Recent Massive Star Formation in NGC1068 Derived from UV/H$\alpha$ Comparisons

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Session 77 -- Starbursts
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[77.04] Recent Massive Star Formation in NGC1068 Derived from UV/H$\alpha$ Comparisons

S. G. Neff (LASP/GSFC/NASA), M. N. Fanelli (NRC/LASP/GSFC), L. J. Roberts, K. P. Cheng (HSTX/GSFC), P. M. N. Hintzen (CSULB/LASP/GSFC/NASA), E. P. Smith, A. M. Smith, T. P. Stecher (LASP/GSFC/NASA), R. C. Bohlin (STScI), R. W. O'Connell (UVa), M. S. Roberts (NRAO)

We present broadband ultraviolet images of NGC1068 made at 152 and 249nm with 3 arcsec resolution. The images were made using the Ultraviolet Imaging Telescope (UIT) in December 1990. The UV images are compared with ground-based H$\alpha$ images having comparable resolution.

Although the overall brightness distributions are similar in the UV and in H$\alpha$, we find significant differences when we compare the images in detail. In particular, the extremely luminous UV region 10 arcsec NW of the active nucleus is unremarkable in H$\alpha$. Several other regions are observed with differing UV and H$\alpha$ appearances.

We use the UV/H$\alpha$ ratio in different regions to explore the recent star formation history of NGC1068. Although both the UV light and the H$\alpha$ emission trace the young stellar population, the H$\alpha$ tends to fade more quickly as the ionizing stars evolve off the main sequence; the UV continuum emission lasts longer, as it originates from a broader range of stellar ages and temperatures. Thus, the `knots' which are UV-bright but H$\alpha$-weak are probably regions currently containing young stars with intermediate masses (5-15 $M_{\sun}$). Alternatively, the `knots' may have formed with a truncated IMF.

We compare our observations to population synthesis models of Fanelli \it et al. \rm to determine ages of the star- forming regions and to constrain the IMF in selected regions of NGC1068.

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