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Register a new userThe impact of galaxy harassment on the globular cluster systems of early-type cluster dwarf galaxies
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The dynamics of globular cluster systems (GCSs) around galaxies are often used to assess the total enclosed mass, and even to constrain the dark matter distribution. The globular cluster system of a galaxy is typically assumed to be in dynamical equilibrium within the potential of the host galaxy. However cluster galaxies are subjected to a rapidly evolving and, at times, violently destructive tidal field. We investigate the impact of the harassment on the dynamics of GCs surrounding early type cluster dwarfs, using numerical simulations. We find that the dynamical behaviour of the GCS is strongly influenced by the fraction of bound dark matter f_{DM} remaining in the galaxy. Only when f_{DM} falls to ~15%, do stars and GCs begin to be stripped. Still the observed GC velocity dispersion can be used to measure the true enclosed mass to within a factor of 2, even when f_{DM} falls as low as ~3%. This is possible partly because unbound GCs quickly separate from the galaxy body. However even the distribution of {it{bound}} GCs may spatially expand by a factor of 2-3. Once f_{DM} falls into the <3% regime, the galaxy is close to complete disruption, and GCS dynamics can no longer be used to reliably estimate the enclosed mass. In this regime, the remaining bound GCS may spatially expand by a factor of 4 to 8. It may be possible to test if a galaxy is in this regime by measuring the dynamics of the stellar disk. We demonstrate that if a stellar disk is rotationally supported, it is likely that a galaxy has sufficient dark matter, that the dynamics of the GCS can be used to reliably estimate the enclosed mass.
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We present a photometric study of the early-type dwarf galaxy population of the Centaurus cluster, aiming at investigating the galaxy luminosity function (LF) and galaxy scaling relations down to the regime of galaxies with M_V~-10 mag. On deep VLT/FORS1 V- and I-band images of the central part of the cluster, we identify cluster dwarf-galaxy candidates using both morphological and surface brightness selection criteria. Photometric and structural parameters of the candidates are derived from analysis of their surface brightness profiles. Fundamental scaling relations, such as the colour-magnitude and the magnitude-surface brightness relation, are used to distinguish the cluster from the background. We find a flat LF with a slope of alpha = -1.14 pm 0.12 for M_V>-14 mag, when fitting a power law to the completeness-corrected galaxy number counts. When plotting the central surface brightness of a Sersic model vs. the galaxy magnitude, we find a continuous relation for magnitudes -20<M_V<-10 mag, with only the brightest core galaxies deviating from this relation, in agreement with previous studies of other clusters. In a size-luminosity diagram of early-type galaxies from a range of environments, we observe that R_eff slowly decreases with decreasing luminosity for -21<M_V<-13 mag and decreases more rapidly at fainter magnitudes. This trend continues to the ultra-faint Local Group dwarf galaxies (M_V~-4 mag). The continuous central surface brightness vs. absolute magnitude relation and the smooth relation in the size-luminosity diagram over a wide range of magnitudes are consistent with the interpretation of dwarf galaxies and more massive elliptical galaxies being one family of objects with gradually changing structural properties. The most massive core galaxies and the rare cE galaxies are the only exceptions.
This paper presents deep high quality photometry of globular cluster (GC) systems belonging to five early-type galaxies covering a range of mass and environment. Photometric data were obtained with the Gemini North and Gemini South telescopes in the filter passbands g, r, and i. The combination of these filters with good seeing conditions allows an excellent separation between GC candidates and unresolved field objects. Bimodal GC colour distributions are found in all five galaxies. Most of the GC systems appear bimodal even in the (g -r) vs (r -i) plane. A population of resolved/marginally resolved GC and Ultra Compact Dwarf candidates was found in all the galaxies. A search for the so-called blue tilt in the colour-magnitude diagrams reveals that NGC 4649 clearly shows that phenomenon although no conclusive evidence was found for the other galaxies in the sample. This blue tilt translates into a mass-metallicity relation given by Z propto M^0.28pm0.03 . This dependence was found using a new empirical (g -i) vs [Z/H] relation which relies on an homogeneous sample of GC colours and metallicities. This paper also explores the radial trends in both colour and surface density for the blue (metal-poor) and red (metal-rich) GC subpopulations. As usual, the red GCs show a steeper radial distribution than the blue ones. Evidence of galactocentric colour gradients is found in some of the GC systems, being more significant for the two S0 galaxies in the sample. Red GC subpopulations show similar colours and gradients to the galaxy halo stars in their inner region. A GC mean colour-galaxy luminosity relation, consistent with [Z/H] propto L_B ^0.26pm0.08, is present for the red GCs. An estimate of the total GC populations and specific frequency SN values is presented for NGC 3115, NGC 3379, NGC 3923 and NGC 4649.
We present a study of globular clusters (GCs) in 17 relatively nearby early-type galaxies, based on deep HST/WFPC2 F555W and F814W images. We compare color distributions, cluster sizes and luminosity functions with those of GCs in the Milky Way. In nearly all cases, a KMM test returns a high confidence level for the hypothesis that a sum of two Gaussians provides a better fit to the observed color distribution than a single Gaussian, although histograms of the V-I distribution are not always obviously bimodal. The blue and red peak colors both correlate with absolute host galaxy B band magnitude and central velocity dispersion (at about the 2-3 sigma level), but we see no clear correlation with host galaxy V-I or J-K color. Red GCs are generally smaller than blue GCs by about 20%. The size difference is seen at all radii and exists also in the Milky Way and Sombrero (M104) spiral galaxies. Fitting t5 functions to the luminosity functions of blue and red GC populations separately, we find that the V-band turn-over of the blue GCs is generally brighter than that of the red ones by about 0.3 mag, as expected if the two GC populations have similar ages and mass distributions but different metallicities. Brighter than M_V ~ -7.5, the luminosity functions (LFs) are well approximated by power-laws with an exponent of about -1.75. This is similar to the LF for young star clusters, suggesting that young and old globular clusters form by the same basic mechanism. We discuss scenarios for GC formation and conclude that our data appear to favor ``in-situ models in which all GCs in a galaxy formed after the main body of the proto-galaxy had assembled into a single potential well.
We analyze the kinematics of six Virgo cluster dwarf early-type galaxies (dEs) from their globular cluster (GC) systems. We present new Keck/DEIMOS spectroscopy for three of them and reanalyze the data found in the literature for the remaining three. We use two independent methods to estimate the rotation amplitude (Vmax) and velocity dispersion (sigma_GC) of the GC systems and evaluate their statistical significance by simulating non-rotating GC systems with the same number of GC satellites and velocity uncertainties. Our measured kinematics agree with the published values for the three galaxies from the literature and, in all cases, some rotation is measured. However, our simulations show that the null hypothesis of being non-rotating GC systems cannot be ruled out. In the case of VCC1861, the measured Vmax and the simulations indicate that it is not rotating. In the case of VCC1528, the null hypothesis can be marginally ruled out, thus, it might be rotating although further confirmation is needed. In our analysis, we find that, in general, the measured Vmax tends to be overestimated and the measured sigma_GC tends to be underestimated by amounts that depend on the intrinsic Vmax/sigma_GC, the number of observed GCs (N_GC), and the velocity uncertainties. The bias is negligible when N_GC>~20. In those cases where a large N_GC is not available, it is imperative to obtain data with small velocity uncertainties. For instance, errors of <2km/s lead to Vmax<10km/s for a system that is intrinsically not rotating.
We present a photometric study of the globular cluster systems of the Fornax cluster galaxies NGC 1374, NGC 1379, and NGC 1387. The data consists of images from the wide-field MOSAIC Imager of the CTIO 4-m telescope, obtained with Washington C and Kron-Cousins R filters. The images cover a field of 36 x 36 arcmin, corresponding to 200 x 200 kpc at the Fornax distance. Two of the galaxies, NGC 1374 and NGC 1379, are low-luminosity ellipticals while NGC 1387 is a low-luminosity lenticular. Their cluster systems are still embedded in the cluster system of NGC 1399. Therefore the use of a large field is crucial and some differences to previous work can be explained by this. The colour distributions of all globular cluster systems are bimodal. NGC 1387 presents a particularly distinct separation between red and blue clusters and an overproportionally large population of red clusters. The radial distribution is different for blue and red clusters, red clusters being more concentrated towards the respective galaxies. The different colour and radial distributions point to the existence of two globular cluster subpopulations in these galaxies. Specific frequencies are in the range S_N= 1.4-2.4, smaller than the typical values for elliptical galaxies. These galaxies might have suffered tidal stripping of blue globular clusters by NGC 1399.
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