Hydrogen Millimeter Recombination Lines as a Tracer of Extragalactic Star Formation Rates

When studying the structure and evolution of distant galaxies, the total star formation rate (SFR) of a given galaxy is an important observable. It is related to the gas, dust, and stellar mass content of a galaxy, and is strongly related to the ultraviolet (UV) radiation field, which affects the physical and chemical state of the star-forming interstellar gas. The SFR can be inferred from several observational tracers, such as UV emission (from massive stars), far-infrared (FIR) emission (from dust heated by massive stars), radio synchrotron emission (from relativistic electrons accelerated in supernovae), or hydrogen recombination line emission (in ionized gas around massive stars, the so-called HII regions). All have their advantages and disadvantages. A promising SFR tracer that has not yet been extensively studied for distant galaxies is the millimeter-wavelength hydrogen recombination line (mm-RL) emission. In this work, I investigate how the physical conditions in HII regions affect the amplification and attenuation of mm-RL emission. We harvest the Atacama Large Millimeter-submillimeter Array (ALMA) data archive and use our own IRAM 30m telescope observations to assemble mm-RL detections towards nearby (within a few hundred Mpc) star-forming galaxies. Our ALMA archive sample consists of the 20 brightest infrared star-forming galaxies, while the IRAM 30m observations target six nearby star-forming galaxies with prior cm-wavelength RL detections. We detect mm RL emission from 9 galaxies in the ALMA sample, while no lines are detected in the IRAM 30m sample. We compare the implied SFR of these galaxies when derived from the mm-RL emission and from their FIR emission, and find a linear correlation that is consistent with measurements of individual massive star-forming regions in our Milky Way galaxy, stretching over eight orders of magnitude. We find that several sources show mm-RL emission suffering from maser amplification, and that maser amplification cannot be ruled out even for galactic sources. We evaluate the quality of mm-RL emission as a reliable SFR tracer and suggest that mm-RL emission, in combination with FIR emission, can be used to trace the recent star formation history.