The Brightest Young Star Clusters in NGC 5253

Abstract

The nearby dwarf starburst galaxy NGC 5253 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 NGC 5253 with wavelength coverage from 1500 Å to 1.9 μm in 13 filters. These include Hα, Pβ, and Pα, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily wellsampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the nine optically brightest clusters (M V < −8.8) and the two young radio nebula clusters. The clusters have ages ∼1-15 Myr and masses ∼1 × 10 4 -2.5 × 10 5 M e . The clusters' spatial location and ages indicate that star formation has become more concentrated toward the radio nebula over the last ∼15 Myr. The most massive cluster is in the radio nebula; with a mass ∼2.5 × 10 5 M e 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 near-IR 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.

The very massive star content of the nuclear star clusters in NGC 5253

“…1, it passes through cluster #5 and cluster #11 is on the edge of the slit. Clusters #1 and 2 (Calzetti et al 2015) lie 2.5 arcsec south-west of cluster #5 and correspond to the peak of the UV emission in NGC 5253 and are denoted UV-1 by Meurer et al (1995). The slit width was 1.…”

“…The agreement is excellent by just applying this scaling. We did not correct for the different reddenings because the color excesses of #5 and R136a are similar with values of 0.46 (Calzetti et al 2015) and 0.42 (Crowther et al 2016).…”

“…In a recent paper, Calzetti et al (2015) present a detailed analysis of the two nuclear clusters (#5 and #11 in their terminology) using HST imaging from the far ultraviolet to the infra-red. By fitting their spectral energy distributions (SED), they find that the two clusters are extremely young with ages of only 1 ± 1 Myr.…”

“…In NGC 5253, clusters #5 and #11 contribute about 40-50% of the total ionization of the galaxy, with the majority coming from cluster #11. However, 50% of the ionizing photon rate is unaccounted for in the region of the supernebula (Calzetti et al 2015). Turner et al (2015) find that they need to invoke a top-heavy initial mass function (IMF) for the two nuclear clusters to provide sufficient Lyman ionizing photons given the age constraint of 4 Myr and an upper limit to the combined cluster mass (derived from the dynamical mass of the gas in the region).…”

“…These studies find that the degree of N and He enrichment depends on rotational mixing and mass loss. In (Calzetti et al 2015) are indicated, as well as clusters #1,2 corresponding to UV-1 (Meurer et al 1995). this paper, we explore whether VMS and/or fast-rotating stars can explain the N enrichment associated with the nuclear clusters in NGC 5253.…”