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Embedded AGN and star formation in the central 80 pc of IC 3639

Embedded AGN and star formation in the central 80 pc of IC 3639
Embedded AGN and star formation in the central 80 pc of IC 3639
[Abridged] Methods: We use interferometric observations in the $N$-band with VLTI/MIDI to resolve the mid-IR nucleus of IC 3639. The origin of the nuclear infrared emission is determined from: 1) the comparison of the correlated fluxes from VLTI/MIDI with the fluxes measured at subarcsec resolution (VLT/VISIR, VLT/ISAAC); 2) diagnostics based on IR fine-structure line ratios, the IR continuum emission, IR bands produced by polycyclic aromatic hydrocarbons (PAH) and silicates; and 3) the high-angular resolution spectral energy distribution.

Results: The unresolved flux of IC 3639 is $90 \pm 20\, \rm{mJy}$ at $10.5\, \rm{\mu m}$, measured with three different baselines in VLTI (UT1-UT2, UT3-UT4, and UT2-UT3; $46$-$58\, \rm{m}$), making this the faintest measurement so far achieved with mid-IR interferometry. The correlated flux is a factor of $3$-$4$ times fainter than the VLT/VISIR total flux measurement. The observations suggest that most of the mid-IR emission has its origin on spatial scales between $10$ and $80\, \rm{pc}$ ($40$-$340\, \rm{mas}$). A composite scenario where the star formation component dominates over the AGN is favoured by the diagnostics based on ratios of IR fine-structure emission lines, the shape of the IR continuum, and the PAH and silicate bands.

Conclusions: A composite AGN-starburst scenario is able to explain both the mid-IR brightness distribution and the IR spectral properties observed in the nucleus of IC 3639. The nuclear starburst would dominate the mid-IR emission and the ionisation of low-excitation lines (e.g. [NeII]$_{12.8 \rm{\mu m}}$) with a net contribution of $\sim 70\%$. The AGN accounts for the remaining $\sim 30\%$ of the mid-IR flux, ascribed to the unresolved component in the MIDI observations, and the ionisation of high-excitation lines (e.g. [NeV]$_{14.3 \rm{\mu m}}$ and [OIV]$_{25.9 \rm{\mu m}}$).
0004-6361
Fernandez-Ontiveros, J.A.
23f1427a-3263-40c4-927e-a1d7f5f9ddc8
Tristram, K.R.W.
37fab926-5cfc-4c23-8f75-52fafaa518b6
Hönig, S.
be0bb8bc-bdac-4442-8edc-f735834f3917
Gandhi, P.
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9
Weigelt, G.
bd0be979-0519-484f-b489-e3e44467bf57
Fernandez-Ontiveros, J.A.
23f1427a-3263-40c4-927e-a1d7f5f9ddc8
Tristram, K.R.W.
37fab926-5cfc-4c23-8f75-52fafaa518b6
Hönig, S.
be0bb8bc-bdac-4442-8edc-f735834f3917
Gandhi, P.
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9
Weigelt, G.
bd0be979-0519-484f-b489-e3e44467bf57

Fernandez-Ontiveros, J.A., Tristram, K.R.W., Hönig, S., Gandhi, P. and Weigelt, G. (2017) Embedded AGN and star formation in the central 80 pc of IC 3639. Astronomy & Astrophysics. (doi:10.1051/0004-6361/201731906).

Record type: Article

Abstract

[Abridged] Methods: We use interferometric observations in the $N$-band with VLTI/MIDI to resolve the mid-IR nucleus of IC 3639. The origin of the nuclear infrared emission is determined from: 1) the comparison of the correlated fluxes from VLTI/MIDI with the fluxes measured at subarcsec resolution (VLT/VISIR, VLT/ISAAC); 2) diagnostics based on IR fine-structure line ratios, the IR continuum emission, IR bands produced by polycyclic aromatic hydrocarbons (PAH) and silicates; and 3) the high-angular resolution spectral energy distribution.

Results: The unresolved flux of IC 3639 is $90 \pm 20\, \rm{mJy}$ at $10.5\, \rm{\mu m}$, measured with three different baselines in VLTI (UT1-UT2, UT3-UT4, and UT2-UT3; $46$-$58\, \rm{m}$), making this the faintest measurement so far achieved with mid-IR interferometry. The correlated flux is a factor of $3$-$4$ times fainter than the VLT/VISIR total flux measurement. The observations suggest that most of the mid-IR emission has its origin on spatial scales between $10$ and $80\, \rm{pc}$ ($40$-$340\, \rm{mas}$). A composite scenario where the star formation component dominates over the AGN is favoured by the diagnostics based on ratios of IR fine-structure emission lines, the shape of the IR continuum, and the PAH and silicate bands.

Conclusions: A composite AGN-starburst scenario is able to explain both the mid-IR brightness distribution and the IR spectral properties observed in the nucleus of IC 3639. The nuclear starburst would dominate the mid-IR emission and the ionisation of low-excitation lines (e.g. [NeII]$_{12.8 \rm{\mu m}}$) with a net contribution of $\sim 70\%$. The AGN accounts for the remaining $\sim 30\%$ of the mid-IR flux, ascribed to the unresolved component in the MIDI observations, and the ionisation of high-excitation lines (e.g. [NeV]$_{14.3 \rm{\mu m}}$ and [OIV]$_{25.9 \rm{\mu m}}$).

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1711.01268 - Accepted Manuscript
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Accepted/In Press date: 3 November 2017
e-pub ahead of print date: 13 November 2017

Identifiers

Local EPrints ID: 415587
URI: http://eprints.soton.ac.uk/id/eprint/415587
ISSN: 0004-6361
PURE UUID: 5b114313-bec8-4ba4-b7d0-1d2990a5d2c2
ORCID for P. Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 15 Nov 2017 17:30
Last modified: 07 Oct 2020 02:06

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Contributors

Author: J.A. Fernandez-Ontiveros
Author: K.R.W. Tristram
Author: S. Hönig
Author: P. Gandhi ORCID iD
Author: G. Weigelt

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