Journal Article

The Kennicutt–Schmidt star formation relation at <i>z</i>∼ 2

Desika Narayanan, Thomas J. Cox, Christopher C. Hayward and Lars Hernquist

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 1, pages 287-294
Published in print March 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI:
The Kennicutt–Schmidt star formation relation at z∼ 2

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Recent observations of excited CO emission lines from z∼ 2 disc galaxies have shed light on the SFR ∝ρN relation at high z via observed ΣSFR–ΣαCOJ=2−1 and ΣSFR–ΣαCOJ=3−2 relations. Here, we describe a novel methodology for utilizing these observations of high-excitation CO to derive the underlying Schmidt (SFR ∝ρN) relationship. To do this requires an understanding of the potential effects of differential CO excitation with SFR. If the most heavily star-forming galaxies have a larger fraction of their gas in highly excited CO states than the lower SFR galaxies, then the observed molecular Kennicutt–Schmidt index, α, will be less than the underlying SFR ∝ρN index, N. Utilizing a combination of SPH models of galaxy evolution and molecular line radiative transfer, we present the first calculations of CO excitation in z∼ 2 disc galaxies with the aim of developing a mapping between various observed ΣSFR–ΣαCO relationships and the underlying SFR ∝ρN relation. We find that even in relatively luminous z∼ 2 discs, differential excitation does indeed exist, resulting in α < N for highly excited CO lines. This means that an observed (e.g.) ΣSFR–ΣαCOJ=3−2 relation does not map linearly to a ΣSFR–ΣαH2 relation. We utilize our model results to provide a mapping from α to N for the range of Schmidt indices N= 1–2. By comparing to recent observational surveys, we find that the observed and ΣSFR–ΣαCOJ=3−2 relations suggest that an underlying SFR ∝ρ1.5 relation describes z∼ 2 disc galaxies.

Keywords: ISM: molecules; galaxies: formation; galaxies: high-redshift; galaxies: ISM; galaxies: starburst; galaxies: star formation; cosmology: theory

Journal Article.  7086 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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