UIT: Ultraviolet Surface Photometry of the Spiral Galaxy M74 (NGC 628)

Ultraviolet photometry, obtained from Ultraviolet Imaging Telescope (UIT) images at 1520 A [far-UV;magnitudes m(152)] and 2490 A [near-UV; magnitudes m(249)], of the spiral galaxy M74 (NGC 628) is compared with Hα, R, V, and B surface photometry and with models. M74's surface brightness profiles have a central peak with an exponential falloff; the exponential scale lengths of the profiles increase with decreasing wavelength for the broad-band images. The slope of the continuum- subtracted Hα profile is intermediate between those of far-UV and near-UV profiles, consistent with the related origins of Hα and UV emission in extreme Population I material. M74's color profiles all become bluer with increasing radius. The [m(152) - m(249)] color as measured by UIT averages near 0.0 (the color of an A0 star) over the central 20" radius and decreases from ~- 0.2 to ~-0.4 from 20" to 200". The spiral arms are the dominant component of the surface photometry colors; interarm regions are slightly redder. In the UV, M74's nuclear region resembles its disk/spiral arm material in colors and morphology, unlike galaxies such as M81. No UV "bulge" is apparent. The m(152) - m(249) colors and models of M74's central region clearly demonstrate that there is no significant population of O or B stars present in the central 10". M74's UV morphology and [m(152) - m(249)] color profiles are similar to those of M33, although M74 is ~0.5 mag redder. M81 has a smooth UV bulge which is much redder than the nuclear regions of M74 and M33. M74 is ~0.4 mag bluer than M81 in its outer disk, although M81 has bright UV sources only in spiral arms more than 5 kpc from its center. We investigate possible explanations for the color profiles of the galaxies and the differences among the galaxies: abundances; reddening due to internal dust; IMF variations, and the history of formation of the dominant generations of stars. Abundance and IMF variations do not produce large enough m(152) - m(249) or UV - V color differences. Comparing model UV/optical colors with those of M74 shows that M74's disk has undergone significant star formation over the past 500 Myr, and that either the star-formation history or the extinction varies systematically across M74's disk. Realistic reddening models show that the effects of dust alone are not sufficient to produce the variations within M74. However, evolutionary models produced by combining data from stellar atmosphere models and observed spectra generate UV - V colors which are compatible with the observations, provided relatively recent star formation is present in the stellar population. Comparison of M74, M33, and M81 (UV - V) colors shows that M74 colors range from the bluest of M33's colors to the bluest of M81's. All material with these colors in all three galaxies has the morphological appearance of spiral arm/disk material. The failure of reddening models to cover the range of colors, and the known abundance range in such material, leads to the conclusion that star-formation history varies significantly as a function of radius in these galaxies, and that such variation is required to explain the range of colors observed in M74, M33, and M81.