HST/WFPC2 F606W

1. 27K The Temperature Distribution of Dense Gas in Starburst Cores 30K Jürgen Ott CSIRO Australia Telescope National Facility 24K Axel Weiss IRAM, Spain Christian Henkel MPIfR, Germany Fabian Walter MPIA, Germany 24K 28K contact: Juergen.Ott@csiro.au NGC 253 26K 34K HST/WFPC 814W Figure 1:Left: A WFPC2 image of NGC253. The inner core is blown-up in the central image. Contours of the ammonia (3,3) emission (super-resolution) are overlaid in red and the 12mm continuum in green. Six clumps can be identified. In the right panel we show a temperature map determined by the NH3 (1,1) and (2,2) lines. Note the temperature difference between the eastern and the western parts (starburst center marked by the white contours of the 12mm continuum). A temperature gradient is also visible in the western clump along the north-south direction with the largest temperatures toward the south. HST/WFPC2 F606W Position-velocity (maj. axis) HST/WFPC2 F606W HST/WFPC2 F606W 44K 32K HST/WFPC2 F814W 113K ? Position- velocity (maj. axis) 99K ? 36K HST/WFPC2 F814W According to Solomon et al. (1992) and Gao & Solomon (2004) dense gas tracers (volume densities: > 104 cm-3) are excellent measures of star formation activity – much better than tracers of less dense gas like CO. This fact can be easily understood as that the dense gas contains the fuel for the actual star formation process. One of the dense gas tracers is the most abundant polyatomic molecule in space: ammonia (NH3). Its particular tetrahedral structure allows the tunneling of the nitrogen atom through the plane defined by the three hydrogen atoms (see picture below). The resulting inversion lines are easily measured at 1cm in different rotational states which are only excited at volume densities >104 cm-3. Furthermore, the inversion lines define an excellent and easy-to-use thermometer of the dense gas phase. We present a survey of the para-NH3 (1,1) and (2,2) inversion lines of nearby southern starburst galaxies observed with the Australia Telescope Compact Array (ATCA). In some cases we also observed the ortho-NH3 (3,3) and (6,6) lines. The observations are the first interferometric maps to determine the rotational temperatures. The starburst sample is presented in Table 1 and our temperature determinations are listed in Table 2. In the figures, we show the ammonia maps and spectra for NGC 253 (Figs. 1 & 2), NGC 1365 (Fig. 3), Arp 220 (Fig. 4), NGC 4945 (Fig. 5), M 83 (Fig. 6), and Cen A (Fig. 7). In general, the emission components exhibit rotational temperatures of ~30K which correspond to kinetic temperatures of ~50K. Absorption components may be up to Trot~100K. The dynamics of the dense gas cannot be entirely described by simplistic models such as rotating molecular rings. We only detect ammonia in the starburst centers of the galaxies despite of the rather large primary beam of the ATCA of ~2’ (synthesized beams: ~5-10”). The largest concentrations of NH3 are found where dust components dim most of the optical and near-infrared light, in particular at the vortices of prominent dusty absorption lanes which may be the endpoints of spiral or bar-like features. Table 1: The sample of southern starburst galaxies for which we searched for ammonia with the ATCA. CenA is the most nearby massive radio galaxy. The star formation rates of the galaxies were determined via their overall far-infrared luminosities. For most of the galaxies AGN activity of different strengths can be observed. We detected ammonia in all objects but Circinus and NGC 1808. The last column lists whether the NH3 is detected in emission or absorption. Note that the 1cm radio continuum emission, which was observed simul-taneously with the ATCA, is also an excellent measure of star formation rates. References: Gao, Y. & Solomon, P.M., 2004, ApJS, 152, 63 Solomon, P.M., Downes, D., & Redford, S.J.E, 1992, ApJ, 387, 55 position-velocity cut (maj. axis) abundance map Table 2: The rotational temperatures determined for the individual starburst galaxies in emission (column 2) and absorption (column 3) components. The ammonia (1,1) and (2,2) emission can be fitted by more or less uniform rotational temperatures of ~30K which correspond to kinetic temperatures of ~50K of the dense molecular gas. The rotational temperatures of the absorption components, however, can be up to ~100K. The 4th column shows the ammonia column densities and the last column the ammonia masses. 2MASS J-K NGC 1365 Figure 2:Left two panels: Along the major axis of NGC 253 (see also Fig. 1) we determined the position-velocity diagram shown in the second panel. Both, the systemic velocity and the position of the starburst center of NGC 253 are marked as dashed lines. The dynamics of the dense gas is quite complicated and an solid-body rotating ring may only be inferred for the inner two clumps. Right two panels: The ammonia abundance of NGC 253. White colors mark a rather low abundance relative to molecular hydrogen while red colors depict a high abundance varying from 3 to 8x10-8. While the ammonia abundance toward the east is relatively uniform, an abundance gradient is visible stretching from the west toward the center. The abundances are also marked as contours in both images. The very right image is a 2MASS J-K color image and the positions of very high absorption are displayed in yellow. The NH3 abundance is lowest where the strongest absorption features are visible. This may be an effect of the NH3 freezing out on dust grains. Figure 3: Ammonia (3,3) emission overlaid as red contours on a HST/WFPC2 image of NGC 1365. The green contours mark the 1cm continuum emission which is extended in NGC 1365. We determined the rotational NH3 (1,1)-(2,2) temperatures at 4 positions (5 components in velocity space; (1,1) spectra: solid lines, (2,2) spectra: dotted lines) to 24-30K. At the lower left a position-velocity cut along the major axis is shown with dashed lines of the systemic velocity and the starburst center on top. Note that this diagram is consistent with a coherent ring of dense molecular gas feeding the starburst. The morphology, however, is asymmetric and resembles more like the endpoints of spiral or bar-like features coincident with dust lanes. NGC 4945 Figure 5: NH3 (1,1) emission of the bright, nearby starburst galaxy NGC 4945 overlaid as red contours on a HST/WFPC2 image. Green contours again mark the 1cm continuum emission of the starburst center. In addition to two ammonia emission components, we were able to identify two absorption components (spectra: NH3(1,1): solid lines, (2,2): dotted lines). The rotational temperatures are marked in the right hand spectra and are 30-45K with the exception of one of the absorption components for which Trot~113K. Like in the case of Cen A (Fig. 7 below) we produced a bin by bin temperature spectrum of the NH3 absorption which we show at the bottom. At the bottom left a position-velocity cut along the major axis is displayed. The dashed lines mark the location of the starburst center and the systemic velocity of NGC 4945. The non—systemic velocity absorption component (absorption: black, emission: yellow) indicates non-circular motions of the dense gas. Arp 220 Figure 4: 12mm continuum emission of Arp 220 overlaid as green contours on a WFPC2 image. We detect no emission but two absorption components in the (2,2) inversion line of ammonia (dotted lines in the spectrum). However, we only find the corresponding (1,1) absorption for the high velocity component (solid line). The rotational temperature of this component is 99K. The NH3 velocities can all be attributed to the western nucleus of Arp 220. Cen A M 83 Figure 7: Blow-up of a WFPC2 image of Cen A overlaid with green contours of the 1cm continuum emission. Both, the NH3 (1,1) (solid line in the spectrum) and (2,2) (dotted line) inversion transitions are clearly detected in absorption. However, the velocity structure of the components are surprisingly different. The main (1,1) component at low velocities correspond to a relatively weak (2,2) feature. The situation is the opposite at velocities of ~550 km s-1. Gaussian fits did not converge and we computed a rotational temperature per velocity bin instead which we show left to the spectrum. A tentative temperature gradient is visible with Trot~25-30K at v<450 km s-1 and rising to ~80K at v~500 km s-1. At higher velocities, rotational temperatures are uncertain on the basis of the NH3 (1,1) and (2,2) inversion lines. Figure 6: The core of the nearby face-on starburst galaxy M83. A blow-up of a WFPC2 images is shown with 1cm continuum as green contours and ammonia (1,1) emission as red contours. The continuum emission marks the starburst center. Two clumps of dense gas are detected at positions where optical dust absorption features are connected. A rotational temperature of 24K was determined for the northern component [(1,1) and (2,2) spectrum: solid and dotted lines, respectively].