UV Intercomparisons of M33, M74, and M81 using UIT
Session 76 -- Spiral and Bar Structure in Galaxies
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [76.01] UV Intercomparisons of M33, M74, and M81 using UIT


We compare UIT near-UV ($\lambda_{eff}$=2490\AA) and far-UV ($\lambda_{eff}$=1520\AA) images and annular surface photometry of the nearby spiral galaxies M33, M74, and M81. In the UV, M33 (Scd) is a somewhat chaotic collection of arms consisting of extended patterns of pointlike and nebulous sources surrounding a nuclear source which is not at a clear center of symmetry. M74 (Sc) has a better defined, more symmetrical UV spiral structure with the nuclear source at its center. In both galaxies the nuclear sources are only moderately bright, and have UV appearances similar to bright arm knots. M81 (Sab) has strikingly different UV morphology, with a dominant, smooth central source surrounded by a ring of spiral arms; the region between the central bulge and the arms is faint in the UV and has no isolated sources. M81's spiral arms are smoother than those of M33 and M74, and individual \hii regions stand out less strongly.

Each galaxy's nuclear region is significantly redder (1-4 mag) in UV$-$V colors than its outer edges. The three galaxies' colors occupy a continuum from the bluest outer regions of M33 to the reddest central regions of M81, with M74 filling a gap'' between the reddest M33 and bluest M81 colors. All material in M74's color range has the appearance of spiral arm material, although some lies within the nuclear regions of M33 and M74.

The \maone-V and \mbone-V colors are reproduced well by evolutionary models generated from stellar atmosphere models and observed spectra, provided some relatively recent star formation is present. Appropriate evolutionary models range from constant star formation, which reproduces M33's bluest colors, to 10-Gyr-old decaying-rate models with decay timescales of 1-2 Gyr, which approximate the colors of M81's bulge. However, the range of observed colors implies that star formation history varies significantly as a function of radius in the individual galaxies, as well as among the three galaxies.