A Nearby Galaxy in the Deep-Ultraviolet: Voyager 2 Observations of M33 from Lyα to the Lyman Limit

Observations of high-redshift galaxies in the emitted-wavelength regime around Lyα have renewed interest in the appearance of normal galaxies in the deep ultraviolet. This paper presents an analysis of spectrophotometry of the local Sc galaxy Messier 33 (NGC 598) obtained by Voyager 2, covering the range from below the Lyman limit to about 1250 A. The scanning nature of the observation preserves one dimension of spatial resolution across the galaxy. M33 exhibits net Lyα emission at equivalent width ≈ 100 A and observed flux 1.8 ± 0.4 × 10-9 ergs cm-2 s-1. The line emission is broadly concentrated toward the center, but there is no peak coincident with the giant H II region NGC 604. This may suggest that Lyα photons from spirals escape preferentially from the diffuse interstellar medium rather than from individual luminous H II regions. The global Lyα/Hα ratio is at least 3. We must see Lyα at the radial velocity range of M33 through substantial structure in the foreground H I of the Milky Way, since applying the mean column density averaged over large scales would give negligible transmission. Therefore, the actual Lyα intensity must be larger than measured. The continuum shows the Lyβ/O VI/C II λ 1030 and Lyγ/C III λ 980 complexes in absorption and a decline in flux toward the Lyman limit. The dominant sources of radiation between 912 and 1216 A are B stars, which is not surprising, since this is where their output peaks. The global spectrum declines only slightly in Fλ from 1500 to 1000 A then declines sharply toward the Lyman limit. The intrinsic stellar spectrum must rise again shortward of the Lyman limit in order to match the intensity of recombination lines such as Hα. The spectrum is very well fitted by a weighted sum of B-star reference spectra, which would correspond to a current main-sequence mass function of N(M) ∝ M-4.7±0.3, although the important role of evolved B stars means that this fit does not necessarily correspond to the actual mass distribution. The stellar population in NGC 604 has bluer deep-UV colors than the rest of the galaxy, and the continuum in the Voyager passband follows the trend of flatter gradients for shorter wavelengths found from UV Imaging Telescope imagery. The continuum shape is a close match for that shown in Hopkins Ultraviolet Telescope observations of two starburst systems, as is expected from the short lifetimes of stars responsible for the deep-UV radiation. A simple comparison of the continuum and line properties of M33 with those of star-forming galaxies at high redshifts indicates that it is both fainter and much larger than the objects we can now detect in the early universe, pointing out how strongly our sample is (and will remain for some time) biased in favor of objects that are both compact and have absolutely high star formation rates. However, the detection of Lyα in M33, with nontrivial metallicity, bolsters the argument that many of the objects with such emission at high redshift are star-forming galaxies rather than those powered by active nuclei. The starburst system Mrk 357 at z = 0.053 has significant Lyα as well, from an archival IUE observation, providing a further example of such emission from star-forming systems. Tabulated properties are shown for nearby B stars observed with the UV Spectrometer systems on Voyagers 1 and 2 used here as reference objects for crude spectral synthesis.