Detection of H-alpha emission from the Magellanic Stream: evidence for an extended gaseous Galactic halo

Abstract

We have detected faint, diffuse H$\alpha emissionfromseveralpointsalongtheMagellanicStream,usingtheRutgersFabry--PerotInterferometerattheCTIO1.5-mtelescope.AtpointsontheleadingedgesoftheHIcloudsMSII,MSIII,andMSIV,wedetectH$αemission of surface brightness $0.37\pm 0.02$ Rayleighs, $0.21\pm 0.04$ R, and $0.20\pm 0.02$ R respectively, corresponding to emission measures of 1.0 to 0.5 \cmsixpc. We have observed several positions near the MS IV concentration, and find that the strongest emission is on the sharp leading-edge density gradient. There is less emission at points away from the gradient, and halfway between MS III and MS IV the H$\alpha surfacebrightnessis$< 0.04$R.WeattributetheH$αemission at cloud leading edges to heating of the Stream clouds by ram pressure from ionized gas in the halo of the Galaxy. These observations suggest that ram pressure from halo gas plays a large role in stripping the Stream out of the Magellanic Clouds. They also suggest the presence of a relatively large density of gas, $n_{\mathrm{H}}\sim 10^{-4}c{m}^{-3}$, in the Galactic halo at $\sim 50$ kpc radius, and far above the Galactic plane, $b\sim -80°$. This implies that the Galaxy has a very large baryonic, gaseous extent, and supports models of Lyman-$αand metal-line QSO absorption lines in which the absorption systems reside in extended galactic halos.

15 pages, aaspp latex, + 1 table & 3 figures. Accepted in A.J. Also available from http://www.physics.rutgers.edu/~bweiner/astro/papers/

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