UIT Ultraviolet Surface Photometry of the Spiral Galaxy M74

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Session 31 -- Star-Forming Galaxies
Display presentation, Tuesday, 9:30-6:30, Heller Lounge Room

[31.03] UIT Ultraviolet Surface Photometry of the Spiral Galaxy M74

\newcommand{\mbone}{m$_{152}$} \newcommand{\maone}{m$_{249}$} \newcommand{\ha}{H$\alpha$ {}} R.H.Cornett, M.R.Greason, J.D.Offenberg (Hughes STX), R.C. Bohlin (STSciI), K.P. Cheng (NRC/GSFC), R.W. O'Connell (U.Va.), M.R. Roberts (NRAO), A.M. Smith,E.P. Smith (NASA/GSFC), R.J. Angione, F.D. Talbert (San Diego State U.), T.P. Stecher (NASA/GSFC)

UV photometry from Ultraviolet Imaging Telescope (UIT) images at 1520\AA~ (magnitudes \mbone) and 2490\AA~ (\maone) of the spiral galaxy M74 (NGC628) is compared with \ha, R, V, and B surface photometry and models. M74's surface brightness profiles have central peaks with exponential falloffs; the profiles' exponential scale lengths increase with decreasing continuum wavelength. The slope of the continuum-subtracted \ha profile is between those of FUV and NUV profiles, consistent with related origins of \ha and UV emission in extreme Population I material.

M74's color profiles have small gradients, all becoming bluer with increasing radius. The UIT color (\mbone$-$\maone) averages near 0.0, the color of an A0 star, over the central 20 arcsec radius, and slopes from $\sim$-0.2 to $\sim$-0.4 from 20 to 200 arcsec. Spiral arms dominate surface photometry colors; interarm regions are slightly redder. In the UV, M74's nuclear region resembles disk/spiral arm material in color and morphology, unlike M81. (\mbone$-$\maone) colors and models of M74's central region clearly demonstrate that there are no O or B stars in the central 10 arcsec.

M74's (\mbone$-$\maone) profile is similar to M33's but is $\sim$0.5 mag redder. M74 is $\sim$0.4 mag bluer than M81 in its outer disk. We investigate explanations for both the color profiles and the differences among the galaxies. M74's \maone$-$V and \mbone$-$V color profiles cannot be explained by a disk of uniform color behind a screen of dust with a known reddening function, distributed like the neutral gas with a fixed gas-to-dust ratio. Known abundance variations could produce the observed color gradient in M74; however, evolutionary cluster models show that sensible time parameters, including star formation start time and exponential decay rate, also produce the observed colors of M74, M33, and M81.

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