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Register a new userA High-Velocity Cloud Impact Forming a Supershell in the Milky Way
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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.
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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%.
(Abridged) We present a new high-resolution (7 km/s FWHM) echelle spectrum of 3C 351 obtained with STIS. 3C 351 lies behind the low-latitude edge of high-velocity cloud Complex C, and the new spectrum provides accurate measurements of O I, Si II, Al II, Fe II, and Si III absorption lines at the velocity of the HVC. We use collisional and photoionization models to derive ionization corrections; in both models we find that the overall metallicity Z = 0.1 - 0.3 Z_{solar} in Complex C, but nitrogen must be underabundant. The iron abundance indicates that Complex C contains very little dust. The absorbing gas probably is not gravitationally confined. The gas could be pressure-confined by an external medium, but alternatively we may be viewing the leading edge of the HVC, which is ablating and dissipating as it plunges into the Milky Way. O VI column densities observed with FUSE toward nine QSOs/AGNs behind Complex C support this conclusion: N(O VI) is highest near 3C 351, and the O VI/H I ratio increases substantially with decreasing latitude, suggesting that the lower-latitude portion of the cloud is interacting more vigorously with the Galaxy. The other sight lines through Complex C show some dispersion in metallicity, but with the current uncertainties, the measurements are consistent with a constant metallicity throughout the HVC. However, all of the Complex C sight lines require significant nitrogen underabundances. Finally, we compare the 3C 351 sight line to the sight line to the nearby QSO H1821+643 to search for evidence of outflowing Galactic fountain gas that could be mixing with Complex C. We find that the intermediate-velocity gas detected toward 3C 351 and H1821+643 has a higher metallicity and may well be a fountain/chimney outflow from the Perseus spiral arm.
X-shooter and ISIS WHT spectra of the starforming galaxy PHL 293B also known as A2228-00 and SDSS J223036.79-000636.9 are presented in this paper. We find broad (FWHM = 1000km/s) and very broad (FWZI = 4000km/s) components in the Balmer lines, narrow absorption components in the Balmer series blueshifted by 800km/s, previously undetected FeII multiplet (42) absorptions also blueshifted by 800km/s, IR CaII triplet stellar absorptions consistent with [Fe/H] < -2.0 and no broad components or blushifted absorptions in the HeI lines. Based on historical records, we found no optical variability at the 5 sigma level of 0.02 mag between 2005 and 2013 and no optical variability at the level of 0.1mag for the past 24 years. The lack of variability rules out transient phenomena like luminous blue variables or SN IIn as the origin of the blue shifted absorptions of HI and FeII. The evidence points to either a young and dense expanding supershell or a stationary cooling wind, in both cases driven by the young cluster wind.
We confirm, quantify, and provide a table of the coherent velocity substructure of the Milky Way disk within 2 kpc of the Sun towards the Galactic anticenter, with 0.2 kpc resolution. We use the radial velocities of ~340,000 F-type stars obtained with the Guoshoujing Telescope (also known as the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, LAMOST), and proper motions derived from the PPMXL catalog. The PPMXL proper motions have been corrected to remove systematic errors by subtracting the average proper motions of galaxies and QSOs that have been confirmed in the LAMOST spectroscopic survey, and that are within 2.5 degrees of the stars position. We provide the resulting table of systematic offsets derived from the PPMXL proper motion measurements of extragalactic objects identified in the LAMOST spectroscopic survey. Using the corrected phase- space stellar sample, we find statistically significant deviations in the bulk disk velocity of 20 km/s or more in the three dimensional velocities of Galactic disk stars. The bulk velocity varies significantly over length scales of half a kpc or less. The rotation velocity of the disk increases by 20 km/s from the Suns position to 1.5 kpc outside the solar circle. Disk stars in the second quadrant, within 1 kpc of the Sun, are moving radially towards the Galactic center and vertically towards a point a few tenths of a kpc above the Galactic plane; looking down on the disk, the stars appear to move in a circular streaming motion with a radius of order 1 kpc.
The Leading Arm of the Magellanic System is a tidally formed HI feature extending $sim 60arcdeg$ from the Magellanic Clouds ahead of their direction of motion. Using atomic hydrogen (HI) data from the Galactic All Sky-Survey (GASS), supplemented with data from the Australia Telescope Compact Array, we have found evidence for an interaction between a cloud in the Leading Arm and the Galactic disk where the Leading Arm crosses the Galactic plane. The interaction occurs at velocities permitted by Galactic rotation, which allows us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the Leading Arm crosses the Galactic Plane at a Galactic radius of $Rapprox 17$ kpc.
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