# Millimeter-wave Line Ratios and Sub-beam Volume Density Distributions

## Abstract

We explore the use of mm-wave emission line ratios to trace molecular gas density when observations integrate over a wide range of volume densities within a single telescope beam. For observations targeting external galaxies, this case is unavoidable. Using a framework similar to that of Krumholz and Thompson, we model emission for a set of common extragalactic lines from lognormal and power law density distributions. We consider the median density of gas that produces emission and the ability to predict density variations from observed line ratios. We emphasize line ratio variations because these do not require us to know the absolute abundance of our tracers. Patterns of line ratio variations have the potential to illuminate the high-end shape of the density distribution, and to capture changes in the dense gas fraction and median volume density. Our results with and without a high-density power law tail differ appreciably; we highlight better knowledge of the probability density function (PDF) shape as an important area. We also show the implications of sub-beam density distributions for isotopologue studies targeting dense gas tracers. Differential excitation often implies a significant correction to the naive case. We provide tabulated versions of many of our results, which canmore »

- Authors:

- Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States)
- Observatorio Astronmico Nacional (IGN), C/Alfonso XII, 3, E-28014 Madrid (Spain)
- Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, D-85748 Garching (Germany)
- Institute für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, D-69120 Heidelberg (Germany)
- Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, D-69120 Heidelberg (Germany)
- National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 (United States)
- Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago (Chile)
- Department of Astronomy, Laboratory for Millimeter-wave Astronomy, and Joint Space Institute, University of Maryland, College Park, MD 20742 (United States)
- Max Planck Institute für Astronomie, Königstuhl 17, D-69117, Heidelberg (Germany)
- CNRS, IRAP, 9 av. du Colonel Roche, BP 44346, F-31028 Toulouse cedex 4 (France)
- Department of Physics, University of Alberta, Edmonton, AB (Canada)

- Publication Date:

- OSTI Identifier:
- 22663907

- Resource Type:
- Journal Article

- Resource Relation:
- Journal Name: Astrophysical Journal; Journal Volume: 835; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; BEAMS; CAPTURE; CORRECTIONS; DENSITY; DISTRIBUTION; EMISSION; EXCITATION; GALAXIES; MOLECULES; POWER DENSITY; PROBABILITY; PROBABILITY DENSITY FUNCTIONS; STARS; TELESCOPES; VARIATIONS

### Citation Formats

```
Leroy, Adam K., Gallagher, Molly, Usero, Antonio, Schruba, Andreas, Bigiel, Frank, Kruijssen, J. M. Diederik, Schinnerer, Eva, Kepley, Amanda, Blanc, Guillermo A., Bolatto, Alberto D., Cormier, Diane, Jiménez-Donaire, Maria J., Hughes, Annie, and Rosolowsky, Erik.
```*Millimeter-wave Line Ratios and Sub-beam Volume Density Distributions*. United States: N. p., 2017.
Web. doi:10.3847/1538-4357/835/2/217.

```
Leroy, Adam K., Gallagher, Molly, Usero, Antonio, Schruba, Andreas, Bigiel, Frank, Kruijssen, J. M. Diederik, Schinnerer, Eva, Kepley, Amanda, Blanc, Guillermo A., Bolatto, Alberto D., Cormier, Diane, Jiménez-Donaire, Maria J., Hughes, Annie, & Rosolowsky, Erik.
```*Millimeter-wave Line Ratios and Sub-beam Volume Density Distributions*. United States. doi:10.3847/1538-4357/835/2/217.

```
Leroy, Adam K., Gallagher, Molly, Usero, Antonio, Schruba, Andreas, Bigiel, Frank, Kruijssen, J. M. Diederik, Schinnerer, Eva, Kepley, Amanda, Blanc, Guillermo A., Bolatto, Alberto D., Cormier, Diane, Jiménez-Donaire, Maria J., Hughes, Annie, and Rosolowsky, Erik. Wed .
"Millimeter-wave Line Ratios and Sub-beam Volume Density Distributions". United States.
doi:10.3847/1538-4357/835/2/217.
```

```
@article{osti_22663907,
```

title = {Millimeter-wave Line Ratios and Sub-beam Volume Density Distributions},

author = {Leroy, Adam K. and Gallagher, Molly and Usero, Antonio and Schruba, Andreas and Bigiel, Frank and Kruijssen, J. M. Diederik and Schinnerer, Eva and Kepley, Amanda and Blanc, Guillermo A. and Bolatto, Alberto D. and Cormier, Diane and Jiménez-Donaire, Maria J. and Hughes, Annie and Rosolowsky, Erik},

abstractNote = {We explore the use of mm-wave emission line ratios to trace molecular gas density when observations integrate over a wide range of volume densities within a single telescope beam. For observations targeting external galaxies, this case is unavoidable. Using a framework similar to that of Krumholz and Thompson, we model emission for a set of common extragalactic lines from lognormal and power law density distributions. We consider the median density of gas that produces emission and the ability to predict density variations from observed line ratios. We emphasize line ratio variations because these do not require us to know the absolute abundance of our tracers. Patterns of line ratio variations have the potential to illuminate the high-end shape of the density distribution, and to capture changes in the dense gas fraction and median volume density. Our results with and without a high-density power law tail differ appreciably; we highlight better knowledge of the probability density function (PDF) shape as an important area. We also show the implications of sub-beam density distributions for isotopologue studies targeting dense gas tracers. Differential excitation often implies a significant correction to the naive case. We provide tabulated versions of many of our results, which can be used to interpret changes in mm-wave line ratios in terms of adjustments to the underlying density distributions.},

doi = {10.3847/1538-4357/835/2/217},

journal = {Astrophysical Journal},

number = 2,

volume = 835,

place = {United States},

year = {Wed Feb 01 00:00:00 EST 2017},

month = {Wed Feb 01 00:00:00 EST 2017}

}