A Wide Field CCD Survey for Low Surface Brightness Galaxies.II.Color Distributions, Stellar Populations, and Missing Baryons
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Abstract
We have performed a digital survey for low surface brightness (LSB) galaxies in the spiral-rich Cancer and Pegasus clusters as well as the low density regime defined by the Great Wall. A total of 127 galaxies of angular diameter larger than 15 arcseconds were found with μβ(0)≥22.0 mag/arcsec2, 119 of which were previously unidentified. Structural parameters (μβ(0),α,r25, etc) and colors (Johnson/Cousins U, B, V, 1, & R, when possible) were determined for all galaxies. Paper I of this series described structural parameters; here we focus the discussion on the stellar populations of these newly discovered galaxies as implied by their observed broad-band colors. The colors of the survey galaxies range continuously from very blue (U-B=-0.56, B-V=0.37) to very red (U-B=0.65, V-I=2.2), and include a group of old galaxies which show evidence for recent star formation. This survey is also the first to discover a significant population of LSB galaxies which have red colors. Since galaxies must fade and redden, the absence of red LSB disks in previous surveys has been puzzling. Their recovery in this survey suggests that photographic selection from blue plates, which forms the basis of previous LSB surveys, carries with it strong selection effects. The continuous range of colors, from very blue to very red, that we observe for this sample clearly shows that LSB galaxies at the present epoch define a wide range of evolutionary states. Consistent with other surveys we find a significant number of galaxies with μβB(0)≥23.0 mag/arcsec-2 which suggests that the space density of galaxies as a function of μβB(0) is not strongly peaked. To more rigorously test this hypothesis we compare the actual surface brightness distribution from our survey with that from two different types of Monte-Carlo based sky images, one with an underlying fiat surface brightness distribution and one with an underlying Gaussian distribution (falling off at 24.0 mag arcsec-2). We show that there is no way of distinguishing between the flat distribution and the Gaussian one as the proper description of the underlying surface brightness distribution for this survey beyond 24.0 mag arcsec-2, in spite of the fact that our survey limit is actually 26.0 mag arcsec-2. This demonstrates that an even deeper and more difficult to obtain isophotal limit is required to more accurately determine the space density of very LSB galaxies as well as to define the faintest possible central surface brightnesses that galactic disks can have.