Misc,
SImulator of GAlaxy Millimeter/submillimeter Emission (SIGAME): CO emission from massive z=2 main sequence galaxies
(2015)cite arxiv:1507.00012Comment: 29 pages, 19 figures; submitted to MNRAS; Comments welcome.
Abstract
We present SIGAME (SImulator of GAlaxy Molecular Emission), a new numerical
code designed to simulate the 12CO rotational line emission spectrum of
galaxies. Using sub-grid physics recipes to post-process the outputs of
smoothed particle hydrodynamics (SPH) simulations, a molecular gas phase is
condensed out of the initial hot and partly ionised SPH gas and distributed in
Giant Molecular Cloud (GMCs). The GMCs are subjected to far-UV radiation fields
and cosmic ray ionisation rates which scale with the local star formation rate
volume density, thereby ensuring that the thermal state of the gas is directly
coupled to the in situ star formation conditions. Level populations as well as
line radiative transport of the CO rotational lines are solved for with the 3-D
radiative transfer code LIME. We have applied SIGAME to cosmological SPH
simulations of three disk galaxies at z=2 with stellar masses in the range
~(0.5-2)x10^11 Msun and star formation rates ~40-140 Msun/yr, for which we
predict a low-excitation gas with CO intensity peaks at the CO J=3-2 transition
and total CO(3-2) luminosities within the range of observations of
corresponding star-forming galaxies at z~1-2.5. Global CO-H2 conversion factors
(alpha_CO) range from 1.4 to 1.6 Msun*pc^2/(K*km/s), i.e. about a third of the
Galactic value. On resolved scales, the model galaxies display an increase in
CO(J-(J-1))/CO(1-0) brightness temperature line ratios at J>=3, but a decrease
in alpha_CO towards the central regions, in agreement with observations of
nearby galaxies. Adopting a steeper GMC radial density profile or a more
shallow mass spectrum leads to increased alpha_CO factors, though still below
the Galactic value. The inclusion of high pressure (P_ext/k_B>10^4K/cm^3)
environments descreases line ratios at high-J.
