Isolated OB associations in stripped H I gas clouds

HST ACS HRC images in UV (F250W), V (F555W), and I (F814W) resolve three isolated OB associations that lie up to 30 kpc from the stellar disk of the S0 galaxy NGC 1533. Previous narrowband Ha imaging and optical spectroscopy showed these objects as unresolved intergalactic H II regions having Hα luminosities consistent with single early-type O stars. These young stars lie in stripped H I gas with column densities ranging from 1.5 to 2.5 × 10 ²⁰ cm^-2 and velocity dispersions near 30 km s ^-1. Using the HST broadband colors and magnitudes along with previously determined Ha luminosities, we place limits on the masses and ages of each association, considering the importance of stochastic effects for faint (M_V > -8) stellar populations. The upper limits to their stellar masses range from 600 to 7000 M_⊙, and ages range from 2 to 6 Myr. This analysis includes an updated calculation of the conversion factor between the ionizing luminosity and the total number of main-sequence O stars contained within an H II region. The photometric properties and sizes of the isolated associations and other objects in the HRC fields are consistent with those of Galactic stellar associations, open clusters, and/or single O and B stars. We interpret the age-size sequence of associations and clustered field objects as an indication that these isolated associations are most likely rapidly dispersing. Furthermore, we consider the possibility that these isolated associations represent the first generation of stars in the H I ring surrounding NGC 1533. This work suggests star formation in the unique environment of a galaxy's outermost gaseous regions proceeds similarly to that within the Galactic disk and that star formation in tidal debris may be responsible for building up a younger halo component.