A new method for obtaining the star formation law in galaxies

We present a new observational method to evaluate the star formation law as formulated by Schmidt: the power-law expression assumed to relate the rate of star formation in a volume of space to the local total gas volume density. Volume densities in the clouds surrounding an OB association are determined with a simple model which considers atomic hydrogen as a photodissociation product on cloud surfaces. The photodissociating flux incident on the cloud is computed from the far-UV luminosity of the OB association and the geometry. We have applied this "PDR Method" to a sample of star-forming regions in M33 using VLA 21-cm data for the HI and GALEX imagery in the far-UV. It provides an estimate of the total volume density of hydrogen (atomic + molecular) in the gas clouds surrounding the young star cluster. A logarithmic graph of the cluster UV luminosity versus the surrounding gas density is a direct measure of the star formation law. However, this plot is severely affected by observational selection, rendering large areas of the diagram inaccessible to the data. An ordinary least-squares regression fit therefore gives a strongly biased result. Its slope primarily reflects the boundary defined when the 21-cm line becomes optically thick, no longer reliably measuring the HI column density. We use a maximum-likelihood statistical approach which can deal with truncated and skewed data, taking into account the large uncertainties in the derived total gas densities. The exponent we obtain for the Schmidt law in M33 is 1.4 \pm 0.2.Comment: Accepted for publication in Ap

A photodissociation model for the morphology of the HI near OB associations in M33

We present an approach for analysing the morphology and physical properties of Hi features near giant OB asso- ciations in M33, in the context of a model whereby the Hi excess arises from photodissociation of the molecular gas in remnants of the parent Giant Molecular Clouds (GMCs). Examples are presented here in the environs of NGC604 and CPSDPZ204, two prominent Hii regions in M33. These are the first results of a detailed analysis of the environs of a large number of OB associations in that galaxy. We present evidence for "diffusion" of the far-UV radiation from the OB association through a clumpy remnant GMC, and show further that enhanced CO(1-0) emission appears preferentially associated with GMCs of higher volume density.Comment: Accepted to Ap

Atomic Hydrogen produced in M33 Photodissociation Regions

We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far-UV photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess far-UV emission that reaches the GMC from the OB association, and the excess 21-cm radio HI emission produced after these far-UV photons convert H2 into HI on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H2 in GMCs, and is especially sensitive to a range of density well below the critical density for CO(1-0) emission. We describe our "PDR method" in more detail and apply it using GALEX far-UV and VLA 21-cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80 pc resolution. The cloud densities we derive range from 1 to 500 cm-3, with no clear dependence on galactocentric radius; these results are generally similar to those obtained earlier in M81, M83, and M101 using the same method.Comment: Accepted for publication in MNRAS. 25 figures, 16 tables, including online-only material

Ultrafast absorption kinetics of NADH in folded and unfolded conformations

The non-radiative energy transfer is shown to occur on a ~3ps time scale for NADH in the folded form in H2O. Addition of methanol thermodynamically favours the open form, for which energy transfer does not occur