Unveiling Extragalactic Star Formation Using Radio Recombination Lines: An EVLA 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 EVLA observations of the RRL and radio continuum emission at 33 GHz from NGC 253, a nearby nuclear starburst galaxy. We detect the previously unobserved H58a and H59a RRLs and make simultaneous sensitive measurements of the continuum. We measure integrated line fluxes of $44.3\pm 0.7$ W m${}^{-2}$ and $39.9\pm 0.8$ W m${}^{-2}$ for the H58a and H59a lines, respectively. The thermal gas in NGC 253 is kinematically complex with multiple velocity components. We constrain the density of the thermal gas to $1.4-4\times 10^4$ cm${}^{-3}$ and estimate an ionizing photon flux of $1\times 10^{53}$ s${}^{-1}$. We use the RRL kinematics and the derived ionizing photon flux to show that the nuclear region of NGC 253 is not gravitationally bound, which is consistent with the outflow of gas inferred from the X-ray and Halpha measurements. The line profiles, fluxes, and kinematics of the H58a and H59a 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 VLA. These observations demonstrate both the power of the EVLA and the future potential of extragalactic RRL studies with the EVLA.

Accepted for publication in the ApJL EVLA Special Issue. Version with high resolution figures at http://www.astro.virginia.edu/~aak8t/data/rrl/ms.ps

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• Theory of Star Formation
• The ACS Nearby Galaxy Survey Treasury
• The Expanded Very Large Array -- a New Telescope for New Science
• The nucleus of NGC 253 and its massive stellar clusters at parsec scales
• VLA H53alpha and H92alpha line observations of the central region of NGC 253
• On the Ionization of Luminous WMAP Sources in the Galaxy : Constraints from He Recombination Line Observations with the GBT
• Detection of the H92alpha recombination line from the starbursts in the Circinus galaxy and NGC 1808
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Cited by in corpus (13)

• ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253
• ALCHEMI: an ALMA Comprehensive High-resolution Extragalactic Molecular Inventory. Survey presentation and first results from the ACA array
• ALMA observations of 99 GHz free-free and H40$\alpha$ line emission from star formation in the centre of NGC 253
• Survey of Water and Ammonia in Nearby galaxies (SWAN): Resolved Ammonia Thermometry, Water and Methanol Masers in the Nuclear Starburst of NGC 253
• Interpreting the Low Frequency Radio Spectra of Starburst Galaxies: A Pudding of Strömgren Spheres
• Spectral energy distribution and radio halo of NGC 253 at low radio frequencies
• A physically motivated `charge-exchange method' for measuring electron temperatures within HII regions
• Hydrogen Radio Recombination Line Emission from M51 and NGC628
• Unveiling Kinematic Structure in the Starburst Heart of NGC 253
• Radio recombination lines from obscured quasars with the SKA
• JVLA Observations of IC342: Probing Star Formation in the Nucleus
• Low-Frequency Radio Recombination Lines Away From the Inner Galactic Plane
• A Spectral Analysis of the Centimeter Regime of Nearby Galaxies: RRLs, Excited OH, and NH${}_3$