The Carbon-to-H2, CO-to-H2 conversion factors, and carbon abundance on kiloparsec scales in nearby galaxies

ABSTRACT Using the atomic carbon [C i] ($^{3} \rm P_{1} \rightarrow {\rm ^3 P}_{0}$) and [C i] ($^{3} \rm P_{2} \rightarrow {\rm ^3 P}_{1}$) emission {hereafter [C i] (1 − 0) and [C i] (2 − 1), respectively} maps observed with the Herschel Space Observatory, and CO (1 − 0), H i, infrared and submm maps from literatures, we estimate the [C i]-to-H2 and CO-to-H2 conversion factors of α[C i](1 − 0), α[C i](2 − 1), and αCO at a linear resolution $\sim 1\,$kpc scale for six nearby galaxies of M 51, M 83, NGC 3627, NGC 4736, NGC 5055, and NGC 6946. This is perhaps the first effort, to our knowledge, in calibrating both [C i]-to-H2 conversion factors across the spiral disks at spatially resolved $\sim 1\,$kpc scale though such studies have been discussed globally in galaxies near and far. In order to derive the conversion factors and achieve these calibrations, we adopt three different dust-to-gas ratio (DGR) assumptions that scale approximately with metallicity taken from precursory results. We find that for all DGR assumptions, the α[C i](1 − 0), α[C i](2 − 1), and αCO are mostly flat with galactocentric radii, whereas both α[C i](2 − 1) and αCO show decrease in the inner regions of galaxies. And the central αCO and α[C i](2 − 1) values are on average ∼2.2 and 1.8 times lower than its galaxy averages. The obtained carbon abundances from different DGR assumptions show flat profiles with galactocentric radii, and the average carbon abundance of the galaxies is comparable to the usually adopted value of 3 × 10−5. We find that both metallicity and infrared luminosity correlate moderately with the αCO, whereas only weakly with either the α[C i](1 − 0) or carbon abundance, and not at all with the α[C i](2 − 1).

Thick disc molecular gas fraction in NGC 6946

Several recent studies reinforce the existence of a thick molecular disc in galaxies along with the dynamically cold thin disc. Assuming a two-component molecular disc, we model the disc of NGC 6946 as a four-component system consisting of stars, H i, thin disc molecular gas, and thick disc molecular gas in vertical hydrostatic equilibrium. Following, we set up the joint Poisson-Boltzmann equation of hydrostatic equilibrium and solve it numerically to obtain a three-dimensional density distribution of different baryonic components. Using the density solutions and the observed rotation curve, we further build a three-dimensional dynamical model of the molecular disc and consecutively produce simulated CO spectral cubes and spectral width profiles. We find that the simulated spectral width profiles distinguishably differ for different assumed thick disc molecular gas fractions. Several CO spectral width profiles are then produced for different assumed thick disc molecular gas fractions and compared with the observed one to obtain the best fit thick disc molecular gas fraction profile. We find that the thick disc molecular gas fraction in NGC 6946 largely remains constant across its molecular disc with a mean value of 0.70 ± 0.09. We also estimate the amount of extra-planar molecular gas in NGC 6946. We find $\sim 50\%$ of the total molecular gas is extra-planar at the central region, whereas this fraction reduces to ∼ 15% at the edge of the molecular disc. With our method, for the first time, we estimate the thick disc molecular gas fraction as a function of radius in an external galaxy with sub-kpc resolution.

Resolving the Disc–Halo Degeneracy – II: NGC 6946

ABSTRACT The mass-to-light ratio (M/L) is a key parameter in decomposing galactic rotation curves into contributions from the baryonic components and the dark halo of a galaxy. One direct observational method to determine the disc M/L is by calculating the surface mass density of the disc from the stellar vertical velocity dispersion and the scale height of the disc. Usually, the scale height is obtained from near-IR studies of edge-on galaxies and pertains to the older, kinematically hotter stars in the disc, while the vertical velocity dispersion of stars is measured in the optical band and refers to stars of all ages (up to ∼10 Gyr) and velocity dispersions. This mismatch between the scale height and the velocity dispersion can lead to underestimates of the disc surface density and a misleading conclusion of the submaximality of galaxy discs. In this paper, we present the study of the stellar velocity dispersion of the disc galaxy NGC 6946 using integrated star light and individual planetary nebulae as dynamical tracers. We demonstrate the presence of two kinematically distinct populations of tracers that contribute to the total stellar velocity dispersion. Thus, we are able to use the dispersion and the scale height of the same dynamical population to derive the surface mass density of the disc over a radial extent. We find the disc of NGC 6946 to be closer to maximal with the baryonic component contributing most of the radial gravitational field in the inner parts of the galaxy (Vmax(bar) = 0.76(±0.14)Vmax).

H i scale height in spiral galaxies

ABSTRACT We model the galactic discs of seven nearby large spiral galaxies as three-component systems consist of stars, molecular gas, and atomic gas in vertical hydrostatic equilibrium. We set up the corresponding joint Poisson–Boltzmann equation and solve it numerically to estimate the 3D distribution of H i in these galaxies. While solving the Poisson–Boltzmann equation, we do not consider a constant H i velocity dispersion (σHI); rather, we develop an iterative method to self-consistently estimate the σHI profile in a galaxy by using the observed second-moment profile of the H i spectral cube. Using the density solutions, we determine the H i vertical scale height in our galaxies. We find that the H i discs flare in a linear fashion as a function of radius. H i scale height in our galaxies is found to vary between a few hundred parsecs at the centre to ∼1–2 kpc at the outskirts. We estimate the axial ratio of the H i discs in our sample galaxies and find a median ratio of 0.1, which is much lower than what is found for dwarf galaxies, indicating much thinner H i discs in spiral galaxies. Very low axial ratios in three of our sample galaxies (NGC 5055, NGC 6946, and NGC 7331) suggest them to be potential superthin galaxies. Using the H i distribution and the H i hole sizes in NGC 6946, we find that most of the H i holes in this galaxy are broken out into the circumgalactic medium and this breaking out is more effective in the inner radii as compared to the outer radii.