SImulator of GAlaxy Millimetre/submillimetre Emission (SIGAME): CO emission from massive z=2 main-sequence galaxies

Karen P. Olsen, Thomas R. Greve, Christian Brinch, Jesper Sommer-Larsen, Jesper Rasmussen, Sune Toft, Andrew Zirm

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Abstract

We present sígame (SImulator of GAlaxy Millimetre/submillimetre Emission), a new numerical code designed to simulate the 12CO rotational line 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 hot and partly ionized SPH gas. The gas is subjected to far-UV radiation fields and cosmic ray ionization rates which are set to scale with the local star formation rate volume density. Level populations and radiative transport of the CO lines are solved with the 3D radiative transfer code lime. We have applied sígame to cosmological SPH simulations of three disc galaxies at z = 2 with stellar masses in the range ∼0.5–2 × 1011 M and star formation rates ∼40–140 M yr−1. Global CO luminosities and line ratios are in agreement with observations of disc galaxies at z ∼ 2 up to and including J = 3–2 but falling short of the few existing J=5–4 observations. The central 5 kpc regions of our galaxies have CO 3 − 2/1 − 0 and 7 − 6/1 − 0 brightness temperature ratios of ∼0.55–0.65 and ∼0.02–0.08, respectively, while further out in the disc the ratios drop to more quiescent values of ∼0.5 and <0.01. Global CO-to-H2 conversion (αCO) factors are ≃1.5M⊙pc−2(Kkms−1)−1, i.e. ∼2–3 times below the typically adopted values for disc galaxies, and αCO increases with radius, in agreement with observations of nearby galaxies. Adopting a top-heavy Giant Molecular Cloud (GMC) mass spectrum does not significantly change the results. Steepening the GMC density profiles leads to higher global line ratios for Jup ≥ 3 and CO-to-H2 conversion factors [≃3.6M⊙pc−2(Kkms−1)−1].
Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society
Volume457
Issue number3
Pages (from-to)3306-3333
ISSN0035-8711
DOIs
Publication statusPublished - 2016

Bibliographical note

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2016 The authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Keywords

  • radiative transfer
  • methods: numerical
  • ISM: clouds
  • ISM: lines and bands
  • galaxies: high-redshift
  • galaxies: ISM

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