Title: THE X-FACTOR IN GALAXIES. II. THE MOLECULAR-HYDROGEN-STAR-FORMATION RELATION

Abstract

There is ample observational evidence that the star formation rate (SFR) surface density, Sigma_SFR, is closely correlated with the surface density of molecular hydrogen, Sigma_H2. This empirical relation holds both for galaxy-wide averages and for individual >=kpc sized patches of the interstellar medium (ISM), but appears to degrade substantially at a sub-kpc scale. Identifying the physical mechanisms that determine the scale-dependent properties of the observed Sigma_H2-Sigma_SFR relation remains a challenge from a theoretical perspective. To address this question, we analyze the slope and scatter of the Sigma_H2-Sigma_SFR relation using a set of cosmological, galaxy formation simulations with a peak resolution of ~100 pc. These simulations include a chemical network for molecular hydrogen, a model for the CO emission, and a simple, stochastic prescription for star formation that operates on ~100 pc scales. Specifically, star formation is modeled as a Poisson process in which the average SFR is directly proportional to the present mass of H2. The predictions of our numerical model are in good agreement with the observed Kennicutt-Schmidt and Sigma_H2-Sigma_SFR relations. We show that observations based on CO emission are ill suited to reliably measure the slope of the latter relation at low (<20 M_sun pc^-2) H2 surface densitiesmore » on sub-kpc scales. Our models also predict that the inferred Sigma_H2-Sigma_SFR relation steepens at high H2 surface densities as a result of the surface density dependence of the CO/H2 conversion factor. Finally, we show that on sub-kpc scales most of the scatter in the relation is a consequence of discreteness effects in the star formation process. In contrast, variations of the CO/H2 conversion factor are responsible for most of the scatter measured on super-kpc scales.« less

Authors:

Feldmann, Robert; Gnedin, Nickolay Y.; Kravtsov, Andrey V.

Publication Date:

Mon Oct 08 00:00:00 EDT 2012

Research Org.:

Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)

Sponsoring Org.:

USDOE Office of Science (SC), High Energy Physics (HEP)

OSTI Identifier:

1433878

Report Number(s):

FERMILAB-PUB-12-113-A-PPD-T; arXiv:1204.3910
Journal ID: ISSN 0004-637X; 1111191

DOE Contract Number:  

AC02-07CH11359

Resource Type:

Journal Article

Journal Name:

Astrophysical Journal

Additional Journal Information:

Journal Volume: 758; Journal Issue: 2; Journal ID: ISSN 0004-637X

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS