Previous abstract Next abstract
Recent UV rocket imagery show 22 bright starburst knots within the irregular galaxy, NGC 4449. IUE low resolution spectra collected from new and archival data for two starburst knots (knot 16, \& 21) and a ridgeline (knot 18) within NGC 4449 reveal the presence of strong spectral lines characteristic of young hot stars with ages less than 12 million years (Myrs). Using a new population synthesis code and a corresponding spectral library of O and B stars constructed from high dispersion data, ages for these starburst knots can be deduced accurately to within 1 to 2 million years by matching the observed spectral features using this library. This population synthesis technique is implemented for the three bright knots observed in NGC 4449. Knot 18 exhibits prominent Si IV (1393, 1402 \AA) and C IV ( 1548, 1550 \AA) P Cyni profiles characteristic of OB stars at age 8 Myrs. Knot 16 shows weakened C IV but prominent Fe III (1900 \AA) absorption indicating an age of 11 Myrs. Knot 21 reveals a population of stars older than stars found in knots 16 and 18. For a 30 Doradus extinction law, a nominal extinction of E(B-V) = 0.03 is derived for knots 16 \& 18 by fitting the UV continuum. This unrealistically low extinction value suggests a very flat UV extinction law for NGC 4449. The new population synthesis technique represents a new powerful tool for determining the ages of starburst knots. This work is the first step of a more extensive study to determine the recent star formation history of NGC 4449.
Saturday program listing