Construction of a Self-consistent Reference Dataset
Spanning 8th to 18th mag.
Christopher Ke-shih Young
, PASA, 18 (2), in press.
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Title/Abstract Page: Total Magnitudes of Virgo
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Introduction
Although galaxy total magnitudes are required for a large number of astronomical applications, they are notoriously difficult to estimate accurately or even consistently. There are a very large number of reasons for this unfortunate state of affairs. One major problem is that atmospheric seeing effects can distort the luminosity profiles of galaxies very sigificantly. Under such conditions, profile extrapolation based on standard growth curves generally leads to spurious results, as demonstrated by Young et al. (1998). This problem is of course most acute when dealing with very distant galaxies and/or observations made under poor seeing conditions. Another serious problem is that it is often technically difficult (or at least impractical) to determine reliably the luminosity profiles of many galaxies out to large enough radial distances so as to be able to avoid large extrapolations. The situation is of course most adverse when dealing with very low surface-brightness objects for which very deep photometry is really essential. By contrast, one of the greatest difficulties to be overcome in the CCD photometry of bright galaxies is how to determine the level of the sky accurately, when the target galaxies are often large enough to fill entire CCD frames. Although wide-field photographic photometry does not suffer from this hazard, photographic emulsions are deficient in terms of their dynamic ranges. In order to prevent the saturation of photographic emulsions, only short-exposure or `shallow' photographic photometry is therefore possible for bright galaxies. In the case of the Virgo direction, the sheer diversity of objects we are confronted with poses what may well be the ultimate challenge to anyone trying to construct a self-consistent and reliable total-magnitude scale. Not only do cluster members systematically catalogued to date span an apparent brightness range of 8th-18th mag., but a full complement of different morphological types is present too. Furthermore, on account of the relatively loose clustering of member galaxies and Virgo's uniquely large angular extent on the sky, backgound galaxies begin to dominate the galaxy number counts at 16th mag. In this paper, we isolate several sources of total-magnitude measurements and photometry for Virgo galaxies that we have good reason to believe are reliable. In some cases, we are able to adopt the published magnitude measurements without modification, whilst in other cases, it was necessary to derive new total-magnitude values based on existing photometry. Papers II, III and IV will deal with the magnitude scales of: the Catalog of Galaxies and Clusters of Galaxies of Zwicky et al. (Volume I: 1961, Volume II: 1963), the Reference Catalogs of Bright Galaxies of de Vaucouleurs et al. (RC2: 1976, RC3: 1991), and the Virgo Cluster Catalog (VCC) of Binggeli et al. (1985) respectively; whilst further papers in the series will deal with the magnitude scales of smaller datasets.Next Section: Faint galaxy sample (18th-14th
Title/Abstract Page: Total Magnitudes of Virgo
Previous Section: Total Magnitudes of Virgo
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