The Distribution of Atomic Hydrogen Around Two Irregular Galaxies

We present radio interferometric observations of two irregular galaxies that were candidates for having unusually extended HI emission. The galaxies, UGC 199 and DDO 26, otherwise appeared to be normal, low-luminosity systems with modest star-formation rates. To a detection limit of 2--3E19 /cm^2 at a resolution of about 50", however, the HI around neither galaxy is unusually extended compared to other irregulars. The HI around UGC 199 appears as a regular, symmetric distribution with regular rotation and a maximum rotation speed of about 80 km/s. By contrast, the HI around DDO 26 shows a concentration into two blobs with an arm in the outer parts to the northwest and some additional emission to the northwest of that. The kinematical major axis is approximately 75 deg from the HI and optical morphological axis which is unusual for Im galaxies. In addition the velocity field in the outer parts of the galaxy is messy and the velocity profiles at the two HI peaks are broad. We suggest that DDO 26 has been perturbed externally or may be two dwarfs in the process of merging.Comment: To be published in AJ, May 200

Measuring star formation rates in blue galaxies

The problems associated with measurements of star formation rates in galaxies are briefly reviewed, and specific models are presented for determinations of current star formation rates from H alpha and Far Infrared (FIR) luminosities. The models are applied to a sample of optically blue irregular galaxies, and the results are discussed in terms of star forming histories. It appears likely that typical irregular galaxies are forming stars at nearly constant rates, although a few examples of systems with enhanced star forming activity are found among HII regions and luminous irregular galaxies

Ionized interstellar froth in irregular galaxies

The warm interstellar medium of galaxies is a complicated place. It is often full of holes, neutral and ionized loops and shells, and diffuse ionized gas. Deep H alpha images of Magellanic-type irregular galaxies also reveal complex spatial structures consisting of loops and filaments in the interstellar gas outside of the boundaries of traditional HII regions. Researchers refer to these ionized structures as froth. Such structures could mark paths over which newly produced heavy elements are dispersed in irregular galaxies, and they could be the signatures of a feedback process related to star formation. In order to investigate the physical nature of the froth, researchers obtained narrow-band images and high and low dispersion spectra from Kitt Peak National Observatory (KPNO) and deep blue-passband plates from the Canada-France-Hawaii Observatory (CFHO)

Star Formation Properties of a Large Sample of Irregular Galaxies

We present the results of Halpha imaging of 94 irregular galaxies, including types Im, BCD, and Sm. The sample spans a large range in galactic parameters including integrated absolute magnitude (MV=-9 to -19), average surface brightness (20 to 27 mag/arcsec2), current star formation activity (0 to 1.3 Msun/yr/kpc2), and relative gas content (0.02 to 5 Msun/LB). The Halpha images were used to measure integrated star formation rates and extents of star formation and to compare azimuthally-averaged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 10^4 with 10% Im galaxies and one Sm system having no measurable star formation at the present time. The BCDs fall at the high end of the range of rates. We find no correlation between star formation activity and proximity to other catalogued galaxies. The cases with strong gradients in the L(Halpha)/LV ratios and with high central star formation rate densities, which include most of the BCDs, require a significant fraction of their gas to have migrated to the center in the last Gyr. We discuss possible torques that could have caused this without leaving an obvious signature, including dark matter bars and past interactions or mergers with small galaxies or HI clouds. There is now substantial evidence for these processes among many surveys of BCDs. The star formation process itself does not appear to differ much among BCD, Im and Sm types. There is evidence in the distribution function for Halpha surface brightness, which is always approximately log-normal, that the turbulent Mach numbers are about the same in these systems.Comment: 82 pages, 21 figures, accepted by Astronomical Journal, Vol. 128, November 200

A Comparison of Young Star Properties with Local Galactic Environment for LEGUS/LITTLE THINGS Dwarf Irregular Galaxies

We have explored the role environmental factors play in determining characteristics of young stellar objects in nearby dwarf irregular and blue compact dwarf galaxies. Star clusters are characterized by concentrations, masses, and formation rates; OB associations by mass and mass surface density; O stars by their numbers and near-ultraviolet absolute magnitudes; and H ii regions by Hα surface brightnesses. These characteristics are compared to surrounding galactic pressure, stellar mass density, H i surface density, and star formation rate (SFR) surface density. We find no trend of cluster characteristics with environmental properties, implying that larger-scale effects are more important in determining cluster characteristics or that rapid dynamical evolution erases any memory of the initial conditions. On the other hand, the most massive OB associations are found at higher pressure and H i surface density, and there is a trend of higher H ii region Hα surface brightness with higher pressure, suggesting that a higher concentration of massive stars and gas is found preferentially in regions of higher pressure. At low pressures we find massive stars but not bound clusters and OB associations. We do not find evidence for an increase of cluster formation efficiency as a function of SFR density. However, there is an increase in the ratio of the number of clusters to the number of O stars with increasing pressure, perhaps reflecting an increase in clustering properties with SFR