Detection of Hα emission from the Magellanic Stream: Evidence for an extended gaseous galactic halo

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 observed position, there is no variation in intensity over the 7′ field of the Fabry-Perot. At points on the leading edges of the H I clouds MS II, MS III, and MS IV, we detect Ha emission of surface brightness 0.37±0.02 Rayleighs, 0.21±0.04 R, and 0.20±0.02 R respectively, corresponding to emission measures of 1.0 to 0.5 cm^-6 pc. 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 Hα surface brightness is <0.04 R. We attribute the Ha emission 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_H∼ 10^-4 cm³, in the Galactic halo at ∼50 kpc radius, and far above the Galactic plane, |b|∼80°. This implies that the Galaxy has a very large baryonic, gaseous extent, and supports models of Lyman-α and metal-line QSO absorption lines in which the absorption systems reside in extended galactic halos.