Monthly Notices of the Royal Astronomical Society
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.
“The Kennicutt-Schmidt Star Formation Relation at z~2 Narayanan, D., Cox, T.J., Hayward, C., Hernquist, L., MNRAS 2011, 412, 287