The Neutral Hydrogen Companions of Actively Star Forming Dwarf Galaxies

By Christopher Lee Taylor

Under the supervision of Dr. Evan D. Skillman

ABSTRACT

This thesis is a search for intergalactic HI clouds which have not originated in galaxies. Using the idea that galaxy-galaxy interactions can trigger massive star formation, we employ the Very Large Array D-configuration to survey the vicinity of star forming dwarf galaxies (HII galaxies) with no known interaction partners. Testing the projects feasibility, we surveyed nine HII galaxies for HI companions. Four galaxies have companions, supporting the hypothesis that the bursts of star formation in these galaxies can be triggered by previously undetected companions.

 

VLC C-configuration observations of five HII galaxies are combined with optical images to study the relationship of the HI distribution to bursts of massive star formation. The observed radial profile of HI density is more sharply peaked in HII galaxies than in low surface brightness galaxies (LSBGs), consistent with LSBGs being quiescent HII galaxies, if the HI in an LSBG can be concentrated at its center.

 

Following the pilot study, the VLA is used to survey a complete, volume limited sample of HII galaxies. We detect 20 of the 21 galaxies, finding 16 companions around 12 galaxies. The frequency of occurrence is 0.57, statistically consistent with a lower limit of 0.37. Four companions have no optical counterparts and may be intergalactic HI clouds. Our data imply that the HII galaxies are dominated by dark matter.

 

Deep optical observations of the candidate intergalactic HI clouds extend the detection threshold down to ~ 25 mag arsec^-2. The low detection limits rule out stellar components comparable to Local Group dwarf spheroidals. We estimate luminosity upper limits for the undetected objects and calculate lower limit mass-to-light ratios. Conservative values for M_tot/L_B range from 90 to 1900, greater than or equal to any known galaxies.

 

Our possible detection of intergalactic HI clouds is consistent with previous efforts, which were mostly incapable of detecting objects like ours. We show that the HI clouds may be related to the Ly-alpha absorption systems, and have properties that would be expected of proto-galaxies, although we find no evidence that the clouds will eventually become galaxies. We also discuss inferences about dark matter using the HI clouds.