M. Hancock

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

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 knots far-ultraviolet flux due to stellar evolution. The star formation rate calculated from knots younger than 20 Myr is ~6 M⊙ yr-1, suggesting that the star formation rate throughout the galaxy may be as high as 30 M⊙ 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.

Star-forming Knots in the UV-bright Interacting Galaxies NGC 3395 and NGC 3396

We have obtained ultraviolet and visible wavelength images for the central regions of the interacting galaxies NGC 3395 and NGC 3396, using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The images show many isolated sources of ultraviolet emission produced by young star-forming regions (knots). The FWHM of most of the knots in both galaxies is ≤20 pc. Far-UV flux distributions for the complete sample of knots can be fitted with a power law with exponent α = -1.20 ± 0.35 for NGC 3395 and a power law with exponent α = -0.69 ± 0.21 for NGC 3396. Comparison with models from Leitherer et al. indicates that the ages of most of the knots are ≤80 Myr. Reddening of the knots ranges from E(B-V) = 0.0 to E(B-V) = 0.3 mag, indicating variable amounts of dust in these regions. Almost all the knots have masses less than 106 M⊙. Many of the knots are probably bound and at least six knots are good proto–globular cluster candidates. There are no significant differences in the fluxes, sizes, and ages of the knots in the two galaxies. The average mass of the knots in NGC 3395 is an order of magnitude less than the average mass of the knots in NGC 3396. There is no obvious correlation between the age of a knot and its position the galaxy.

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

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 A in the UV and 2900–10270 A 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⊙ 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⊙, 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.

Hubble Space Telescope Observations of Star-forming Regions in NGC 3994/3995*

We obtained ultraviolet and optical images of star-forming regions in the interacting galaxy system NGC 3994/3995 using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We describe the characteristics of 56 detected knots, a majority of which are younger than 20 Myr old. There is no discernible pattern of knot age with respect to position within the galaxies. The knots have masses and radii ranging from 7.2 × 103 to 4.4 × 106 M⊙ and from 4 to 23 pc, respectively. Using a conservative criterion, we find that ~15% of these knots may be proto-globular clusters; the percentage of proto-globular clusters may be as high as ~70%. The UV flux distribution of the knots in NGC 3995 can be fitted with a power law with α = -0.72 ± 0.11, with no turnover detected brightward of the completeness limit.