2021MNRAS.503.2187C

We report Keck-NIRSPEC observations of the Brackett α 4.05 µm recombination line across the two candidate embedded super star clusters (SSCs) in NGC 1569. These SSCs power a bright H II region and have been previously detected as radio and mid-infrared sources. Supplemented with high-resolution VLA mapping of the radio continuum along with IRTF-TEXES spectroscopy of the [S IV] 10.5 µm line, the Brackett α data provide new insight into the dynamical state of gas ionized by these forming massive clusters. Near-infrared sources detected in 2 µm images from the slit-viewing Camera are matched with Gaia sources to obtain accurate celestial coordinates and slit positions to within ∼0_.^′′1. Br α is detected as a strong emission peak powered by the less luminous infrared source, MIR1 (L_IR ∼ 2 x 10⁷ L_☉). The second candidate SSC MIR2 is more luminous (L_IR >= 4 x 10⁸ L_☉) but exhibits weak radio continuum and Br α emission, suggesting the ionized gas is extremely dense (n_e >= 10⁵ cm^–3), corresponding to hypercompact H II regions around newborn massive stars. The Br α and [S IV] lines across the region are both remarkably symmetric and extremely narrow, with observed line widths Δv ≃ 40 km s^–1, full width at half-maximum. This result is the first clear evidence that feedback from NGC 1569's youngest giant clusters is currently incapable of rapid gas dispersal, consistent with the emerging theoretical paradigm in the formation of giant star clusters.