Faint Fuzzy Stuff:
The Missing Baryons?
Author: Karen O'Neil
Published:Ph.D. Thesis, University of Oregon, Eugene (1997).
ABSTRACT:
In recent years evidence has been mounting that a substantial amount of baryonic matter appears to be "missing" from our current inventory of the Universe. 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 determine if such galaxies exist, three observational surveys, employing different observing techniques, were undertaken to search for previously undiscovered and uncatalogued galaxies with the aim of discovering these "missing" repositories of baryonic matter.
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 MacDonald Observatory 0.8m telescope and a 2048x2048 CCD camera. 127 galaxies with Bµ(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 Bµ(0) to 24.0 Bµ(0), at which point a sharp drop-off is observed, though galaxies were seen 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, provoking the development of a computer model to examine the effects starburst activity has on galaxies and to better understand the range of colors found.
The other two surveys employed space-based telescopes to determine if non-terrestrial observing techniques were effective in uncovering diffuse galaxies. The second project 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. The second project 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 of these galaxies, which include 3 luminous ellipticals, 2 bonafide dwarf ellipticals and 5 galaxies of small angular size that may or may not be members of the Fornax cluster. The third survey employed the Hubble Space Telescope Wide Field Planetary Camera-2 to search for LSB galaxies surrounding known galaxies 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.
All three survey methods proved successful, showing that
much of the
baryonic matter in the universe could lie in galaxies whose small angular size
and/or low surface brightness has resulted in their remaining undetected until
recently (if at all).