A Spectroscopic Study of the Star-Forming Properties of the Center of NGC 4194

Abstract

We have obtained Space Telescope Imaging Spectrograph long-slit spectra of the central region of the advanced merger NGC 4194. The spectra cover the wavelength ranges 1150-1750 Å in the UV and 2900-10270 Å in the visible. Results from the study of the properties of 14 star-forming regions (knots) are presented. If the [N II] contribution is 40% of the combined Hα + [N II] flux, then the average E(B-V)=0.7 mag. The metal abundances are approximately solar, with individual knot abundances ranging from log(O/H)+12=8.1+/-0.5 to 8.9+/-0.4. The Hα luminosities of the 14 observed knots yield a total star formation rate (SFR) of ~46 M_solar yr^-1. The sizes of the H II regions associated with the knots were determined from L(Hβ) and range from ~28 to ~119 pc when a filling factor of 0.1 is assumed. The sizes are a factor of ~2.15 smaller for a filling factor of 1. Using Starburst99, the EW(Hα + [N II]), and EW(Hβ), we estimate the ages of the star-forming regions to be 5.5-10.5 Myr. From ground-based spectra the effective temperatures of the H II regions are found to be ~11,000 K, and the electron densities are determined to be ~530 cm^-3. We find that eight of the knots probably formed with a Salpeter initial mass function truncated at an upper mass of 30 M_solar, and one of the knots likely formed with a standard Salpeter initial mass function. We suggest that the knots in our sample are the precursors of globular clusters. Two of the knots are in a region of flowing gas and are among the most massive, are the largest in radius, have the highest SFR, and are among the youngest of the knots.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.