The radio-infrared nebula in II Zw 40: clusters forming in colliding elongated clouds

ABSTRACT II Zw 40 is a starburst dwarf and merger product, and holds a radio-infrared supernebula excited by thousands of embedded OB stars. We present here observations of three aspects of the supernebula: maps of the K and KU radio continuum that trace dense ionized gas with spatial resolution ${\sim} 0.1^{\prime \prime }$, a spectral data cube of the [S iv]$10.5\,\mu$m emission line that measures the kinematics of the ionized gas with velocity resolution 4.5 km s$^{-1}$, and an ALMA (Atacama Large Millimeter Array) spectral cube of the CO(3-2) line that probes the dense warm molecular gas with spatial and velocity resolution comparable to the ionized gas. The observations suggest that the supernebula is the overlap, collision or merger of two star clusters, each associated with an elongated molecular cloud. We accordingly modelled the supernebula with simulations of colliding clusters. The model that best agrees with the data is a grazing collision that has distorted the gas and stars to create the distinctive structures observed. These models may have wide applicability in the cluster-rich regions of young starbursts.