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Young massive clusters (YMCs) are the most massive star clusters forming innearby galaxies and are thought to be a young analogue to the globularclusters. Understanding the formation process of YMCs leads to looking intovery efficient star formation in high-redshift galaxies suggested by recentJWST observations. We investigate possible observational signatures of theirformation stage, particularly when the mass of a cluster is increasing viaaccretion from a natal molecular cloud. To this end, we study the broad-bandcontinuum emission from ionized gas and dust enshrouding YMCs, whose formationis followed by recent radiation-hydrodynamics simulations. We performpost-process radiative transfer calculations using simulation snapshots andfind characteristic spectral features at radio and far-infrared frequencies. Weshow that a striking feature is long-lasting, strong free-free emission from a$\sim$ 10pc-scale HII region with a large emission measure of $\gtrsim 10^7\mathrm{cm}^{-6} \ \mathrm{pc}$, corresponding to the mean electron density of$\gtrsim 10^3~\mathrm{cm}^{-3}$. There is a turnover feature below $\sim$ 10GHz, a signature of the optically-thick free-free emission, often found inGalactic ultra-compact HII regions. These features come from the peculiar YMCformation process, where the cluster's gravity effectively traps photoionizedgas for a long duration and enables continuous star formation within thecluster. Such large and dense HII regions show distinct distribution on thedensity-size diagram, apart from the standard sequence of Galactic HII regions.This is consistent with the observational trend inferred for extragalactic HIIregions associated with YMCs.

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