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Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies

Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies
Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies

We investigate the relation between the detection of the 11.3  μm polycyclic aromatic hydrocarbon (PAH) feature in the nuclear (∼24-230 pc) regions of 22 nearby Seyfert galaxies and the properties of the cold molecular gas. For the former we use ground-based (0.3-0.6″ resolution) mid-infrared (mid-IR) spectroscopy. The cold molecular gas is traced by ALMA and NOEMA high (0.2-1.1″) angular resolution observations of the CO(2-1) transition. Galaxies with a nuclear detection of the 11.3 μm PAH feature contain more cold molecular gas (median 1.6 × 107 MO ) and have higher column densities (N(H2) = 2 × 1023 cm-2) over the regions sampled by the mid-IR slits than those without a detection. This suggests that molecular gas plays a role in shielding the PAH molecules in the harsh environments of Seyfert nuclei. Choosing the PAH molecule naphthalene as an illustration, we compute its half-life in the nuclear regions of our sample when exposed to 2.5 keV hard X-ray photons. We estimate shorter half-lives for naphthalene in nuclei without a 11.3 μm PAH detection than in those with a detection. The Spitzer/IRS PAH ratios on circumnuclear scales (∼4″ ∼ 0.25-1.3 kpc) are in between model predictions for neutral and partly ionized PAHs. However, Seyfert galaxies in our sample with the highest nuclear H2 column densities are not generally closer to the neutral PAH tracks. This is because in the majority of our sample galaxies, the CO(2-1) emission in the inner ∼4″ is not centrally peaked and in some galaxies traces circumnuclear sites of strong star formation activity. Spatially resolved observations with the MIRI medium-resolution spectrograph on the James Webb Space Telescope will be able to distinguish the effects of an active galactic nucleus (AGN) and star formation on the PAH emission in nearby AGN.

Galaxies: ISM, Galaxies: Seyfert, Galaxies: nuclei, Radio lines: galaxies
0004-6361
Alonso-Herrero, A.
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Pereira-Santaella, M.
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Rigopoulou, D.
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Garcia-Bernete, I.
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Dominguez-Fernandez, A.J.
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Combes, F.
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Davies, R.I.
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Diaz-Santos, T.
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Esparza-Arrendondo, D.
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Gonzalez-Martin, O.
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Hernan-Caballero, A.
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Hicks, E.K.S.
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Hoenig, S.F.
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Levenson, N.A.
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Ramos Almeida, C.
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Roche, P.F.
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Rosario, D.
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Alonso-Herrero, A.
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Pereira-Santaella, M.
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Rigopoulou, D.
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Garcia-Bernete, I.
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Dominguez-Fernandez, A.J.
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Combes, F.
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Davies, R.I.
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Diaz-Santos, T.
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Esparza-Arrendondo, D.
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Gonzalez-Martin, O.
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Hernan-Caballero, A.
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Hicks, E.K.S.
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Hoenig, S.F.
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Levenson, N.A.
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Ramos Almeida, C.
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Roche, P.F.
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Rosario, D.
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Alonso-Herrero, A., Pereira-Santaella, M., Rigopoulou, D., Garcia-Bernete, I., Dominguez-Fernandez, A.J., Combes, F., Davies, R.I., Diaz-Santos, T., Esparza-Arrendondo, D., Gonzalez-Martin, O., Hernan-Caballero, A., Hicks, E.K.S., Hoenig, S.F., Levenson, N.A., Ramos Almeida, C., Roche, P.F. and Rosario, D. (2020) Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies. Astronomy & Astrophysics, 639, [A43]. (doi:10.1051/0004-6361/202037642).

Record type: Article

Abstract

We investigate the relation between the detection of the 11.3  μm polycyclic aromatic hydrocarbon (PAH) feature in the nuclear (∼24-230 pc) regions of 22 nearby Seyfert galaxies and the properties of the cold molecular gas. For the former we use ground-based (0.3-0.6″ resolution) mid-infrared (mid-IR) spectroscopy. The cold molecular gas is traced by ALMA and NOEMA high (0.2-1.1″) angular resolution observations of the CO(2-1) transition. Galaxies with a nuclear detection of the 11.3 μm PAH feature contain more cold molecular gas (median 1.6 × 107 MO ) and have higher column densities (N(H2) = 2 × 1023 cm-2) over the regions sampled by the mid-IR slits than those without a detection. This suggests that molecular gas plays a role in shielding the PAH molecules in the harsh environments of Seyfert nuclei. Choosing the PAH molecule naphthalene as an illustration, we compute its half-life in the nuclear regions of our sample when exposed to 2.5 keV hard X-ray photons. We estimate shorter half-lives for naphthalene in nuclei without a 11.3 μm PAH detection than in those with a detection. The Spitzer/IRS PAH ratios on circumnuclear scales (∼4″ ∼ 0.25-1.3 kpc) are in between model predictions for neutral and partly ionized PAHs. However, Seyfert galaxies in our sample with the highest nuclear H2 column densities are not generally closer to the neutral PAH tracks. This is because in the majority of our sample galaxies, the CO(2-1) emission in the inner ∼4″ is not centrally peaked and in some galaxies traces circumnuclear sites of strong star formation activity. Spatially resolved observations with the MIRI medium-resolution spectrograph on the James Webb Space Telescope will be able to distinguish the effects of an active galactic nucleus (AGN) and star formation on the PAH emission in nearby AGN.

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2005.11959 - Accepted Manuscript
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Accepted/In Press date: 5 May 2020
e-pub ahead of print date: 7 July 2020
Published date: 7 July 2020
Keywords: Galaxies: ISM, Galaxies: Seyfert, Galaxies: nuclei, Radio lines: galaxies

Identifiers

Local EPrints ID: 442458
URI: http://eprints.soton.ac.uk/id/eprint/442458
ISSN: 0004-6361
PURE UUID: a2337839-cf90-4f86-8602-c830f554cb01

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Date deposited: 15 Jul 2020 16:32
Last modified: 25 Nov 2021 17:42

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Contributors

Author: A. Alonso-Herrero
Author: M. Pereira-Santaella
Author: D. Rigopoulou
Author: I. Garcia-Bernete
Author: A.J. Dominguez-Fernandez
Author: F. Combes
Author: R.I. Davies
Author: T. Diaz-Santos
Author: D. Esparza-Arrendondo
Author: O. Gonzalez-Martin
Author: A. Hernan-Caballero
Author: E.K.S. Hicks
Author: S.F. Hoenig
Author: N.A. Levenson
Author: C. Ramos Almeida
Author: P.F. Roche
Author: D. Rosario

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