Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userEvolution of Molecular and Atomic Gas Phases in the Milky Way
192
0
0.0
(
0
)
Authors
Jin Koda
Ask ChatGPT about the research
No Arabic abstract
We analyze radial and azimuthal variations of the phase balance between the molecular and atomic ISM in the Milky Way. In particular, the azimuthal variations -- between spiral arm and interarm regions -- are analyzed without any explicit definition of spiral arm locations. We show that the molecular gas mass fraction, i.e., fmol=H2/ (HI+H2) in mass, varies predominantly in the radial direction: starting from ~100% at the center, remaining ~>50% (~>60%) to R~6kpc, and decreasing to ~10-20% (~50%) at R=8.5 kpc when averaged over the whole disk thickness (in the mid plane). Azimuthal, arm-interarm variations are secondary: only ~20%, in the globally molecule-dominated inner MW, but becoming larger, ~40-50%, in the atom-dominated outskirts. This suggests that in the inner MW, the gas stays highly molecular (fmol>50%) as it goes from an interarm region, into a spiral arm, and back into the next interarm region. Stellar feedback does not dissociate molecules much, and the coagulation and fragmentation of molecular clouds dominate the evolution of the ISM at these radii. The trend differs in the outskirts, where the gas phase is globally atomic (fmol<50%). The HI and H2 phases cycle through spiral arm passage there. These different regimes of ISM evolution are also seen in external galaxies (e.g., LMC, M33, and M51). We explain the radial gradient of fmol by a simple flow continuity model. The effects of spiral arms on this analysis are illustrated in Appendix.
rate research
Read More
Throughout the Milky Way, molecular clouds typically appear filamentary, and mounting evidence indicates that this morphology plays an important role in star formation. What is not known is to what extent the dense filaments most closely associated with star formation are connected to the surrounding diffuse clouds up to arbitrarily large scales. How are these cradles of star formation linked to the Milky Ways spiral structure? Using archival Galactic plane survey data, we have used multiple datasets in search of large-scale, velocity-coherent filaments in the Galactic plane. In this paper, we present our methods employed to identify coherent filamentary structures first in extinction and confirmed using Galactic Ring Survey data. We present a sample of seven Giant Molecular Filaments (GMFs) that have lengths of order $sim$100 pc, total masses of 10$^4$ - 10$^5$ M$_{odot}$, and exhibit velocity coherence over their full length. The GMFs we study appear to be inter-arm clouds and may be the Milky Way analogues to spurs observed in nearby spiral galaxies. We find that between 2 and 12% of the total mass (above $sim$10$^{20}$ cm$^{-2}$) is dense (above 10$^{22}$ cm$^{-2}$), where filaments near spiral arms in the Galactic midplane tend to have higher dense gas mass fractions than those further from the arms.
Recent submillimeter and far-infrared wavelength observations of absorption in the rotational ground-state lines of various simple molecules against distant Galactic continuum sources have opened the possibility of studying the chemistry of diffuse molecular clouds throughout the Milky Way. In order to calculate abundances, the column densities of molecular and atomic hydrogen, HI, must be known. We aim at determining the atomic hydrogen column densities for diffuse clouds located on the sight lines toward a sample of prominent high-mass star-forming regions that were intensely studied with the HIFI instrument onboard Herschel. Based on Jansky Very Large Array data, we employ the 21 cm HI absorption-line technique to construct profiles of the HI opacity versus radial velocity toward our target sources. These profiles are combined with lower resolution archival data of extended HI emission to calculate the HI column densities of the individual clouds along the sight lines. We employ Bayesian inference to estimate the uncertainties of the derived quantities. Our study delivers reliable estimates of the atomic hydrogen column density for a large number of diffuse molecular clouds at various Galactocentric distances. Together with column densities of molecular hydrogen derived from its surrogates observed with HIFI, the measurements can be used to characterize the clouds and investigate the dependence of their chemistry on the molecular fraction, for example.
We use a model of the Galactic fountain to simulate the neutral-hydrogen emission of the Milky Way Galaxy. The model was developed to account for data on external galaxies with sensitive HI data. For appropriate parameter values, the model reproduces well the HI emission observed at Intermediate Velocities. The optimal parameters imply that cool gas is ionised as it is blasted out of the disc, but becomes neutral when its vertical velocity has been reduced by ~30 per cent. The parameters also imply that cooling of coronal gas in the wakes of fountain clouds transfers gas from the virial-temperature corona to the disc at ~2 Mo/yr. This rate agrees, to within the uncertainties with the accretion rate required to sustain the Galaxys star formation without depleting the supply of interstellar gas. We predict the radial profile of accretion, which is an important input for models of Galactic chemical evolution. The parameter values required for the model to fit the Galaxys HI data are in excellent agreement with values estimated from external galaxies and hydrodynamical studies of cloud-corona interaction. Our model does not reproduce the observed HI emission at High Velocities, consistent with High Velocity Clouds being extragalactic in origin. If our model is correct, the structure of the Galaxys outer HI disc differs materially from that used previously to infer the distribution of dark matter on the Galaxys outskirts.
We investigate data from the Galactic Effelsberg--Bonn HI Survey (EBHIS), supplemented with data from the third release of the Galactic All Sky Survey (GASS III) observed at Parkes. We explore the all sky distribution of the local Galactic HI gas with $|v_{rm LSR}| < 25 $ kms$^{-1}$ on angular scales of 11 to 16. Unsharp masking (USM) is applied to extract small scale features. We find cold filaments that are aligned with polarized dust emission and conclude that the cold neutral medium (CNM) is mostly organized in sheets that are, because of projection effects, observed as filaments. These filaments are associated with dust ridges, aligned with the magnetic field measured on the structures by Planck at 353 GHz. The CNM above latitudes $|b|>20^circ$ is described by a log-normal distribution, with a median Doppler temperature $T_{rm D} = 223$ K, derived from observed line widths that include turbulent contributions. The median neutral hydrogen (HI) column density is $N_{rm HI} simeq 10^{19.1},{rm cm^{-2}}$. These CNM structures are embedded within a warm neutral medium (WNM) with $N_{rm HI} simeq 10^{20} {rm cm^{-2}}$. Assuming an average distance of 100 pc, we derive for the CNM sheets a thickness of $< 0.3$ pc. Adopting a magnetic field strength of $B_{rm tot} = (6.0 pm 1.8)mu$G, proposed by Heiles & Troland 2005, and assuming that the CNM filaments are confined by magnetic pressure, we estimate a thickness of 0.09 pc. Correspondingly the median volume density is in the range $ 14 < n < 47 {rm cm^{-3}}$.
The role of large-scale stellar feedback in the formation of molecular clouds has been investigated observationally by examining the relationship between HI and 12CO(J=1-0) in supershells. Detailed parsec-resolution case studies of two Milky Way supershells demonstrate an enhanced level of molecularisation over both objects, and hence provide the first quantitative observational evidence of increased molecular cloud production in volumes of space affected by supershell activity. Recent results on supergiant shells in the LMC suggest that while they do indeed help to organise the ISM into over-dense structures, their global contribution to molecular cloud formation is of the order of only ~10%.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
