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Поисковые слова: arp 220
Temperature distribution of dense molecular gas

The temperature distribution of dense molecular gas in starburst cores

Dense molecular gas is the fuel for star formation in galaxies. Its physical state is described by two parameters, temperature and density. The new 12- and 3-mm receivers at the Compact Array cover the frequencies of a large number of molecular lines and are therefore ideal to determine the physical gas conditions via multi-transition observations. In this article we present the first Compact Array results on temperature measurements of dense gas in nearby starburst galaxies using the lowest ammonia inversion lines.

Star formation (SF) is not uniform over the evolution of galaxies. Times of low-level star formation are interrupted by shorter episodes of very high SF rates (SFRs). Those starburst events are witnessed in starburst galaxies. Indeed, starburst galaxies are the most prominent contributors for SF in the local and distant universe. Their study may therefore provide important clues for a better understanding of the SF history of the universe. High mass stars are formed abundantly in starburst phases and their short lifetimes imply feedback of energy and metals to the interstellar and even to the intergalactic medium in the form of strong stellar winds and supernovae explosions. In turn, metals are important for the cooling and heating mechanisms of the interstellar/intergalactic gas which regulate the formation of subsequent stellar populations.

Recently, Gao & Solomon (2004) compared the SFRs of nearby starburst galaxies to the integrated luminosities of the CO and the HCN molecules. They concluded that HCN, a tracer for dense molecular gas (densities > 104 cm-3) is very tightly correlated with the SFRs determined by the far infrared luminosities. The correlation is weaker when CO luminosities, which trace less dense gas, are used instead of HCN. This important result prompted us to investigate the properties of dense molecular gas in more detail and to determine its physical state in a sample of nearby, southern starburst galaxies. The brightest tracers of high density regions are transitions of molecules like HCO+ or CS or those involved in the cyanide chemistry of molecular clouds: HCN, HNC, HCO+, and ammonia (NH3).

Ammonia is an excellent and easy-to-use thermometer of the dense molecular gas. This is due to its specific tetrahedral structure where the nitrogen atom tunnels through the plane defined by the three hydrogen atoms. This effect can be observed in the 12-mm band as inversion lines emitted by metastable rotational levels. The weighted line strengths follow a Boltzmann distribution from which a rotational temperature can be determined (e.g. Ungerechts, Walmsley & Winnewisser 1986; Henkel et al. 2000).

We detected ammonia with the Compact Array in the nearby starburst galaxies NGC 253, M 83, NGC 4945, NGC 1365, and the prototypical ultraluminous infrared merger, Arp 220. In addition, ammonia was also successfully observed in the most nearby, massive elliptical radio galaxy, Centaurus A. In the ATNF annual report 2003 we showed the first ammonia temperature maps of NGC 253. Here we present results for the other galaxies in the sample.

Figure 1: An HST/WFPC2 F606W optical image of the core of NGC 1365. Prominent dust absorption features are visible by their lighter colours. Overlaid as contours are the 12-mm continuum emission (light grey) as well as the ammonia (3, 3) emission (black). Spectra of ammonia (1, 1) (solid lines) and (2, 2) (dotted lines) are shown for three different positions. Gaussian fits are displayed for all detections. Rotational temperatures of the dense gas were computed from the weighted (1, 1) to (2, 2) line ratios.

In Figure 1, both the 12-mm continuum emission as well as the ammonia (3, 3) inversion line is overlaid on an optical HST image of the barred spiral galaxy NGC 1365 (distance ~18 Mpc, Compact Array resolution 5 тАУ 13 arcsec). The 12-mm continuum emission is mainly caused by thermal free-free radiation and is a good indicator for the location and the rate of active SF. In NGC 1365, SF is largest in the central region which is surrounded by prominent dust lanes seen as absorption features in optical images. We were able to determine the rotational ammonia temperatures of the dense molecular gas in four distinct molecular complexes to 24 тАУ 27 K [using NH3 (1, 1) and (2, 2)]. This corresponds to kinetic temperatures of ~50 K. The ammonia column densities are of order 1013 cm-2 which adds up to a NH3 mass of 35 Msun.

Figure 2: The ammonia content of M83. The image/contours are the same as in Figure 1 with the exception that the HST image was observed in the F814W filter and that we display NH3 (1, 1) instead of (3, 3). Most of the dense gas is stored where optical dust features connect. Note that current star-formation (traced by 12-mm continuum emission) is mainly observed at the dense gas-stellar population interface.

With a mere 1.5 Msun, the lowest amount of NH3 is observed in M 83 (Figure 2; distance = 4.5 Mpc). This face-on starburst galaxy exhibits its current SF mainly at the interface between a molecular ring and its stellar interior. The main NH3 components are observed where features of optical dust absorption connect. Such vertices appear to form the prime reservoirs of dense gas in all the galaxies of our survey. Those regions may therefore be the end-points of gas streaming along the bar toward the nucleus before the material collapses into stars. Due to limitations in signal-to-noise, a temperature determination is possible only toward the north of M 83. At this location we again derive a rotational temperature of ~25 K, which corresponds to a kinetic temperature of ~50 K.

Figure 3: Ammonia (1, 1) emission within the core of NGC 4945 (see Figure 2 for details). One absorption component is observed offset to the systemic velocity of this galaxy. This is an indication for non-circular motions of the dense molecular gas.

Ammonia in emission as well as in absorption is detected in NGC 4945 (Figure 3; distance ~3.6 Mpc). The components are asymmetric with respect to the nuclear starburst core of this galaxy. While one emission and an absorption component coincide with the starburst centre, the emission extends further toward the south-west. The second absorption component is observed at a velocity offset to the systemic velocity of NGC 4945. This indicates the presence of non-circular motions of the dense gas. The temperatures of the emission and absorption components are similar to those in NGC 1365.


Figure 4: The ammonia (1, 1) (solid) and (2, 2) (dotted) spectra of the closest, most massive elliptical radio galaxy, Centaurus A as well as for the most nearby, ultraluminous far-infrared galaxy, Arp 220. Two velocity components are detected in both galaxies (for Arp 220 the dotted Gaussian fits are displayed for the individual components as well as for their sum).

Finally, we present the ammonia spectra of Centaurus A and Arp 220 in Figure 4. Both objects are only observed in absorption. Centaurus A (3.7 Mpc) is the most nearby, massive elliptical radio galaxy and the temperature distribution is very ambiguous due to the very different line shapes of the NH3 (1, 1) and (2, 2) components. Our scheduled observations of higher ammonia transitions will certainly help in the interpretation of this unexpected result. Arp 220 (distance = 78 Mpc) is the most nearby, ultraluminous infrared galaxy. In fact it is a merger of two galaxies and is commonly viewed as the archetype of a major merger in the hierarchical galaxy formation scenarios at larger look-back times. The (FIR) SFR of Arp 220 is ~30 Msun yr-1. We observe two velocity-components in the NH3 (2, 2) absorption spectrum and a single one in the NH3 (1, 1) spectrum. The ammonia absorption can be attributed to the western nucleus of Arp 220. For the high-velocity component, we derive a very large rotational temperature of ~100 K. Due to the very broad NH3 lines which partially overlap, however, this value is relatively uncertain.

In summary, we conclude that ammonia is a powerful diagnostic of temperatures of the dense molecular gas in starburst galaxies. We derive relatively uniform NH3 rotational temperatures of ~25 тАУ 30K [using para-NH3 (1, 1) and (2, 2)], which correspond to kinetic temperatures of ~50K. In a continuing observing campaign, these data will be combined with HCN and HNC observations. High resolution Compact Array observations of all three species will eventually reveal the density of the molecular cores and hence complement the parameters of the physical state of this important gas phase in protostellar evolution.

References

Gao, Yu & Solomon P.M., 2004, ApJ, 606, 271
Henkel, C. et al., 2000, A&A, 361, 45
Ungerechts, H., Walmsley, C. M. & Winnewisser, G., 1986, A&A, 157, 207

Juergen Ott (CSIRO ATNF), Axel Weiss (IRAM, Granada, Spain), Christian Henkel (MPIfR, Bonn, Germany) and Fabian Walter (MPIA, Heidelberg, Germany)
(Juergen.Ott@csiro.au)

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