Far-Ultraviolet Observations of M83 Using the Hopkins Ultraviolet Telescope
Session 20 -- Astro 2
Display presentation, Tuesday, June 13, 1995, 9:20am - 6:30pm

## [20.03] Far-Ultraviolet Observations of M83 Using the Hopkins Ultraviolet Telescope

J.-C. Liu, A.F. Davidsen (JHU), H.C. Ferguson (STScI)

We present a spectrum of the center of the starburst galaxy M83 (NGC~5236) measured through a 20" diameter aperture with the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mission in March 1995. The spectral region shortward of 1200{\AA}, sampled for the first time in a starburst galaxy, is rich in both interstellar and stellar absorption features. The O{\sc vi} absorption line is strong (EW $\simeq$ 10~{\AA}), and is accompanied by P-Cygni emission.

The spectral-energy distribution in the Far-UV is used to calculate the star-formation rate and the age of the starburst, with results consistent with those derived from IUE and imaging studies at longer wavelengths. For the HUT spectrum the best fit star-formation rate is $0.7 M_\odot{\rm yr^{-1}}$, with an initial-mass function slope $x = -1.5$ , an upper mass limit of $100 M_\odot$, and an age of $9 \times 10^6 {\rm yrs}$.

While the results are sensitive to the detailed shape of the extinction curve, an extrapolation ''obscuration'' curve of the Kinney et al (1994) to the Lyman limit works reasonably well, with an inferred value of $E(B-V)$ (0.34) consistent with that derived from IUE spectra. Access to the region below 1200{\AA} provides interesting information on the relative distributions of dust, stars, and gas in M83. In the Milky Way, individual hot stars with $E(B-V) \geq$0.2 invariably show $H_2$ absorption features. The center of M83 has strong molecular CO emission, and therefore presumably molecular hydrogen. However, absorption due to $H_2$ is not obvious in the HUT spectrum, suggesting that most of the emission seen by HUT comes from stars that are not within the molecular gas, but are nevertheless behind significant amounts of dust and atomic gas.

This work was supported by NASA contract NAS 5-27000 to the Johns Hopkins University.