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Bulge Evolution in Face-On Spiral and Low Surface Brightness Galaxies
It is an observational fact that bulges of spiral galaxies contain ahigh fraction of old and metal-rich stars. Following this observationalfact, we have investigated the colors of 21 bulges hosted by a selectedsample of high surface brightness spiral galaxies and low surfacebrightness galaxies observed in the B and R optical bands and the J andKs near-IR bands. Using structural parameters derived fromthese observations, we obtain evidence that bulges could be formed bypure disk evolution (secular evolution), in agreement with suggestionsby some authors. The color profiles, especially the near-IR ones, shownull or almost null color gradients, supporting the hypothesis that thedisk stellar populations are similar to those present in the bulgeand/or that some bulges can be understood as disks with enhanced stellardensity (or pseudobulges). In the optical, half of the galaxies presentan inverse color gradient, giving additional evidence in favor ofsecular evolution for the sample investigated. The comparison of theobserved colors with those obtained from spectrophotometric models ofgalaxy evolution suggests that bulges of the selected sample have solarand subsolar metallicities and are independent of the current stellarformation rate. Also, we obtain evidence suggesting that galaxieshosting small bulges tend to be systematically metal-poor compared tothose with larger bulges. These results are being checked more carefullywith high signal-to-noise ratio spectroscopy.
| High and Low Surface Brightness Galaxies in the Local Universe. V. Optical and H I Properties
Using optical spectra of the nuclei of 250 galaxies chosen from theAutomated Plate Measuring Facility (APM) survey of UK Schmidt plates, wederive synthetic B-V and V-R colors, estimates of reddening from Balmerdecrements, metallicity via oxygen abundance, and star formation ratefrom the Hα luminosity. We also present mass-to-light ratios andgas mass fractions from 21 cm H I measurements of 238 APM surveygalaxies, 101 of which also have optical spectra. This sample ofgalaxies spans a large range in surface brightness, from high surfacebrightness irregular galaxies down to the kind of low surface brightness(LSB) galaxies that are missing from most magnitude-limited catalogs.The generally blue global colors are best explained by a combination oflow metallicity and active star formation, primarily because of thetremendous scatter in metallicity versus B-V. Optical spectra show thepresence of G and K main-sequence stars in the nuclei of these galaxies,and the stellar mass-to-light ratios and gas mass fractions show thegalaxies as a whole to be gas-rich. LSB galaxies are most likely toexperience sporadic low levels of star formation over gigayeartimescales.
| Low Surface Brightness Galaxies in the Local Universe. I. The Catalog
Data are presented for 693 galaxies identified in a large new survey forlow surface brightness galaxies in the nearby universe (z <~ 0.1).The survey covers 786 square degrees centered on the equator, and itextends significantly the surface brightness range of galaxy surveys inwhich there are a substantial number of galaxies with redshifts. Thedata are derived from the Automated Plate Measuring machine scans ofsurvey plates from the UK Schmidt Telescope and from follow-upobservations at radio and optical wavelengths. Accurate positions, totalB magnitudes, surface brightness parameters, and angular sizes aretabulated for each galaxy. Radial velocities, optical luminosities, andneutral hydrogen masses are listed for a subset of the sample. Findingcharts are also presented for those objects having a large enoughangular size that the scans from survey plates provide somemorphological information. The selection function and the luminosityfunction that can be derived from the survey are discussed in twocompanion papers.
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