Dense gas in nearby galaxies: XV. Hot ammonia in NGC253, Maffei2 and IC342

The detection of NH3 inversion lines up to the (J,K)=(6,6) level is reported toward the central regions of the nearby galaxies NGC253, Maffei2, and IC342. The observed lines are up to 406K (for (J,K)=(6,6)) and 848K (for the (9,9) transition) above the ground state and reveal a warm (T_kin= 100 - 140 K) molecular component toward all galaxies studied. The tentatively detected (J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC342. Toward NGC253, IC342 and Maffei2 the global beam averaged NH3 abundances are 1-2 10^-8, while the abundance relative to warm H2 is around 10^-7. The temperatures and NH3 abundances are similar to values found for the Galactic central region. C-shocks produced in cloud-cloud collisions can explain kinetic temperatures and chemical abundances. In the central region of M82, however, the NH3 emitting gas component is comparatively cool (~ 30K). It must be dense (to provide sufficient NH3 excitation) and well shielded from dissociating photons and comprises only a small fraction of the molecular gas mass in M82. An important molecular component, which is warm and tenuous and characterized by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions (PDRs) can explain both the high fraction of warm H_2 in M82 and the observed chemical abundances.Comment: 11 pages, 3 Figures, 5 Table

First detection of ammonia in M82

We report the detection of the (J,K) = (1,1), (2,2), and (3,3) inversion lines of ammonia (NH3) towards the south--western molecular lobe in M82. The relative intensities of the ammonia lines are characterized by a rotational temperature of T_rot=29+/-5 K which implies an average kinetic temperature of T_kin~60 K. A Gaussian decomposition of the observed spectra indicates increasing kinetic temperatures towards the nucleus of M82, consistent with recent findings based on CO observations. The observations imply a very low NH3 abundance relative to H2, X(NH3)~5x10^(-10). We present evidence for a decreasing NH3 abundance towards the central active regions in M82 and interpret this abundance gradient in terms of photodissociation of NH3 in PDRs. The low temperature derived here from NH3 also explains the apparent underabundance of complex molecules like CH_3OH and HNCO, which has previously been reported.Comment: 4 pages, 4 figures, accepted by ApJ

Collisional excitation of far-infrared line emissions from warm interstellar carbon monoxide (CO)

Motivated by recent observations with Herschel/PACS, and the availability of new rate coefficients for the collisional excitation of CO (Yang et al. 2010), the excitation of warm astrophysical CO is revisited with the use of numerical and analytic methods. For the case of an isothermal medium, results have been obtained for a wide range of gas temperatures (100 to 5000 K) and H2 densities (1E+3 to 1E+9 cm-3), and presented in the form of rotational diagrams, in which the logarithm of the column density per magnetic substate, log (N[J]/g[J]), is plotted for each state, as a function of its energy, E[J]. For rotational transitions in the wavelength range accessible to Herschel/PACS, such diagrams are nearly linear when n(H2) > 1E+8 cm-3. When log10(n[H2]) = 6.8 to 8, they exhibit significant negative curvature, whereas when log10(n[H2]) < 4.8 the curvature is uniformly positive throughout the PACS-accessible range. Thus, the observation of a positively-curved CO rotational diagram does not NECESSARILY require the presence of multiple temperature components. Indeed, for some sources observed with Herschel/PACS, the CO rotational diagrams show a modest positive curvature that can be explained by a single isothermal component. Typically, the required physical parameters are H2 densities in the 1E+4 to 1E+5 cm-3 range and temperatures, T, close to the maximum at which CO can survive. Other sources exhibit rotational diagrams with more curvature than can be accounted for by a single temperature component. For the case of a medium with a power-law distribution of gas temperatures, with dN/dT proportional to T to the power -b, results have been obtained for H2 densities 1E+3 to 1E+9 cm-3 and power-law indices, b, in the range 1 to 5; such a medium can account for a CO rotational diagram that is more positively curved than any resulting from an isothermal medium.Comment: Accepted for publication in the Astrophysical Journa

Spatially Resolved Chemistry in Nearby Galaxies I. The Center of IC 342

We have imaged emission from the millimeter lines of eight molecules--C2H, C34S, N2H+, CH3OH, HNCO, HNC, HC3N, and SO--in the central half kpc of the nearby spiral galaxy IC 342. The 5" (~50 pc) resolution images were made with OVRO. Using these maps we obtain a picture of the chemistry within the nuclear region on the sizescales of individual GMCs. Bright emission is detected from all but SO. There are marked differences in morphology for the different molecules. A principal component analysis is performed to quantify similarities and differences among the images. This analysis reveals that while all molecules are to zeroth order correlated, that is, they are all found in dense molecular clouds, there are three distinct groups of molecules distinguished by the location of their emission within the nuclear region. N2H+, C18O, HNC and HCN are widespread and bright, good overall tracers of dense molecular gas. C2H and C34S, tracers of PDR chemistry, originate exclusively from the central 50-100 pc region, where radiation fields are high. The third group of molecules, CH3OH and HNCO, correlates well with the expected locations of bar-induced orbital shocks. The good correlation of HNCO with the established shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has been uncertain, is indeed produced by processing of grains. HC3N is observed to correlate tightly with 3mm continuum emission, demonstrating that the young starbursts are the sites of the warmest and densest molecular gas. We compare our HNC images with the HCN images of Downes et al. (1992) to produce the first high resolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity across the nucleus and the correlation of both of these gas tracers with the star formation is excellent. (Abridged).Comment: 54 pages including 10 figures and 8 tables. Accepted for publication in Ap

Hot gas and dust in a protostellar cluster near W3(OH

We used the IRAM Interferometer to obtain sub-arcsecond resolution observations of the high-mass star-forming region W3(OH) and its surroundings at a frequency of 220 GHz. With the improved angular resolution, we distinguish 3 peaks in the thermal dust continuum emission originating from the hot core region about 6 arcsec (0.06 pc) east of W3(OH). The dust emission peaks are coincident with known radio continuum sources, one of which is of non-thermal nature. The latter source is also at the center of expansion of a powerful bipolar outflow observed in water maser emission. We determine the hot core mass to be 15 solar masses based on the integrated dust continuum emission. Simultaneously many molecular lines are detected allowing the analysis of the temperature structure and the distribution of complex organic molecules in the hot core. From HNCO lines, spanning a wide range of excitation, two 200 K temperature peaks are found coincident with dust continuum emission peaks suggesting embedded heating sources within them.Comment: 12 pages, 3 figure

Dense Gas in Nearby Galaxies: XVII. The Distribution of Ammonia in NGC253, Maffei2 and IC342

The central few 100 pc of galaxies often contain large amounts of molecular gas. The chemical and physical properties of these extragalactic star formation regions differ from those in galactic disks, but are poorly constrained. This study aims to develop a better knowledge of the spatial distribution and kinetic temperature of the dense neutral gas associated with the nuclear regions of three prototypical spiral galaxies, NGC253, IC342, and Maffei2. VLA CnD and D configuration measurements have been made of three ammonia (NH3) inversion transitions. The (J,K)=(1,1) and (2,2) transitions of NH3 were imaged toward IC342 and Maffei2. The (3,3) transition was imaged toward NGC253. The entire flux obtained from single-antenna measurements is recovered for all three galaxies observed. Derived lower limits to the kinetic temperatures determined for the giant molecular clouds in the centers of these galaxies are between 25 and 50K. There is good agreement between the distributions of NH3 and other H2 tracers, such as rare CO isotopologues or HCN, suggesting that NH3 is representative of the distribution of dense gas. The "Western Peak" in IC342 is seen in the (6,6) line but not in lower transitions, suggesting maser emission in the (6,6) transition.Comment: 13 pages, 8 figures, latex format, accepted by A&

Tracing shocks and photodissociation in the Galactic center region

We present a systematic study of the HNCO, C18O, 13CS, and C34S emission towards 13 selected molecular clouds in the Galactic center region. The molecular emission in these positions are used as templates of the different physical and chemical processes claimed to be dominant in the circumnuclear molecular gas of galaxies. The relative abundance of HNCO shows a variation of more than a factor of 20 amo ng the observed sources. The HNCO/13CS abundance ratio is highly contrasted (up to a factor of 30) between the shielded molecular clouds mostly affected by shocks, where HNCO is released to gas-phase from grain mantles, and those pervaded by an intense UV radiation field, where HNCO is photo-dissociated and CS production favored via ion reactions. We propose the relative HNCO to CS abundance ratio as a highly contrasted diagnostic tool to distinguish between the influence of shocks and/or the radiation field in the nuclear regions of galaxies and their relation to the evolutionary state of their nuclear star formation bursts.Comment: 25 pages, 5 figures, Accepted for publication in Ap

Kinetic temperatures toward X1/X2 orbit interceptions regions and Giant Molecular Loops in the Galactic center region

Context: It is well known that the kinetic temperatures, Tkin, of the molecular clouds in the Galactic center region are higher than in typical disk clouds. However, the Tkin of the molecular complexes found at higher latitudes towards the giant molecular loops in the central region of the Galaxy is so far unknown. The gas of these high latitude molecular clouds (hereafter referred to as halo clouds) is located in a region where the gas in the disk may interact with the gas in the halo in the Galactic center region. Aims: To derive Tkin in the molecular clouds at high latitude and understand the physical process responsible for the heating of the molecular gas both in the Central Molecular Zone (the concentration of molecular gas in the inner 500 pc) and in the giant molecular loops. Methods: We measured the metastable inversion transitions of NH3 from (1,1) to (6,6) toward six positions selected throughout the Galactic central disk and halo. We used rotational diagrams and large velocity gradient modeling to estimate the kinetic temperatures toward all the sources. We also observed other molecules like SiO, HNCO, CS, C34S, C18O, and 13CO, to derive the densities and to trace different physical processes (shocks, photodissociation, dense gas) expected to dominate the heating of the molecular gas. Results: We derive for the first time Tkin of the high latitude clouds interacting with the disk in the Galactic center region. We find high rotational temperatures in all the observed positions. We derive two kinetic temperature components (150 K and 40 K) for the positions in the Central Molecular Zone, and only the warm kinetic temperature component for the clouds toward the giant molecular loops. The fractional abundances derived from the different molecules suggest that shocks provide the main heating mechanism throughout the Galactic center, also at high latitudesComment: accepted for publication in A&A 06/09/201