ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions - IV. Radio recombination lines and evolution of star formation efficiencies

We report the detection of radio recombination line (RRL) H-40 alpha towards 75 sources, with data obtained from ACA (Atacama Compact 7 m Array) observations in the ATOMS (ALMA Three-millimeter Observations of Massive Star-forming regions) survey of 146 active Galactic star-forming regions. We calculated ionized gas mass and star formation rate (SFR) with H-40 alpha, line emission. The mass of ionized gas is significantly smaller than molecular gas mass, indicating that ionized gas is negligible in the star-forming clumps of the ATOMS sample. The SFR estimated with RRL H-40 alpha, agrees well with that calculated with the total bolometric luminosity (L-bol) when SFR >= 5 M-circle dot Myr(-1), suggesting that millimetre RRLs could well sample the upper part of the initial mass function and thus be good tracers of SFR. We also study the relationships between L-bo1 and the molecular line luminosities (L-bo1) of CS J = 2-1 and HC3N J = 11-10 for all the 146 ATOMS sources. The L-bo1-L'(mol) correlations of both the CS J = 2-1 and HC3N J = 11-10 lines appear approximately linear and these transitions have success in predicting Lbol similar to that of more commonly used transitions. The L-bol-to-L'(mol) ratios or SFR-to-mass ratios (star formation efficiency) do not change with galactocentric distances (R-GC). Sources with H-40 alpha emission (or H II regions) show higher L-bol-to-L'(mol )ratios than those without H-40 alpha emission, which may be an evolutionary effect.

Automated summary

This study reported the detection of H-40 alpha radio recombination line (RRL) towards 75 sources and calculated the ionized gas mass and star formation rate (SFR). The results showed that ionized gas is negligible in the star-forming clumps of the ATOMS sample. The SFR estimated with RRL H-40 alpha agrees well with the total bolometric luminosity (L-bol) when SFR ≥ 5 M-circle dot Myr(-1), indicating that millimetre RRLs are good tracers of SFR. Additionally, the correlations between CS J = 2-1, HC3N J = 11-10 molecular line luminosities (L-bo1) and L-bol were found to be approximately linear.