The Brightest Young Star Clusters in NGC 5253
  The nearby dwarf starburst galaxy NGC5253 hosts a number of young, massive
star clusters, the two youngest of which are centrally concentrated and
surrounded by thermal radio emission (the `radio nebula'). To investigate the
role of these clusters in the starburst energetics, we combine new and archival
Hubble Space Telescope images of NGC5253 with wavelength coverage from 1500 Ang
to 1.9 micron in 13 filters. These include H-alpha, P-beta, and P-alpha, and
the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV
Survey). The extraordinarily well-sampled spectral energy distributions enable
modeling with unprecedented accuracy the ages, masses, and extinctions of the 9
optically brightest clusters (M_V < -8.8) and the two young radio nebula
clusters. The clusters have ages ~1-15 Myr and masses ~1x10^4 - 2.5x10^5 M_sun.
The clusters' spatial location and ages indicate that star formation has become
more concentrated towards the radio nebula over the last ~15 Myr. The most
massive cluster is in the radio nebula; with a mass 2.5x10^5 M_sun and an age
~1 Myr, it is 2-4 times less massive and younger than previously estimated. It
is within a dust cloud with A_V~50 mag, and shows a clear nearIR excess, likely
from hot dust. The second radio nebula cluster is also ~1 Myr old, confirming
the extreme youth of the starburst region. These two clusters account for about
half of the ionizing photon rate in the radio nebula, and will eventually
supply about 2/3 of the mechanical energy in present-day shocks. Additional
sources are required to supply the remaining ionizing radiation, and may
include very massive stars.
