2013ApJ...767...53B

The nearby dwarf galaxy II Zw 40 hosts an intense starburst. At the center of the starburst is a bright compact radio and infrared source, thought to be a giant dense H II region containing ~14, 000 O stars. Radio continuum images suggest that the compact source is actually a collection of several smaller emission regions. We accordingly use the kinematics of the ionized gas to probe the structure of the radio-infrared emission region. With TEXES on the NASA-IRTF we measured the 10.5 µm [S IV] emission line with effective spectral resolutions, including thermal broadening, of ∼25 and ∼3 km/s and spatial resolution ∼1''. The line profile shows two distinct, spatially coextensive, emission features. The stronger feature is at galactic velocity and has FWHM 47 km/s. The second feature is ∼44 km/s redward of the first and has FWHM 32 km/s. We argue that these are two giant embedded clusters, and estimate their masses to be ~3x10⁵ M_☉ and ~1.5x10⁵ M_☉. The velocity shift is unexpectedly large for such a small spatial offset. We suggest that it may arise in a previously undetected kinematic feature remaining from the violent merger that formed the galaxy.