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Register a new userThe Globular Cluster System of NGC 5128 I. Survey and Catalogs
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We present the results of a photometric and spectroscopic survey of the globular cluster system of NGC 5128 (Centaurus A), a galaxy whose proximity makes it an important target for early-type galaxy studies. We imaged three fields in UBVRI that extend 50 and 30 kpc along the major and minor axes, respectively. We used both color and size information to develop efficient selection criteria for differentiating between star clusters and foreground stars. In total, we obtained new velocities for 138 globular clusters, nearly tripling the number of known clusters, and bringing the confirmed total in NGC 5128 to 215. We present a full catalog of all known GCs, with their positions, photometry, and velocities. In addition, we present catalogs of other objects observed, such as foreground stars, background galaxies, three Galactic white dwarfs, seven background QSOs, and 52 optical counterparts to known X-ray point sources. We also report an observation of the cluster G169, in which we confirm the existence of a bright emission line object. This object, however, is unlikely to be a planetary nebula, but may be a supernova remnant.
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We present a study of the nearby post-merger giant elliptical galaxy, NGC 5128 (Centaurus A), in which we use the properties of its globular cluster (GC) and planetary nebula (PN) systems to constrain its evolution. Using photometric and spectroscopic data for 215 GCs presented in Paper I, we study trends in age, metallicity, and kinematics for the GC system. We confirm that the GC metallicity distribution is bimodal, and show that these two sub-populations have different properties. Using spectral line index measurements of the brightest clusters, the metal-poor GCs have old ages like the Milky Way globular clusters, while the metal-rich GCs have H-beta line-strengths that could be interpreted as a mean age of ~5 (+3/-2) Gyr. Both populations appear to have [Mg/Fe] ratios consistent with that of the Galactic GC system, although this quantity is not very well-constrained. The kinematics of the metal-rich GCs are similar to those of the planetary nebulae, exhibiting significant rotation about a misaligned axis, while the metal-poor GCs have a higher velocity dispersion and show a weaker kinematic correlation with the field stars. The total gravitating mass of NGC 5128 derived from the GCs is in excellent agreement with the value derived from stellar (PN) kinematics. We suggest that these and other data support a picture in which the main body of NGC 5128 was formed 3-8 Gyr ago by the dissipational merger of two unequal-mass disk galaxies supplemented by the continual accretion of both gas-rich and gas-poor satellites.
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 the measurement of radial velocities for globular clusters in M60, giant elliptical galaxy in the Virgo cluster. Target globular cluster candidates were selected using the Washington photometry based on the deep $16arcmin times 16arcmin$ images taken at the KPNO 4m and using the $VI$ photometry derived from the HST/WFPC2 archive images. The spectra of the target objects were obtained using the Multi-Object Spectrograph (MOS) at the Canada-France-Hawaii Telescope (CFHT). We have measured the radial velocity for 111 objects in the field of M60: 93 globular clusters (72 blue globular clusters with $1.0le(C-T_1)<1.7$ and 21 red globular clusters with $1.7le(C-T_1)<2.4$), 11 foreground stars, 6 small galaxies, and the nucleus of M60. The measured velocities of the 93 globular clusters range from $sim 500$ km s$^{-1}$ to $sim 1600$ km s$^{-1}$, with a mean value of $1070_{-25}^{+27}$ km s$^{-1}$, which is in good agreement with the velocity of the nucleus of M60 ($v_{rm gal}=1056$ km s$^{-1}$). Combining our results with data in the literature, we present a master catalog of radial velocities for 121 globular clusters in M60. The velocity dispersion of the globular clusters in the master catalog is found to be $234_{-14}^{+13}$ km s$^{-1}$ for the entire sample, $223_{-16}^{+13}$ km s$^{-1}$ for 83 blue globular clusters, and $258_{-31}^{+21}$ km s$^{-1}$ for 38 red globular clusters.
We present HST/ACS $g$ and $z$ photometry and half-light radii $R_{rm h}$ measurements of 360 globular cluster (GC) candidates around the nearby S0 galaxy NGC 3115. We also include Subaru/Suprime-Cam $g$, $r$, and $i$ photometry of 421 additional candidates. The well-established color bimodality of the GC system is obvious in the HST/ACS photometry. We find evidence for a blue tilt in the blue GCs, wherein the blue GCs get redder as luminosity increases, indicative of a mass-metallicity relationship. We find a color gradient in both the red and blue subpopulations, with each group of clusters becoming bluer at larger distances from NGC 3115. The gradient is of similar strength in both subpopulations, but is monotonic and more significant for the blue clusters. On average, the blue clusters have ~10% larger $R_{rm h}$ than the red clusters. This average difference is less than is typically observed for early-type galaxies but does match that measured in the literature for M104, suggesting that morphology and inclination may affect the measured size difference between the red and blue clusters. However, the scatter on the $R_{rm h}$ measurements is large. We also identify 31 clusters more extended than typical GCs, which we consider ultra-compact dwarf (UCD) candidates. Many of these objects are fainter than typical UCDs. While it is likely that a significant number will be background contaminants, six of these UCD candidates are spectroscopically confirmed. To explore low-mass X-ray binaries in the GC system, we match our ACS and Suprime-Cam detections to corresponding Chandra X-ray sources. We identify 45 X-ray - GC matches, 16 among the blue subpopulation and 29 among the red subpopulation. These X-ray/GC coincidence fractions are larger than is typical for most GC systems, probably due to the increased depth of the X-ray data compared to previous studies of GC systems.
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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