The Globular Cluster Systems in Virgo and M85

Abstract

This thesis presents a study of the globular clusters (GCs) in the Virgo cluster and M85 based on the comprehensive photometric and spectroscopic surveys. The GCs are of great use to trace the formation and evolution history of their host environments. We investigate the origin of intracluster GCs (IGCs) in the Virgo cluster as well as the GCs in the merger remnant galaxy M85. We present the results of a wide-field spectroscopic survey of GCs in the Virgo cluster. We obtain spectra for 201 GCs and 55 ultracompact dwarfs (UCDs) using Hectospec on the Multiple-Mirror Telescope and derive their radial velocities. We identify 46 genuine IGCs, not associated with any Virgo galaxies, using the 3D GMM test on the spatial and radial velocity distribution. They are located at a projected distance 200 kpc < R < 500 kpc from the center of M87. The radial velocity distribution of these IGCs shows two peaks, one at vr =1023 km s−1, associated with the Virgo main body, and another at vr = 36 km s−1, associated with the infalling structure. The velocity dispersion of the IGCs in the Virgo main body is sigma_IGC ∼ 314 km s−1, which is smoothly connected to the velocity dispersion profile of M87 GCs but is much lower than that of dwarf galaxies in the same survey field, sigma_dwarf ∼ 608 km s−1. The UCDs are more centrally concentrated on massive galaxies–M87, M86, and M84. The radial velocity dispersion of the UCD system is much smaller than that of dwarf galaxies. Our results confirm the large-scale distribution of Virgo IGCs indicated by previous photometric surveys. The color distribution of the confirmed IGCs shows a bimodality similar to that of M87 GCs. This indicates that most IGCs are stripped off dwarf galaxies and some off massive galaxies in the Virgo. M85 is one of nearby merger remnant galaxies that underwent merging events a few Gyrs ago. The GCs in M85 are surveyed with the MegaCam attached at 3.6m Canada-French-Hawaii telescope. We obtain ugi-band images of 1◦× 1◦ field around M85.We identify 1318 GC candidates among point sources in the survey region using color and magnitude criteria. The GC candidates are well concentrated on M85 but show more extended spatial distribution than the galaxy light. The effective radius of the entire GC system is about 5.′54, corresponding to 29 kpc. The color distribution of the GC candidates shows two peaks of (g−i)0 = 0.65 and 0.87. We detect the green GC population in the inner region R < 2' in addition to blue and red GC populations. The radial number density profile and the surface number density map of the blue GC system show more extended features than those of red GC system. The effective radii of the blue and red GC systems are 7.'41 and 2.'31, respectively. The spatial distributions of both blue and red GC systems are elongated, which are similar to galaxy light. This feature differs from that shown in typical early-type galaxies. We suggest that the presence of the green GCs and the elongated spatial distribution of the blue and red GC systems are merger-induced features. We present the first spectroscopic study of GCs in the merger remnant galaxy M85 using two instruments: Gemini Multi-Object Spectrograph (GMOS) attached at the 8.1m Gemini-North telescope and Hectospec on the 6.5m Multiple-Mirror Telescope. We obtain the spectra for 20 GCs as well as the nucleus of M85 using the Gemini/GMOS. The radial velocities of the GCs range from 396 km s−1 to 1127 km s−1, which is consistent with that of the M85 nucleus (vr = 743 ± 8 km s−1). We find a signal of rotation of M85 GC system, and estimate their rotation-corrected radial velocity dispersion as 154 km s−1. The ages and metallicities of the GCs are derived from Lick indices and full spectrum fitting. We detect the intermediate-age population of which mean age is 4.0 ± 2.0 Gyr with solar metallicities ([Fe/H] ∼ –0.19), comprising 50% of the observed GCs, as well as old population. This suggests that M85 experienced a wet merging event about 4 Gyrs ago. In addition, we obtain the spectra for 53 GCs including 11 IGCs using the MMT/Hectospec. The radial velocities of the GCs range from ∼ 400 km s−1 to ∼ 1600km s−1. We classify the GC sample into three categories, blue and red GCs in M85, and IGCs. We select the GCs that belong to M85 with two criteria: vr < 1200 km s−1 and R < 20', and the other GCs are considered as IGCs. The color criterion (g−i)0 = 0.8 is used to divide the M85 GCs into blue and red ones. All red GCs are located within R < 6', while the blue ones are distributed to R ∼ 20'. We investigate the kinematics of the GC system of M85. The entire M85 GC system shows overall rotation as shown in the GC sample of GMOS observation. The red GC system shows more clear rotation feature than the blue GC system despite the small sample. The rotation axis of the GC system of M85 is clearly different from that of the central galaxy light. We derive the rotation-corrected radial velocity dispersion of two GC systems: sigma_r = 149 ± 20 km s−1 for blue GCs and sigma_r = 51 ± 14 km s−1 for red GCs. The rotation-corrected velocity dispersion of red GC system is much lower than that of blue GC system. The strong rotation feature of the GC system and kinematically decoupled GC system from the stellar light are suspected to be caused by merging events. The kinematics of the M85 GCs and IGCs are clearly different from each other. The mean radial velocities of M85 GCs and IGCs are 765 ± 27 km s−1 and 1012 ± 108 km s−1, respectively. The radial velocity dispersion of the IGCs is 354 ± 78 km s−1, which is higher than that of the M85 GCs and similar to that of dwarf galaxies in the survey region (sigma = 330±95 km s−1). This indicates that the IGCs are governed by the gravitational potential of the galaxy cluster.