Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies

Abstract

Context. Extragalactic observations allow the study of molecular chemistry and excitation under physical conditions which may differ greatly from those found in the Milky Way. The compact, obscured nuclei (CON) of luminous infrared galaxies (LIRG) combine large molecular columns with intense infrared (IR), ultra-violet (UV), and X-radiation and represent ideal laboratories for the study of the chemistry of the interstellar medium (ISM) under extreme conditions. Aims. Our aim was to obtain for the first time a multi-band spectral scan of a LIRG, and to derive molecular abundances and excitation to be compared to other Galactic and extragalactic environments. Methods. We obtained an ALMA Cycle 0 spectral scan of the dusty LIRG NGC 4418, spanning a total of 70.7 GHz in bands 3, 6, and 7. We use a combined local thermal equilibrium (LTE) and non-LTE (NLTE) fit of the spectrum in order to identify the molecular species and to derive column densities and excitation temperatures. We derive molecular abundances and compare them with other Galactic and extragalactic sources by means of a principal component analysis. Results. We detect 317 emission lines from a total of 45 molecular species, including 15 isotopic substitutions and 6 vibrationally excited variants. Our LTE/NLTE fit find kinetic temperatures from 20 to 350 K, and densities between 10 5 and 10 7 cm −3 . The spectrum is dominated by vibrationally excited HC 3 N, HCN, and HNC, with vibrational temperatures from 300 to 450 K. We find that the chemistry of NCG 4418 is characterized by high abundances of HC 3 N, SiO, H 2 S, and c-HCCCH but a low CH 3 OH abundance. A principal component analysis shows that NGC 4418 and Arp 220 share very similar molecular abundances and excitation, which clearly set them apart from other Galactic and extragalactic environments. Conclusions. Our spectral scan confirms that the chemical complexity in the nucleus of NGC 4418 is one of the highest ever observed outside our Galaxy. The similar molecular abundances observed toward NCG 4418 and Arp 220 are consistent with a hot gas-phase chemistry, with the relative abundances of SiO and CH 3 OH being regulated by shocks and X-ray driven dissociation. The bright emission from vibrationally excited species confirms the presence of a compact IR source, with an effective diameter smaller than 5 pc and brightness temperatures higher than 350 K. The molecular abundances and the vibrationally excited spectrum are consistent with a young AGN/starburst system. We suggest that NGC 4418 may be a template for a new kind of chemistry and excitation, typical of CON. Because of the narrow line widths and bright molecular emission, NGC 4418 is the ideal target for further studies of the chemistry in CONs.

The unbearable opaqueness of Arp220

“…Observations of HC 3 N in the vibrationally excited levels probe T vib = 350 K between vibrational levels with T ex = 35 K measured within the ground-vibrational state (Martín et al 2011). This is again quite similar to the excitation conditions measured in NGC 4418 (Costagliola & Aalto 2010;Sakamoto et al 2010), as has been noted by Costagliola et al (2015). A comprehensive explanation on the excitation of HCN vibrational levels was presented by Sakamoto et al (2010) for NGC 4418, which also applies to the conditions we observe in Arp 220.…”

“…Line contribution to the continuum emission has been shown to be a problem in Arp 220 (Martín et al 2011), where the spectra are confusion limited because of both the prolific molecular emission and broad line widths. This becomes even more of a problem at high frequency, as has also been observed toward the LIRG NGC 4418 (Costagliola et al 2015), and it hinders finding line-free spectral regions to extract continuum information. Although the spectral lines presented in this paper were observed in the upper sideband of the two setups, we used the continuum windows available in both bands to better account for the spectral index (α ∼ 3) at these frequencies.…”

HC3N observations of nearby galaxies

“…The ALMA observations by Costagliola et al (2015) even reported the HC 3 N J = 32−31 rotational transition, and some of the vibrationally excited HC 3 N lines. The latest highresolution line surveys in a number of very nearby galaxies (Meier & Turner 2005 and Meier et al (2014Meier et al ( , 2015 show that the derived HC 3 N abundances (on ∼100 pc, approximately GMC scales) are approximately several 10 −10 (relative to H 2 ), which is approximately an order of magnitude lower than the abundance of HCN and some other molecules.…”

“…In very recent line surveys of a small number of local active galaxies (AGN and/or starbursts, Aladro et al 2015;Costagliola et al 2015), several HC 3 N transitions in 3 mm band (HC 3 N J = 10−9, J = 11−10 and J = 12−11) were detected. The ALMA observations by Costagliola et al (2015) even reported the HC 3 N J = 32−31 rotational transition, and some of the vibrationally excited HC 3 N lines.…”

COMPLEX HYDROCARBON CHEMISTRY IN INTERSTELLAR AND SOLAR SYSTEM ICES REVEALED: A COMBINED INFRARED SPECTROSCOPY AND REFLECTRON TIME-OF-FLIGHT MASS SPECTROMETRY ANALYSIS OF ETHANE (C2H6) AND D6-ETHANE (C2D6) ICES EXPOSED TO IONIZING RADIATION

“…The C 3 H 4 isomer methylacetylene (CH 3 CCH; IE = 10.36 eV) isomer has been detected in a few astronomical surveys toward SgrB2, PKS 1830-211, L1544, and tentatively in NCG 4418 (Belloche et al 2013;Muller et al 2014;Vastel et al 2014;Costagliola et al 2015). Methylacetylene and its isomer allene (H 2 CCCH 2 ; IE = 9.69 eV) have been reactants in attempts to untangle the chemistry taking place in Titanʼs atmosphere (Vakhtin et al 2001;Goulay et al 2007;Zhang et al 2009).…”