Nearby Gas-Rich Low Surface Brightness Galaxies

We examine the Fisher-Tully cz<1000 km/s galaxy sample to determine whether it is a complete and representative sample of all galaxy types, including low surface brightness populations, as has been recently claimed. We find that the sample is progressively more incomplete for galaxies with (1) smaller physical diameters at a fixed isophote and (2) lower HI masses. This is likely to lead to a significant undercounting of nearby gas-rich low surface brightness galaxies. However, through comparisons to other samples we can understand how the nearby galaxy counts need to be corrected, and we see some indications of environmental effects that probably result from the local high density of galaxies.Comment: 12 page, 2 figures, to appear in Ap

On the Structural Differences between Disk and Dwarf Galaxies

Gas-rich dwarf and disk galaxies overlap in numerous physical quantities that make their classification subjective. We report the discovery of a separation between dwarfs and disks into two unique sequences in the mass (luminosity) versus scale length plane. This provides an objective classification scheme for late-type galaxies that only requires optical or near-IR surface photometry of a galaxy. Since the baryonic Tully-Fisher relation for these samples produces a continuous relation between baryonic mass and rotational velocity, we conclude that the difference between dwarfs and disks must be because of their distribution of stellar light such that dwarfs are more diffuse than disk galaxies. This structural separation may be due to a primordial difference between low and high mass galaxies or produced by hierarchical mergers where disks are built up from dwarfs. Structural differences between dwarf and disk galaxies may also be driven by the underlying kinematics where the strong rotation in disks produces an axial symmetric object that undergoes highly efficient star formation in contrast to the lower rotation, more disordered motion of dwarfs that produces a diffuse, triaxial object with a history of inefficient star formation.Comment: 16 pages, 2 figures, AJ in press, AASTeX5.

The Effects of Starburst Activity on Low Surface Brightness Disk Galaxies

Although numerous simulations have been done to understand the effects of intense bursts of star formation on high surface brightness galaxies, few attempts have been made to understand how localized starbursts would affect both the color and surface brightness of low surface brightness (LSB) galaxies. To remedy this, we have run 53 simulations involving bursts of star formation activity on LSB galaxies, varying both the underlying galaxy properties and the parameters describing the starbursts. We discovered that although changing the total color of a galaxy was fairly straightforward, it was virtually impossible to alter a galaxy's central surface brightness and thereby remove it from the LSB galaxy classification without placing a high (and fairly artificial) threshold for the underlying gas density. The primary effect of large amounts of induced star formation was to produce a centralized core (bulge) component which is generally not observed in LSB galaxies. The noisy morphological appearance of LSB galaxies as well as their noisy surface brightness profiles can be reproduced by considering small bursts of star formation that are localized within the disk. The trigger mechanism for such bursts is likely distant/weak tidal encounters. The stability of disk central surface brightness to these periods of star formation argues that the large space density of LSB galaxies at z = 0 should hold to substantially higher redshifts.Comment: 38 pages, 5 figures, 4 tables, tarred and compressed Also available on http://guernsey.uoregon.edu/~kare

The Stellar Populations of Low Surface Brightness Galaxies

Near-infrared (NIR) K' images of a sample of five low surface brightness disc galaxies (LSBGs) were combined with optical data, with the aim of constraining their star formation histories. Both red and blue LSBGs were imaged to enable comparison of their stellar populations. For both types of galaxy strong colour gradients were found, consistent with mean stellar age gradients. Very low stellar metallicities were ruled out on the basis of metallicity-sensitive optical-NIR colours. These five galaxies suggest that red and blue LSBGs have very different star formation histories and represent two independent routes to low B band surface brightness. Blue LSBGs are well described by models with low, roughly constant star formation rates, whereas red LSBGs are better described by a `faded disc' scenario.Comment: 5 pages LaTeX; 2 embedded figures; MNRAS Letters, Accepte