Unveiling extragalactic star formation using radio recombination lines: An expanded very large array pilot study with NGC 253

Abstract

Radio recombination lines (RRLs) are powerful, extinction-free diagnostics of the ionized gas in young, star-forming regions. Unfortunately, these lines are difficult to detect in external galaxies. We present the results of Expanded Very Large Array (EVLA) observations of the RRL and radio continuum emission at 33 GHz from NGC253, a nearby nuclear starburst galaxy. We detect the previously unobserved H58α and H59α RRLs and make simultaneous sensitive measurements of the continuum. We measure integrated line fluxes of 44.3 0.7 W m-2 and 39.9 0.8 W m-2 for the H58α and H59α lines, respectively. The thermal gas in NGC253 is kinematically complex with multiple velocity components. We constrain the density of the thermal gas to (1.4-4) × 104 cm-3 and estimate an ionizing photon flux of 1 × 1053 s-1. We use the RRL kinematics and the derived ionizing photon flux to show that the nuclear region of NGC253 is not gravitationally bound, which is consistent with the outflow of gas inferred from the X-ray and Hα measurements. The line profiles, fluxes, and kinematics of the H58α and H59α lines agree with those of RRLs at different frequencies confirming the accuracy of the previous, more difficult, high-frequency observations. We find that the EVLA is an order of magnitude more efficient for extragalactic RRL observations than the Very Large Array. These observations demonstrate both the power of the EVLA and the future potential of extragalactic RRL studies with the EVLA. © 2011. The American Astronomical Society. All rights reserved.