Brightest young star clusters in LEGUS: host to Extremely Massive Stars?

19 Sep

Screen Shot 2015-09-19 at 12.59.23 PMThe 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.

To investigate the role of these clusters in the starburst energetics, we use Hubble Space Telescope images of NGC5253 combined with data of the 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 brightest clusters.The clusters have ages ~1-15 Myr and masses ~10,000 – 250,000 solar masses.

The most massive cluster is in the radio nebula; with a mass 250,000 solar masses and an age ~1 Myr,  2-4 times less massive and younger than previously estimated. 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.

Calzetti & the LEGUS collaboration (2015), Accepted for publication in the Astrophysical Journal,