CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 “conversion factor,” XCO, in different environments. In the Milky Way disk, we recommend a conversion factor XCO = 2×10 20 cm −2 (K km s −1 ) −1 with ±30% uncertainty. Studies of other “normal galaxies” return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that XCO increases with decreasing metallicity, turning up sharply below metallicity ≈ 1/3–1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, XCO appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower XCO is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments. Subject headings: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM

/pdf/the-co-to-h2-conversion-factor-26tvppkvj7.pdf

The CO-to-H2 Conversion Factor

Astronomical data analysis software and systems

We present the largest galaxies as seen in the near-infrared (1–2 μm), imaged with the Two Micron All Sky Survey (2MASS), ranging in angular size from 1' to 15. We highlight the 100 largest in the sample. The galaxies span all Hubble morphological types, including elliptical galaxies, normal and barred spirals, and dwarf and peculiar classes. The 2MASS Large Galaxy Atlas provides the necessary sensitivity and angular resolution to examine in detail morphologies in the near-infrared, which may be radically different from those in the optical. Internal structures such as spirals, bulges, warps, rings, bars, and star formation regions are resolved by 2MASS. In addition to large mosaic images, the atlas includes astrometric, photometric, and shape global measurements for each galaxy. A comparison of fundamental measures (e.g., surface brightness, Hubble type) is carried out for the sample and compared with the Third Reference Catalogue. We further showcase NGC 253 and M51 (NGC 5194/5195) to demonstrate the quality and depth of the data. The atlas represents the first uniform, all-sky, dust-penetrated view of galaxies of every type, as seen in the near-infrared wavelength window that is most sensitive to the dominant mass component of galaxies. The images and catalogs are available through the NASA/IPAC Extragalactic Database and Infrared Science Archive and are part of the 2MASS Extended Source Catalog.

2MASS large galaxy atlas

Aims. The goal of this paper is to analyse the behaviour of the
 gas-to-dust mass ratio (G/D) of local Universe galaxies over a wide metallicity range. We
 especially focus on the low-metallicity part of the G/D vs metallicity relation and
 investigate several explanations for the observed relation and scatter.Methods. We assembled a total of 126 galaxies, covering a 2 dex
 metallicity range and with 30% of the sample with 12 + log(O/H)≤ 8.0. We homogeneously determined the dust
 masses with a semi-empirical dust model including submm constraints. The atomic and
 molecular gas masses have been compiled from the literature. We used two XCO scenarios to
 estimate the molecular gas mass: the Galactic conversion factor, XCO,MW, and a
 XCO that depends on the metallicity
 XCO,Z
 (∝Z-2). We
 modelled the observed trend of the G/D with metallicity using two simple power laws (slope
 of –1 and free) and a broken power law. Correlations with morphological type, stellar
 masses, star formation rates, and specific star formation rates are also discussed. We
 then compared the observed evolution of the G/D with predictions from several chemical
 evolution models and explored different physical explanations for the observed scatter in
 the G/D values.Results. We find that out of the five tested galactic parameters,
 metallicity is the main physical property of the galaxy driving the observed G/D. The G/D
 versus metallicity relation cannot be represented by a single power law with a slope of –1
 over the whole metallicity range. The observed trend is steeper for metallicities lower
 than ~8.0. A large scatter is
 observed in the G/D values for a given metallicity: in metallicity bins of ~0.1 dex, the dispersion around the mean
 value is ~0.37 dex. On
 average, the broken power law reproduces the observed G/D best compared to the two power
 laws (slope of –1 or free) and provides estimates of the G/D that are accurate to a factor
 of 1.6. The good agreement of observed values of the G/D and its scatter with respect to
 metallicity with the predicted values of the three tested chemical evolution models allows
 us to infer that the scatter in the relation is intrinsic to galactic properties,
 reflecting the different star formation histories, dust destruction efficiencies, dust
 grain size distributions, and chemical compositions across the sample. Conclusions. Our results show that the chemical evolution of
 low-metallicity galaxies, traced by their G/D, strongly depends on their local internal
 conditions and individual histories. The large scatter in the observed G/D at a given
 metallicity reflects the impact of various processes occurring during the evolution of a
 galaxy. Despite the numerous degeneracies affecting them, disentangling these various
 processes is now the next step.

/pdf/gas-to-dust-mass-ratios-in-local-galaxies-over-a-2-dex-43hzotihph.pdf

Gas-to-dust mass ratios in local galaxies over a 2 dex metallicity range

We present results of an extensive mapping survey of N2H+(1-0) in about 60 low mass cloud cores already mapped in the NH3(1,1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution about 1.5 times finer than the previous ammonia observations. Cores with stars typically have map sizes about a factor of two smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been found with NH3 data. ``Local'' velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complexmotions not interpretable as simple solid body rotation. Integrated intensity profiles of starless cores present a ``central flattening'' and are consistent with a spherically symmetric density law n ~ r^{-1.2} for r < ~0.03 pc and n ~ r^{-2} at larger r. Cores with stars are better modelled with single density power laws with n ~ r^{-2}. Line widths change across the core but we did not find a general trend. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~ 10 coherence lengths. The corresponding value of the coherence length, ~ 0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased ``coherence'' of line widths on small scales. Despite of the finer angular resolution, the majority of N2H+ and NH3 maps show a similar ``simple'' structure, with single peaks and no elongation.

Dense Cores in Dark Clouds. XIV. N2H+(1-0) maps of dense cloud cores

For the Antennae interacting galaxy pair, we have obtained high quality, fully sampled $^{12}$CO J=1-0, 2-1 and 3-2 maps of the regions surrounding the nuclei and the area of overlap between the two galaxies, with an angular resolution of 15" or 1.5 kpc. The $^{12}$CO J=1-0, 2-1, 3-2 emission all peak in the overlap region, which contains ~ 4 x 10^9 solar mass of molecular gas mass if the conventional X factor is used. Line emission at $^{13}$CO J=2-1 and 3-2 is detected at selected points in the two nuclei and the overlap region. Both the ^{12}CO/^{13}CO J=2-1 and 3-2 integrated intensity ratios are remarkably high in the overlap region. This is the first published case in which such high $^{12}$CO/$^{13}$CO J=2-1 and 3-2 ratios are found outside a galactic nucleus. Detailed LVG modeling indicates that the $^{12}$CO and ^{13}CO emission originate in different spatial components. Assuming a CO-to-H2 abundance ratio of $10^{-4}$, the X factor given by the LVG model is an order of magnitude lower than the conventional value for molecular clouds in the Milky Way. Accordingly, we suggest the possibility that the strong CO emission in the overlap region of the Antennae galaxies is associated with increased radiative efficiency, possibly caused by a large velocity dispersion within the individual molecular clouds. A comparison of the CO J=3-2 emission with the SCUBA 850 um continuum in the Antennae galaxies shows that the CO line emission contributes globally 46% of the 850 um continuum flux and that the ratio of $^{12}$CO J=3-2 to SCUBA 850 um flux varies by a factor of two across the system.

https://arxiv.org/pdf/astro-ph/0301126.pdf

A Multi-Transistion CO Study of the "Antennae" Galaxies NGC4038/9

For the Antennae interacting galaxy pair, we have obtained high-quality, fully sampled 12CO J = 1-0 and 3-2 maps of the regions surrounding the nuclei and the area of overlap between the two galaxies. The maps possess an angular resolution of 15'' or 1.5 kpc, so far the highest resolution maps available at both the J = 1-0 and 3-2 transitions. In addition, 12CO J = 2-1 data have been obtained for the positions of the two nuclei, as well as in part of the overlap region with 20'' angular resolution. The 12CO J = 1-0, 2-1, and 3-2 emissions all peak in an off-nucleus region adjacent to where the two disks overlap. Use of the conventional X factor yields ~4 × 109 M☉ molecular gas mass in the overlap region. It is difficult to understand how such a large amount of molecular gas can be accumulated in this region given the relatively short lifetime of molecular clouds and the limited period of time for this region to form. Line emission at 13CO J = 2-1 and 3-2 is detected at selected points in the two nuclei and the overlap region. Both the 12CO/13CO J = 2-1 and 3-2 integrated intensity ratios are remarkably high in the overlap region. This is the first published case in which such high 12CO/13CO J = 2-1 and 3-2 ratios are found outside a galactic nucleus. Detailed large velocity gradient (LVG) modeling indicates that the 12CO and 13CO emissions originate in different spatial components. The 12CO emission may originate within a nonvirialized low-density gas component with a large gradient in velocity. Assuming a CO-to-H2 abundance ratio of 10-4, the X factor given by the LVG model is an order of magnitude lower than the conventional value for molecular clouds in the Milky Way, but it scales inversely as the assumed value for the CO-to-H2 abundance ratio. Accordingly, we suggest the possibility that the strong CO emission in the overlap region of the Antennae galaxies is associated with increased radiative efficiency, possibly caused by a large velocity dispersion within the individual molecular clouds. A comparison of the CO J = 3-2 emission with the SCUBA 850 μm continuum in the Antennae galaxies shows that the CO line emission contributes globally 46% of the 850 μm continuum flux and that the ratio of 12CO J = 3-2 to SCUBA 850 μm flux varies by a factor of 2 across the system. After correcting for the 12CO J = 3-2 contamination, the dust emission at 850 μm detected by SCUBA is consistent with the thermal emission from a single warm dust component with a mass of 1.7 × 107 M☉.

https://iopscience.iop.org/article/10.1086/368353/pdf

A Multitransition CO Study of the Antennae Galaxies NGC 4038/9

We present a statistical study on the [C I]($^{3} \\rm P_{1} \\rightarrow {\\rm ^3 P}_{0}$), [C I] ($^{3} \\rm P_{2} \\rightarrow {\\rm ^3 P}_{1}$) lines (hereafter [C I] (1$-$0) and [C I] (2$-$1), respectively) and the CO (1$-$0) line for a sample of (ultra)luminous infrared galaxies [(U)LIRGs]. We explore the correlations between the luminosities of CO (1$-$0) and [C I] lines, and find that $L'_\\mathrm{CO(1-0)}$ correlates almost linearly with both $L'_ \\mathrm{[CI](1-0)}$ and $L'_\\mathrm{[CI](2-1)}$, suggesting that [C I] lines can trace total molecular gas mass at least for (U)LIRGs. We also investigate the dependence of $L'_\\mathrm{[CI](1-0)}$/$L'_\\mathrm{CO(1-0)}$, $L'_\\mathrm{[CI](2-1)}$/$L'_\\mathrm{CO(1-0)}$ and $L'_\\mathrm{[CI](2-1)}$/$L'_\\mathrm{[CI](1-0)}$ on the far-infrared color of 60-to-100 $\\mu$m, and find non-correlation, a weak correlation and a modest correlation, respectively. Under the assumption that these two carbon transitions are optically thin, we further calculate the [C I] line excitation temperatures, atomic carbon masses, and the mean [C I] line flux-to-H$_2$ mass conversion factors for our sample. The resulting $\\mathrm{H_2}$ masses using these [C I]-based conversion factors roughly agree with those derived from $L'_\\mathrm{CO(1-0)}$ and CO-to-H$_2$ conversion factor.

Neutral Carbon Emission in Luminous Infrared Galaxies: The [C i] Lines as Total Molecular Gas Tracers*

We present a statistical study on the [C I]($^{3} \rm P_{1} \rightarrow {\rm ^3 P}_{0}$), [C I] ($^{3} \rm P_{2} \rightarrow {\rm ^3 P}_{1}$) lines (hereafter [C I] (1$-$0) and [C I] (2$-$1), respectively) and the CO (1$-$0) line for a sample of (ultra)luminous infrared galaxies [(U)LIRGs]. We explore the correlations between the luminosities of CO (1$-$0) and [C I] lines, and find that $L'_\mathrm{CO(1-0)}$ correlates almost linearly with both $L'_ \mathrm{[CI](1-0)}$ and $L'_\mathrm{[CI](2-1)}$, suggesting that [C I] lines can trace total molecular gas mass at least for (U)LIRGs. We also investigate the dependence of $L'_\mathrm{[CI](1-0)}$/$L'_\mathrm{CO(1-0)}$, $L'_\mathrm{[CI](2-1)}$/$L'_\mathrm{CO(1-0)}$ and $L'_\mathrm{[CI](2-1)}$/$L'_\mathrm{[CI](1-0)}$ on the far-infrared color of 60-to-100 $\mu$m, and find non-correlation, a weak correlation and a modest correlation, respectively. Under the assumption that these two carbon transitions are optically thin, we further calculate the [C I] line excitation temperatures, atomic carbon masses, and the mean [C I] line flux-to-H$_2$ mass conversion factors for our sample. The resulting $\mathrm{H_2}$ masses using these [C I]-based conversion factors roughly agree with those derived from $L'_\mathrm{CO(1-0)}$ and CO-to-H$_2$ conversion factor.

Neutral Carbon Emission in luminous infrared galaxies The \CI\ Lines as Total Molecular Gas Tracers

The Taffy Galaxies system, UGC 12914/5, contains huge amounts of molecular gas in the bridge region between the receding spirals after a direct collision. $2 - 9 \\times 10^9$M$_\\odot$ of molecular gas is present between the galaxies, more than the CO emission from the entire Milky Way! Such dense gas can only be torn off by collisions between dense clouds, in this case with relative velocities of about 800 $\\kms$, such that the remnant cloud acquires an intermediate velocity and is left in the bridge after separation of the colliding galaxies. We suggest that after ionization in the collision front, the gas cooled and recombined very quickly such that the density remained high and the gas left the colliding disks in molecular form.

/pdf/a-molecular-gas-bridge-between-the-taffy-galaxies-3xx24u5v9c.pdf

Ming Zhu

Papers

A Multi-Transistion CO Study of the "Antennae" Galaxies NGC4038/9

A Multitransition CO Study of the Antennae Galaxies NGC 4038/9

Neutral Carbon Emission in Luminous Infrared Galaxies: The [C i] Lines as Total Molecular Gas Tracers*

Neutral Carbon Emission in luminous infrared galaxies The \CI\ Lines as Total Molecular Gas Tracers

A molecular gas bridge between the Taffy galaxies