Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.5115w&link_type=abstract
American Astronomical Society, 185th AAS Meeting, #51.15; Bulletin of the American Astronomical Society, Vol. 26, p.1397
Astronomy and Astrophysics
Astronomy
Scientific paper
We have detected faint ha emission from several points along the Magellanic Stream, using the Rutgers Fabry--Perot Interferometer at the CTIO 1.5-m telescope. The sources of the emission are diffuse; at each surveyed position, there is little variation in intensity over the 7' field of the Fabry--Perot. At points on the leading edges of H I concentrations within the clouds MS II, MS III, and MS IV, we detect emission with surface brightness ranging from 0.2 to 0.5 Rayleighs (1 R = 10(6) / 4pi photons cmsqssr), or EM of 0.7 to 2 cm(-6) pc. In each field the ha velocity agrees with the 21 cm velocity. We have surveyed several positions near the MS IV concentration, and find that the strongest emission is on the sharp leading-edge density gradient. The emission decreases at points away from the gradient, and halfway between MS III and MS IV the ha surface brightness is down to ≲ 0.03 R. We interpret this correlation of ha intensity with cloud leading edges as caused by shocks, as the material of the Stream collides with an ambient medium of ionized gas in the halo of the Galaxy. These observations suggest that ram pressure from halo gas plays the dominant role in stripping the Stream out of the Magellanic Clouds. The observed ha flux implies the presence of relatively large amounts of gas -- n_H ~ 10(-3) {cm}(-3) -- in the Galactic halo at large radius, 40 to 50 kpc, and far above the Galactic plane, at b = -70deg to -90deg . Such large coronal gas densities are difficult to reconcile with X-ray and cooling timescale constraints if the gas is a smooth ionized halo, but may be compatible with a lower-density ionized halo which contains a population of partially-ionized clouds with low column-density H I. This accords well with models of Lyman-alpha and metal-line quasar absorption lines in which the absorption systems are resident in extended galactic halos.
Weiner Benjamin J.
Williams Ted B.
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