The geometry of the gas surrounding the Central Molecular Zone: on the origin of localized molecular clouds with extreme velocity dispersions
ABSTRACT Observations of molecular gas near the Galactic Centre (|l| < 10°, |b| < 1°) reveal the presence of a distinct population of enigmatic compact clouds that are characterized by extreme velocity dispersions ($\Delta v \gt 100\, {\rm km\, s^{-1}}$). These extended velocity features are very prominent in the data cubes and dominate the kinematics of molecular gas just outside the Central Molecular Zone (CMZ). The prototypical example of such a cloud is Bania Clump 2. We show that similar features are naturally produced in simulations of gas flow in a realistic barred potential. We analyse the structure of the features obtained in the simulations and use this to interpret the observations. We find that the features arise from collisions between material that has been infalling rapidly along the dust lanes of the Milky Way bar and material that belongs to one of the following two categories: (i) material that has ‘overshot’ after falling down the dust lanes on the opposite side; (ii) material which is part of the CMZ. Both types of collisions involve gas with large differences in the line-of-sight velocities, which is what produces the observed extreme velocity dispersions. Examples of both categories can be identified in more »
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Award ID(s):
Publication Date:
NSF-PAR ID:
10184735
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
488
Issue:
4
Page Range or eLocation-ID:
4663 to 4673
ISSN:
0035-8711
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5. ABSTRACT In the cold neutral medium, high out-of-equilibrium temperatures are created by intermittent dissipation processes, including shocks, viscous heating, and ambipolar diffusion. The high-temperature excursions are thought to explain the enhanced abundance of CH+ observed along diffuse molecular sightlines. Intermittent high temperatures should also have an impact on H2 line luminosities. We carry out simulations of magnetohydrodynamic (MHD) turbulence in molecular clouds including heating and cooling, and post-process them to study H2 line emission and hot-gas chemistry, particularly the formation of CH+. We explore multiple magnetic field strengths and equations of state. We use a new H2 cooling function for $n_{\text{H}}\le 10^5\, {\text{cm}}^{-3}$, $T\le 5000\, {\text{K}}$, and variable H2 fraction. We make two important simplifying assumptions: (i) the H2/H fraction is fixed everywhere and (ii) we exclude from our analysis regions where the ion–neutral drift velocity is calculated to be greater than 5 km s−1. Our models produce H2 emission lines in accord with many observations, although extra excitation mechanisms are required in some clouds. For realistic root-mean-square (rms) magnetic field strengths (≈10 μG) and velocity dispersions, we reproduce observed CH+ abundances. These findings contrast with those of Valdivia et al. (2017) Comparison of predicted dust polarization with observations by Planck suggests thatmore »