We present HST /NICMOS observations ( 1.1 - 2.2 \mu m ) and 1.9 - 4.1 \mu m spectroscopy of the central region of the dwarf galaxy NGC 5253 .
The HST /NICMOS observations reveal the presence of a nuclear double star cluster separated by \simeq 0.3 - 0.4 \arcsec or 6 - 8 pc ( for a distance d = 4.1 Mpc ) .
The double star cluster , also a bright double source of Pa \alpha emission , appears to be coincident with the double radio nebula detected at 1.3 cm .
The eastern near-infrared star cluster ( C1 ) is identified with the youngest optical cluster , whereas the western star cluster ( C2 ) , although it is almost completely obscured in the optical , becomes the brightest star cluster in the central region of NGC 5253 at wavelengths longer than 2 \mu m. Both clusters are extremely young with ages of approximately 3.5 million years old .
C2 is more massive than C1 by a factor of 6 to 20 ( M _ { C 2 } = 7.7 \times 10 ^ { 5 } -2.6 \times 10 ^ { 6 } { M } _ { \odot } , for a Salpeter IMF in the mass range 0.1 - 100 { M } _ { \odot } ) .
Analysis of the circumnuclear spectrum excluding C1 and C2 , as well as of a number of other near-infrared selected clusters with a range of ( young ) ages , suggests that the star formation was triggered across the central regions of the galaxy .
We have also modelled the nuclear UV to mid-infrared spectral energy distribution ( SED ) of NGC 5253 and found that the infrared part is well modelled with a highly obscured ( A _ { V } = 17 mag ) young starburst with a stellar mass consistent with our photometric estimates for C1 and C2 .
The SED model predicts a moderately bright polycyclic aromatic hydrocarbon ( PAH ) feature at 3.3 \mu m that is not detected in our nuclear L -band spectrum .
NGC 5253 ’ s low metallicity and a top-heavy IMF likely combine to suppress the 3.3 { \mu m } PAH emission that is commonly seen in more massive starburst systems .