A Galaxy in Transition: Structure, Globular Clusters, and Distance of the Star-Forming S0 Galaxy NGC 1533 in Dorado

Abstract

We use two-band imaging data from the Advanced Camera for Surveys on board the Hubble Space Telescope for a detailed study of NGC1533, an SB0 galaxy in the Dorado group surrounded by a ring of HI. NGC1533 appears to be completing a transition from late to early type: it is red, but not quite dead. Faint spiral structure becomes visible following galaxy subtraction, and luminous blue stars can be seen in isolated areas of the disk. Dust is visible in the color map in the region around the bar, and there is a linear color gradient throughout the disk. We determine an accurate distance from the surface brightness fluctuations (SBF) method, finding m-M = 31.44\pm0.12 mag, or d = 19.4\pm1.1 Mpc. We then study the globular cluster (GC) colors, sizes, and luminosity function (GCLF). Estimates of the distance from the median of the GC half-light radii and from the peak of the GCLF both agree well with the SBF distance. The GC specific frequency is S_N=1.3\pm0.2, typical for an early-type disk galaxy. The color distribution is bimodal, as commonly observed for bright galaxies. There is a suggestion of the redder GCs having smaller sizes, but the trend is not significant. The sizes do increase significantly with galactocentric radius, in a manner more similar to the Milky Way GC system than to those in Virgo. This difference may be an effect of the steeper density gradients in loose groups as compared to galaxy clusters. Additional studies of early-type galaxies in low density regions can help determine if this is indeed a general environmental trend.