Large-scale Map of Millimeter-wavelength Hydrogen Radio Recombination Lines around a Young Massive Star Cluster

TL;DR

This study presents the first large-scale millimeter-wavelength hydrogen recombination line map of a giant H II region around a young massive star cluster, revealing an expanding ionized shell interacting with molecular gas and triggering new star formation.

Contribution

First large-scale mm-RRL map of W43-Main, combining multi-wavelength data to analyze ionized gas dynamics and star formation processes.

Findings

Detected an expanding ionized shell driven by the star cluster.

Pressure broadening dominates near the cluster, dynamical broadening near the shell edge.

Ionized gas motion appears to trigger new star formation at the shell's periphery.

Abstract

We report the first map of large-scale (10 pc in length) emission of millimeter-wavelength hydrogen recombination lines (mm-RRLs) toward the giant H II region around the W43-Main young massive star cluster (YMC). Our mm-RRL data come from the IRAM 30 m telescope and are analyzed together with radio continuum and cm-RRL data from the Karl G. Jansky Very Large Array and HCO$^{+}$ 1-0 line emission data from the IRAM 30 m. The mm-RRLs reveal an expanding wind-blown ionized gas shell with an electron density ~70-1500 cm$^{-3}$ driven by the WR/OB cluster, which produces a total Ly$\alpha$ photon flux of 1.5 x 10$^{50}$ s$^{-1}$. This shell is interacting with the dense neutral molecular gas in the W43-Main dense cloud. Combining the high spectral and angular resolution mm-RRL and cm-RRL cubes, we derive the two-dimensional relative distributions of dynamical and pressure broadening of the ionized gas emission and find that the RRL line shapes are dominated by pressure broadening (4-55 km s$^{-1}$) near the YMC and by dynamical broadening (8-36 km s$^{-1}$) near the shell's edge. Ionized gas clumps hosting ultra-compact H II regions found at the edge of the shell suggest that large-scale ionized gas motion triggers the formation of new star generation near the periphery of the shell.