arXiv subjects

C. J. Grillmair

Publications and source records attributed to C. J. Grillmair.

22 records · Page 2Linked to original sources

arXiv1996

Velocity Dispersions and Mass-to-Light Ratios for 9 Globular Clusters in M31

We present internal velocity dispersion determinations from high-resolution spectroscopic observations of a sample of nine globular clusters in M31. Comprehensive numerical simulations are used to show that the typical uncertainty of our velocity dispersion measurements is about 5%. Using these new velocity dispersions together with structural parameters derived from HST observations, we estimate the mass-to-light ratios of these clusters and find that they are typical of those measured for Galactic clusters. We show relations between velocity dispersion, luminosity and physical scales for globular clusters belonging to the Galaxy, the Magellanic clouds, Fornax, M31, and Centaurus A. The mean relations and the degree of scatter are similar in all galaxies. This reveals remarkable similarities, in term of structure and mass-to-light ratio, between the globular clusters belonging to these different galaxies. We briefly discuss the possible use of individual globular clusters as extragalactic distance indicators.

astro-ph↗

arXiv1996

The Centers of Early-Type Galaxies with HST II: Empirical Models and Structural Parameters

We present a set of structural parameters for the central parts of 57 early-type galaxies observed with the Planetary Camera of the Hubble Space Telescope. These parameters are based on a new empirical law that successfully characterizes the centers of early type galaxies. This empirical law assumes that the surface brightness profile is a combination of two power laws with different slopes gamma and beta for the inner and outer regions. Conventional structural parameters such as core radius and central surface brightness are replaced by break radius r_b, where the transition between power-law slopes takes place, and surface brightness mu_b at that radius. An additional parameter alpha describes the sharpness of the break. The structural parameters are derived using a chi-squared minimization process applied to the mean surface brightness profiles. The resulting model profiles generally give very good agreement to the observed profiles out to the radius of 10 arcseconds imaged by the Planetary Camera. Exceptions include galaxies which depart from pure power-laws at large radius, those with strong nuclear components, and galaxies partly obscured by dust. The uncertainties in the derived parameters are estimated using Monte-Carlo simulations which test the stability of solutions in the face of photon noise and the effects of the deconvolution process. The covariance of the structural parameters is examined by computing contours of constant chi squared in multi-dimensional parameter space.

astro-ph↗

arXiv1996

Hubble Space Telescope Observations of Globular Clusters in M31 II: Structural Parameters

We analyze post-refurbishment Hubble Space Telescope images of four globular clusters in M31. The ability to resolve stars to below the horizontal branch permits us to use star counts to extend the surface brightness profiles determined using aperture photometry to almost 5 orders of magnitude below the central surface density. Three of the resulting cluster profiles are reasonably well-fit using single-mass King models, with core and tidal radii typical of those seen in Galactic globular clusters. We confirm an earlier report of the discovery of a cluster which has apparently undergone core collapse. Three of the four clusters show departures in their outskirts from King model behavior which, based on recent results for Galactic globulars, may indicate the presence of tidal tails.

astro-ph↗

arXiv1995

Globular Clusters with Tidal Tails: Deep Two-Color Star Counts

We examine the outer structure of 12 Galactic globular clusters using star-count analyses. Deep, two-color, photographic photometry is used to select and count stars with colors and magnitudes consistent with cluster-specific, color-magnitude sequences. The resulting reduction in the number of contaminating foreground stars allows us to push the star counts to significantly lower surface densities than has previously been possible. We find that most of our sample clusters show extra-tidal wings in their surface density profiles. The form of the surface density profiles is consistent with recent numerical studies of the tidal stripping of globular clusters. Two-dimensional surface density maps for several clusters are consistent with the expected appearance of tidal tails, with allowance for the effects of orbit shape, orbital phase, and orientation of our line of sight. We identify the extra-tidal material with stars which are still in the process of being removed from the clusters. The extra-tidal stars effectively limit the accuracy to which we can resolve the ``tidal'' radii of globular clusters. However, by tracing the orbital paths of their parent clusters, these stars may also facilitate a renewed attack on the problem of determining globular cluster orbits and the shape of the Galactic potential field.

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