Constraining cloud parameters using high density gas tracers in galaxies
arXiv:1609.02567 · doi:10.1051/0004-6361/201424594
Abstract
Far-infrared molecular emission is an important tool used to understand the excitation mechanisms of the gas in the inter-stellar medium of star-forming galaxies. In the present work, we model the emission from rotational transitions with critical densities n >~ 10^4 cm-3. We include 4-3 < J <= 15-14 transitions of CO and 13CO, in addition to J <= 7-6 transitions of HCN, HNC, and HCO+ on galactic scales. We do this by re-sampling high density gas in a hydrodynamic model of a gas-rich disk galaxy, assuming that the density field of the interstellar medium of the model galaxy follows the probability density function (PDF) inferred from the resolved low density scales. We find that in a narrow gas density PDF, with a mean density of ~10 cm-3 and a dispersion σ= 2.1 in the log of the density, most of the emission of molecular lines, emanates from the 10-1000 cm-3 part of the PDF. We construct synthetic emission maps for the central 2 kpc of the galaxy and fit the line ratios of CO and 13CO up to J = 15-14, as well as HCN, HNC, and HCO+ up to J = 7-6, using one photo-dissociation region (PDR) model. We attribute the goodness of the one component fits for our model galaxy to the fact that the distribution of the luminosity, as a function of density, is peaked at gas densities between 10 and 1000 cm-3. We explore the impact of different log-normal density PDFs on the distribution of the line-luminosity as a function of density, and we show that it is necessary to have a broad dispersion, corresponding to Mach numbers >~ 30 in order to obtain significant emission from n > 10^4 cm-3 gas. Such Mach numbers are expected in star-forming galaxies, LIRGS, and ULIRGS. By fitting line ratios of HCN(1-0), HNC(1-0), and HCO+(1-0) for a sample of LIRGS and ULIRGS using mechanically heated PDRs, we constrain the Mach number of these galaxies to 29 < M < 77.
13 pages, 11 figures
References in corpus (13)
- Diagnostics of irradiated dense gas in galaxy nuclei. II. A grid of XDR and PDR models
- On the Density Distribution in Star-forming Interstellar Clouds
- A 2 mm spectral line survey of the starburst galaxy NGC 253
- The Relationship Between Molecular Gas Tracers and Kennicutt-Schmidt Laws
- Mechanical feedback in the molecular ISM of luminous IR galaxies
- The Role of Galactic Winds on Molecular Gas Emission from Galaxy Mergers
- Understanding star formation in molecular clouds II. Signatures of gravitational collapse of IRDCs
- Herschel's view of the large-scale structure in the Chamaeleon dark clouds
- APEX mapping of H3O+ in the Sgr B2 region
- Diagnostics of the molecular component of PDRs with mechanical heating. II: line intensities and ratios
- Molecular gas heating in Arp 299
- Spatially Resolved Chemistry in Nearby Galaxies III. Dense Molecular Gas in the Inner Disk of the LIRG IRAS 04296+2923
- Constraining cloud parameters using high density gas tracers in galaxies