We describe a population of small, high velocity, atomic hydrogen clouds, loops, and filaments found above and below the disk near the Galactic Center. The objects have a mean radius of 15 pc, velocity widths of $sim 14$ km/s and are observed at $|z|
$ heights up to 700 pc. The velocity distribution of the clouds shows no signature of Galactic rotation. We propose a scenario where the clouds are associated with an outflow from a central star-forming region at the Galactic Center. We discuss the clouds as entrained material traveling at $sim 200$ km/s in a Galactic wind.
We investigate the influence of large-scale stellar feedback on the formation of molecular clouds in the Large Magellanic Cloud (LMC). Examining the relationship between HI and 12CO(J=1-0) in supergiant shells (SGSs), we find that the molecular fract
ion in the total volume occupied by SGSs is not enhanced with respect to the rest of the LMC disk. However, the majority of objects (~70% by mass) are more molecular than their local surroundings, implying that the presence of a supergiant shell does on average have a positive effect on the molecular gas fraction. Averaged over the full SGS sample, our results suggest that ~12-25% of the molecular mass in supergiant shell systems was formed as a direct result of the stellar feedback that created the shells. This corresponds to ~4-11% of the total molecular mass of the galaxy. These figures are an approximate lower limit to the total contribution of stellar feedback to molecular cloud formation in the LMC, and constitute one of the first quantitative measurements of feedback-triggered molecular cloud formation in a galactic system.
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 supe
rshells 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%.
We present the pilot results of the `MAGMO project, targeted observations of ground-state hydroxyl masers towards sites of 6.7-GHz methanol maser emission in the Carina-Sagittarius spiral arm tangent, Galactic longitudes 280 degrees to 295 degrees. T
he `MAGMO project aims to determine if Galactic magnetic fields can be traced with Zeeman splitting of masers associated with star formation. Pilot observations of 23 sites of methanol maser emission were made, with the detection of ground-state hydroxyl masers towards 11 of these and six additional offset sites. Of these 17 sites, nine are new detections of sites of 1665-MHz maser emission, three of them accompanied by 1667-MHz emission. More than 70% of the maser features have significant circular polarization, whilst only ~10% have significant linear polarization (although some features with up to 100% linear polarization are found). We find 11 Zeeman pairs across six sites of high-mass star formation with implied magnetic field strengths between -1.5 mG and +3.8 mG and a median field strength of +1.6 mG. Our measurements of Zeeman splitting imply that a coherent field orientation is experienced by the maser sites across a distance of 5.3+/-2.0 kpc within the Carina-Sagittarius spiral arm tangent.
We present a survey of atomic hydrogen HI) emission in the direction of the Galactic Center conducted with the CSIRO Australia Telescope Compact Array (ATCA). The survey covers the area -5 deg < l < +5, -5 deg < b <+5 deg over the velocity range -309
< v_{LSR} < 349 km/s with a velocity resolution of 1 km/s. The ATCA data are supplemented with data from the Parkes Radio Telescope for sensitivity to all angular scales larger than the 145 arcsec angular resolution of the survey. The mean rms brightness temperature across the field is 0.7 K, except near (l,b)=(0 deg, 0 deg) where it increases to ~2 K. This survey complements the Southern Galactic Plane Survey to complete the continuous coverage of the inner Galactic plane in HI at ~2 arcmin resolution. Here we describe the observations and analysis of this Galactic Center survey and present the final data product. Features such as Banias Clump 2, the far 3 kiloparsec arm and small high velocity clumps are briefly described.
We present an in-depth case study of three molecular clouds associated with the walls of the Galactic supershells GSH 287+04-17 and GSH 277+00+36. These clouds have been identified in previous work as examples in which molecular gas is either being f
ormed or destroyed due to the influence of the shells. 12CO(J=1-0), 13CO(J=1-0) and C18O(J=1-0) mapping observations with the Mopra telescope provide detailed information on the distribution and properties of the molecular gas, enabling an improved discussion of its relationship to the wider environment in which it resides. We find that massive star formation is occurring in molecular gas likely formed in-situ in the shell wall, at a Galactic altitude of ~200 pc. This second-generation star formation activity is dominating its local environment; driving the expansion of a small HII region which is blistering out of the atomic shell wall. We also find new morphological evidence of disruption in two smaller entrained molecular clouds thought to pre-date the shells. We suggest that at the present post-interaction epoch, the lifetime of this surviving molecular material is no longer strongly determined by the shells themselves.
Using data from the Galactic All-Sky Survey, we have compared the properties and distribution of HI clouds in the disk-halo transition at the tangent points in mirror-symmetric regions of the first quadrant (QI) and fourth quadrant (QIV) of the Milky
Way. Individual clouds are found to have identical properties in the two quadrants. However, there are 3 times as many clouds in QI as in QIV, their scale height is twice as large, and their radial distribution is more uniform. We attribute these major asymmetries to the formation of the clouds in the spiral arms of the Galaxy, and suggest that the clouds are related to star formation in the form of gas that has been lifted from the disk by superbubbles and stellar feedback, and fragments of shells that are falling back to the plane.
We present parsec-scale resolution observations of the atomic and molecular ISM in two Galactic supershells, GSH 287+04-17 and GSH 277+00+36. HI synthesis images from the Australia Telescope Compact Array are combined with 12CO(J=1-0) data from the N
ANTEN telescope to reveal substantial quantities of molecular gas closely associated with both shells. These data allow us to confirm an enhanced level of molecularization over the volumes of both objects, providing the first direct observational evidence of increased molecular cloud production due to the influence of supershells. We find that the atomic shell walls are dominated by cold gas with estimated temperatures and densities of T ~ 100 K and n0 ~ 10 cm-3. Locally, the shells show rich substructure in both tracers, with molecular gas seen elongated along the inner edges of the atomic walls, embedded within HI filaments and clouds, or taking the form of small CO clouds at the tips of tapering atomic `fingers. We discuss these structures in the context of different formation scenarios, suggesting that molecular gas embedded within shell walls is well explained by in-situ formation from the swept up medium, whereas CO seen at the ends of fingers of HI may trace remnants of molecular clouds that pre-date the shells. A preliminary assessment of star formation activity within the shells confirms ongoing star formation in the molecular gas of both GSH 287+04-17 and GSH 277+00+36.
Using a recent catalogue of extragalactic Faraday rotation derived from the NRAO VLA Sky Survey we have found an agreement between Faraday rotation structure and the HI emission structure of a High Velocity Cloud (HVC) associated with the Leading Arm
of the Magellanic System. We suggest that this morphological agreement is indicative of Faraday rotation through the HVC. Under this assumption we have used 48 rotation measures through the HVC, together with estimates of the electron column density from H-alpha measurements and QSO absorption lines to estimate a strength for the line-of-sight component of the coherent magnetic field in the HVC of <B_{||}> > 6 {rm mu G}$. A coherent magnetic field of this strength is more than sufficient to dynamically stabilize the cloud against ram pressure stripping by the Milky Way halo and may also provide thermal insulation for the cold cloud. We estimate an upper limit to the ratio of random to coherent magnetic field of $B_{r}/B_{||} < 0.8$, which suggests that the random field does not dominate over the coherent field as it does in the Magellanic Clouds from which this HVC likely originates.
We present new parsec-scale resolution data from a multi-phase study of the ISM in the walls of Galactic supershells. HI synthesis images and CO survey data reveal a wealth of substructure, including dense-tipped fingers and extended molecular clouds
embedded in shell walls. We briefly consider formation scenarios for these features, and suggest that both the interaction of an expanding shell with pre-existing dense clouds, as well as in-situ formation of CNM and molecular gas, are likely to be relevant. Future work will also examine the role of instabilities in structure formation and breakup, and will investigate the presence of high-altitude gas associated with supershells and chimneys.
