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

Extragalactic CS survey

We present a coherent and homogeneous multi-line study of the CS molecule in nearby (D$<$10Mpc) galaxies. We include, from the literature, all the available observations from the $J=1-0$ to the $J=7-6$ transitions towards NGC 253, NGC 1068, IC 342, Henize~2-10, M~82, the Antennae Galaxies and M~83. We have, for the first time, detected the CS(7-6) line in NGC 253, M~82 (both in the North-East and South-West molecular lobes), NGC 4038, M~83 and tentatively in NGC 1068, IC 342 and Henize~2-10. We use the CS molecule as a tracer of the densest gas component of the ISM in extragalactic star-forming regions, following previous theoretical and observational studies by Bayet et al. (2008a,b and 2009). In this first paper out of a series, we analyze the CS data sample under both Local Thermodynamical Equilibrium (LTE) and non-LTE (Large Velocity Gradient-LVG) approximations. We show that except for M~83 and Overlap (a shifted gas-rich position from the nucleus NGC 4039 in the Antennae Galaxies), the observations in NGC 253, IC 342, M~82-NE, M~82-SW and NGC 4038 are not well reproduced by a single set of gas component properties and that, at least, two gas components are required. For each gas component, we provide estimates of the corresponding kinetic temperature, total CS column density and gas density.Comment: 17 pages, 16 figures, 3 tables, Accepted to Ap

A new intermediate mass protostar in the Cepheus A HW2 region

We present the discovery of the first molecular hot core associated with an intermediate mass protostar in the CepA HW2 region. The hot condensation was detected from single dish and interferometric observations of several high excitation rotational lines (from 100 to 880K above the ground state) of SO2 in the ground vibrational state and of HC3N in the vibrationally excited states v7=1 and v7=2. The kinetic temperature derived from both molecules is 160K. The high-angular resolution observations (1.25'' x 0.99'') of the SO2 J=28(7,21)-29(6,24) line (488K above the ground state) show that the hot gas is concentrated in a compact condensation with a size of 0.6''(430AU), located 0.4'' (300AU) east from the radio-jet HW2. The total SO2 column density in the hot condensation is 10E18cm-2, with a H2 column density ranging from 10E23 to 6 x 10E24cm-2. The H2 density and the SO2 fractional abundance must be larger than 10E7cm-3 and 2 x 10E-7 respectively. The most likely alternatives for the nature of the hot and very dense condensation are discussed. From the large column densities of hot gas, the detection of the HC3N vibrationally excited lines and the large SO2 abundance, we favor the interpretation of a hot core heated by an intermediate mass protostar of 10E3 Lo. This indicates that the CepA HW2 region contains a cluster of very young stars

The largest oxigen bearing organic molecule repository

We present the first detection of complex aldehydes and isomers in three typical molecular clouds located within 200pc of the center of our Galaxy. We find very large abundances of these complex organic molecules (COMs) in the central molecular zone (CMZ), which we attribute to the ejection of COMs from grain mantles by shocks. The relative abundances of the different COMs with respect to that of CH3OH are strikingly similar for the three sources, located in very different environments in the CMZ. The similar relative abundances point toward a unique grain mantle composition in the CMZ. Studying the Galactic center clouds and objects in the Galactic disk having large abundances of COMs, we find that more saturated molecules are more abundant than the non-saturated ones. We also find differences between the relative abundance between COMs in the CMZ and the Galactic disk, suggesting different chemical histories of the grain mantles between the two regions in the Galaxy for the complex aldehydes. Different possibilities for the grain chemistry on the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an important role in the grain chemistry. With these new detections, the molecular clouds in the Galactic center appear to be one of the best laboratories for studying the formation of COMs in the Galaxy.Comment: 20 pages, 4 figures, accepted in Ap