Atomic and Molecular Gas in the Starburst Galaxy NGC4945

Spatial and kinematical correlations between HI and CO (2-1) emission of the southern spiral galaxy NGC4945 are studied with a common angular (23 arcsec) and velocity resolution (7 km/s). The 21cm continuum emission is also observed. The HI kinematics yield a galaxy mass of 1.4x10^{11} Mo within a radius of 380 arcsec, with molecular and neutral atomic gas each contributing 2%. Nuclear HI absorption at velocities 80 km/s higher than systemic indicates gas flowing towards the centre. HI features at each end of the major axis (R ~ 600 arcsec) are interpreted as spiral arms that are viewed tangentially and that also cause prominent emission features in the radio continuum, HI, and CO further inside the galaxy. A central elongated region showing non-circular motions is interpreted as a bar which fuels the nuclear starburst. HI and CO position-velocity data have been analysed using linear resonance theory and possible locations of resonances are identified.Comment: 16 pages, Latex file, 9 Postscript Figures, aa.cls and psfig.sty included. Accepted by Astronomy and Astrophysic

A Dense Gas Trigger for OH Megamasers

HCN and CO line diagnostics provide new insight into the OH megamaser (OHM) phenomenon, suggesting a dense gas trigger for OHMs. We identify three physical properties that differentiate OHM hosts from other starburst galaxies: (1) OHMs have the highest mean molecular gas densities among starburst galaxies; nearly all OHM hosts have = 10^3-10^4 cm^-3 (OH line-emitting clouds likely have n(H2) > 10^4 cm^-3). (2) OHM hosts are a distinct population in the nonlinear part of the IR-CO relation. (3) OHM hosts have exceptionally high dense molecular gas fractions, L(HCN)/L(CO)>0.07, and comprise roughly half of this unusual population. OH absorbers and kilomasers generally follow the linear IR-CO relation and are uniformly distributed in dense gas fraction and L(HCN), demonstrating that OHMs are independent of OH abundance. The fraction of non-OHMs with high mean densities and high dense gas fractions constrains beaming to be a minor effect: OHM emission solid angle must exceed 2 pi steradians. Contrary to conventional wisdom, IR luminosity does not dictate OHM formation; both star formation and OHM activity are consequences of tidal density enhancements accompanying galaxy interactions. The OHM fraction in starbursts is likely due to the fraction of mergers experiencing a temporal spike in tidally driven density enhancement. OHMs are thus signposts marking the most intense, compact, and unusual modes of star formation in the local universe. Future high redshift OHM surveys can now be interpreted in a star formation and galaxy evolution context, indicating both the merging rate of galaxies and the burst contribution to star formation.Comment: 5 pages, 3 figures, 1 table, accepted by ApJ Letter

The Circumnuclear Molecular Gas in the Seyfert Galaxy NGC4945

We have mapped the central region of NGC 4945 in the $J=2\to1$ transition of $^{12}$CO, $^{13}$CO, and C$^{18}$O, as well as the continuum at 1.3 mm, at an angular resolution of 5\farc \times 3\farc with the Submillimeter Array. The relative proximity of NGC 4945 (distance of only 3.8 Mpc) permits a detailed study of the circumnuclear molecular gas and dust in a galaxy exhibiting both an AGN (classified as a Seyfert 2) and a circumnuclear starburst in an inclined ring with radius $\sim$2\farcs5 ($\sim$50 pc). We find that all three molecular lines trace an inclined rotating disk with major axis aligned with that of the starburst ring and large-scale galactic disk, and which exhibits solid-body rotation within a radius of $\sim$5\farc ($\sim$95 pc). We infer an inclination for the nuclear disk of $62^{\circ} \pm 2^{\circ}$, somewhat smaller than the inclination of the large-scale galactic disk of $\sim$$78^{\circ}$. The continuum emission at 1.3 mm also extends beyond the starburst ring, and is dominated by thermal emission from dust. If it traces the same dust emitting in the far-infrared, then the bulk of this dust must be heated by star-formation activity rather than the AGN. We discover a kinematically-decoupled component at the center of the disk with a radius smaller than 1\farcs4 (27 pc), but which spans approximately the same range of velocities as the surrounding disk. This component has a higher density than its surroundings, and is a promising candidate for the circumnuclear molecular torus invoked by AGN unification models.Comment: 13 pages, 10 figures,accepted by Ap

CH 3 GHz Observations of the Galactic Center

A 3 $\times$ 3 map of the Galactic Center was made at 9\arcmin resolution and 10\arcmin spacing in the CH $^2\Pi_{1/2}$, J=1/2, F=1-1 transition at 3335 MHz. The CH emission shows a velocity extent that is nearly that of the CO(1-0) line, but the CH line profiles differ markedly from the CO. The 3335 MHz CH transition primarily traces low-density molecular gas and our observations indicate that the mass of this component within $\sim$ 30 pc of the Galactic Center is $\sim$ 9 $\times$ 10$^6$ M$_\odot$. The CO-H$_2$ conversion factor obtained for the low-density gas in the mapped region is greater than that thought to apply to the dense molecular gas at the Galactic Center. In addition to tracing the low-density molecular gas at the Galactic Center, the CH spectra show evidence of emission from molecular clouds along the line of sight both in the foreground and background. The scale height of these clouds ranges from 27 - 109 pc, consistent with previous work based on observations of molecular clouds in the inner Galaxy.Comment: 29 pages, 12 figure

Supersonic water masers in 30 Doradus

We report on extremely high velocity molecular gas, up to -80 km/s relative to the ambient medium, in the giant star-formation complex 30 Doradus in the Large Magellanic Cloud (LMC), as observed in new 22 GHz H2O maser emission spectra obtained with the Mopra radio telescope. The masers may trace the velocities of protostars, and the observed morphology and kinematics indicate that current star formation occurs near the interfaces of colliding stellar-wind blown bubbles. The large space velocities of the protostars and associated gas could result in efficient mixing of the LMC. A similar mechanism in the Milky Way could seed the galactic halo with relatively young stars and gas.Comment: 11 pages plus 1 PS and 1 EPS figure, uses AASTeX preprint style; accepted for publication in Astrophysical Journal Letter

Abundances and Isotope Ratios in the Magellanic Clouds: The Star Forming Environment of N113

With the goal of deriving the physical and chemical conditions of star forming regions in the Large Magellanic Cloud (LMC), a spectral line survey of the prominent star forming region N113 is presented. The observations cover parts of the frequency range from 85 GHz to 357 GHz and include 63 molecular transitions from a total of 16 species, among them spectra of rare isotopologues. Maps of selected molecular lines as well as the 1.2 mm continuum distribution are also presented. Molecular abundances in the core of the complex are found to be consistent with a photon dominated region (PDR) that is nitrogen deficient, with the potential exception of N2H+. Densities range from 5x10^3 cm-3 for CO to almost 10^6 for CS and HCN, indicating that only the densest regions provide sufficient shielding even for some of the most common species. An ortho- to para-H_2CO ratio of ~3 hints at H_2CO formation in a warm (>=40 K) environment. Isotope ratios are 12C/13C ~ 49+-5, 16O/18O ~ 2000+-250, 18O/17O ~ 1.7+-0.2 and 32S/34S ~ 15. Agreement with data from other star forming clouds shows that the gas is well mixed in the LMC . The isotope ratios do not only differ from those seen in the Galaxy. They also do not form a continuation of the trends observed with decreasing metallicity from the inner to the outer Galaxy. This implies that the outer Galaxy, is not providing a transition zone between the inner Galaxy and the metal poor environment of the Magellanic Clouds. A part of this discrepancy is likely caused by differences in the age of the stellar populations in the outer Galaxy and the LMC.Comment: 50 pages, 13 figures, accepted for publication in Ap