Observational signatures of forming young massive clusters: continuum emission from dense Hii regions

Young massive clusters (YMCs) are the most massive star clusters forming in nearby galaxies and are thought to be a young analogue to the globular clusters. Understanding the formation process of YMCs leads to looking into very efficient star formation in high-redshift galaxies suggested by recent JWST observations. We investigate possible observational signatures of their formation stage, particularly when the mass of a cluster is increasing via accretion from a natal molecular cloud. To this end, we study the broad-band continuum emission from ionized gas and dust enshrouding YMCs, whose formation is followed by recent radiation hydrodynamics simulations. We perform post-process radiative transfer calculations using simulation snapshots and find characteristic spectral features at radio and far-infrared frequencies. We show that a striking feature is long-lasting, strong free-free emission from a similar to 10-pc-scale Hii region with a large emission measure of greater than or similar to 10(7) cm(-6) pc, corresponding to the mean electron density of greater than or similar to 10(3) cm(-3). There is a turnover feature below similar to 10 GHz, a signature of the optically thick free-free emission, often found in Galactic ultracompact Hii regions. These features come from the peculiar YMC formation process, where the cluster's gravity effectively traps photoionized gas for a long duration and enables continuous star formation within the cluster. Such large and dense Hii regions show distinct distribution on the density-size diagram, apart from the standard sequence of Galactic Hii regions. This is consistent with the observational trend inferred for extragalactic Hii regions associated with YMCs.