Thick Molecular Disk Research Articles

Thick disc molecular gas fraction in NGC 6946

Abstract 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.

Theoretical modelling of two-component molecular discs in spiral galaxies

As recent observations of the molecular discs in spiral galaxies point to the existence of a diffuse, low-density thick molecular disc along with the prominent thin one, we investigate the observational signatures of this thick disc by theoretically modelling two-component molecular discs in a sample of eight nearby spiral galaxies. Assuming a prevailing hydrostatic equilibrium, we set up and solved the joint Poisson’s-Boltzmann equation to estimate the three-dimensional distribution of the molecular gas and the molecular scale height in our sample galaxies. The molecular scale height in a two-component molecular disc is found to vary between 50 and 300 pc, which is higher than what is found in a single-component disc. We find that this scale height can vary significantly depending on the assumed thick disc molecular gas fraction. We also find that the molecular gas flares as a function of the radius and follows a tight exponential law with a scale length of (0.48 ± 0.01)r25. We used the density solutions to produce the column density maps and spectral cubes to examine the ideal observing conditions to identify a thick molecular disc in galaxies. We find that unless the molecular disc is an edge-on system and imaged with a high spatial resolution (≲100 pc), it is extremely hard to identify a thick molecular disc in a column density map. The spectral analysis further reveals that at moderate to high inclination (i ≳ 40°), spectral broadening can fictitiously introduce the signatures of a two-component disc into the spectral cube of a single-component disc. Hence, we conclude that a low inclination molecular disc imaged with high spatial resolution would serve as the ideal site for identifying the thick molecular disc in galaxies.

Relationship between the line width of the atomic and molecular ISM in M33

ABSTRACT We investigate how the spectral properties of atomic (H i) and molecular (H2) gas, traced by CO(2−1) , are related in M33 on 80 pc scales. We find the H i and CO(2−1) velocity at peak intensity to be highly correlated, consistent with previous studies. By stacking spectra aligned to the velocity of H i peak intensity, we find that the CO line width (σHWHM = 4.6 ± 0.9 ${\rm km\, s^{-1}}$ ; σHWHM is the effective Gaussian width) is consistently smaller than the H i line width (σHWHM = 6.6 ± 0.1 ${\rm km\, s^{-1}}$), with a ratio of ∼0.7, in agreement with Druard et al. The ratio of the line widths remains less than unity when the data are smoothed to a coarser spatial resolution. In other nearby galaxies, this line width ratio is close to unity which has been used as evidence for a thick, diffuse molecular disc that is distinct from the thin molecular disc dominated by molecular clouds. The smaller line width ratio found here suggests that M33 has a marginal thick molecular disc. From modelling individual lines of sight, we recover a strong correlation between H i and CO line widths when only the H i located closest to the CO component is considered. The median line width ratio of the line-of-sight line widths is 0.56 ± 0.01. There is substantial scatter in the H i –CO(2−1) line width relation, larger than the uncertainties, that results from regional variations on <500 pc scales, and there is no significant trend in the line widths, or their ratios, with galactocentric radius. These regional line width variations may be a useful probe of changes in the local cloud environment or the evolutionary state of molecular clouds.

Molecular scale height in spiral galaxies

Having to have low thermal energy, the molecular gas in galaxies is expected to settle in a thin disc near the midplane. However, contradicting this understanding, recent studies have revealed considerably thick molecular discs in nearby spiral galaxies. To understand this apparent discrepancy, we theoretically model the molecular discs in a sample of eight nearby spiral galaxies and estimate their molecular scale heights (Half Width at Half Maxima (HWHM)). We assume that the baryonic discs are in vertical hydrostatic equilibrium under their mutual gravity in the external force field of the dark matter halo. We set up the joint Poisson's-Boltzman equation of hydrostatic equilibrium and numerically solve it to obtain the three-dimensional molecular gas distribution and determine the scale heights in our sample galaxies. We find that the scale heights follow a universal exponential law with a scale length of $0.46 \pm 0.01 \ r_{25}$. The molecular scale heights in our sample galaxies are found to vary between 50-200 pc depending on the galaxy and radius. Using the density solutions, we build dynamical models of the molecular discs and produce molecular column density maps. These model maps found to match to the observed ones reasonably well. We further incline the dynamical models to an inclination of 90$^o$ to estimate the expected observed thickness of the molecular discs. Interestingly it is found that at edge-on orientation, our sample galaxies under hydrostatic assumption can easily produce a few kpc thick observable molecular disc.

The interaction of the outflow with the molecular disk in the Active Galactic Nucleus of NGC 6951

AbstractContext: we present a study of the central 200 pc of NGC 6951, in the optical and NIR, taken with the Gemini North Telescope integral field spectrographs, with resolution of ~ 0”.1 Methods: we used a set of image processing techniques, as the filtering of high spatial and spectral frequencies, Richardson-Lucy deconvolution and PCA Tomography (Steiner et al.2009) to map the distribution and kinematics of the emission lines. Results: we found a thick molecular disk, with the ionization cone highly misaligned.

Improving the triaxial bulge model of M31

A detailed hydrodynamical model of the gas flow in the triaxial gravitational potential of the bulge of the Andromeda galaxy (M31) has recently been proposed by Berman, and shown to provide excellent agreement with the CO emission-line velocities observed along its major axis. In the present paper, we confirm the validity of that model by showing that it can also reproduce the CO velocities observed off the major axis – a much more robust test. The CO observations, however, tend to span a wider range of velocities than a direct application of the original model of Berman would suggest. This situation can be improved significantly if the molecular disc is made thicker, a requirement already encountered in dynamical simulations of other spiral galaxies, and typically attributed to a broadening of the molecular layer in galactic fountain-like processes. In the central regions of M31, however, it is unclear whether there actually is a thick molecular disc, or whether broadening the molecular layer is merely an artificial theoretical means of accounting for some disc warping. Other effects not included in the model, such as hydraulic jumps, might also contribute to a widening of the range of velocities.

A 2000M☉Rotating Molecular Disk around NGC 6334A

We present millimeter and centimeter wave spectroscopic observations of the H II region NGC 6334A. We have mapped the source in several transitions of CO, CS, and NH3. The molecular emission shows a distinct flattened structure in the east-west direction. This structure is probably a thick molecular disk or torus (2.2 × 0.9 pc) responsible for the bipolarity of the near-infrared (NIR) and radio continuum emission which extends in two lobes to the north and south of the shell-like H II region. The molecular disk is rotating from west to east (ω ≈ 2.4 km s-1 pc-1) about an axis approximately parallel to the radio and NIR emission lobes. By assuming virial equilibrium, we find that the molecular disk contains ~2000 M☉. Single-component gas excitation model calculations show that the molecular gas in the disk is warmer and denser (Tk ≈ 60 K, n ≈ 3000 cm-3) than the gas to the north and south (Tk ≈ 50 K, n ≈ 400 cm-3). High resolution (~5'') NH3 (3, 3) images of NGC 6334A reveal several small (~0.1 pc) clumps, one of which lies southwest of the radio continuum shell, and is spatially coincident with a near-infrared source, IRS 20. A second NH3 clump is coincident with an H2O maser and the center of a molecular outflow. The dense gas tracers, CS J = 5 → 4 and 7 → 6, peak near IRS 20 and the H2O maser, not at NGC 6334A. IRS 20 has a substantial far-infrared (FIR) luminosity LFIR ~ 105 L☉, which indicates the presence of an O 7.5 star but has no detected radio continuum (F6 cm < 0.02 Jy). The combination of dense gas, a large FIR luminosity and a lack of radio continuum can best be explained if IRS 20 is a protostar. A third clump of NH3 emission lies to the west of IRS 20 but is not associated with any other molecular or continuum features. The star formation activity in the region has moved west of NGC 6334A to IRS 20 and the H2O maser position. We suggest that NGC 6334A, IRS 20, and the H2O maser spot are part of a protocluster of stars which is condensing from the massive molecular disk. The similarity between the structure around NGC 6334A and other large (r ~ 1 pc), massive (M ~ 103 M☉), rotating disks (K3-50A and G10.6-0.4) suggests that this may be a common mechanism by which open clusters form.

Detection of a thick molecular disk in the galaxy

Detection of a thick molecular disk in the galaxy