June 12, 2002
It has been suggested that high velocity clouds may be distributed throughout the Local Group and are therefore not in general associated with the Milky Way galaxy. With the aim of testing this hypothesis, we have made observations in the H-alpha line of high velocity clouds selected as the most likely candidates for being at larger than average distances. We have found H-alpha emission from 4 out of 5 of the observed clouds, suggesting that the clouds under study are being illuminated by a Lyman continuum flux greater than that of the metagalactic ionizing radiation. Therefore, it appears likely that these clouds are in the Galactic halo and not distributed throughout the Local Group.
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November 17, 1998
Optical emission lines provide an important new window on the HVCs. Recent studies of the H-alpha line reveal that ionized gas is pervasively associated with the neutral hydrogen in HVCs. The Wisconsin H-Alpha Mapper (WHAM) instrument has so far detected H-alpha from high-velocity clouds in the M, A, and C complexes. We find a close spatial correspondence between the neutral and ionized portions of the HVCs with some evidence that the ionized gas envelopes the neutral part of...
September 5, 2001
Optical emission lines have now been detected from about 20 high velocity clouds. These emission lines -- primarily H-alpha, secondarily [N II] and [S II] -- are very faint and diffuse, spread over the surfaces of the clouds. We compile emission line measurements and present a model in which the H-alpha is recombination caused by photoionizing radiation escaping the Milky Way. In such a model, we infer HVC distances of 5--30 kpc. The photoionization model fails to explain the...
October 1, 2001
We present some developments in determining H-alpha distances to high-velocity clouds (HVCs) in the Galactic halo. Until recently, it was difficult to assess the nature and origin of HVCs because so little was known about them. But now several HVCs have reliable distance bounds derived from the stellar absorption technique, and more than a dozen have abundance measurements. In addition, twenty or more HVCs have been detected in H-alpha (and a few in optical forbidden lines). ...
November 24, 1999
We report the discovery of molecular hydrogen absorption in a Galactic high-velocity cloud (HVC) in the direction of the Large Magellanic Cloud. For the same HVC we derive an iron abundance which is half of the solar value. Thus, all evidence points to a Galactic origin for high-velocity cloud complex in front of the LMC.
February 15, 2006
In this article I review recent observations of the gaseous halos of galaxies and the intergalactic medium at low redshift. In the first part I discuss distribution, metal content, and physical properties of the Galactic intermediate- and high-velocity clouds and the hot halo of the Milky Way. Recent absorption and emission measurements show that the Galaxy's tidal interaction with the Magellanic Clouds, the infall of low-metallicity gas, as well as the circulation of gas as ...
September 4, 2000
Tantalizing evidence has been presented supporting the suggestion that a large population of extragalactic gas clouds permeates the Local Group, a population which has been associated with the Galactic High-Velocity Clouds (HVCs). We comment on both the strengths and weaknesses of this suggestion, informally referred to as the Blitz/Spergel picture. Theoretical predictions for the spatial and kinematic distributions, metallicities, distances, and emission properties of Blitz/...
July 15, 1998
The first observations of the recently completed Wisconsin H-Alpha Mapper (WHAM) facility include a study of emission lines from high velocity clouds in the M, A, and C complexes, with most of the observations on the M I cloud. We present results including clear detections of H-alpha emission from all three complexes with intensities ranging from 0.06 R to 0.20 R. In every observed direction where there is significant high velocity H I gas seen in the 21 cm line we have found...
July 29, 2003
We present deep Halpha spectroscopy towards several high-velocity clouds (HVCs) which vary in structure from compact (CHVCs) to the Magellanic Stream. The clouds range from being bright (~640 mR) to having upper limits on the order of 30 to 70 mR. The Halpha measurements are discussed in relation to their HI properties and distance constraints are given to each of the complexes based on f_esc = 6% of the ionizing photons escaping normal to the Galactic disk (f_escs = 1 - 2% w...
September 17, 1998
We have recently identified several high velocity (V < -100 km/s) clouds in the directions of Mrk 509 and PKS 2155-304 that have unusual ionization properties. The clouds exhibit strong C IV absorption with little or no detectable low ion (C II, Si II) absorption or H I 21cm emission. As the closest known analog to the outer diffuse halos of damped Ly-alpha absorbers and the low H I column density metal line absorption systems seen in the spectra of high redshift quasars, the...
October 17, 2003
I examine some of the evidence relevant to the idea that high-velocity clouds (HVCs) are gas clouds distributed throughout the Local Group, as proposed by Blitz et al. (1999) and Braun & Burton (1999). This model makes several predictions: a) the clouds have low metallicities; b) there should be no detectable Halpha emission; c) analogues near other galaxies should exist; and d) many faint HVCs in the region around M31 can be found. Low metallicities are indeed found in sever...