Ultraviolet Imaging Telescope Observations of M82
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Session 23 -- Galaxies I
Oral presentation, Wednesday, January 12, 10:15-11:45, Salon IV Room (Crystal Gateway)

[23.03] Ultraviolet Imaging Telescope Observations of M82

Gregory S. Hennessy (UVa)

We present vacuum ultraviolet images of the prototype starburst galaxy M82 obtained with the Ultraviolet Imaging Telescope during the Astro--1 mission in December 1990. The Mid--Ultraviolet (MUV, $\sim$ 250 nm) emission is bright along the major axis of the galaxy, and there is a MUV plume coinciding with the H$\alpha$ filaments. Many of the morphological features seen at visible wavelengths have MUV counterparts. Surface photometry shows that the brightness profile in the outer halo of M82 follows a de Vaucouleurs profile, suggesting that the outer region of M82 is similar to the relaxed spheroids of spiral galaxies. No statistically significant far--ultraviolet (FUV, $\sim$ 160 nm) flux is present. Large MUV--V color gradients are present, with the color changes in the inner 120\arcsec{} being best explained by reddening from dust in a model where the dust and stars are mixed. Color maps of M82 are presented, showing the large effect that dust has on the continuum colors. The reddest colros are concentrated to an irregular and (presumably caused by dust) which extends about 4\arcmin{} (3.7 kpc) along, but slightly south of, the major axis. We tetect no scattered MUV light from the cool HI clouds in the outer regions of M82, which appears to be consistent with a normal gas--to--dust ratio and reasonable assumptions about the central brightness of the galaxy Estimates of the extinction law of M82 were made in two locations in the galaxy. The extinction in the main dust lane of M82 is consistent with the mean galactic reddening law in the optical, but there is much less extinction in the MUV than predicted by the mean galactic law. The extinction law in the eastern dust lane systematically shows more extinction than the mean galactic law. The scattering plume is also present in visible bands, after subtraction a model galactic brightness, although not as prominently as in the MUV. We present a model for this scattering plume.

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