The near-infrared structure of M33

We present near-infrared mosaics of J, H, and K emission from the central 15' x 30' in M33 which map the stellar distribution of its disk and spheroidal components. Detailed examination of individual OB associations and population synthesis models show that recent massive star formation contributes significantly to near infrared emission, which must be considered in deriving the stellar mass distribution. The scale length of the exponential disk is less than 6', shorter than optical scale lengths in part because of a radial gradient in extinction. We identify two grand-design spiral arms with an underlying stellar component in the inner 9'; these terminate near the radius at which the rotation curve begins to flatten. A weak, bar-shaped excess in the stellar mass distribution is present in the inner 1. 5'; it is unclear whether this traces the inward extension of the spiral arms or a true bar. We find the spheroidal light is reasonably fitted by a single r¹/4^ distribution with a scale length much longer than previously reported for the central spheroidal component. Including a compact bulgelike component does not significantly improve the fit to the surface photometry indicating that the contribution of a compact bulge component to the spheroidal light must be very low. In previous studies of M33, the ratio of the number of globular clusters to spheroid luminosity was anomalously high. Our more extended spheroid results in a ratio closer to that obtained for other spiral galaxies, although it is still high.