Densitometry and Thermometry of Starburst Galaxies

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA ;
e-mail: jmangum@nrao.edu
² Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, Box 389, University of Colorado, Boulder, CO 80309-0389, USA ;
e-mail: jdarling@origins.colorado.edu
³ Max-Planck-Institüt für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany ;
e-mail: kmenten@mpifr-bonn.mpg.de; chenkel@mpifr-bonn.mpg.de
⁴ Harvard University, Cambridge, MA 02138, USA ;
e-mail: mmacgreg@fas.harvard.edu

Abstract

With a goal toward deriving the physical conditions in external galaxies, we present a survey of formaldehyde (H₂CO) and ammonia (NH₃) emission and absorption in a sample of starburst galaxies using the Green Bank Telescope. By extending well-established techniques used to derive the spatial density in star formation regions in our own Galaxy, we show how the relative intensity of the 1₁₀−1₁₁ and 2₁₁−2₁₂ K-doublet transitions of H₂CO can provide an accurate densitometer for the active star formation environments found in starburst galaxies (cf. Mangum et al. 2008). Similarly, we employ the well-established technique of using the relative intensities of the (1,1), (2,2), and (4,4) transitions of NH₃ to derive the kinetic temperature in starburst galaxies. Our measurements of the kinetic temperature constrained spatial density in our starburst galaxy sample represent the first mean density measurements made toward starburst galaxies. We note a disparity between kinetic temperature measurements derived assuming direct coupling to dust and those derived from our NH₃ measurements which points to the absolute need for direct gas kinetic temperature measurements using an appropriate molecular probe. Finally, our spatial density measurements point to a rough constancy to the spatial density (10^4.5 to 10^5.5 cm^-3) in our starburst galaxy sample. This implies that the Schmidt-Kennicutt relation between L_IR and M_dense: (1) Is a measure of the dense gas mass reservoir available to form stars, and (2) Is not directly dependent upon a higher average density driving the star formation process in the most luminous starburst galaxies.