Dark Molecular Gas in Simulations of z ∼ 0 Disk Galaxies

Qi Li, Desika Narayanan, Romeel S Dave, Mark R. Krumholz

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

The H2 mass of molecular clouds has traditionally been traced by the CO(J = 1-0) rotational transition line. This said, CO is relatively easily photodissociated and can also be destroyed by cosmic rays, thus rendering some fraction of molecular gas to be "CO-dark." We investigate the amount and physical properties of CO-dark gas in two z ∼ 0 disk galaxies and develop predictions for the expected intensities of promising alternative tracers ([C i] 609 μm and [C ii] 158 μm emission). We do this by combining cosmological zoom simulations of disk galaxies with thermal-radiative-chemical equilibrium interstellar medium (ISM) calculations to model the predicted H i and H2 abundances and CO (J = 1-0), [C i] 609 μm, and [C ii] 158 μm emission properties. Our model treats the ISM as a collection of radially stratified clouds whose properties are dictated by their volume and column densities, the gas-phase metallicity, and the interstellar radiation field (ISRF) and CR ionization rates. Our main results follow. Adopting an observationally motivated definition of CO-dark gas, i.e., H2 gas with W CO < 0.1 K km s-1, we find that a significant amount (50%) of the total H2 mass lies in CO-dark gas, most of which is diffuse gas, poorly shielded due to low dust column density. The CO-dark molecular gas tends to be dominated by [C ii], though [C i] also serves as a bright tracer of the dark gas in many instances. At the same time, [C ii] also tends to trace neutral atomic gas. As a result, when we quantify the conversion factors for the three carbon-based tracers of molecular gas, we find that [C i] suffers the least contamination from diffuse atomic gas and is relatively insensitive to secondary parameters.

Original languageEnglish (US)
Article number73
JournalAstrophysical Journal
Volume869
Issue number1
DOIs
StatePublished - Dec 10 2018
Externally publishedYes

Fingerprint

disk galaxies
molecular gases
gases
gas
simulation
tracers
monatomic gases
interstellar radiation
tracer
neutral gases
molecular clouds
radiation distribution
metallicity
chemical equilibrium
cosmic rays
contamination
physical properties
dust
vapor phases
ionization

Keywords

  • astrochemistry
  • galaxies: ISM
  • ISM: molecules
  • methods: numerical

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Dark Molecular Gas in Simulations of z ∼ 0 Disk Galaxies. / Li, Qi; Narayanan, Desika; Dave, Romeel S; Krumholz, Mark R.

In: Astrophysical Journal, Vol. 869, No. 1, 73, 10.12.2018.

Research output: Contribution to journalArticle

Li, Qi ; Narayanan, Desika ; Dave, Romeel S ; Krumholz, Mark R. / Dark Molecular Gas in Simulations of z ∼ 0 Disk Galaxies. In: Astrophysical Journal. 2018 ; Vol. 869, No. 1.
@article{b1d1a1c608444c87be43e2fa7863f9f6,
title = "Dark Molecular Gas in Simulations of z ∼ 0 Disk Galaxies",
abstract = "The H2 mass of molecular clouds has traditionally been traced by the CO(J = 1-0) rotational transition line. This said, CO is relatively easily photodissociated and can also be destroyed by cosmic rays, thus rendering some fraction of molecular gas to be {"}CO-dark.{"} We investigate the amount and physical properties of CO-dark gas in two z ∼ 0 disk galaxies and develop predictions for the expected intensities of promising alternative tracers ([C i] 609 μm and [C ii] 158 μm emission). We do this by combining cosmological zoom simulations of disk galaxies with thermal-radiative-chemical equilibrium interstellar medium (ISM) calculations to model the predicted H i and H2 abundances and CO (J = 1-0), [C i] 609 μm, and [C ii] 158 μm emission properties. Our model treats the ISM as a collection of radially stratified clouds whose properties are dictated by their volume and column densities, the gas-phase metallicity, and the interstellar radiation field (ISRF) and CR ionization rates. Our main results follow. Adopting an observationally motivated definition of CO-dark gas, i.e., H2 gas with W CO < 0.1 K km s-1, we find that a significant amount (50{\%}) of the total H2 mass lies in CO-dark gas, most of which is diffuse gas, poorly shielded due to low dust column density. The CO-dark molecular gas tends to be dominated by [C ii], though [C i] also serves as a bright tracer of the dark gas in many instances. At the same time, [C ii] also tends to trace neutral atomic gas. As a result, when we quantify the conversion factors for the three carbon-based tracers of molecular gas, we find that [C i] suffers the least contamination from diffuse atomic gas and is relatively insensitive to secondary parameters.",
keywords = "astrochemistry, galaxies: ISM, ISM: molecules, methods: numerical",
author = "Qi Li and Desika Narayanan and Dave, {Romeel S} and Krumholz, {Mark R.}",
year = "2018",
month = "12",
day = "10",
doi = "10.3847/1538-4357/aaec77",
language = "English (US)",
volume = "869",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "IOP Publishing Ltd.",
number = "1",

}

TY - JOUR

T1 - Dark Molecular Gas in Simulations of z ∼ 0 Disk Galaxies

AU - Li, Qi

AU - Narayanan, Desika

AU - Dave, Romeel S

AU - Krumholz, Mark R.

PY - 2018/12/10

Y1 - 2018/12/10

N2 - The H2 mass of molecular clouds has traditionally been traced by the CO(J = 1-0) rotational transition line. This said, CO is relatively easily photodissociated and can also be destroyed by cosmic rays, thus rendering some fraction of molecular gas to be "CO-dark." We investigate the amount and physical properties of CO-dark gas in two z ∼ 0 disk galaxies and develop predictions for the expected intensities of promising alternative tracers ([C i] 609 μm and [C ii] 158 μm emission). We do this by combining cosmological zoom simulations of disk galaxies with thermal-radiative-chemical equilibrium interstellar medium (ISM) calculations to model the predicted H i and H2 abundances and CO (J = 1-0), [C i] 609 μm, and [C ii] 158 μm emission properties. Our model treats the ISM as a collection of radially stratified clouds whose properties are dictated by their volume and column densities, the gas-phase metallicity, and the interstellar radiation field (ISRF) and CR ionization rates. Our main results follow. Adopting an observationally motivated definition of CO-dark gas, i.e., H2 gas with W CO < 0.1 K km s-1, we find that a significant amount (50%) of the total H2 mass lies in CO-dark gas, most of which is diffuse gas, poorly shielded due to low dust column density. The CO-dark molecular gas tends to be dominated by [C ii], though [C i] also serves as a bright tracer of the dark gas in many instances. At the same time, [C ii] also tends to trace neutral atomic gas. As a result, when we quantify the conversion factors for the three carbon-based tracers of molecular gas, we find that [C i] suffers the least contamination from diffuse atomic gas and is relatively insensitive to secondary parameters.

AB - The H2 mass of molecular clouds has traditionally been traced by the CO(J = 1-0) rotational transition line. This said, CO is relatively easily photodissociated and can also be destroyed by cosmic rays, thus rendering some fraction of molecular gas to be "CO-dark." We investigate the amount and physical properties of CO-dark gas in two z ∼ 0 disk galaxies and develop predictions for the expected intensities of promising alternative tracers ([C i] 609 μm and [C ii] 158 μm emission). We do this by combining cosmological zoom simulations of disk galaxies with thermal-radiative-chemical equilibrium interstellar medium (ISM) calculations to model the predicted H i and H2 abundances and CO (J = 1-0), [C i] 609 μm, and [C ii] 158 μm emission properties. Our model treats the ISM as a collection of radially stratified clouds whose properties are dictated by their volume and column densities, the gas-phase metallicity, and the interstellar radiation field (ISRF) and CR ionization rates. Our main results follow. Adopting an observationally motivated definition of CO-dark gas, i.e., H2 gas with W CO < 0.1 K km s-1, we find that a significant amount (50%) of the total H2 mass lies in CO-dark gas, most of which is diffuse gas, poorly shielded due to low dust column density. The CO-dark molecular gas tends to be dominated by [C ii], though [C i] also serves as a bright tracer of the dark gas in many instances. At the same time, [C ii] also tends to trace neutral atomic gas. As a result, when we quantify the conversion factors for the three carbon-based tracers of molecular gas, we find that [C i] suffers the least contamination from diffuse atomic gas and is relatively insensitive to secondary parameters.

KW - astrochemistry

KW - galaxies: ISM

KW - ISM: molecules

KW - methods: numerical

UR - http://www.scopus.com/inward/record.url?scp=85058449807&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85058449807&partnerID=8YFLogxK

U2 - 10.3847/1538-4357/aaec77

DO - 10.3847/1538-4357/aaec77

M3 - Article

AN - SCOPUS:85058449807

VL - 869

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1

M1 - 73

ER -