We report the discovery of significant localized structures in the projected two-dimensional (2D) spatial distributions of the Globular Cluster (GC) systems of the ten brightest galaxies in the Virgo Cluster. We use catalogs of GCs extracted from the
HST ACS Virgo Cluster Survey (ACSVCS) imaging data, complemented, when available, by additional archival ACS data. These structures have projected sizes ranging from $sim!5$ arcsec to few arc-minutes ($sim!1$ to $sim!25$ kpc). Their morphologies range from localized, circular, to coherent, complex shapes resembling arcs and streams. The largest structures are preferentially aligned with the major axis of the host galaxy. A few relatively smaller structures follow the minor axis. Differences in the shape and significance of the GC structures can be noticed by investigating the spatial distribution of GCs grouped by color and luminosity. The largest coherent GC structures are located in low-density regions within the Virgo cluster. This trend is more evident in the red GC population, believed to form in mergers involving late-type galaxies. We suggest that GC over-densities may be driven by either accretion of satellite galaxies, major dissipationless mergers or wet dissipation mergers. We discuss caveats to these scenarios, and estimate the masses of the potential progenitors galaxies. These masses range in the interval $10^{8.5}!-!10^{9.5}$ solar masses, larger than those of the Local Group dwarf galaxies.
We report significant inhomogeneities in the projected two-dimensional (2D) spatial distributions of Low-Mass X-ray Binaries (LMXBs) and Globular Clusters (GCs) of the intermediate mass elliptical galaxy NGC4278. In the inner region of NGC4278, a sig
nificant arc-like excess of LMXBs extending south of the center at ~50 in the western side of the galaxy can be associated to a similar over-density of the spatial distribution of red GCs from~Brassington et al. (2009). Using a recent catalog of GCs produced by Usher et al.(2013) and covering the whole field of the NGC4278 galaxy, we have discovered two other significant density structures outside the D25 isophote to the W and E of the center of NGC4278, associated to an over-density and an under-density respectively. We discuss the nature of these structures in the context of the similar spatial inhomogeneities discovered in the LMXBs and GCs populations of NGC4649 and NGC4261, respectively. These features suggest streamers from disrupted and accreted dwarf companions.
We report significant anisotropies in the projected two-dimensional (2D) spatial distributions of Globular Clusters (GCs) of the giant Virgo elliptical galaxy NGC4649 (M60). Similar features are found in the 2D distribution of low-mass X-ray binaries
(LMXBs), both associated with GCs and in the stellar field. Deviations from azimuthal symmetry suggest an arc-like excess of GCs extending north at 4-15 kpc galactocentric radii in the eastern side of major axis of NGC4649. This feature is more prominent for red GCs, but still persists in the 2D distribution of blue GCs. High and low luminosity GCs also show some segregation along this arc, with high-luminosity GCs preferentially located in the southern end and low-luminosity GCs in the northern section of the arc. GC-LMXBs follow the anisotropy of red-GCs, where most of them reside; however, a significant overdensity of (high-luminosity) field LMXBs is present to the south of the GC arc. These results suggest that NGC4649 has experienced mergers and/or multiple accretions of less massive satellite galaxies during its evolution, of which the GCs in the arc may be the fossil remnant. We speculate that the observed anisotropy in the field LMXB spatial distribution indicates that these X-ray binaries may be the remnants of a star formation event connected with the merger, or maybe be ejected from the parent red GCs, if the bulk motion of these clusters is significantly affected by dynamical friction. We also detect a luminosity enhancement in the X-ray source population of the companion spiral galaxy NGC4647. We suggest that these may be younger high mass X-ray binaries formed as a result of the tidal interaction of this galaxy with NGC4649.
We investigate the radial distribution of the low-mass X-ray binary (LMXB) population in the elliptical galaxy NGC 4649, using Chandra and Hubble data to separate the field and globular cluster (GC) populations. GCs with LMXBs have the same radial di
stribution as the parent red and blue GCs. The radial profile of field LMXBs follows the V-band profile within the D25 of NGC 4649. Using the spatial information provided by our data, we find that the global galaxy-wide relations between cumulative number and luminosity of LMXBs and the integrated stellar mass hold on local scales within D25. An excess of field LMXBs with respect to the V-light is observed in the galaxys outskirt, which may be partially due to unidentified GC sources or to a rejuvenated field LMXB population caused by past merging interactions.
We present a new method for the determination of the two-dimensional (2D) projected spatial distribution of globular clusters (GCs) in external galaxies. This method is based on the K-Nearest Neighbor density estimator of Dressler (1980), complemente
d by MonteCarlo simulations to establish the statistical significance of the results. We apply this method to NGC4261, a test galaxy where significant 2D anisotropy in the GC distribution has been reported. We confirm that the 2D distribution of GC is not azimuthally isotropic. Moreover, we demonstrate that the 2D distribution departures from the average GC radial distribution results in highly significant spiral-like or broken shell features. Overall, the same perturbations are found in red and blue GCs, but with some differences. In particular, we observe a central feature, roughly aligned with the minor axis of NGC4261, composed of red and most luminous GCs. Blue and fainter GCs are more frequent at large radial distances and follow the spiral-like features of the overall density structure. These results suggest a complex merging history for NGC4261.
We report the discovery of a new ultraluminous X-ray source (ULX) associated with a globular cluster in the elliptical galaxy NGC 4649. The X-ray source was initially detected with a luminosity below 5 x 10^38 erg/s, but in subsequent observations 7
and 11 years later it had brightened substantially to 2 - 3 x 10^39 erg/s. Over the course of six separate observations it displayed significant spectral variability, in both continuum slope and absorption column. Short-term variability in the X-ray flux was also present in at least one observation. The properties of this object appear consistent with a stellar-mass black hole accreting at super-Eddington rates (i.e. in the ultraluminous accretion state), although a highly super-Eddington neutron star cannot be excluded. The coincidence of an increase in absorption column with a possible enhancement in short-term variability in at least one observation is suggestive of a clumpy radiatively-driven wind crossing our line-of-sight to the object
We report the spectral and temporal variability properties of 18 candidate transient and potential transient (TC and PTC) sources detected in deep multi-epoch Chandra observation of the nearby elliptical galaxies, NGC 3379, NGC 4278 and NGC 4697. Onl
y one source can be identified with a background counterpart, leaving 17 TCs + PTCs in the galaxies. Of these, 14 are in the galaxy field, supporting the theoretical picture that the majority of field X-ray binaries (XRBs) will exhibit transient accretion for >75% of their lifetime. Three sources are coincident with globular clusters (GCs), including two high-luminosity candidate black hole (BH) XRBs, with Lx=5.4E38 erg/s, and Lx=2.8E39 erg/s, respectively. The spectra, luminosities and temporal behavior of these 17 sources suggest that the transient population is heterogeneous, including neutron star (NS) and BH XRBs in both normal and high-rate accretion modes, and super soft sources containing white dwarf binaries. Our TC and PTC detections are noticeably fewer that the number expected from the populations synthesis (PS) models of Fragos et al. (2009), tailored to our new Chandra pointings of NGC 4278. We attribute this discrepancy to the PS assumption that the transient population is composed of NS XRBs, as well as differences between the statistical analysis and error estimates used in the model and our observations.
The analysis of a deep (579 ks) Chandra ACIS pointing of the elliptical galaxy NGC4278, which hosts a low luminosity AGN and compact radio emission, allowed us to detect extended emission from hot gas out to a radius of sim 5 kpc, with a 0.5--8 keV l
uminosity of 2.4x10^{39} erg/s. The emission is elongated in the NE-SW direction, misaligned with respect to the stellar body, and aligned with the ionized gas, and with the Spitzer IRAC 8mum non-stellar emission. The nuclear X-ray luminosity decreased by a factor of sim 18 since the first Chandra observation in 2005, a dimming that enabled the detection of hot gas even at the position of the nucleus. Both in the projected and deprojected profiles, the gas shows a significantly larger temperature (kT=0.75 keV) in the inner sim 300 pc than in the surrounding region, where it stays at sim 0.3 keV, a value lower than expected from standard gas heating assumptions. The nuclear X-ray emission is consistent with that of a low radiative efficiency accretion flow, accreting mass at a rate close to the Bondi one; estimates of the power of the nuclear jets require that the accretion rate is not largely reduced with respect to the Bondi rate. Among possibile origins for the central large hot gas temperature, such as gravitational heating from the central massive black hole and a recent AGN outburst, the interaction with the nuclear jets seems more likely, especially if the latter remain confined, and heat the nuclear region frequently. The unusual hot gas distribution on the galactic scale could be due to the accreting cold gas triggering the cooling of the hot phase, a process also contributing to the observed line emission from ionize gas, and to the hot gas temperature being lower than expected; alternatively, the latter could be due to an efficiency of the type Ia supernova energy mixing lower than usually adopted.
The VAO (Virtual Astronomical Observatory) Science Council (VAO-SC) met on July 27-28, 2011 at the Harvard-Smithsonian Center for Astrophysics in Cambridge MA, to review the VAO performance during its first year of operations. In this meeting the VAO
demonstrated the new tools for astronomers that are being released in September 2011 and presented plans for the second year of activities, resulting from studies conducted during the first year. This document contains the recommendations of the VAO-SC for the second year of activity of the VAO.
We report a detailed spectral analysis of the population of low-mass X-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 s
tellar field. The range of (0.3-8 keV) $L_X$ for these sources suggests that they may be black hole binaries (BHBs). Comparison of our results with simulations allows us to discriminate between disk and power-law dominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the $L_X sim T^4_{in}$ relation of Galactic BHBs are in the 5-15 $M_{odot}$ range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges 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 $N_H$ 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 uncover a previously unreported $L_X - R_G$ correlation (where $R_G$ 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 $L_X - R_G$ 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.
