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Register a new userThe Supershell-Molecular Cloud Connection in the Milky Way and Beyond
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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%.
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Neutral atomic hydrogen (HI) gas in interstellar space is largely organized into filaments, loops, and shells, the most prominent of which are supershells. These gigantic structures requiring $gtrsim 3 times 10^{52}$ erg to form are generally thought to be produced by either the explosion of multiple supernovae (SNe) in OB associations or alternatively by the impact of high-velocity clouds (HVCs) falling to the Galactic disk. Here we report the detection of a kiloparsec (kpc)-size supershell in the outskirts of the Milky Way with the compact HVC 040+01$-$282 (hereafter CHVC040) at its geometrical center using the Inner-Galaxy Arecibo L-band Feed Array HI 21-cm survey data. The morphological and physical properties of both objects suggest that CHVC040, which is either a fragment of a nearby disrupted galaxy or a cloud originated from an intergalactic accreting flow, collided with the disk $sim 5$ Myrs ago to form the supershell. Our result shows that some compact HVCs can survive their trip through the Galactic halo and inject energy and momentum into the Milky Way disk.
The accumulation, compression and cooling of the ambient interstellar medium (ISM) in large-scale flows powered by OB cluster feedback can drive the production of dense molecular clouds. We review the current state of the field, with a strong focus on the explicit modelling and observation of the neutral interstellar medium. Magneto-hydrodynamic simulations of colliding ISM flows provide a strong theoretical framework in which to view feedback-driven cloud formation, as do models of the gravitational fragmentation of expanding shells. Rapid theoretical developments are accompanied by growing body of observational work that provides good evidence for the formation of molecular gas via stellar feedback - both in the Milky Way and the Large Magellanic Cloud. The importance of stellar feedback compared to other major astrophysical drivers of dense gas formation remains to be investigated further, and will be an important target for future work.
We study the time evolution of molecular clouds across three Milky Way-like isolated disc galaxy simulations at a temporal resolution of 1 Myr, and at a range of spatial resolutions spanning two orders of magnitude in spatial scale from ~10 pc up to ~1 kpc. The cloud evolution networks generated at the highest spatial resolution contain a cumulative total of ~80,000 separate molecular clouds in different galactic-dynamical environments. We find that clouds undergo mergers at a rate proportional to the crossing time between their centroids, but that their physical properties are largely insensitive to these interactions. Below the gas disc scale-height, the cloud lifetime obeys a scaling relation of the form $tau_{rm life} propto ell^{-0.3}$ with the cloud size $ell$, consistent with over-densities that collapse, form stars, and are dispersed by stellar feedback. Above the disc scale-height, these self-gravitating regions are no longer resolved, so the scaling relation flattens to a constant value of ~13 Myr, consistent with the turbulent crossing time of the gas disc, as observed in nearby disc galaxies.
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.
We apply an analytic theory for environmentally-dependent molecular cloud lifetimes to the Central Molecular Zone of the Milky Way. Within this theory, the cloud lifetime in the Galactic centre is obtained by combining the time-scales for gravitational instability, galactic shear, epicyclic perturbations and cloud-cloud collisions. We find that at galactocentric radii $sim 45$-$120$ pc, corresponding to the location of the 100-pc stream, cloud evolution is primarily dominated by gravitational collapse, with median cloud lifetimes between 1.4 and 3.9 Myr. At all other galactocentric radii, galactic shear dominates the cloud lifecycle, and we predict that molecular clouds are dispersed on time-scales between 3 and 9 Myr, without a significant degree of star formation. Along the outer edge of the 100-pc stream, between radii of 100 and 120 pc, the time-scales for epicyclic perturbations and gravitational free-fall are similar. This similarity of time-scales lends support to the hypothesis that, depending on the orbital geometry and timing of the orbital phase, cloud collapse and star formation in the 100-pc stream may be triggered by a tidal compression at pericentre. Based on the derived time-scales, this should happen in approximately 20 per cent of all accretion events onto the 100-pc stream.
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