LIGHTING THE DARK MOLECULAR GAS: H₂ AS A DIRECT TRACER

Abstract

Robust knowledge of molecular gas mass is critical for understanding star formation in galaxies. The H 2 molecule does not emit efficiently in the cold interstellar medium, hence the molecular gas content of galaxies is typically inferred using indirect tracers. At low metallicity and in other extreme environments, these tracers can be subject to substantial biases. We present a new method of estimating total molecular gas mass in galaxies directly from pure mid-infrared rotational H 2 emission. By assuming a power-law distribution of H 2 rotational temperatures, we can accurately model H 2 excitation and reliably obtain warm (T100 K) H 2 gas masses by varying only the power law's slope. With sensitivities typical of Spitzer/IRS, we are able to directly probe the H 2 content via rotational emission down to ∼80 K, accounting for ∼15% of the total molecular gas mass in a galaxy. By extrapolating the fitted power-law temperature distributions to a calibrated single lower cutoff temperature, the model also recovers the total molecular content within a factor of ∼2.2 in a diverse sample of galaxies, and a subset of broken powerlaw models performs similarly well. In ULIRGs, the fraction of warm H 2 gas rises with dust temperature, with some dependency on α CO . In a sample of five low-metallicity galaxies ranging down to [ ] + = 12 log O H 7.8, the model yields molecular masses up to ∼100×larger than implied by CO, in good agreement with other methods based on dust mass and star formation depletion timescale. This technique offers real promise for assessing molecular content in the early universe where CO and dust-based methods may fail.

Powerful H2 Line Cooling in Stephan’s Quintet. II. Group-wide Gas and Shock Modeling of the Warm H2 and a Comparison with [C ii] 157.7 μm Emission and Kinematics

“…Because of the need to include many lines in this fitting, the modeling does not extend as far to the east as the two-temperature fitting, and so the extension in the direction of NGC 7319 is less obvious, although the spreading of the contours in the mid-shock region shows a bulge in the direction of the "bridge" marked in Figure 3. The values of the power-law index in Figure 8 are almost all lower than the mean found for normal galaxies of 4.84±0.61 (Togi & Smith 2016), and they fall in the asymmetric tail in the distribution found in their study of SINGS galaxies. The tail is populated by SINGS galaxies containing nuclei of Seyferts or LINERs.…”

“…Several ultraluminous infrared galaxies, which are local mergers, with warm dust color temperatures have nondetections of S(0) but detection with good signal-to-noise ratio for high-J rotational lines (Higdon et al 2006;Stierwalt et al 2014). In these galaxies we need to extrapolate our power-law model to 80 K to recover the total molecular gas mass in order to be consistent with their dynamical mass estimates (Togi & Smith 2016). …”

“…In those cases, where the warm H 2 mass is dominated by hotter gas, the ratio of warm to total H 2 mass drops to 0.1. An alternative method for estimating to total molecular gas down to some lower temperature has been presented by Togi & Smith (2016). The method relies on extrapolating the powerlaw index derived from the measured rotational lines to much lower temperatures under the assumption that a single powerlaw index applies to all the gas.…”

“…T ℓ can be thought of as the lowest (asymptotic) temperature found in the range of possible temperatures needed to explain the excitation of the gas. If T ℓ is well constrained (see later), and the single power-law approximation is valid, molecular gas masses can be explored down to temperatures as low as ∼50 K, significantly lower than those temperatures that excite the rotational transitions (>80-100 K; Togi & Smith 2016). Keeping the upper temperature, T u , fixed at 2000 K and varying T ℓ and n, we fit the H 2 excitation diagram for each square region in the grid of our mapped area of the Quintet.…”

“…We follow the approach of Togi & Smith (2016) by assuming a continuous power-law temperature distribution for the H 2 , and use this to fit the excitation diagrams and calculate the total H 2 gas mass in the ISM by extrapolating to lower temperatures. We assume that the column density of H 2 molecules is distributed according to a power-law function with respect to temperature, dN ∝ -T n dT, where dN is the number of molecules in the temperature range T to T + dT.…”