arXiv subjects

A. Dressler

Publications and source records attributed to A. Dressler.

59 records · Page 4Linked to original sources

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↗

arXiv1995

A COMPARATIVE STUDY OF MORPHOLOGICAL CLASSIFICATIONS OF APM GALAXIES

We investigate the consistency of visual morphological classifications of galaxies by comparing classifications for 831 galaxies from six independent observers. The galaxies were classified on laser print copy images or on computer screen produced from scans with the Automated Plate Measuring (APM) machine. Classifications are compared using the Revised Hubble numerical type index T. We find that individual observers agree with one another with rms combined dispersions of between 1.3 and 2.3 type units, typically about 1.8 units. The dispersions tend to decrease slightly with increasing angular diameter and, in some cases, with increasing axial ratio $(b/a)$. The agreement between independent observers is reasonably good but the scatter is non-negligible. In spite of the scatter the Revised Hubble T system can be used to train an automated galaxy classifier, e.g. an Artificial Neural Network, to handle the large number of galaxy images that are being compiled in the APM and other surveys.

astro-ph↗

arXiv1994

Galaxies, Human Eyes and Artificial Neural Networks

Quantitative morphological classification of galaxies is important for understanding the origin of type frequency and correlations with environment. But galaxy morphological classification is still mainly done visually by dedicated individuals, in the spirit of Hubble's original scheme, and its modifications. The rapid increase in data on galaxy images at low and high redshift calls for re-examination of the classification schemes and for new automatic methods. Here we show results from the first systematic comparison of the dispersion among human experts classifying a uniformly selected sample of over 800 digitised galaxy images. These galaxy images were then classified by six of the authors independently. The human classifications are compared with each other, and with an automatic classification by Artificial Neural Networks (ANN). It is shown that the ANNs can replicate the classification by a human expert to the same degree of agreement as that between two human experts.

astro-ph↗

arXiv1994

The Optical Redshift Survey: Sample Selection and the Galaxy Distribution

This is the first in a series of papers describing the {\it Optical Redshift Survey} (ORS), a redshift survey of optically selected galaxies covering 98\% of the sky above $|b| = 20^\circ$ (8.09 ster). The survey is drawn from the UGC, ESO, and ESGC galaxy catalogues, and contains two sub-samples, one complete to a $B$ magnitude of 14.5, the other complete to a $B$ major axis diameter of $1.9^\prime$. The entire sample consists of 8457 objects, of which redshifts are now available for 8286; 171 objects remain without measured redshifts. The ORS provides the most detailed and homogeneous sampling of the large-scale galaxy distribution to date in these areas. The density field of bright optical galaxies is well-defined to $8000 \kms$, and is dominated by the Virgo, Telescopium-Pavo-Indus, Hydra-Centaurus, Pisces-Perseus, and Coma-A1367 Superclusters. The dense sampling provided by ORS allows a detailed analysis of the galaxy density field, and will be used to test its dependence on morphology and other galaxy parameters.

astro-ph↗

arXiv1993

A Family of Models for Spherical Stellar Systems

We describe a one-parameter family of models of stable spherical stellar systems in which the phase-space distribution function depends only on energy. The models have similar density profiles in their outer parts ($ρ\propto r^{-4}$) and central power-law density cusps, $ρ\propto r^{3-η}$, $0<η\le 3$. The family contains the Jaffe (1983) and Hernquist (1990) models as special cases. We evaluate the surface brightness profile, the line-of-sight velocity dispersion profile, and the distribution function, and discuss analogs of King's core-fitting formula for determining mass-to-light ratio. We also generalize the models to a two-parameter family, in which the galaxy contains a central black hole; the second parameter is the mass of the black hole. Our models can be used to estimate the detectability of central black holes and the velocity-dispersion profiles of galaxies that contain central cusps, with or without a central black hole.

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