HI properties of Low Surface Brightness dwarf and blue compact dwarf galaxies

Original paper can be found at: http://www.astrosociety.org/pubs/cs/156-194.html--Copyright Astronomical Society of the PacificWe present high resolution (~15") VLA HI observations of one LSB dwarf galaxy and 5 Blue Compact Dwarfs. Previous works have emphasized the difference in the peak column density of the HI between the two types of galaxy. However, the peak column density is typically only a factor of 2-4 greater in BCDs than in LSB dwarfs, suggesting that the difference between the two is one of degree, not quality. The HI morphologies of the two types are often quite similar

A survey of H II galaxies to detect H I companion objects

Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society [Full text of this article is not available in the UHRA]Peer reviewe

A Survey of Low Surface Brightness Dwarf Galaxies to Detect H i-rich Companions

Original article can be found at: http://adsabs.harvard.edu/ Copyright American Astronomical Society. DOI: 10.1086/192358 [Full text of this article is not available in the UHRA]We have conducted a survey for H I-rich companions in the vicinity of low surface brightness (LSB) dwarf galaxies, objects that are characterized by low rates of massive star formation. This survey complements our earlier survey of dwarf galaxies exhibiting high star formation rates (H II galaxies). Four of the 17 LSB dwarfs in the sample have nearby, H I-rich companions, and a total of five companions were detected. The companion frequency is therefore {rho} = 0.24, and we determine a lower limit on the true rate of 0.08. This is a much lower rate of occurrence than was detected around the H II galaxies, {rho} = 0.57, which implies a lower limit of {rho} > 0.37. Because the two surveys were carried out under nearly identical circumstances, this difference is genuine and not the result of any relative bias between the two samples. That H II galaxies have companions more than twice as often as LSB dwarfs provides evidence that the bursts of massive star formation in H II galaxies are being triggered by galaxy-galaxy interactions.Peer reviewe

An H i/Optical Atlas of H II Galaxies and Their Companions

Original article can be found at: http://adsabs.harvard.edu/ Copyright American Astronomical Society. DOI: 10.1086/192193 [Full text of this article is not available in the UHRA]We have conducted a directed search for intergalactic H I clouds, using H II galaxies as pointers to fields on the sky likely to contain such clouds. This extends our previous survey (Taylor et al. 1993) by using the VLA to map in the 21 cm line of H I a complete, volume-limited sample of H II galaxies. We detected 20 of the 21 galaxies in our sample and find that 14 have nearby H I companions. Some H II galaxies have multiple companions, and we have discovered a total of 19 companions, or a frequency of occurrence of companions of 0.67. This detection rate is statistically consistent with a lower limit of the companion frequency of 0.46, assuming that no companions are present outside of the spatial and velocity ranges searched by our VLA observations. The companion population is not a homogeneous one but is comprised of H II galaxies, cataloged and previously uncataloged dwarfs, and several objects for which there are no known optical counterparts. These latter objects may be intergalactic H I clouds. Here we present our H I maps and R-band optical images of the H II galaxy and companion systems, along with physical properties determined from these data. We compare our sample to samples of H II galaxies from the literature, finding that the literature samples over-represent the more massive, more luminous H II galaxies. Our total mass estimates imply that H II galaxies are dominated by dark matter, in agreement with observations of dwarf irregular galaxies. We also find a tendency for H II galaxies to be characterized by dense central H I concentrations surrounded by less dense envelopes. Detailed analysis of the H II galaxy/companion systems will appear in subsequent papers.Peer reviewe

Star formation thresholds in H II galaxies with H I companions

Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society. DOI: 10.1086/116910 [Full text of this article is not available in the UHRA]We present high resolution Very Large Array (VLA) 21 cm line observations of five H II galaxies combined with previous lower resolution data from Taylor et al. (1993) and optical broadband R and H-alpha Charge Coupled Device (CCD) images of the systems. Following Kennicutt (1989) we calculated the threshold H I surface density for star formation for the H II galaxies and compared the location and shape of this predicted threshold density contour with the optical shape of the galaxies. We find generally a good correlation between these two, although a constant density contour of 1021/sq cm fits the images of the optical galaxies equally as well. The H I synthesis observations have revealed that the H II galaxies have sharply peaked H I radial profiles, in contrast to the relatively flattened profiles of low surface brightness (LSB) galaxies, suggesting that large central concentrations of gas are a necessary condition for the occurrence of bursts of massive star formation seen in H II galaxies. These observations are consistent with the hypothesis that LSB galaxies represent the quiescent phase of H II galaxies, if a suitable mechanism exists (such as galaxy interactions) to cause H I to concentrate at the center of LSB galaxies prior to the onset of the burst of star formation. However, it is noted that the H II galaxies (and dwarf galaxies in general) span a relatively large range in mass. Since many properties correlate with mass (e.g., gas mass fraction), we point out that great care needs to be taken in choosing the proper comparison samples of LSB and H II galaxies.Peer reviewe