We present HST /STIS optical and Gemini/NIFS near-IR IFU spectroscopy , and archival HST imaging of the triplet of super star clusters ( A1 , A2 and A3 ) in the core of the M82 starburst .
Using model fits to the STIS spectra , and the weakness of red supergiant CO absorption features ( appearing at \sim 6 Myr ) in the NIFS H-band spectra , the ages of A2 and A3 are 4.5 \pm 1.0 Myr .
A1 has strong CO bands , consistent with our previously determined age of 6.4 \pm 0.5 Myr .
The photometric masses of the three clusters are 4– 7 \times 10 ^ { 5 } M _ { \odot } , and their sizes are R _ { eff } = 159 , 104 , 59 mas ( \sim 2.8 , 1.8 , 1.0 pc ) for A1,2 and 3 .
The STIS spectra yielded radial velocities of 320 \pm 2 , 330 \pm 6 , and 336 \pm 5 km s ^ { -1 } for A1,2 , and 3 , placing them at the eastern end of the x _ { 2 } orbits of M82 ’ s bar .
Clusters A2 and A3 are in high density ( 800–1000 cm ^ { -3 } ) environments , and like A1 , are surrounded by compact H ii regions .
We suggest the winds from A2 and A3 have stalled , as in A1 , due to the high ISM ambient pressure .
We propose that the 3 clusters were formed in-situ on the outer x _ { 2 } orbits in regions of dense molecular gas subsequently ionized by the rapidly evolving starburst .
The similar radial velocities of the 3 clusters and their small projected separation of \sim 25 pc suggest that they may merge in the near future unless this is prevented by velocity shearing .