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Wading Through the Magellanic Stream (1Sep95) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 43, September 1995) The Magellanic Stream is an enigmatic filament of neutral hydrogen, stretching over 100^o in a continuous arc behind the Magellanic Clouds, tracing their orbit around the Milky Way. Up till now, it has been detected only in 2-cm emission and in optical absorption against the background galaxy Fairall 9. No stars are known to be associated with the Stream, consequently, its distance is unknown (although one end of the Stream is connected to the SMC). The most viable theories for the origin of the Stream are tidal models, in which the Stream is torn out of the Magellanic Clouds by tidal interactions between the Clouds and the Galaxy, or ram-pressure models, in which the Stream is stripped out of the Clouds by gas postulated to exist in the Galactic halo. Clearly these scenarios are highly dependent on such unknowns as the orbit of the Magellanic Clouds and therefore the outer rotation curve of the Galaxy, as well the properties of the putative halo gas. Ben Weiner and Ted Williams (Rutgers) observed the Stream with the Rutgers Fabry-Perot interferometer on the CTIO 1.5-meter telescope in August 1994, as part of a program to search for faint Ha emission from Galactic high-velocity clouds and the Magellanic Stream. Their goal was to set limits on the ionizing flux emergent from the Galactic disk and the extragalactic ionizing flux. The Fabry-Perot provides high sensitivity to diffuse emission lines, because it combines high spectral resolution (0.75 A) with a very large collecting area, effectively 20,000 arcsec^2. On the second night of their observing run, the signature of an emission line appeared in their first 15 minute exposure, on the leading edge of the Magellanic Stream cloud MS IV. This leading edge is the location of a sharp density gradient in H I. The emission was unexpectedly strong--0.2 Rayleighs (1 Rayleigh = 10^6 photons cm^-2 s^-1, corresponding to an emission measure (EM) of 0.5 cm^-6 pc. This is ~10 times greater than expected from limits on the diffuse ionizing flux. Weiner and Williams undertook a short survey to constrain the origin of this Ha emission. The results are shown in the Figure. The spectra at three positions, on the leading edges of clouds MS II, MS III, and MS IV of the Stream, show Ha emission of EM 1, 0.5, and 0.5 cm^-6 pc. The fourth position, and three other positions (not shown), which are not on cloud leading edges, show only marginal evidence for Ha emission, of EM 0.15 cm^-6 pc or less. Where is the energy to power this emission coming from? The association with the cloud leading edges suggests that the Ha emission is caused by bow shocks in the Magellanic Stream as it moves through a Galactic corona of low-density gas. This is reinforced by the shape of the HI density contours on the leading edges of the LMC and the clouds of the Magellanic Stream, which resemble bow shocks. Most other sources, such as photoionization, can be ruled out. The only other source that might be able to contribute is thermal conduction from hot gas (T ~10^7K) in the Galactic halo, although this does not explain the association with leading edges. Shocks and thermal conduction in the Magellanic Stream require the presence of an ambient medium of hot ionized gas in the Galactic halo, at a Galactic radius of r ~50 kpc, well above the Galactic plane, at b = -70 to -90, implying that baryons associated with the Galaxy extend to large radii. If, as seems likely, the emission is due to shocks, the density of the corona is on the order of n_H ~10^-4 cm^-3 at r ~50 kpc. This suggests that ram pressure plays an important role in tearing the Stream out of the Magellanic Clouds. The density agrees with models in which part of the soft X-ray background comes from a hot Galactic corona. Such a large corona also accords with models of QSO absorption line systems where the absorbers are resident in extended halos around normal galaxies. [Figure not included] Spectra (object field minus sky field) of four positions on the Magellanic Stream, obtained with the Rutgers Fabry-Perot and CTIO 1.5-m. The solid lines are fits to the data made by LOWESS, a robust smoothing technique. The inverted triangle indicates the wavelength corresponding to the velocity of H I (heliocentric) from 21 cm measurements. The agreement confirms that the emission is H associated with the Stream. (a) MS II field, on the MS II cloud leading edge. The residuals around 6563 are due to incomplete object-sky cancellation of the geocoronal H line. (b) MS III, leading edge field. (c) MS IV, leading edge field. (d) MS II, not on a cloud leading edge.
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