Looking Closely at Medusa: Star-forming Knots at the Center of NGC 4194

Abstract

We report high-resolution ultraviolet and visible-wavelength imaging of the blue compact galaxy NGC 4194 (the Medusa) using the Hubble Space Telescope. A complete sample of 38 UV-bright knots is identified. These knots produce 20% of the ultraviolet emission from the area of the galaxy observed. The bright end of the knot flux distribution can be fitted by a power law with exponent α=-1.47+/-0.27. The complete flux distribution is Gaussian. Comparison with starburst evolutionary tracks indicates that two-thirds of the knots are younger than 20 Myr. There is one concentration of very young knots less than 5 Myr old. The knot masses are large, comparable to those found in other major merger galaxies. The mass distribution is a power law with exponent -1.85+/-0.24. The difference between the far-ultraviolet flux distribution and the mass distribution can be understood by the rapid decrease in a knot's far-ultraviolet flux due to stellar evolution. The star formation rate calculated from knots younger than 20 Myr is ~6 M_solar yr^-1, suggesting that the star formation rate throughout the galaxy may be as high as 30 M_solar yr^-1. Estimates of knot stability suggest that at least half the knots are bound protoclusters and will become globular clusters in several gigayears. The location and masses of individual star-forming regions in NGC 4194 and the rate of star formation are typical of major merger galaxies.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.