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Register a new userA SLUGGS and Gemini/GMOS combined study of the elliptical galaxy M60: wide-field photometry and kinematics of the globular cluster system
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We present new wide-field photometry and spectroscopy of the globular clusters (GCs) around NGC 4649 (M60), the third brightest galaxy in the Virgo cluster. Imaging of NGC 4649 was assembled from a recently-obtained HST/ACS mosaic, and new Subaru/Suprime-Cam and archival CFHT/MegaCam data. About 1200 sources were followed up spectroscopically using combined observations from three multi-object spectrographs: Keck/DEIMOS, Gemini/GMOS and MMT/Hectospec. We confirm 431 unique GCs belonging to NGC 4649, a factor of 3.5 larger than previous datasets and with a factor of 3 improvement in velocity precision. We confirm significant GC colour bimodality and find that the red GCs are more centrally concentrated, while the blue GCs are more spatially extended. We infer negative GC colour gradients in the innermost 20 kpc and flat gradients out to large radii. Rotation is detected along the galaxy major axis for all tracers: blue GCs, red GCs, galaxy stars and planetary nebulae. We compare the observed properties of NGC 4649 with galaxy formation models. We find that formation via a major merger between two gas-poor galaxies, followed by satellite accretion, can consistently reproduce the observations of NGC 4649 at different radii. We find no strong evidence to support an interaction between NGC 4649 and the neighbouring spiral galaxy NGC 4647. We identify interesting GC kinematic features in our data, such as counter-rotating subgroups and bumpy kinematic profiles, which encode more clues about the formation history of NGC 4649.
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We present a photometric study of the globular clusters in the giant elliptical galaxy M60 in the Virgo cluster, based on deep, relatively wide field Washington CT_1 CCD images. The color-magnitude diagram reveals a significant population of globular clusters in M60, and a large number of young luminous clusters in NGC 4647, a small companion spiral galaxy north-west of M60. The color distribution of the globular clusters in M60 is clearly bimodal, with a blue peak at (C-T_1)=1.37, and a red peak at (C-T_1)=1.87. We derive two new transformation relations between the (C-T_1)_0 color and [Fe/H] using the data for the globular clusters in our Galaxy and M49. Using these relations we derive the metallicity distribution of the globular clusters in M60, which is also bimodal: a dominant metal-poor component with center at [Fe/H]=-1.2, and a weaker metal-rich component with center at [Fe/H]=-0.2. The radial number density profile of the globular clusters is more extended than that of the stellar halo, and the radial number density profile of the blue globular clusters is more extended than that of the red globular clusters. The number density maps of the globular clusters show that the spatial distribution of the blue globular clusters is roughly circular, while that of the red globular cluster is elongated similarly to that of the stellar halo. We estimate the total number of the globular clusters in M60 to be 3600+/-500$,and the specific frequency to be S_N=3.8+/-0.4. The mean color of the bright blue globular clusters gets redder as they get brighter in both the inner and outer region of M60. This blue tilt is seen also in the outer region of M49, the brightest Virgo galaxy. Implications of these results are discussed.
We present Sloan g and i imaging from the GMOS instrument on the Gemini North telescope for the globular cluster (GC) system around the Virgo galaxy NGC 4649 (M60). Our three pointings, taken in good seeing conditions, cover an area of about 90 sq. arcmins. We detect 2,151 unresolved sources. Applying colour and magnitude selection criteria to this source list gives 995 candidate GCs that is greater than 90% complete to a magnitude of i = 23.6, with little contamination from background galaxies. We find fewer than half a dozen potential Ultra Compact Dwarf galaxies around NGC 4649. Foreground extinction from the nearby spiral NGC 4647 is limited to be A_V < 0.1. We confirm the bimodality in the GC colour distribution found by earlier work using HST/WFPC2 imaging. As is commonly seen in other galaxies, the red GCs are concentrated towards the centre of the galaxy, having a steeper number density profile than the blue GC subpopulation. The varying ratio of red-to-blue GCs with radius can largely explain the overall GC system colour gradient. The underlying galaxy starlight has a similar density profile slope and colour to the red GCs. This suggests a direct connection between the galaxy field stars and the red GC subpopulation. We estimate a total GC population of 3700 +/- 900, with the uncertainty dominated by the extrapolation to larger radii than observed. This total number corresponds to a specific frequency S_N = 4.1 +/- 1.0. Future work will present properties derived from GMOS spectra of the NGC 4649 GCs.
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.
We present preliminary results of a wide field study of the globular cluster system of NGC4594, the Sombrero galaxy. The galaxy was observed in B, V, and R using the Wide Field Imager on the ESO 2.2m telescope. Using color and shape criteria to select a sample of highly probable globular cluster candidates, we measured the radial density profile of clusters out to 40 (100 Kpc) in the galaxy halo. The colors are consistent with the bimodal color distribution observed in previous studies. The red cluster candidates show a clear central concentration relative to the blue clusters. The population of red clusters does not appear significantly flattened, thus indicating that they are associated to the galaxy bulge rather than to the disk.
A detailed imaging analysis of the globular cluster (GC) system of the Sombrero galaxy (NGC 4594) has been accomplished using a six-image mosaic from the Hubble Space Telescope Advanced Camera for Surveys. The quality of the data is such that contamination by foreground stars and background galaxies is negligible for all but the faintest 5% of the GC luminosity function (GCLF). This enables the study of an effectively pure sample of 659 GCs until ~2 mags fainter than the turnover magnitude, which occurs at M_V=-7.60+/-0.06 for an assumed m-M=29.77. Two GC metallicity subpopulations are easily distinguishable, with the metal-poor subpopulation exhibiting a smaller intrinsic dispersion in color compared to the metal-rich subpopulation. Three new discoveries include: (1) A metal-poor GC color-magnitude trend. (2) Confirmation that the metal-rich GCs are ~17% smaller than the metal-poor ones for small projected galactocentric radii (less than ~2 arcmin). However, the median half-light radii of the two subpopulations become identical at ~3 arcmin from the center. This is most easily explained if the size difference is the result of projection effects. (3) The brightest (M_V < -9.0) members of the GC system show a size-magnitude upturn where the average GC size increases with increasing luminosity. Evidence is presented that supports an intrinsic origin for this feature rather than a being result from accreted dwarf elliptical nuclei. In addition, the metal-rich GCs show a shallower positive size-magnitude trend, similar to what is found in previous studies of young star clusters.
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