PHIBSS: Exploring the Dependence of the CO-H$_2$ Conversion Factor on Total Mass Surface Density at ${\it z} < 1.5$

Timothy Carleton; Michael C. Cooper; Alberto D. Bolatto; Frederic Bournaud; Françoise Combes; Jonathan Freundlich; Santiago Garcia-Burillo; Reinhard Genzel; Roberto Neri; Linda J. Tacconi; Karin M. Sandstrom; Benjamin J. Weiner; Axel Weiss

doi:10.1093/mnras/stx390

paper

PHIBSS: Exploring the Dependence of the CO-H $_{2}$ Conversion Factor on Total Mass Surface Density at $z < 1.5$

arXiv:1611.04587 · doi:10.1093/mnras/stx390

Abstract

We present an analysis of the relationship between the CO-H $_{2}$ conversion factor ( $α_{CO}$ ) and total mass surface density ( $Σ_{tot}$ ) in star-forming galaxies at $z < 1.5$ . Our sample, which is drawn from the IRAM Plateau de Bure HIgh- $z$ Blue Sequence Survey (PHIBSS) and the CO Legacy Database for GASS (COLD GASS), includes 'normal,' massive star-forming galaxies that dominate the evolution of the cosmic star formation rate (SFR) at this epoch and probe the $Σ_{tot}$ regime where the strongest variation in $α_{CO}$ is observed. We constrain $α_{CO}$ via existing CO observations, measurements of the star formation rate, and an assumed molecular gas depletion time ( $t_{dep}$ = $M_{gas}$ /SFR) --- the latter two of which establish the total molecular gas mass independent of the observed CO luminosity. For a broad range of adopted depletion times, we find that $α_{CO}$ is independent of total mass surface density, with little deviation from the canonical Milky Way value. This runs contrary to a scenario in which $α_{CO}$ decreases as surface density increases within the extended clouds of molecular gas that potentially fuel clumps of star formation in $z \sim 1$ galaxies, similar to those observed in local ULIRGs. Instead, our results suggest that molecular gas, both at $z \sim 0$ and $z \sim 1$ , is primarily in the form of self-gravitating molecular clouds. While CO observations suggest a factor of $\sim 3$ reduction in the average molecular gas depletion time between $z \sim 0$ and $z \sim 1$ , we find that, for typical galaxies, the structure of molecular gas and the process of star formation at $z \sim 1$ is otherwise remarkably similar to that observed in local star-forming systems.

Accepted to MNRAS. 16 pages, 8 figures

PHIBSS: Exploring the Dependence of the CO-H $_{2}$ Conversion Factor on Total Mass Surface Density at $z < 1.5$

Abstract

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Cited by in corpus (23)

PHIBSS: Exploring the Dependence of the CO-H2​ Conversion Factor on Total Mass Surface Density at z<1.5

Abstract

References in corpus (122)

Cited by in corpus (23)

PHIBSS: Exploring the Dependence of the CO-H $_{2}$ Conversion Factor on Total Mass Surface Density at $z < 1.5$