Super Star Clusters in M82

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

[77.03] Super Star Clusters in M82

R.W. O'Connell (UVa), J.S. Gallagher, III (UWisc), D.A. Hunter (Lowell Obs), W.N. Colley (UVa)

We discuss Hubble Space Telescope Planetary Camera images in the V and I bands of the central $1.1\,^\prime$ of the prototype starburst galaxy M82. Included are the optical knots A and C, which are coincident with the brightest diffuse X-ray emission and the base of the H$\alpha$-bright filaments. We deconvolved the summed images using the Lucy-Richardson algorithm. The HST observations reveal a remarkable complex of ``super'' star clusters, whose presence was only hinted at with ground-based data. Region A, which in the best ground-based images consists of 3 contiguous unresolved clusters, each $\sim 2\,^{\prime\prime}$ (30 pc) in diameter and with $V \sim\, 13.8$, is resolved into over 3 dozen compact subclusters. There are more than 50 such objects in the central 300 pc of M82. Their diameters (FWHM) range from $<$1 to 4 pc. After correction for extinction, which is uncertain but here assumed to be $A_V = 3.0$, their absolute magnitudes are $M_{V,0} = -10.5$ to $-13$, and their surface brightnesses range up to $2\times10^5\, {\rm L}_{{\rm V}, \odot} {\rm pc}^{-2}$. Most of these objects are more extreme than the brightest compact cluster in the Local Group, R136a in the LMC 30 Dor complex.

Comparison of the I and V band images shows that the clusters have large color differences. Their (V--I)'s range from 1.5 to 4.0. This could be partly due to variations in the red supergiant populations, but it is undoubtedly mostly caused by variable extinction. We compare the HST imaging to ground-based optical and thermal infrared maps and suggest that the regions we see are in the outer shell of the intense star-forming core of M82, which mostly lies behind layers with high extinction. The cluster complex exhibits distorted morphologies, which may be produced by strong disturbances from the galaxy's superwind.

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