2015A&A...579A.101A

We aim to better understand the imprints that the nuclear activity in galaxies leaves in the molecular gas. 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 galaxies hosting an active galactic nucleus (AGN) M51, NGC1068, and NGC7469, and the ultra-luminous infrared galaxies (ULIRGs) Arp 220 and Mrk 231. Assuming local thermodynamic equilibrium (LTE), we calculated the column densities of 27 molecules and 10 isotopologues (or their upper limits in case of non-detections). 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. Hα 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 each group of galaxies (starbursts, AGNs and ULIRGs), as well as the differences among galaxies within the same group. Suitable species for characterising and comparing starburst galaxies are CH₃OH and HNCO as tracers of large-scale shocks, which dominate early to intermediate starburst stages, and CH₃CCH, c-C₃H₂, and HCO as tracers of UV fields, which control the intermediate-to-old or post starburst phases. M83 shows signs of a shock-dominated environment. NGC253 is characterised by both strong shocks and some UV fields. M82 stands out for its bright photo-dissociated region tracers, which indicate an UV field-dominated environment. Regarding AGNs, the abundances of HCN and CN (previously claimed as enhanced in AGNs) in M51 are similar to those in starburst galaxies, while the HCN/HCO⁺ ratio is high in M51 and NGC1068, but not in NGC7469. We did not find a correlation between the HCN/CS ratio (recently claimed as a possible starburst/AGN discriminator) and the AGN activity. However, a high enough spatial resolution to separate their circumnuclear disks from the surrounding star-forming regions is needed to find molecular abundance trends in AGNs. High abundances of H¹³CN and HC₃N, as well as a similarity between the column densities of ¹³CO and C¹⁸O, are representative of the molecular interstellar medium in the ULIRGs. Furthermore, the chemistry of Arp220 points towards a more starburst-dominated environment, while that of Mrk231 more resembles the AGNs of our sample.