Faint Fuzzy Stuff: The Missing Baryons

Author: Karen O'Neil

Published: To be published in PASP, December, 1997


In recent years evidence has been mounting that a substantial amount of baryonic matter appears to be "missing" from our current inventory of galaxies, groups and clusters. This implies that significant repositories of baryonic matter are awaiting discovery. The most likely form of this repository is a very diffuse galaxy whose existence is effectively masked by the brightness and noise of the night sky background. In order to better determine the space density of these galaxies, three observational surveys, employing different observing techniques, were undertaken to search for previously undiscovered and uncatalogued galaxies.

The first, and most extensive, survey was a Low Surface Brightness (LSB) galaxy survey of the Cancer and Pegasus galaxy clusters and the low density regime defined by the Great Wall. The survey was undertaken over a four year period using the University of Texas McDonald Observatory 0.8m telescope and a 2048x2048 CCD camera. 127 galaxies with µB(0) > 22.0 mag arcsec-2 were found, 119 of which were previously unidentified. Structural parameters and colors were determined for the galaxies. The central surface brightness distribution of the galaxies is flat from 22.0 µB(0) to 24.0 µB(0), at which point a sharp drop-off is observed, though galaxies were detected down to 25.5 mag arcsec-2. The colors of the galaxies in the Texas survey range continuously from very blue through very red, and include a group of old galaxies which show evidence for recent star formation. This survey for LSB galaxies is the first to discover a significant population of objects with red colors that may be the signature of actual disk fading. This is the expected route to low disk surface brightness but until this survey, it had not been detected. To better understand the nature of the colors, a computer model was developed to examine the effects starburst activity has on galaxies of low surface brightness.

The other two surveys employed space-based telescopes to determine if non-terrestrial observing techniques were more effective or efficient in detecting intrinsically diffuse galaxies. One survey examined an Astro-1 Ultraviolet Imaging Telescope deep image of the Fornax cluster for purposes of determining the efficacy of vacuum ultraviolet imaging in terms of the recovery of very faint and/or diffuse galaxies. A 1000 second near-UV image detected 10 galaxies, which include 3 luminous ellipticals, 2 bonafide dwarf ellipticals and 5 very faint galaxies of small angular size that may or may not be members of the Fornax cluster. The other survey employed the Hubble Space Telescope Wide Field Planetary Camera-2 to search for additional LSB galaxies surrounding known LSB dwarfs in the Virgo cluster. Four fields were surveyed (160" x 160" each) and 215 galaxies were found, the majority of which ] were previously uncatalogued. Structural parameters were found for all the galaxies, showing they range from bright ellipticals through very diffuse LSB galaxies. Some of these are likely to be Virgo cluster members while others are surely background.

In sum, all three survey methods proved successful, in that significant numbers of very diffuse galaxies were detected. Construction of the luminosity and/or space density functions of LSB galaxies shows that indeed much of the baryonic matter in the universe could lie in galaxies whose low surface brightness has resulted in their remaining undetected until recently. Since surface brightness selection effects are still present in all our surveys, it is clear we have yet to probe the very faint end of disk diffuseness.