FORMALDEHYDE DENSITOMETRY of STARBURST GALAXIES: DENSITY-INDEPENDENT GLOBAL STAR FORMATION Jeffrey G

Draft version November 8, 2018 Preprint typeset using LATEX style emulateapj v. 5/2/11 FORMALDEHYDE DENSITOMETRY OF STARBURST GALAXIES: DENSITY-INDEPENDENT GLOBAL STAR FORMATION Jeffrey G. Mangum National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA Jeremy Darling Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, Box 389, University of Colorado, Boulder, CO 80309-0389, USA Christian Henkel1 Max Planck InstitütfürRadioastronomie, Auf dem Hügel69, 53121 Bonn, Germany and Karl M. Menten Max Planck InstitütfürRadioastronomie, Auf dem Hügel69, 53121 Bonn, Germany Draft version November 8, 2018 ABSTRACT Accurate techniques that allow for the derivation of the spatial density in star formation regions are rare. A technique that has found application for the derivation of spatial densities in Galactic star formation regions utilizes the density-sensitive properties of the K-doublet transitions of formaldehyde (H2CO). In this paper, we present an extension of our survey of the formaldehyde 110−111 (λ = 6:2 cm) and 211 − 212 (λ = 2:1 cm) K-doublet transitions of H2CO in a sample of 56 starburst systems (Mangum et al. 2008). We have extended the number of galaxies in which both transitions have been detected from 5 to 13. We have improved our spatial density measurements by incorporating kinetic temperatures based upon NH3 measurements of 11 of the galaxies with a total of 14 velocity components in our sample (Mangum et al. 2013). Our spatial density measurements lie in a relatively narrow range from 104:5 to 105:5 cm−3. This implies that the Schmidt-Kennicutt relation between LIR and Mdense: (1) is an indication 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. We have also used our H2CO measurements to derive two separate measures of the dense gas mass which are generally smaller, in many cases by a factor of 102 − 103, than those derived using HCN. This disparity suggests that H2CO traces a denser, more compact, component of the giant molecular clouds in our starburst galaxy sample. 2 We also report measurements of the rotationally-excited λ = 6:3 cm Π1=2J = 1=2 state of OH and the H111α radio recombination line taken concurrently with our H2CO 110 − 111 measurements. Subject headings: ISM: molecules, galaxies: ISM, galaxies: nuclei, galaxies: starbursts, radio lines: galaxies 1. INTRODUCTION mainly nearby galaxies have been studied; see the re- Studies of the abundant CO molecule toward external views of Henkel, Baan, & Mauersberger (1991, for early galaxies have revealed the large scale structure of the work) and Omont (2007). molecular mass in these objects. Unfortunately, high op- Formaldehyde (H2CO) has proven to be a reliable den- arXiv:1302.3526v2 [astro-ph.CO] 5 Mar 2013 tical depth tends to limit the utility of the abundant CO sity and kinetic temperature probe in Galactic molecu- molecule as a probe of the spatial density and kinetic lar clouds (Mangum & Wootten 1993; Mangum et al. temperature in dense gas environments, quantities nec- 1993; Ginsburg et al. 2011). Existing measurements of essary to assess the possibility and conditions of star for- the H2CO 110 − 111 and 211 − 212 K-doublet transitions mation (Liszt et al. 2010). Emission from less-abundant, in a wide variety of galaxies by Baan et al. (1986, 1990, higher-dipole moment molecules is better-suited to the 1993), and Araya et al. (2004) have mainly concentrated task of deriving the spatial density and kinetic tem- on measurements of the 110−111 transition. By collecting perature within dense gas environments. A variety of a consistent set of measurements of both K-doublet tran- molecules which trace dense molecular environments in sitions we are able to derive the spatial density within the extragalactic star formation regions which comprise this study. Using the unique density selectivity of the K- [email protected] doublet transitions of H CO we have measured the spa- [email protected] 2 [email protected] tial density in a sample of galaxies exhibiting starburst [email protected] phenomena and/or high infrared luminosity (Mangum 1 Also Astronomy Department, Faculty of Science, King Ab- et al. 2008). In the present work we have nearly tripled dulaziz University, P. O. Box 80203, Jeddah, Saudi Arabia 2 Mangum et al. the number of galaxies within which the spatial density Bank Telescope (GBT) during the periods 2006/06/07- can be derived from five to thirteen. In eleven of these 14 (reported in Mangum et al. 2008), 2007/01/16-04/22, galaxies with a total of fourteen velocity components we 2007/11/05-11/26, 2008/04/14-09/22, and 2011/05/04- have further improved the quality of our spatial density 07/16. Using exactly the same observing setup as was determination by applying kinetic temperatures derived used for the observations presented in Mangum et al. from NH3 measurements (Mangum et al. 2013). In x2 (2008), single-pointing measurements were obtained of we discuss the specific properties of the H2CO molecule the 110 − 111 (4.829660 GHz; Tucker et al. 1971) and that make it a good probe of spatial density. x3 presents a 211 − 212 (14.488479 GHz; Tucker et al. 1972) K-doublet summary of our observations; x4 our H2CO, OH, H111α, transitions of H2CO, the H111α radio recombination and continuum emission measurement results; x5 analy- line (RRL) at 4.744183 GHz, and two (F = 1 − 0 ses of our H2CO, OH, and H111α measurements, includ- and 1 − 1) of the three 6 cm lines of the rotationally- 2 ing Large Velocity Gradient (LVG) model fits and dense excited Π1=2J = 1=2 state of OH at 4.750656(3) and gas mass calculations based on our H2CO measurements. 4.765562(3) GHz (Radford 1968; Lovas 1992) toward a sample of 56 infrared-luminous and/or starburst galax- 2. FORMALDEHYDE AS A HIGH DENSITY PROBE ies (Table 1). Our galaxy sample was chosen to represent The ubiquitous and readily-observed formaldehyde both galaxies with measured H2CO emission or absorp- (H2CO) molecule is a reliable probe of the high den- tion (Baan et al. 1993; Araya et al. 2004) and other galax- sity environs of star formation regions. Pervasive in the ies with substantial molecular emission deduced from interstellar medium, H2CO exhibits a moderate abun- HCN measurements (Gao & Solomon 2004a). The single- dance which appears to vary by less than an order- 2 beam H2CO 110 − 111, H111α, and OH Π1=2J = 1=2 of-magnitude within star formation regions in our own transitions were measured simultaneously using 4 spec- Galaxy (Mangum et al. 1990). As a slightly asymmet- tral windows each with 50 MHz of bandwidth sampled ric rotor molecule, most H2CO rotational transitions are by 16384 channels. All of the 4.8 GHz measurements split into two sub levels. These energy levels are desig- 00 (θB = 153 ) utilized the position switching technique nated by a total angular momentum quantum number, with reference position located 30 arcmin west in az- J, the projection of J along the symmetry axis for a lim- imuth from each galaxy position. Galaxy and reference iting prolate symmetric top, K , and the projection of J −1 integrations were 3 minutes each at 4.8 GHz. The H2CO along the symmetry axis for a limiting oblate symmetric 211 − 212 transition was measured using a dual-beam top, K . This splitting leads to two types of transi- 00 00 +1 (θB = 51 ; beam separation 330 in cross-elevation) tions: the high-frequency ∆J = 1, ∆K−1 = 0, ∆K+1 receiver over 50 MHz of bandwidth sampled by 16384 = −1 \P-branch" transitions and the lower-frequency channels. The dual-beam system at 14.5 GHz allowed for ∆J = 0, ∆K−1 = 0, ∆K+1 = ±1 \Q-branch" transi- both position switching or beam nodding. Galaxy and tions, popularly known as the \K-doublet" transitions reference integrations were 2 minutes each at 14.5 GHz. (see discussion in Mangum & Wootten 1993; McCauley The correlator configurations produced a spectral chan- et al. 2011). The P-branch transitions are only seen in nel width of 3.052 kHz, which is approximately 0.2 and 4 −3 emission in regions where n(H2) & 10 cm . In the K- 0.08 km s−1 at 4.8 and 14.5 GHz, respectively. 5:5 −3 doublet transitions, for n(H2) . 10 cm , the lower For eighteen of the galaxies listed in Table 1 mea- energy states of the 110 − 111 through 514 − 515 transi- surements of one or both K-doublet H2CO transitions tions become overpopulated due to a collisional selection were presented in Mangum et al. (2008). Additional ob- effect (Evans et al. 1975; Garrison et al. 1975). This servations and/or updated analysis (i.e. modified spec- overpopulation cools the J≤ 5 K-doublets to excitation tral smoothing) of the K-doublet H2CO measurements temperatures lower than that of the cosmic microwave of these eighteen galaxies are included in the results pre- background, causing them to appear in absorption. For sented here. 5:5 −3 n(H2) & 10 cm and an assumed kinetic temperature To calibrate the intensity scale of our measurements, of 40 K, this collisional pump is quenched and the J ≤ several corrections need to be considered: 5 K-doublets are driven into emission over a wide range of kinetic temperatures and abundances (see Figure 1 in Opacity:: Historical opacity estimates based on atmo- Mangum et al. 2008). spheric model calculations using ambient pressure, Measurements of the spatial density in galactic and temperature, and relative humidity measurements extragalactic star formation regions often rely upon only indicated that τ at 4.8 and 14.5 GHz was ∼ 0.01 order-of-magnitude or critical density estimates.

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