Star formation rates in the centers of disk galaxies often vastly exceed those at larger radii, whether measured by the surface density of star formation Σ SFR , by the star formation rate per unit gas mass, Σ SFR /Σ, or even by total output. In this paper, we investigate the idea that central starbursts are selfregulated systems, in which the momentum flux injected to the interstellar medium (ISM) by star formation balances the gravitational force confining the ISM gas in the disk. For most starbursts, supernovae are the largest contributor to the momentum flux, and turbulence provides the main pressure support for the predominantly-molecular ISM. If the momentum feedback per stellar mass formed is p * /m * ∼ 3000 km s −1 , the predicted star formation rate is Σ SFR ∼ 2πGΣ 2 m * /p * ∼ 0.1 M ⊙ kpc −2 yr −1 (Σ/100 M ⊙ pc −2 ) 2 in regions where gas dominates the vertical gravity. We compare this prediction with numerical simulations of vertically-resolved disks that model star formation including feedback, finding good agreement for gas surface densities in the range Σ ∼ 10 2 − 10 3 M ⊙ pc −2 . We also compare to a compilation of star formation rates and gas contents from local and high-redshift galaxies (both mergers and normal galaxies), finding good agreement provided that the conversion factor X CO from integrated CO emission to H 2 surface density decreases weakly as Σ and Σ SFR increase. Star formation rates in dense, turbulent gas are also expected to depend on the gravitational free-fall time at the corresponding mean ISM density ρ 0 ; if the star formation efficiency per free-fall time is ε ff (ρ 0 ) ∼ 0.01, the turbulent velocity dispersion driven by feedback is expected to be v z = 0.4 ε ff (ρ 0 )p * /m * ∼ 10 km s −1 , relatively independent of Σ or Σ SFR . Turbulence-regulated starbursts (controlled by kinetic momentum feedback) are part of the larger scheme of self-regulation; primarily-atomic low-Σ outer disks may have star formation regulated by UV heating feedback, whereas regions at extremely high Σ may be regulated by feedback of stellar radiation that is reprocessed into trapped IR.

We investigate the effect of line of sight temperature variations and noise on two commonly used methods to determine dust properties from dust continuum observations of dense cores. One method employs a direct fit to a modified blackbody SED; the other involves a comparison of flux ratios to an analytical prediction. Fitting fluxes near the SED peak produces inaccurate temperature and dust spectral index estimates due to the line of sight temperature (and density) variations. Longer wavelength fluxes in the Rayleigh-Jeans part of the spectrum ( > ∼ 600 µm for typical cores) may more accurately recover the spectral index, but both methods are very sensitive to noise. The temperature estimate approaches the density weighted temperature, or "column temperature," of the source as short wavelength fluxes are excluded. An inverse temperature -spectral index correlation naturally results from SED fitting, due to the inaccurate isothermal assumption, as well as noise uncertainties. We show that above some "threshold" temperature, the temperatures estimated through the flux ratio method can be highly inaccurate. In general, observations with widely separated wavelengths, and including shorter wavelengths, result in higher threshold temperatures; such observations thus allow for more accurate temperature estimates of sources with temperatures less than the threshold temperature. When only three fluxes are available, a constrained fit, where the spectral index is fixed, produces less scatter in the temperature estimate when compared to the estimate from the flux ratio method.

Theoretical and observational investigations have indicated that the abundance of carbon monoxide (CO) is very sensitive to intrinsic properties of the gaseous medium, such as density, metallicity and the background radiation field. CO observations are often employed to study the properties of molecular clouds (MCs), such as mass, morphology and kinematics. It is thus important to understand how well CO traces the total mass, which in MCs is predominantly due to molecular hydrogen (H 2 ). Recent hydrodynamic simulations by Glover & Mac Low have explicitly followed the formation and destruction of molecules in model MCs under varying conditions. These models have confirmed that CO formation strongly depends on the cloud properties. Conversely, the formation of H 2 is primarily determined by the amount of time available for its formation. We apply radiative transfer calculations to these MC models in order to investigate the properties of CO line emission. We focus on integrated CO (J = 1-0) intensities emerging from individual clouds, including its relationship to the total, H 2 and CO column densities, as well as the 'X factor,' the ratio of H 2 column density to CO intensity. Models with high CO abundances have a threshold CO intensity of ≈65 K km s −1 at sufficiently large extinctions (or column densities). Clouds with low CO abundances show no such intensity thresholds. The distributions of total and H 2 column densities are well described as lognormal functions, though the distributions of CO intensities and column densities are usually not lognormal. In general, the probability distribution functions of the integrated intensity do not follow the distribution functions of CO column densities. In the model with Milky Way-like conditions, the X factor is in agreement with the near-constant value determined from observations. In clouds with lower metallicity, lower density or a higher background UV radiation field, the CO abundances are in general lower, and hence the X factor can vary appreciably -sometimes by up to 4 orders of magnitude. In models with high densities, the CO line is fully saturated, so that the X factor is directly proportional to the molecular column density.

We investigate the formation of substructure in spiral galaxies using global MHD simulations, including gas self-gravity. Our models extend previous local models by Kim and Ostriker (2002) by including the full effects of curvilinear coordinates, a realistic log-spiral perturbation, self-gravitational contribution from 5 radial wavelengths of the spiral shock, and variation of density and epicyclic frequency with radius. We show that with realistic Toomre Q values, self-gravity and galactic differential rotation produce filamentary gaseous structures with kpc-scale separations, regardless of the strength -- or even presence -- of a stellar spiral potential. However, the growth of sheared features distinctly associated with the spiral arms, described as spurs or feathers in optical and IR observations of many spiral galaxies, requires a sufficiently strong spiral potential in self gravitating models. Unlike independently-growing ''background'' filaments, the orientation of arm spurs depends on galactic location. Inside corotation, spurs emanate outward, on the convex side of the arm; outside corotation, spurs grow inward, on the concave side of the arm. Based on spacing, orientation, and the relation to arm clumps, it is possible to distinguish ''true spurs'' that originate as instabilities in the spiral arms from independently growing ''background'' filaments. Our models also suggest that magnetic fields are important in preserving grand design spiral structure when gas in the arms fragments via self-gravity into GMCs.Comment: 36 pages, 17 figures, Accepted for publication in ApJ. PDF version with high resolution figures available at http://www.astro.umd.edu/~shetty/Research