We present infrared spectroscopy of the Antennae Galaxies ( NGC 4038/4039 ) with NIRSPEC at the W. M. Keck Observatory .
We imaged the star clusters in the vicinity of the southern nucleus ( NGC 4039 ) in 0 { { } ^ { \prime \prime } } .39 seeing in K-band using NIRSPEC ’ s slit-viewing camera .
The brightest star cluster revealed in the near-IR ( M _ { K } ( 0 ) \simeq - 17.9 ) is insignificant optically , but coincident with the highest surface brightness peak in the mid-IR ( 12 - 18 \mu m ) ISO image presented by Mirabel et al .
( 16 ) .
We obtained high signal-to-noise 2.03 - 2.45 \mu m spectra of the nucleus and the obscured star cluster at R \sim 1900 .

The cluster is very young ( age \sim 4 Myr ) , massive ( M \sim 16 \times 10 ^ { 6 } M _ { \odot } ) , and compact ( density \sim 115 M _ { \odot } pc ^ { -3 } within a 32 pc half-light radius ) , assuming a Salpeter IMF ( 0.1 - 100 M _ { \odot } ) .
Its hot stars have a radiation field characterized by T _ { eff } \sim 39 , 000 K , and they ionize a compact H ii region with n _ { e } \sim 10 ^ { 4 } cm ^ { -3 } .
The stars are deeply embedded in gas and dust ( A _ { V } \sim 9 - 10 mag ) , and their strong FUV field powers a clumpy photodissociation region with densities n _ { H } \gtrsim 10 ^ { 5 } cm ^ { -3 } on scales of \sim 200 pc , radiating L _ { { H } _ { 2 } 1 - 0 ~ { } { S } ( 1 ) } = 9600 L _ { \odot } .