Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H2O, H218O, OH,18OH, O I, HCN, and NH3

González-Alfonso, E.; Fischer, J.; Graciá-Carpio, J.; Sturm, E.; Hailey-Dunsheath, S.; Lutz, D.; Poglitsch, A.; Contursi, A.; Feuchtgruber, H.; Veilleux, Sylvain; Spoon, H. W. W.; Verma, Aprajita; Christopher, N.; Davies, R. I.; Sternberg, A.; Genzel, R.; Tacconi, L. J.

doi:10.1051/0004-6361/201118029

Cited by 153 publications

(200 citation statements)

References 111 publications

(138 reference statements)

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“…With the data at hand it is difficult to separate the two suggested models for CO emission, and we suggest the two interpretations as two equally probable scenarios. Both interpretations are consistent with the molecular inflow detected by González-Alfonso et al (2012) with Herschel observations (see discussion in Sect. 4.3).…”

Section: Outflow/in-flow Scenariosupporting

confidence: 90%

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High-resolution mm and cm study of the obscured LIRG NGC 4418

Costagliola

Aalto

Sakamoto

et al. 2013

A&A

119

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Context. Understanding the nature of the power-source in luminous infrared galaxies (LIRG) is difficult due to their extreme obscuration. Observations at radio and mm wavelengths can penetrate large columns of dust and gas and provide unique insights into the properties of the compact obscured nuclei of LIRGs. Aims. The aim of this study is to constrain the dynamics, structure, and feeding of the compact nucleus of NGC 4418, and to reveal the nature of the main hidden power-source: starburst or active galactic nucleus (AGN). Methods. We obtained high spatial resolution observations of NGC 4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz with the SMA in very extended configuration. We used the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate, and the dust temperature. Emission lines were used to study kinematics through position-velocity diagrams. Molecular emission was studied with population diagrams and by fitting a local thermal equilibrium (LTE) synthetic spectrum. Results. We detect bright 1-mm-line emission from CO, HC 3 N, HNC, and C 34 S and 1.4 GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s −1 . We detect vibrationally excited HC 3 N and HNC, with T vib ∼ 300 K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160, and 300 K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 10 4 M pc −2 is extremely high and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220. Conclusions. Our observations confirm the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC 4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in the ULIRG Arp 220, with an age of 3-10 Myr and a star formation rate >10 M yr −1 . If an AGN is present, it must be extremely Compton-thick.

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Section: Outflow/in-flow Scenariosupporting

confidence: 90%

“…A spherical molecular in-flow in the core of NGC 4418 was first suggested by González-Alfonso et al (2012) to explain the redshifted OH absorption observed by the Herschel satellite. The presence of an atomic and molecular in-flow in NGC 4418 would not be surprising.…”

Section: Atomic and Molecular In-flowmentioning

confidence: 92%

High-resolution mm and cm study of the obscured LIRG NGC 4418

Costagliola

Aalto

Sakamoto

et al. 2013

A&A

119

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“…Herschel PACS observations by González-Alfonso et al (2012), and our combined MERLIN/SMA study ) reveal the presence of redshifted OH and HI absorption, which is interpreted as the signature of a molecular inflow. The presence of a molecular outflow is also suggested by a U-shaped red optical feature along the northwestern semi-minor axis of the galaxy .…”

Section: Ngc 4418: the Prototypical Obscured Lirgmentioning

confidence: 52%

“…A compact IR source of similar temperature and size is also required by models by González-Alfonso et al (2012) in order to fit the mid-IR continuum.…”

Section: Temperature and Density Structurementioning

confidence: 99%

Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies

Costagliola

Sakamoto

Müller

et al. 2015

A&A

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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.

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“…In a recent study (González-Alfonso et al 2012, hereafter Paper I), we have reported on the Herschel/PACS observations of these two sources in H 2 O, H 18 2 O, OH, 18 OH, HCN, and NH 3 , together with a quantitative analysis in which a deconvolution of the continuum into several components enabled us to derive the column densities and abundances of these species associated with each of the components. A chemical dichotomy was found in Paper I: on the one hand, the nuclear regions of both galaxies have high H 2 O abundances (∼10 −5 ), and high HCN and NH 3 column densities, indicating a chemistry characterized by high T gas , T dust , and high density, where the release of molecules from grain mantles to the gas phase has occurred (through evaporation in "hot core" chemistry and sputtering in shocks).…”

Section: Introductionmentioning

confidence: 99%

Excited OH⁺, H₂O⁺, and H₃O⁺in NGC 4418 and Arp 220

González-Alfonso

Fischer

Bruderer

et al. 2013

A&A

Self Cite

107

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We report on Herschel/PACS observations of absorption lines of OH + , H 2 O + and H 3 O + in NGC 4418 and Arp 220. Excited lines of OH + and H 2 O + with E lower of at least 285 and ∼200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density environment of the nuclear regions. Abundance ratios OH + /H 2 O + of 1−2.5 are estimated in the nuclei of both sources. The inferred OH + column and abundance relative to H nuclei are (0.5−1) × 10 16 cm −2 and ∼2 × 10 −8 , respectively. Additionally, in Arp 220, an extended low excitation component around the nuclear region is found to have OH + /H 2 O + ∼ 5−10. H 3 O + is detected in both sources with N(H 3 O + ) ∼ (0.5−2) × 10 16 cm −2 , and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ∼500 K, indicative of formation pumping and/or hot gas. Simple chemical models favor an ionization sequence dominated by H + → O + → OH + → H 2 O + → H 3 O + , and we also argue that the H + production is most likely dominated by X-ray/cosmic ray ionization. The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density ( 10 4 cm −3 ) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ζ > 10 −13 s −1 , a lower limit that is several × 10 2 times the highest current rate estimates for Galactic regions. In Arp 220, our lower limit for ζ is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission, and also with the expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray ionization due to an active galactic nucleus is responsible for the molecular ion production.

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Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H₂O, H₂¹⁸O, OH,¹⁸OH, O I, HCN, and NH₃

Cited by 153 publications

References 111 publications

High-resolution mm and cm study of the obscured LIRG NGC 4418

High-resolution mm and cm study of the obscured LIRG NGC 4418

Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies

Excited OH⁺, H₂O⁺, and H₃O⁺in NGC 4418 and Arp 220

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Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H2O, H218O, OH,18OH, O I, HCN, and NH3

Cited by 153 publications

References 111 publications

High-resolution mm and cm study of the obscured LIRG NGC 4418

High-resolution mm and cm study of the obscured LIRG NGC 4418

Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies

Excited OH+, H2O+, and H3O+in NGC 4418 and Arp 220

Contact Info

Product

Resources

About

Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H₂O, H₂¹⁸O, OH,¹⁸OH, O I, HCN, and NH₃

Excited OH⁺, H₂O⁺, and H₃O⁺in NGC 4418 and Arp 220