CS, HC₃N, and CH₃CCH multi-line analyses toward starburst galaxies

Abstract

Aims. 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 (M 82) and large-scale shocks (NGC 253, IC 342 and Maffei 2), and to relate the chemical and physical properties of the molecular clouds with the evolutionary stage of the nuclear starbursts. Methods. We have carried out multi-transitional observations and analyses of three dense gas molecular tracers, CS, HC 3 N (cyanoacetylene) and CH 3 CCH (methyl acetylene), 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 H 2 density structure in the molecular clouds.Results. The CS and HC 3 N data indicate the presence of density gradients in the molecular clouds. These two molecules show similar excitation conditions, suggesting that they arise from the same gas components. In M 82, CH 3 CCH has the highest fractional abundance determined in a extragalactic source (1.1 × 10 −8 ). Conclusions. The density and the chemical gradients we have found in all galaxies can be explained in the framework of the starburst evolution, which affects the chemistry and the structure of molecular clouds around the galactic nuclei. The young shock-dominated starburst galaxies, like presumably Maffei 2, 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 NGC 253 and IC 342, 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, M 82, 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 HC 3 N and CH 3 CCH.

Aλ= 1.3 mm and 2 mm molecular line survey towards M 82

“…This could be explained if the gas in NGC 253 had a higher kinetic temperature. It seems that in these two galaxies, the spatial distribution of CS is similar to that of ammonia, or at least, is clumpier than those of H 2 CO and CO (Aladro et al 2011). If so, the kinetic temperature in M 82 would be around 60 K, while that of NGC 253 would be between 100 and 120 K. In that case, NGC 253 does not necessarily need to have higher H 2 densities than M 82, as seen by the LVG modelling of the CS emission towards these two galaxies by Aladro et al (2011).…”

“…6). That HC 3 N has a very warm T rot in NGC 253 (73 K) seems to be related to the higher kinetic temperature of this galaxy, since the averaged n H 2 densities are lower in NGC 253 than in M 82 (Aladro et al 2011). The [c-C 3 H 2 /HC 3 N] ratio appears to indicate the evolutionary stage of the bursts of star formation (Fuente et al 2005).…”

“…-CS. Carbon monosulfide is one of the most well-studied dense gas tracers (Bayet et al 2008a) in extragalactic sources, and in particular starburst galaxies (Henkel & Bally 1985;Bayet et al 2008bBayet et al , 2009bAladro et al 2011). The main chemical reactions that lead to the CS formation are detailed in Pineau des Forêts et al (1986) and Drdla et al (1989).…”

Multimolecule ALMA observations toward the Seyfert 1 galaxy NGC 1097

“…Though a small abundance difference of up to a factor of two may be claimed as is suggested by the observed difference in the low-J transition ratio (Aladro et al 2013), the larger difference observed in the high-J ratio is likely due to the excitation of CS. In starburst galaxies, the high-J transitions of CS appear to be associated to a moderately higher excitation temperature gas component (∼15-30 K, Aladro et al 2011b). This component appears to correspond to a dense molecular component (∼10 6 cm −3 ).…”

“…Though the assumption on the excitation temperature will not strongly affect the relative abundances given that most of our observed transitions have similarly low (4−10 K) energies, it might be an issue for the transition of HC 3 N with a significantly higher upper energy level (Table 1). However, the work by Aladro et al (2011b) toward nearby starbursts shows that such subthermal excitation affects equally to other molecular species in their lower J transitions, to those not only at lower energies but also up to transitions with ∼50 K upper energy levels. Additionally the non detection of the two higher energy transitions of HNCO listed in Table 1 and shown in Fig.…”