CS, HC3N and CH3CCH multi-line analyses towards starburst galaxies. The evolution of cloud structures in the central regions of galaxies

We aim to study the properties of the dense molecular gas towards the inner few 100 pc of four nearby starburst galaxies dominated both by photo dissociation regions (M82) and large-scale shocks (NGC253, IC342 and Maffei2), and to relate the chemical and physical properties of the molecular clouds with the evolutionary stage of the nuclear starbursts. We have carried out multi-transitional observations and analyses of three dense gas molecular tracers, CS, HC3N and CH3CCH, using Boltzmann diagrams in order to determine the rotational temperatures and column densities of the dense gas, and using a Large Velocity Gradients model to calculate the H2 density structure in the molecular clouds. The CS and HC3N data indicate the presence of density gradients in the molecular clouds, showing similar excitation conditions, and suggesting that they arise from the same gas components. In M82, CH3CCH has the highest fractional abundance determined in a extragalactic source (10^-8). The density and the chemical gradients found in all galaxies can be explained in the framework of the starburst evolution. The young shock-dominatedstarburst galaxies, like presumably Maffei2, show a cloud structure with a rather uniform density and chemical composition which suggests low star formation activity. Molecular clouds in galaxies with starburst in an intermediate stage of evolution, such as NGC253 and IC342, show clouds with a large density contrast (two orders of magnitude) between the denser regions (cores) and the less dense regions (halos) of the molecular clouds and relatively constant chemical abundance. Finally, the galaxy with the most evolved starburst, M82, has clouds with a rather uniform density structure, large envelopes of atomic/molecular gas subjected to UV photodissociating radiation from young star clusters, and very different chemical abundances of HC3N and CH3CCH.Comment: 14 pages + 1 appendix of 2 pages; 7 figures. Accepted for publication in Astronomy and Astrophysic

A lambda=3 mm molecular line survey of NGC1068. Chemical signatures of an AGN environment

We aimed to study the molecular composition of the interstellar medium (ISM) surrounding an Active Galactic Nucleus (AGN), by making an inventory of molecular species and their abundances, as well as to establish a chemical differentiation between starburst galaxies and AGN. We used the IRAM-30 m telescope to observe the central 1.5-2 kpc region of NGC1068, covering the frequencies between 86.2 GHz and 115.6 GHz. Using Boltzmann diagrams, we calculated the column densities of the detected molecules. We used a chemical model to reproduce the abundances found in the AGN, to determine the origin of each detected species, and to test the influence of UV fields, cosmic rays, and shocks on the ISM. We identified 24 different molecular species and isotopologues, among which HC3N, SO, N2H+, CH3CN, NS, 13CN, and HN13C are detected for the first time in NGC1068. We obtained the upper limits to the isotopic ratios 12C/13C=49, 16O/18O=177 and 32S/34S=5. Our chemical models suggest that the chemistry in the nucleus of NGC1068 is strongly influenced by cosmic rays, although high values of both cosmic rays and far ultraviolet (FUV) radiation fields also explain well the observations. The gas in the nucleus of NGC1068 has a different chemical composition as compared to starburst galaxies. The distinct physical processes dominating galaxy nuclei (e.g. C-shocks, UV fields, X-rays, cosmic rays) leave clear imprints in the chemistry of the gas, which allow to characterise the nucleus activity by its molecular abundances.Comment: 16 pages, 6 figures, 7 tables. Accepted for publication in Astronomy and Astrophysic

Lambda = 3 mm line survey of nearby active galaxies

We used the IRAM 30m telescope to observe the frequency range [86-116]GHz towards the central regions of the starburst galaxies M83, M82, and NGC253, the AGNs M51, NGC1068, and NGC7469, and the ULIRGs Arp220 and Mrk231. Assuming LTE conditions, we calculated the column densities of 27 molecules and 10 isotopologues. Among others, we report the first tentative detections of CH3CHO, HNCO, and NS in M82 and, for the first time in the extragalactic medium, HC5N in NGC253. Halpha recombination lines were only found in M82 and NGC253. Vibrationally excited lines of HC3N were only detected in Arp220. CH3CCH emission is only seen in the starburst-dominated galaxies. By comparison of the fractional abundances among the galaxies, we looked for the molecules that are best suited to characterise the chemistry of starbursts, AGNs and ULIRGs, as well as the differences among galaxies within the same group.Comment: 24 pages, 6 figures, 12 tables. Accepted for publication in Astronomy and Astrophysic

An Extragalactic 12CO J=3-2 survey with the Heinrich-Hertz-Telescope

We present results of a ^{12}CO J = 3-2 survey of 125 nearby galaxies obtained with the 10-m Heinrich-Hertz-Telescope, with the aim to characterize the properties of warm and dense molecular gas in a large variety of environments. With an angular resolution of 22'', ^{12}CO 3-2 emission was detected in 114 targets. Based on 61 galaxies observed with equal beam sizes the ^{12}CO 3-2/1-0 integrated line intensity ratio R_{31} is found to vary from 0.2 to 1.9, with an average value of 0.81. No correlations are found for R_{31} to Hubble type and far infrared luminosity. Possible indications for a correlation with inclination angle and the 60mum/100mum color temperature of the dust are not significant. Higher R_{31} ratios than in ``normal'' galaxies, hinting at enhanced molecular excitation, may be found in galaxies hosting active galactic nuclei. Even higher average values are determined for galaxies with bars or starbursts, the latter being identified by the ratio of infrared luminosity versus isophotal area, log[(L_{FIR}/L_{SUN})/(D_{25}/kpc)^2)] > 7.25. (U)LIRGs are found to have the highest averaged R_{31} value. This may be a consequence of particularly vigorous star formation activity, triggered by galaxy interaction and merger events. The nuclear CO luminosities are slightly sublinearly correlated with the global FIR luminosity in both the ^{12}CO J = 3-2 and the 1-0 lines. The slope of the log-log plots rises with compactness of the respective galaxy subsample, indicating a higher average density and a larger fraction of thermalized gas in distant luminous galaxies. While linear or sublinear correlations for the ^{12}CO J = 3-2 line can be explained, if the bulk of the observed J = 3-2 emission originates from molecular gas with densities below the critical one, the case of the ^{12}CO J = 1-0 line with its small critical density remains a puzzle.Comment: 26 pages, 9 figures, 4 tables, Accepted for publication in The Astrophysical Journal (Part 1

The detection of extragalactic 15^{15}N: Consequences for nitrogen nucleosynthesis and chemical evolution

Detections of extragalactic $^{15}$N are reported from observations of the rare hydrogen cyanide isotope HC$^{15}$N toward the Large Magellanic Cloud (LMC) and the core of the (post-) starburst galaxy NGC 4945. Accounting for optical depth effects, the LMC data from the massive star-forming region N113 infer a $^{14}N/^{15}$N ratio of 111 $\pm$ 17, about twice the $^{12}C/^{13}$C value. For the LMC star-forming region N159HW and for the central region of NGC 4945, $^{14}N/^{15}$N ratios are also $\approx$ 100. The $^{14}N/^{15}$N ratios are smaller than all interstellar nitrogen isotope ratios measured in the disk and center of the Milky Way, strongly supporting the idea that $^{15}$N is predominantly of `primary' nature, with massive stars being its dominant source. Although this appears to be in contradiction with standard stellar evolution and nucleosynthesis calculations, it supports recent findings of abundant $^{15}$N production due to rotationally induced mixing of protons into the helium-burning shells of massive stars.Comment: 15 pages including one postscript figure, accepted for publication by ApJ Letter, further comments: please contact Yi-nan Chi