Comparison of UV and Hα morphologies in the Magellanic Clouds Available to Purchase

Ultraviolet continuum and Hα fluxes measure two critical spectral regions of the emission from hot stars in galaxies. Hα indirectly measures the Lyman continuum’s ionizing flux, while the 1400 Å–3200 Å band, such as that imaged by the Ultraviolet Imaging Telescope (UIT), directly measures non-ionizing flux produced by O, B and A-type stars. The ratio of the two gives a “color” which is extremely sensitive to the age of the young stellar populations: model spectral energy distributions show that the ratio of Lyman continuum photon flux to the UIT 1520 Å bandpass flux decreases monotonically by a factor of 100 during the first 10 Myr of cluster evolution. We compare the ultraviolet morphology of the Magellanic Clouds, from the UIT and a rocket-borne wide-field UIT predecessor, with Hα morphology derived from the “Parking Lot Camera” images made by Bothun and Thompson (1988). In the LMC, the 30 Doradus region stands out as a region of large Hα/UV sandwiched between two regions with much smaller Hα/UV. The Shapley III constellation has UV-bright components (e.g the “fleur-de-lis”) but very little Hα. In the SMC, evolutionary extremes are suggested by the UV-bright clusters NGC 330 (nearly Hα-free) and NGC 346 (surrounded by a bright HII region). We present “color” images in the Hα/UV ratio for the entire LMC and most of the SMC bar. We interpret the images, compare them with other data, and discuss their implications for recent star formation, on large and small scales, in the Magellanic Clouds.