CS in nearby galaxies: Distribution, kinematics, and multilevel studies

As a result of observations at the Institute for Radio Astronomy in the Millimeter Range (IRAM) 30-m telescope, maps of the distribution of the J = 2-1 transition of CS toward the galaxies IC 342 and NGC 253 are presented. The distribution of the CS emission from NGC 253 is consistent with the CO 1-0 line. The distribution of the CS emission from IC 342, however, resembles more that seen in the CO 3-2 line. For the first time, the detection of the isotopic substitution C-34S is reported toward an external galaxy: The C-34S 2-1 line has been detected toward NGC 253 and M 82 and the C-34S line has been detected tentatively toward M 82. Also for the first time, extragalactic CS has been observed in the 3-2 (toward NGC 253, IC 342 and M 82) and 5-4 (NGC 253 and IC 342) transitions

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

Methanol detection in M82

We present a multilevel study of the emission of methanol, detected for the first time in this galaxy, and discuss the origin of its emission. The high observed methanol abundance of a few 10^-9 can only be explained if injection of methanol from dust grains is taken into account. While the overall [CH3OH]/[NH3] ratio is much larger than observed towards other starbursts, the dense high excitation component shows a similar value to that found in NGC 253 and Maffei 2. Our observations suggest the molecular material in M 82 to be formed by dense warm cores, shielded from the UV radiation and similar to the molecular clouds in other starbursts, surrounded by a less dense photodissociated halo. The dense warm cores are likely the location of recent and future star formation within M 82.Comment: Accepted for publication in A&A Letter

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

Ammonia in the hot core W51-IRS2: 12 new maser lines and a maser component with a velocity drift

With the 100-m telescope at Effelsberg, 19 ammonia (NH3) maser lines have been detected toward the prominent massive star forming region W51-IRS2. Eleven of these inversion lines, the (J,K) = (6,2), (5,3), (7,4), (8,5), (7,6), (7,7), (9,7), (10,7), (9,9), (10,9), and (12,12) transitions, are classified as masers for the first time in outer space. All detected masers are related to highly excited inversion doublets. The (5,4) maser originates from an inversion doublet 340 K above the ground state, while the (12,12) transition, at 1450 K, is the most highly excited NH3 maser line so far known. Strong variability is seen not only in ortho- but also in para-NH3 transitions. Bright narrow emission features are observed, for the first time, in (mostly) ortho-ammonia transitions, at V ~ 45 km/s, well separated from the quasi-thermal emission near 60 km/s. These features were absent 25 years ago and show a velocity drift of about +0.2 km/s/yr. The component is likely related to the SiO maser source in W51-IRS2 and a possible scenario explaining the velocity drift is outlined. The 57 km/s component of the (9,6) maser line is found to be strongly linearly polarized. Maser emission in the (J,K) to (J+1,K) inversion doublets is strictly forbidden by selection rules for electric dipole transitions in the ground vibrational state. However, such pairs (and even triplets with (J+2,K)) are common toward W51-IRS2. Similarities in line widths and velocities indicate that such groups of maser lines arise from the same regions, which can be explained by pumping through vibrational excitation. The large number of NH3 maser lines in W51-IRS2 is most likely related to the exceptionally high kinetic temperature and NH3 column density of this young massive star forming region.Comment: Accepted for publication in Astronomy & Astrophysics, 11 pages, 12 postscript figures, 1 tabl