Densitometry and Thermometry of Starburst Galaxies

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
² Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0389, USA
³ Max-Planck-Institüt für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Grningen, The Netherlands
⁶ Leiden Observatory, Leiden University, 2300 RA, Leiden, The Netherlands
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

e-mail: jmangum@nrao.edu

Abstract

With a goal toward deriving the physical conditions in external galaxies, we have conducted a survey and subsequent high spatial resolution imaging of formaldehyde (H₂CO) and ammonia (NH₃) emission and absorption in a sample of starburst galaxies. In this article we present the results from a subset of this survey which focuses on high spatial resolution measurements of volume density- and kinetic temperature-sensitive transitions of the H₂CO molecule. The volume density structure toward the nuclear region of NGC 253 has been derived from θ ≃ 4 arcsec NRAO Very Large Array (VLA) measurements of the 1₁₀ − 1₁₁ and 2₁₁ − 2₁₂ K-doublet transitions of H₂CO. The kinetic temperature structure toward NGC 253 and NGC 4945 has been derived from θ ≃ 0.5 − 1.0 arcsec measurements of the H₂CO 3_K−1K+1 − 2_K−1K+1 (near 218 GHz) and 5_K−1K+1 − 4_K−1K+1 (near 365 GHz) transitions acquired using the Atacama Large Millimeter/submillimeter Array (ALMA). These measurements have allowed us to characterize the dense gas and kinetic temperature structure within these star forming galaxies, which is a first step toward associating dense star-forming gas and the heating processes at work within galaxies.