AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 12. Extra Galactic Star Clusters: From Ancient Globulars to Young Star Clusters
Display, Monday, May 31, 1999, 9:20am-6:30pm, Southwest Exhibit Hall

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[12.05] The Luminosity Function of Young Star Clusters in "The Antennae" Galaxies (NGC~4038/4039)

B. C. Whitmore (STScI), Q. Zhang (STScI \& JHU), C. Leitherer, M. Fall (STScI), F. Schweizer (CIW/DTM), B. W. Miller (Leiden)

The Wide Field and Planetary Camera 2 of the Hubble Space Telescope has been used to obtain high-resolution images of NGC 4038/4039 that go roughly 3 magnitudes deeper in V than previous observations made during Cycle 2. A variety of different techniques are used to decouple the cluster and stellar LFs, which overlap in the range -9 \lesssim MV \lesssim -6. We find that the young-cluster LF is a power law with exponent \alpha = -2.6 ±0.2 for absolute magnitudes brighter than MV = -10.4, but flattens to \alpha = -1.7 ±0.2 in the range -10.4 < MV < -8.0. The stellar LF in the range -9 < MV < -6 is much steeper, with \alpha = -2.9 ±0.1, and is dominated by young red and blue supergiants. The star clusters of the Antennae appear slightly resolved, with median effective radii of 4 ±1 pc, similar to or perhaps slightly larger than those of globular clusters in our Galaxy. However, the radial extents of some of the very young clusters (ages < 10 Myr) are much larger than those of old globular clusters (e.g., the outer radius of Knot S exceeds 450 pc). This may indicate that the tidal forces from the galaxies have not had time to remove some of the outer stars from the young clusters. Age estimates derived from GHRS spectroscopy yield 3 ±1 Myr for Knot~K (just south of the nucleus of NGC 4038) and 7 ±1 Myr for Knot~S in the Western Loop, in good agreement with ages derived from the UBVI colors. Effective gas-outflow velocities from Knots S and K are estimated to be about 25 -- 30 kms-1, based on the above cluster ages and the sizes of the surrounding H\alpha bubbles. However, the measured widths of the interstellar absorption lines suggest dispersion velocities of ~400 kms-1 along the lines of sight to Knots S and K.

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