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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}{sub SFR}, is closely correlated with the surface density of molecular hydrogen, {Sigma}{sub H{sub 2}}. This empirical relation holds both for galaxy-wide averages and for individual {approx}>kpc sized patches of the interstellar medium, but appears to degrade substantially at a sub-kpc scale. Identifying the physical mechanisms that determine the scale-dependent properties of the observed {Sigma}{sub H{sub 2}}-{Sigma}{sub SFR} relation using a set of cosmological, galaxy formation simulations with a peak resolution of {approx}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 {approx}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 H{sub 2}. The predictions of our numerical model are in good agreement with the observed Kennicutt-Schmidt and {Sigma}{sub H{sub 2}}-{Sigma}{sub SFR} relations. We show that observations based on CO emission are ill suited to reliably measure the slope of the latter relation at low ({approx}< 20 M {sub Sun} pc{sup -2}) H{sub 2} surface densities on sub-kpc scales. Our models also predictmore » that the inferred {Sigma}{sub H{sub 2}}-{Sigma}{sub SFR} relation steepens at high H{sub 2} surface densities as a result of the surface density dependence of the CO/H{sub 2} conversion factor. Finally, we show that on sub-kpc scales most of the scatter of the relation is a consequence of discreteness effects of the star formation process. In contrast, variations of the CO/H{sub 2} conversion factor are responsible for most of the scatter measured on super-kpc scales.« less

Authors:
 [1];  [2];  [3]
  1. Department of Astronomy, University of California Berkeley, Berkeley, CA 94720 (United States)
  2. Particle Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510 (United States)
  3. Kavli Institute for Cosmological Physics and Enrico Fermi Institute, University of Chicago, Chicago, IL 60637 (United States)
Publication Date:
OSTI Identifier:
22086507
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 758; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; CARBON MONOXIDE; COMPUTERIZED SIMULATION; DENSITY; GALACTIC EVOLUTION; GALAXIES; HYDROGEN; ION EMISSION; STAR EVOLUTION; STARS; STOCHASTIC PROCESSES; SURFACES

Citation Formats

Feldmann, Robert, Gnedin, Nickolay Y., and Kravtsov, Andrey V., E-mail: feldmann@berkeley.edu, E-mail: feldmann@fnal.gov. THE X-FACTOR IN GALAXIES. II. THE MOLECULAR-HYDROGEN-STAR-FORMATION RELATION. United States: N. p., 2012. Web. doi:10.1088/0004-637X/758/2/127.
Feldmann, Robert, Gnedin, Nickolay Y., & Kravtsov, Andrey V., E-mail: feldmann@berkeley.edu, E-mail: feldmann@fnal.gov. THE X-FACTOR IN GALAXIES. II. THE MOLECULAR-HYDROGEN-STAR-FORMATION RELATION. United States. doi:10.1088/0004-637X/758/2/127.
Feldmann, Robert, Gnedin, Nickolay Y., and Kravtsov, Andrey V., E-mail: feldmann@berkeley.edu, E-mail: feldmann@fnal.gov. Sat . "THE X-FACTOR IN GALAXIES. II. THE MOLECULAR-HYDROGEN-STAR-FORMATION RELATION". United States. doi:10.1088/0004-637X/758/2/127.
@article{osti_22086507,
title = {THE X-FACTOR IN GALAXIES. II. THE MOLECULAR-HYDROGEN-STAR-FORMATION RELATION},
author = {Feldmann, Robert and Gnedin, Nickolay Y. and Kravtsov, Andrey V., E-mail: feldmann@berkeley.edu, E-mail: feldmann@fnal.gov},
abstractNote = {There is ample observational evidence that the star formation rate (SFR) surface density, {Sigma}{sub SFR}, is closely correlated with the surface density of molecular hydrogen, {Sigma}{sub H{sub 2}}. This empirical relation holds both for galaxy-wide averages and for individual {approx}>kpc sized patches of the interstellar medium, but appears to degrade substantially at a sub-kpc scale. Identifying the physical mechanisms that determine the scale-dependent properties of the observed {Sigma}{sub H{sub 2}}-{Sigma}{sub SFR} relation using a set of cosmological, galaxy formation simulations with a peak resolution of {approx}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 {approx}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 H{sub 2}. The predictions of our numerical model are in good agreement with the observed Kennicutt-Schmidt and {Sigma}{sub H{sub 2}}-{Sigma}{sub SFR} relations. We show that observations based on CO emission are ill suited to reliably measure the slope of the latter relation at low ({approx}< 20 M {sub Sun} pc{sup -2}) H{sub 2} surface densities on sub-kpc scales. Our models also predict that the inferred {Sigma}{sub H{sub 2}}-{Sigma}{sub SFR} relation steepens at high H{sub 2} surface densities as a result of the surface density dependence of the CO/H{sub 2} conversion factor. Finally, we show that on sub-kpc scales most of the scatter of the relation is a consequence of discreteness effects of the star formation process. In contrast, variations of the CO/H{sub 2} conversion factor are responsible for most of the scatter measured on super-kpc scales.},
doi = {10.1088/0004-637X/758/2/127},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 758,
place = {United States},
year = {2012},
month = {10}
}