Jonathan Surasa Heiner

The volume densities of giant molecular clouds in M81

HI features near young star clusters in M81 are identified as the photodissociated surfaces of Giant Molecular Clouds (GMCs) from which the young stars have recently formed. The HI column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 × 10 21 cm −2 near R ≈ 7.5 kpc. This trend is similar to that of the radially-averaged HI distribution in this galaxy, and implies a constant area covering factor of ≈ 0.21 for GMCs throughout M81. The incident UV fluxes G 0 of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the HI column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1 < n < 200 cm −3 are derived, with a geometric mean of 17 cm −3. These values are similar to the densities of GMCs in the Galaxy, but somewhat lower than those found earlier for M101 with similar methods. Low values of molecular density in the GMCs of M81 will result in low levels of collisional excitation of the CO(1-0) transition, and are consistent with the very low surface brightness of CO(1-0) emission observed in the disk of M81.H I features near young star clusters in M81 are identified as the photodissociated surfaces of giant molecular clouds (GMCs) from which the young stars have recently formed. The H I column densities of these features show a weak trend, from undetectable values inside R = 3.7 kpc and increasing rapidly to values around 3 × 1021 cm −2 near R ≈ 7.5 kpc. This trend is similar to that of the radially averaged H I distribution in this galaxy, and implies a constant area covering factor of ≈0.21 for GMCs throughout M81. The incident UV fluxes G0 of our sample of candidate PDRs decrease radially. A simple equilibrium model of the photodissociation-reformation process connects the observed values of the incident UV flux, the H I column density, and the relative dust content, permitting an independent estimate to be made of the total gas density in the GMC. Within the GMC this gas will be predominantly molecular hydrogen. Volume densities of 1 < n < 200 cm −3 are derived, with a geometric mean of 17 cm−3. These values are similar to the densities of GMCs in the Galaxy, but somewhat lower than those found earlier for M101 with similar methods. Low values of molecular density in the GMCs of M81 will result in low levels of collisional excitation of the CO(1-0) transition, and are consistent with the very low surface brightness of CO(1-0) emission observed in the disk of M81.

The volume densities of giant molecular clouds in M 83

Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm HI column densities, along with estimates of the local dust abundances, we measure the volume densities of a sample of actively star-forming giant molecular clouds (GMCs) in the nearby spiral galaxy M83 on a typical resolution scale of 170 pc. Our approach is based on an equilibrium model for the cycle of molecular hydrogen formation on dust grains and photodissociation under the influence of the FUV radiation on the cloud surfaces of GMCs. We find a range of total volume densities on the surface of GMCs in M83, namely 0.1-400 cm(-3) inside R(25), 0.5-50 cm(-3) outside R(25). Our data include a number of GMCs in the HI ring surrounding this galaxy. Finally, we discuss the effects of observational selection, which may bias our results.

The JCMT Nearby Galaxies Legacy Survey VI. The distribution of gas and star formation in M81

We present the first complete 12 CO J = 3−2 map of M 81, observed as part of the Nearby Galaxies Legacy Survey being carried out at the James Clerk Maxwell Telescope. We detect nine regions of significant CO emission located at different positions within the spiral arms, and confirm that the global CO emission in the galaxy is low. We combine these data with a new Hα map obtained using the Isaac Newton Telescope and archival H i ,2 4μm, and FUV images to uncover a correlation between the molecular gas and star forming regions in M 81. For the nine regions detected in CO J = 3−2, we combine our CO J = 3−2 data with existing CO J = 1− 0d ata to calculate line ratios. We find that the ratio J = (3−2)/(1−0) is in agreement with the range of typical values found in the literature (0.2−0.8). Making reasonable assumptions, this allows us to constrain the hydrogen density to the range (10 3 −10 4 )c m −3 .W e also estimated the amount of hydrogen produced in photo-dissociation regions near the locations where CO J = 3−2 was detected.