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Register a new userThe Norma Cluster (ACO 3627): I. A Dynamical Analysis of the Most Massive Cluster in the Great Attractor
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P.A. Woudt
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A detailed dynamical analysis of the nearby rich Norma cluster (ACO 3627) is presented. From radial velocities of 296 cluster members, we find a mean velocity of 4871 +/- 54 km/s and a velocity dispersion of 925 km/s. The mean velocity of the E/S0 population (4979 +/- 85 km/s) is offset with respect to that of the S/Irr population (4812 +/- 70 km/s) by `Delta v = 164 km/s in the cluster rest frame. This offset increases towards the core of the cluster. The E/S0 population is free of any detectable substructure and appears relaxed. Its shape is clearly elongated with a position angle that is aligned along the dominant large-scale structures in this region, the so-called Norma wall. The central cD galaxy has a very large peculiar velocity of 561 km/s which is most probably related to an ongoing merger at the core of the cluster. The spiral/irregular galaxies reveal a large amount of substructure; two dynamically distinct subgroups within the overall spiral-population have been identified, located along the Norma wall elongation. The dynamical mass of the Norma cluster within its Abell radius is 1 - 1.1 x 10^15 h^-1_73 M_Sun. One of the cluster members, the spiral galaxy WKK 6176 which recently was observed to have a 70 kpc X-ray tail, reveals numerous striking low-brightness filaments pointing away from the cluster centre suggesting strong interaction with the intracluster medium.
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ATCA 21 cm HI observations of the rich galaxy cluster ACO 3627 in the Great Attractor region are presented. Three fields of 30 diameter located within one Abell radius of ACO 3627 were observed with a resolution of 15 and an rms noise of sim 1 mJy/beam. Only two galaxies were detected in these fields. We compare their HI distribution to new optical R-band images and discuss their velocity fields. The first galaxy is a gas-rich unperturbed spiral whereas the second shows a peculiar HI distribution. The estimated 3-sigma HI mass limit of our observations is sim 7 x 10^8 M_{odot} for a line width of 150 km s^{-1}. The non-detection of a considerable number of luminous spiral galaxies indicates that the spiral galaxies are HI deficient. The low detection rate is comparable to the HI deficient Coma cluster (Bravo-Alfaro et al. 2000). ACO 3627 is a bright X-ray cluster. We therefore suspect that ram pressure stripping is responsible for the HI deficiency of the bright cluster spirals.
We study a sample composed of 28 of the brightest stars in the Arches cluster. We analyze K-band spectra obtained with the integral field spectrograph SINFONI on the VLT. Atmosphere models computed with the code CMFGEN are used to derive the effective temperatures, luminosities, stellar abundances, mass loss rates and wind terminal velocities. We find that the stars in our sample are either H-rich WN7-9 stars (WN7-9h) or O supergiants, two being classified as OIf+. All stars are 2-4 Myr old. There is marginal evidence for a younger age among the most massive stars. The WN7-9h stars reach luminosities as large as 2 x 1e6 Lsun, consistent with initial masses of ~ 120 Msun. They are still quite H-rich, but show both N enhancement and C depletion. They are thus identified as core H-burning objects showing products of the CNO equilibrium at their surface. Their progenitors are most likely supergiants of spectral types earlier than O4-6 and initial masses > 60 Msun. Their winds follow a well defined modified wind momentum - luminosity relation (WLR): this is a strong indication that they are radiatively driven. Stellar abundances tend to favor a slightly super solar metallicity, at least for the lightest metals. We note however that the evolutionary models seem to under-predict the degree of N enrichment.
We independently redetermine the reddening and age of the globular cluster 037-B327 in M31 by comparing independently obtained multicolour photometry with theoretical stellar population synthesis models. 037-B327 has long been known to have a very large reddening value, which we confirm to be E(B-V)=1.360+-0.013, in good agreement with the previous results. We redetermine its most likely age at 12.4+- 3.2 Gyr. 037-B327 is a prime example of an unusually bright early counterpart to the ubiquitous ``super star clusters presently observed in most high-intensity star-forming regions in the local Universe. In order to have survived for a Hubble time, we conclude that its stellar IMF cannot have been top-heavy. Using this constraint, and a variety of SSP models, we determine a photometric mass of M_{GC}=(3.0+-0.5)*10^7, somewhat depending on the SSP models used, the metallicity and age adopted and the IMF representation. This mass, and its relatively small uncertainties, make this object the most massive star cluster of any age in the Local Group. Assuming that the photometric mass estimate thus derived is fairly close to its dynamical mass, we predict that this GC has a (one-dimensional) velocity dispersion of order (72+-13)km/s. As a surviving super star cluster, this object is of prime importance for theories aimed at describing massive star cluster evolution.
The results obtained from the ASCA observations of the cluster of galaxies Abell 3627 are presented. This cluster, located behind the Milky way, was recently found to be a nearby, X-ray bright and very rich cluster. Pointed observations onto the central region of the cluster gave a gas temperature of $sim7$ keV and a metallicity of about 0.2 solar. An offset pointing to a substructure elongated to south-east of the cluster center gave a significantly lower temperature of $sim 5$ keV. The 2--10 keV luminosity within a radius of 40 (1.1 Mpc) is estimated to be $3.7 times 10^{44} erg/s$. The X-ray data imply a cluster mass of about $4times 10^{14}$ $M_odot$ within 40.
We report on the properties of the most massive ultra-compact dwarf galaxy (UCD) in the nearby Virgo Cluster of galaxies using imaging from the Next Generation Virgo Cluster Survey (NGVS) and spectroscopy from Keck/DEIMOS. This object (M59-UCD3) appears to be associated with the massive Virgo galaxy M59 (NGC 4621), has an integrated velocity dispersion of 78 km/s, a dynamical mass of $3.7times10^8 M_odot$, and an effective radius ($R_e$) of 25 pc. With an effective surface mass density of $9.4times10^{10} M_odot/kpc^2$, it is the densest galaxy in the local Universe discovered to date, surpassing the density of the luminous Virgo UCD, M60-UCD1. M59-UCD3 has a total luminosity of $M_{g}=-14.2$ mag, and a spectral energy distribution consistent with an old (14 Gyr) stellar population with [Fe/H]=0.0 and [$alpha$/Fe]=+0.2. We also examine deep imaging around M59 and find a broad low surface brightness stream pointing towards M59-UCD3, which may represent a tidal remnant of the UCD progenitor. This UCD, along with similar objects like M60-UCD1 and M59cO, likely represents an extreme population of tidally stripped galaxies more akin to larger and more massive compact early-type galaxies than to nuclear star clusters in present-day dwarf galaxies.
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