UNVEILING EXTRAGALACTIC STAR FORMATION USING RADIO RECOMBINATION LINES: AN EXPANDED VERY LARGE ARRAY PILOT STUDY WITH NGC 253
- Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325 (United States)
- National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475 (United States)
- National Radio Astronomy Observatory, Pete V. Domenici Science Operations Center, P.O. Box 0, 1003 Lopezville Road, Socorro, NM 87801-0387 (United States)
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 NGC 253, a nearby nuclear starburst galaxy. We detect the previously unobserved H58{alpha} and H59{alpha} RRLs and make simultaneous sensitive measurements of the continuum. We measure integrated line fluxes of 44.3 {+-} 0.7 W m{sup -2} and 39.9 {+-} 0.8 W m{sup -2} for the H58{alpha} and H59{alpha} 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) x 10{sup 4} cm{sup -3} and estimate an ionizing photon flux of 1 x 10{sup 53} s{sup -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 H{alpha} measurements. The line profiles, fluxes, and kinematics of the H58{alpha} and H59{alpha} 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.
- OSTI ID:
- 21565458
- Journal Information:
- Astrophysical Journal Letters, Vol. 739, Issue 1; Other Information: DOI: 10.1088/2041-8205/739/1/L24; ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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