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

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