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RE: NGC 4649
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Title: Deep Chandra Monitoring Observations of NGC 4649: I. Catalogue of Source Properties
Authors: B. Luo, G. Fabbiano, J. Strader, D.-W. Kim, J. P. Brodie, T. Fragos, J. S. Gallagher, A. King, A. Zezas

We present the X-ray source catalogue 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 D_25 ellipse. The observed 0.3-8.0 keV luminosities of these 501 sources range from 9.3x10^{36} erg/s to 5.4x10^{39} erg/s. The 90% detection completeness limit within the D_25 ellipse is 5.5x10^{37} erg/s. 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 D_25 ellipse). There are nine sources with luminosities greater than 10^{39} erg/s, 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 1.5x10^{38} erg/s. The X-ray colours suggest that the majority of the catalogue 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 D_25 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, yet the X-ray source properties are comparable in all three systems.

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Posts: 131433
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Messier 60
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Title: The massive elliptical galaxy NGC 4649 from the perspective of extended gravity
Authors: M. A. Jimenez, G. Garcia, X. Hernandez

Elliptical galaxies are systems where dark matter is usually less necessary to explain observed dynamics than in the case of spiral galaxies, however there are some instances where Newtonian gravity and the observable mass are insufficient to explain their observed structure and kinematics. Such is the case of NGC 4649, a massive elliptical galaxy in the Virgo cluster for which recent studies report a high fraction of dark matter, 0.78 at 4R_e. However this galaxy has been studied within the MOND hypothesis, where a good agreement with the observed values of velocity dispersion is found. In a similar way, we have constructed a self-consistent gravitational equilibrium dynamical model for this galaxy assuming a modified gravity force law, which is equivalent to MOND for aa_{0}. The modified gravity regime will be characterised by centrifugal equilibrium or dispersion velocities which become independent of distance, and which scale with the fourth root of the total baryonic mass, V^{4}\propto(M G a_{0}). We find that the recent detailed observations of the surface brightness profile and the velocity dispersion profile for this galaxy are consistent with the phenomenology expected in MONDian theories of modified gravity, without the need of invoking the presence of any hypothetical dark matter.

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RE: NGC 4649
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Title: A variable ultraluminous X-ray source in a globular cluster in NGC 4649
Authors: T.P. Roberts (1), G. Fabbiano (2), B. Luo (2), D.-W. Kim (2), J. Strader (2,3), M.J. Middleton (1), J.P. Brodie (4), T. Fragos (2), J.S. Gallagher (5), V. Kalogera (6), A.R. King (7), A. Zezas (8) ((1) Durham University, (2) Harvard-Smithsonian Center for Astrophysics, (3) Michigan State University, (4) UCO/Lick Observatory, (5) University of Wisconsin, (6) Northwestern University, (7) University of Leicester, (8) University of Crete)

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

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Title: Deep Chandra Monitoring Observations of NGC 4649: II. Wide-Field Hubble Space Telescope Imaging of the Globular Clusters
Authors: Jay Strader (Michigan State), Giuseppina Fabbiano (CfA), Bin Luo (CfA), Dong-Woo Kim (CfA), Jean P. Brodie (Santa Cruz), Tassos Fragos (CfA), John S. Gallagher (Wisconsin), Vassiliki Kalogera (Northwestern), Andrew King (Leicester), Andreas Zezas (Crete)

We present g and z photometry and size estimates for globular clusters (GCs) in the massive Virgo elliptical NGC 4649 (M60) using a five-pointing Hubble Space Telescope/Advanced Camera for Surveys mosaic. The metal-poor GCs show a monotonic negative metallicity gradient of (-0.43 0.10) dex per dex in radius over the full radial range of the data, out to ~ 24 kpc. There is evidence for substantial colour substructure among the metal-rich GCs. The metal-poor GCs have typical sizes ~ 0.4 pc larger than the metal-rich GCs out to large galactocentric distances (~> 20 kpc), favouring an intrinsic explanation for the size difference rather than projection effects. There is no clear relation between half-light radius and galactocentric distance beyond ~ 15 kpc, suggesting that the sizes of GCs are not generically set by tidal limitation. Finally, we identify ~ 20 candidate ultra-compact dwarfs that extend down to surprisingly faint absolute magnitudes (M_z ~ -8.5), and may bridge the gap between this class and "extended clusters" in the Local Group. Three of the brighter candidates have published radial velocities and can be confirmed as bona fide ultra-compact dwarfs; follow-up spectroscopy will determine the nature of the remainder of the candidates.

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Title: The Globular Cluster System of M60 (NGC 4649). II. Kinematics of the Globular Cluster System
Authors: Ho Seong Hwang (1), Myung Gyoon Lee (1), Hong Soo Park (1), Sang Chul Kim (2), Jang-Hyun Park (2), Young-Jong Sohn (3), Sang-Gak Lee (1), Soo-Chang Rey (4), Young-Wook Lee (3), Ho-Il Kim (2) ((1) Seoul National Univ., (2) KASI, (3) Yonsei Univ., (4) Chungnam National Univ.)

We present a kinematic analysis of the globular cluster (GC) system in the giant elliptical galaxy (gE) M60 in the Virgo cluster. Using the photometric and spectroscopic database of 121 GCs (83 blue GCs and 38 red GCs), we have investigated the kinematics of the GC system. We have found that the M60 GC system shows a significant overall rotation. The rotation amplitude of the blue GCs is slightly smaller than or similar to that of the red GCs, and their angles of rotation axes are similar. The velocity dispersions about the mean velocity and about the best fit rotation curve for the red GCs are marginally larger than those for the blue GCs. Comparison of observed stellar and GC velocity dispersion profiles with those calculated from the stellar mass profile shows that the mass-to-light ratio should be increased as the galactocentric distance increases, indicating the existence of an extended dark matter halo. The entire sample of GCs in M60 is found to have a tangentially biased velocity ellipsoid unlike the GC systems in other gEs. Two subsamples appear to have different velocity ellipsoids. The blue GC system has a modest tangentially biased velocity ellipsoid, while the red GC system has a modest radially biased or an isotropic velocity ellipsoid. From the comparison of the kinematic properties of the M60 GC system to those of other gEs (M87, M49, NGC 1399, NGC 5128, and NGC 4636), it is found that the velocity dispersion of the blue GC system is similar to or larger than that of the red GC system except for M60, and the rotation of the GC system is not negligible. The entire sample of each GC system shows an isotropic velocity ellipsoid except for M60, while the subsamples show diverse velocity ellipsoids. We discuss the implication of these results for the formation models of the GC system in gEs.

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