolution of low - concentration globular clusters on the evolution of globular cluster systems

We investigate the role of dissolution of low-concentration clusters due to mass loss through stellar evolution on the evolution of the properties of globular cluster systems (GCSs) in elliptical galaxies. Our simulations show that, for an initial mass-concentration relationship based on that inferred from Galactic globular clusters, dissolution of low-concentration clusters leads to the disruption of a large number of clusters. A power-law initial globular cluster system mass function (GCMF) similar to that observed in young cluster systems in merging galaxies is transformed by this dissolution into a bell-shaped GCMF with a mean mass similar to that of old GCSs for all the galaxies investigated. Two-body relaxation and dynamical friction, which are also included in our simulations, subsequently lead to an additional significant evolution and disruption of the population of clusters. As shown previously, when these processes act on a bell-shaped GCMF with a mean mass similar to that of old GCSs, they do not significantly alter the value of the mean mass. The final GCMFs are bell-shaped with similar peaks at different radii within galaxies and between different galaxies, in agreement with current observations.We investigate the role of dissolution of low-concentration clusters due to mass loss through stellar evolution on the evolution of the properties of globular cluster systems (GCSs) in elliptical galaxies. Our simulations show that, for an initial mass-concentration relationship based on that inferred from Galactic globular clusters, dissolution of low-concentration clusters leads to the disruption of a large number of clusters. A power-law initial globular cluster system mass function (GCMF) similar to that observed in young cluster systems in merging galaxies is transformed by this dissolution into a bell-shaped GCMF with a mean mass similar to that of old GCSs for all the galaxies investigated. Two-body relaxation and dynamical friction, which are also included in our simulations, subsequently lead to an additional significant evolution and disruption of the population of clusters. As shown previously, when these processes act on a bell-shaped GCMF with a mean mass similar to that of old GCS, they do not significantly alter the value of the mean mass. The final GCMFs are bell-shaped with similar peaks at different radii within galaxies and between different galaxies, in agreement with current observations.

The X-ray spectra of the luminous LMXBs in NGC 3379: Field and globular cluster sources

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× 10 38 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-r ay binaries from these simple models and thereby constrain the properties of the source. Of the eight LMXBs studied, three reside within globular clusters, and one is a confirmed field source. Due to the nature of the luminosity cut all sources are either neu tron star binaries emitting at or above the Eddington luminosity or black hole 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

Transient Low-mass X-ray Binary Populations in Elliptical Galaxies NGC 3379 and NGC 4278

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration, and recurrence time. We apply this prescription to the population synthesis models of field LMXBs presented by Fragos et al., and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC 3379 and NGC 4278, which revealed several transient sources. We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Finally, our comparison suggests that transient LMXBs are very rare in globular clusters (GCs), and thus the number of identified transient LMXBs may be used as a tracer of the relative contribution of field and GC LMXB populations.

Probing the Low-Luminosity X-Ray Luminosity Function in Normal Elliptical Galaxies

We present the first low luminosity (LX > 5 - 10 1036 erg 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 ellipse 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 x 1037 erg s-1 that was recently suggested to be a general feature of LMXB XLFs. In NGC 3379 instead we find a localized excess over the power law XLF at ~4 x 1037 erg s-1, but with a marginal significance of ~1.6s. Because of the small number of luminous sources, we cannot constrain the high luminosity break (at 5 x 1038 erg s-1) found in a large sample of early type galaxies. While the optical luminosities of the two galaxies are similar, their integrated LMXB X-ray luminosities differ by a factor of 4, consistent with the relation between the X-ray to optical luminosity ratio and the globular cluster specific frequency.

Deep Chandra Monitoring Observations of NGC 3379 : Catalog of Source Properties

We present the X-ray source catalog for the Chandra monitoring observations of the elliptical galaxy, NGC 4649. The galaxy has been observed with Chandra ACIS-S3 in six separate pointings, reaching a total exposure of 299 ks. There are 501 X-ray sources detected in the 0.3–8.0 keV band in the merged observation or in one of the six individual observations; 399 sources are located within the D25 ellipse. The observed 0.3–8.0 keV luminosities of these 501 sources range from 9.3 × 10 36 erg s −1 to 5.4 × 10 39 erg s −1 . The 90% detection completeness limit within the D25 ellipse is 5.5 × 10 37 erg s −1 . Based on the surface density of background active galactic nuclei (AGNs) and detection completeness, we expect ≈45 background AGNs among the catalog sources (≈15 within the D25 ellipse). There are nine sources with luminosities greater than 10 39 erg s −1 , which are candidates for ultraluminous X-ray sources. The nuclear source of NGC 4649 is a low-luminosity AGN, with an intrinsic 2.0–8.0 keV X-ray luminosity of 1.5 × 10 38 erg s −1 . The X-ray colors suggest that the majority of the catalog sources are low-mass X-ray binaries (LMXBs). We find that 164 of the 501 X-ray sources show long-term variability, indicating that they are accreting compact objects. We discover four transient candidates and another four potential transients. We also identify 173 X-ray sources (141 within the D25 ellipse) that are associated with globular clusters (GCs) based on Hubble Space Telescope and ground-based data; these LMXBs tend to be hosted by red GCs. Although NGC 4649 has a much larger population of X-ray sources than the structurally similar early-type galaxies, NGC 3379 and NGC 4278, the X-ray source properties are comparable in all three systems.

DISCOVERY OF HOT GAS IN OUTFLOW IN NGC 3379

We report the discovery of a faint ( -->LX ~ 4 ? 1.5 ? 1037 ergs s?1, 0.5-2 keV), outflowing gaseous hot interstellar medium (ISM) in NGC 3379. This represents the lowest X-ray luminosity ever measured from a hot phase of the ISM in a nearby early-type galaxy. The discovery of the hot ISM in a very deep Chandra observation was possible thanks to its unique spectral and spatial signatures, which distinguish it from the integrated stellar X-ray emission, responsible for most of the unresolved emission in the Chandra data. This hot component is found in a region of ~800 pc in radius at the center of the galaxy and has a total mass -->M ~ 3 ? 1 ? 105 M?. Independent theoretical prediction of the characteristics of an ISM in this galaxy, based on the intrinsic properties of NGC 3379, reproduce well the observed luminosity, temperature, and radial distribution and mass of the hot gas, and indicate that the gas is in an outflowing phase, predicted by models but not observed in any system so far.

Near-Infrared Observations of Globular Clusters in NGC 4472, NGC 4594, NGC 3585, and NGC 5813 and Implications for Their Ages and Metallicities

We present near-infrared photometry of the globular cluster systems of the early-type galaxies NGC 4472, NGC 4594, NGC 3585, and NGC 5813. We combine these near-infrared data, obtained with PANIC at the Magellan Baade 6.5 m telescope, with archival optical HST and FORS/VLT data, and use the optical to near-infrared colors to constrain the ages and metallicities of the globular clusters around the target galaxies. For NGC 4472 we have the most extensive near-infrared and optical photometric data set. These colors show that the NGC 4472 globular cluster system has a broad metallicity distribution and that the clusters are predominantly old (i.e., ages of about 10 Gyr or more). This result agrees well with earlier spectroscopic work on NGC 4472, and is evidence that the combination of optical to near-infrared colors can identify predominantly old systems and distinguish these from systems with a substantial intermediate-age component. Based on the smaller sample of combined optical and near-infrared data, NGC 4594 and NGC 3585 appear to have predominantly old globular cluster systems, while that of NGC 5813 may have a more significant age spread. We also match our sample of globular clusters with near-infrared and optical photometry to Chandra X-ray source detections in these galaxies, and discuss how the probability that a globular cluster hosts a low-mass X-ray binary depends on metallicity and age.

The globular cluster mass/low mass X-ray binary correlation: implications for kick velocity distributions from supernovae

Optical and X-ray studies of six nearby galaxies show that the probability a globular cluster will be an X-ray source is consistent with being linearly proportional to its mass. We show that this result is consistent with some recent estimates of the velocity kick distributions for isolated radio pulsars - those which are the sum of two Maxwellians, with the slower distribution at about 100 km s-1 - so long as a large fraction of the retained binaries are in binary systems with other massive stars. We confirm that over a large sample of galaxies, metallicity is clearly a factor in determining whether a globular cluster will contain an X-ray binary, and we estimate the transformations between color and metallicity for a large number of optical filter combinations. We also show that the core interaction rate is roughly linearly proportional to the stellar mass of a globular cluster for the Milky Way when one bins the clusters by mass.