Jana Grcevich

A High-resolution Study of the H I-H2 Transition across the Perseus Molecular Cloud

To investigate the fundamental principles of H2 formation in a giant molecular cloud, we derive the H I and H2 surface density (?H I and ?H2) images of the Perseus molecular cloud on sub-pc scales (~0.4?pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the ?H2 distribution across Perseus. We find a relatively uniform ?H I ~ 6-8 M ??pc?2 for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between ?H2/?H I and ?H I + ?H2. The transition between the H I- and H2-dominated regions occurs at N(H I) + 2N(H2) ~ (8-14)?? 1020?cm?2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of CO-dark gas, f DG ~ 0.3. While significant spatial variations of f DG are found, we do not find a clear correlation with the mean V-band extinction.

Characterizing the turbulent properties of the starless molecular cloud MBM 16

We investigate turbulent properties of the non-star-forming, translucent molecular cloud MBM 16 by applying the statistical technique of a two-dimensional spatial power spectrum (SPS) on the neutral hydrogen (H I) observations obtained by the Galactic Arecibo L-Band Feed Array H I survey. The SPS, calculated over the range of spatial scales from 0.1 to 17 pc, is well represented with a single power-law function, with a slope ranging from –3.3 to –3.7 and being consistent over the velocity range of MBM 16 for a fixed velocity channel thickness. However, the slope varies significantly with the velocity slice thickness, suggesting that both velocity and density contribute to H I intensity fluctuations. By using this variation, we estimate the slope of three-dimensional density fluctuations in MBM 16 to be –3.7 ± 0.2. This is significantly steeper than what has been found for H I in the Milky Way plane, the Small Magellanic Cloud, or the Magellanic Bridge, suggesting that interstellar turbulence in MBM 16 is driven on scales >17 pc and that the lack of stellar feedback could be responsible for the steep power spectrum.

The Gaseous Halo Mask

Support for spiral galaxies harboring an extended cosmologically significant gaseous halo is presented through evidence of the stripping of gas from dwarf galaxies in both observations and local and cosmological simulations. Dwarf galaxies within 270 kpc of the Milky Way and Andromeda are largely devoid of gas, and those with significant amounts of gas are beyond this radius. This is most easily explained by ram pressure stripping as the dwarfs move through perigalacticon. The star formation histories of the dwarf galaxies can also be partially understood in the context of their movement through this halo medium. Since the dwarfs are stripped at distances generally greater than 20 kpc, the gas is unlikely to make it directly into the spiral galaxy disk as cold star formation fuel. Some of the dwarfs’ gas may be integrated into the spiral galaxy’s hot halo and warm clumps may undergo a recooling process close to the disk.