Dust and Recent Star Formation in the Core of NGC 5253

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Abstract

Ultraviolet and optical narrow and broad band images of NGC 5253 obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 are used to derive the properties of the dust distribution and the recent star formation history of this metal-poor dwarf galaxy. Corrections for the effects of dust are important in the center of NGC 5253: dust reddening is markedly inhomogeneous across the galaxy's central 20" region. One of the most obscured regions coincides with the region of highest star formation activity in the galaxy; clouds of more than 9 mag of optical depth at V enshroud a 2.5 Myr old stellar cluster in this area. The ages of the bright clusters in the center of the galaxy are anticorrelated with the amount of dust obscuration the cluster suffers. This result agrees with the expectation that young stellar associations are located in heavily obscured regions, but after only 2-3 Myr they remove/emerge from the parental dust cloud and become almost extinction-free. On average, the continuum emission of the diffuse stellar population is about a factor of 2 less reddened than the ionized gas emission, a behavior typical of starburst galaxies (Calzetti et al. 1994, ApJ, 429, 582). In the case of NGC 5253, this difference originates from the larger scale length of the star distribution relative to the ionized gas: the half light radius of the UV-bright stars is about twice as large as the half light radius of the ionized gas emission. Star formation has been active at least over the past 100 Myr in the central 20" of the galaxy, as indicated by the age distribution of both the blue diffuse stellar population and the bright stellar clusters. The star formation episodes may have been discrete in time, or almost continuous but variable in intensity and spatial extension. The current peak of the star formation is located in a 6" region, more spatially concentrated than the star formation averaged over the past 100 Myr. Its average star formation intensity is 10-5-10-4 M_sun/yr/pc2 for a 0.1-100 M_sun Salpeter IMF, a factor of 10 to 100 times larger than in the galaxy's central 20". This starburst region contains a stellar population ∼5 Myr old and the two youngest (2.5 Myr and ∼3-4 Myr, respectively) of the bright stellar clusters in the galaxy's center. The two clusters contribute between 20% and 65% of the ionizing photons in the starburst, a contribution between 1.3 and 4.3 times larger than the average over the central 20". This is expected if cluster formation is an important mode of star formation in the early phase of a starburst event. The mass of the 2.5 Myr old cluster may be as large as ∼106 M_sun, making this one a Super-Star-Cluster candidate.

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