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Register a new userThe globular cluster system of the young elliptical NGC 6702
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We study the globular cluster (GC) system of the dust-lane elliptical galaxy NGC 6702, using B, V and I-band photometric observations carried out at the Keck telescope. This galaxy has a spectroscopic age of ~2Gyrs suggesting recent star-formation. We find strong evidence for a bimodal GC colour distribution, with the blue peak having a colour similar to that of the Galactic halo GCs. Assuming that the blue GCs are indeed old and metal-poor, we estimate an age of 2-5Gyrs and supersolar metallicity for the red GC subpopulation. Despite the large uncertainties, this is in reasonable agreement with the spectroscopic galaxy age. Additionally, we estimate a specific frequency of S_{N}=2.3pm1.1 for NGC 6702. We predict that passive evolution of NGC 6702 will further increase its specific frequency to S_{N}~2.7 within 10Gyrs, in closer agreement to that of typical present-day ellipticals. We also discuss evidence that the merger/accretion event that took place a few Gyrs ago involved a high gas fraction.
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We present a kinematic analysis of the globular cluster(GC) system in the giant elliptical galaxy (gE) NGC 4636 in the Virgo cluster. Using the photometric and spectroscopic database of 238 GCs, we have investigated the kinematics of the GC system. The NGC 4636 GC system shows weak overall rotation, which is dominated by the red GCs. However, both the blue GCs and red GCs show some rotation in the inner region at R<4.3. The velocity dispersion for all the GCs is derived to be sigma_p = 225{+12-9} km/s. The velocity dispersion for the blue GCs (sig=251 km/s) is slightly larger than that for the red GCs (sig=205 km/s). The velocity dispersions for the blue GCs about the mean velocity and about the best fit rotation curve have a significant variation depending on the galactocentric radius. Comparison of observed stellar and GC velocity dispersion profiles with the velocity dispersion profiles calculated from the stellar mass profile shows that the mass-to-light ratio should increase as the galactocentric distance increases, indicating the existence of an extended dark matter halo. From the comparison of the observed GC velocity dispersion profiles and the velocity dispersion profiles calculated for the X-ray mass profiles in the literature, we find that the orbit of the GC system is tangential, and that the orbit of the red GCs is slightly more tangential than that of the blue GCs. We compare the GC kinematics of NGC 4636 with those of other six gEs, finding that the kinematic properties of the GCs are diverse among gEs. We find several correlations between the kinematics of the GCs and the global parameters of their host galaxies. We discuss the implication of the results for the formation models of the GC system in gEs, and suggest a mixture scenario for the origin of the GCs in gEs.
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 have analysed the globular cluster (GC) system of the nucleated dwarf elliptical galaxy VCC 1087 in the Virgo cluster, based on Keck/LRIS spectroscopy and archival HST/ACS imaging. We estimate VCC 1087 hosts a total population of 77+/-19 GCs, which corresponds to a relatively high V-band specific frequency of 5.8+/-1.4. The g-z color distribution of the GCs shows a blue (metal-poor) peak with a tail of redder (metal-rich) clusters similar in color to those seen in luminous ellipticals. Spectroscopy of a subsample of 12 GCs suggests that the GC system is old and coeval (~10 Gyr), with a fairly broad metallicity distribution (-1.8<[m/H]<-0.8). In contrast, an integrated spectrum of the underlying galaxy starlight reveals that its optical luminosity is dominated by metal-rich, intermediate-aged stars. Radial velocities of the GCs suggest rotation close to the major axis of the galaxy, and this rotation is dynamically significant with (v/sigma)^* >1. A compilation of the kinematics of the GC systems of 9 early-type galaxies shows surprising diversity in the v/sigma parameter for GC systems. In this context, the GC system of VCC 1087 exhibits the most significant rotation to velocity dispersion signature. Modeling the velocity dispersion profile of the GCs and galaxy stars suggest fairly constant mass-to-light ratios of ~3 out to 6.5 kpc. The present observations can entertain both baryonic and non-baryonic solutions, and GC velocities at larger radii would be most valuable with regard to this issue. We discuss the evolution of VCC 1087 in terms of the galaxy ``harassment scenario, and conclude that this galaxy may well be the remains of a faded, tidally perturbed Sc spiral [abridged].
We present a spectroscopic study of the globular clusters (GCs) in the giant elliptical galaxy NGC 4636 in the Virgo cluster. We selected target GC candidates using the Washington photometry derived from the deep CCD images taken at the KPNO 4m. Then we obtained the spectra of 164 target objects in the field of NGC 4636 using the Multi-Object Spectroscopy (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the SUBARU 8.2m Telescope. We have measured the velocities for 122 objects: 105 GCs in NGC 4636, the nucleus of NGC 4636, 11 foreground stars, 2 background galaxies, and 3 probable intracluster GCs in the Virgo cluster. The GCs in NGC 4636 are located in the projected galactocentric radius within 10arcmin (corresponding to 43 kpc). The measured velocities for the GCs range from 300km/s to 1600km/s, with a mean value of 932_{-22}^{+25} km/s, which is in good agreement with the velocity for the nucleus of NGC 4636, 928pm 45 km/s. The velocity dispersion of the GCs in NGC 4636 is derived to be 231_{-17}^{+15} km/s and the velocity dispersion of the blue GCs is slightly larger than that of the red GCs. Combining our results with data in the literature, we produce a master catalog of radial velocities for 238 GCs in NGC 4636. The velocity dispersion of the GCs in the master catalog is found to be 225_{-9}^{+12} km/s for the entire sample, 251_{-12}^{+18} km/s for 108 blue GCs, and 205_{-13}^{+11} km/s for 130 red GCs.
We have studied the Globular Cluster System of the merger galaxy NGC 1316 in Fornax, using CCD BVI photometry. A clear bimodality is not detected from the broadband colours. However, dividing the sample into red (presumably metal- rich) and blue (metal-poor) subpopulations at B-I=1.75, we find that they follow strikingly different angular distributions. The red clusters show a strong correlation with the galaxy elongation, but the blue ones are circularly distributed. No systematic difference is seen in their radial profile and both are equally concentrated. We derive an astonishingly low Specific Frequency for NGC 1316 of only Sn=0.9, which confirms with a larger field a previous finding by Grillmair et al. (1999). Assuming a normal Sn of ~4 for early-type galaxies, we use stellar population synthesis models to estimate in 2 Gyr the age of this galaxy, if an intermediate-age population were to explain the low Sn we observe. This value agrees with the luminosity-weighted mean age of NGC 1316 derived by Kuntschner & Davies (1998) and Mackie & Fabbiano (1998). By fitting t5 functions to the Globular Cluster Luminosity Function (GCLF), we derived the following turnover magnitudes: B=24.69 +/- 0.15, V=23.87 +/- 0.20 and I=22.72 +/- 0.14. They confirm that NGC 1316, in spite of its outlying location, is at the same distance as the core of the Fornax cluster.
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