AGN/starburst connection in action: the half million second RGS spectrum of NGC1365

📝 Abstract

We present a deep (~5.8 days) 0.3-2 keV high-resolution spectrum of NGC1365, collected with the reflection grating spectrometer (RGS) on board XMM-Newton. The spectrum is dominated by strong recombination lines of He- and H-like transitions from carbon to silicon, as well as by L transitions from FeXVII. The continuum is strong, especially in the 10 to 20 Angstrom, range. Formal fits require two optically thin, collisionally ionised plasma components, with temperatures ~300 and ~640 eV. However, they leave the bulk of the forbidden components of the He-alpha OVII and NVI triplets unaccounted for. These features can be explained as being produced by photoionised gas. NGC1365 is therefore the first obscured AGN, whose high-resolution X-ray spectrum requires both collisional ionisation and photoionisation. The relative weakness of photoionisation does not stem from the intrinsic weakness of its AGN, whose X-ray luminosity is ~10^{42} erg/s. We suggest that it may instead come from the line-of-sight from the active nucleus to the NLR being blocked by optically thick matter in the broad line region, at the same time responsible for the large observed variation of the column density obscuring the X-ray active nucleus. Alternatively, NGC1365 could host a remarkably luminous nuclear starburst when compared to the AGN accretion power [abriged].

💡 Analysis

We present a deep (~5.8 days) 0.3-2 keV high-resolution spectrum of NGC1365, collected with the reflection grating spectrometer (RGS) on board XMM-Newton. The spectrum is dominated by strong recombination lines of He- and H-like transitions from carbon to silicon, as well as by L transitions from FeXVII. The continuum is strong, especially in the 10 to 20 Angstrom, range. Formal fits require two optically thin, collisionally ionised plasma components, with temperatures ~300 and ~640 eV. However, they leave the bulk of the forbidden components of the He-alpha OVII and NVI triplets unaccounted for. These features can be explained as being produced by photoionised gas. NGC1365 is therefore the first obscured AGN, whose high-resolution X-ray spectrum requires both collisional ionisation and photoionisation. The relative weakness of photoionisation does not stem from the intrinsic weakness of its AGN, whose X-ray luminosity is ~10^{42} erg/s. We suggest that it may instead come from the line-of-sight from the active nucleus to the NLR being blocked by optically thick matter in the broad line region, at the same time responsible for the large observed variation of the column density obscuring the X-ray active nucleus. Alternatively, NGC1365 could host a remarkably luminous nuclear starburst when compared to the AGN accretion power [abriged].

📄 Content

arXiv:0908.0268v1 [astro-ph.CO] 3 Aug 2009 Astronomy & Astrophysics manuscript no. accepted c⃝ESO 2018 November 16, 2018 AGN/starburst connection in action: the half million second RGS spectrum of NGC 1365 M.Guainazzi1, G.Risaliti,2,3 A.Nucita,1 Junfeng Wang,2 S.Bianchi,4 R.Soria,5 A.Zezas2,6 1European Space Astronomy Centre of ESA, P.O.Box 78, Villanueva de la Ca˜nada, E-28691 Madrid, Spain e-mail: Matteo.Guainazzi@sciops.esa.int 2Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02318 3INAF-Osservatorio di Arcetri, Firenze, I-50125, Italy 4Dipartimento di Fisica, Universit`a degli Studi Roma Tre, via della Vasca Navale 84, I-00146 Roma, Italy 5Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK 6Physics Department, University of Crete, P.O. Box 2208, GR-710 03, Heraklion, Crete, Greece Received ; accepted ABSTRACT Context. High-resolution X-ray observations in the imaging and spectral domain have recently opened a new window on active galactic nuclei (AGN) feedback onto the circumnuclear gas. Spectral diagnostics, as well as the remarkable morphological coincidence between [O iii] and X-rays, point to AGN photoionisation as the dominant ionisation mech- anism on scales as large as a few kpc. Aims. In this paper we extend these studies to the nearby Seyfert 2 galaxy NGC 1365, known to host a circumnuclear ring of intense star formation at ≃1.3 kpc from the nucleus. The main scope of this investigation is to study the con- nection between nuclear activity and star formation in nearby AGN. Methods. We present a deep (≃5.8 days) 0.3–2 keV high-resolution spectrum of NGC 1365, collected with the reflection grating spectrometer (RGS) on board XMM-Newton. Results. The spectrum is dominated by strong recombination lines of He- and H-like transitions from carbon to silicon, as well as by L transitions from Fexvii. The continuum is strong, especially in the 10 to 20˚A range. Formal fits require two optically thin, collisionally ionised plasma components, with temperatures ≃300 and ≃640 eV. However, they leave the bulk of the forbidden components of the He-α Ovii and Nvi triplets unaccounted for. These features can be ex- plained as being produced by photoionised gas. NGC 1365 is therefore the first obscured AGN, whose high-resolution X-ray spectrum requires both collisional ionisation and photoionisation. Conclusions. The relative weakness of photoionisation does not stem from the intrinsic weakness of its AGN, whose X-ray luminosity is ∼> 1042 erg s−1. We suggest that it may instead come from the line-of-sight from the active nucleus to the NLR being blocked by optically thick matter in the broad line region, at the same time responsible for the large observed variation of the column density obscuring the X-ray active nucleus. Alternatively, NGC 1365 could host a remarkably luminous nuclear starburst when compared to the AGN accretion power. Key words. Galaxies: active – Galaxies:Seyferts – Galaxies: starbursts – X-rays:galaxies – X-rays:individual:NGC 1365

1. Ionising the NLRs: the X-ray view Most of the observational constraints on the physics and geometry of narrow line regions (NLRs) in active galactic nuclei (AGN) come from optical spectroscopy and imag- ing. Comparison between spatially resolved spectroscopy of nearby bright obscured AGN and photoionisation models (Ferland & Osterbrock 1986, Osterbrock 1989) and the study of ionisation cones (Pogge 1988, Tadhunter & Tsvetanov 1989) indicate that the AGN high-energy emission is the main source of ionising photons, although the contribution by collisionally ionised plasma also seems to be required to accurately match the spectra (Viegas-Aldrovandi & Contini 1989). A correlation be- tween radio power and line widths (Wilson & Willis 1980, Whittle 1985, Whittle 1992, Ulvestad & Ho 2001) and the good morphological correlation between [OIII] and radio images (Capetti et al. 1996, Axon et al. 1998; see however Send offprint requests to: M.Guainazzi Das et al. 2006 for a different view) raise the possibility that the interaction between radio jets and the NLR gas may affect the ionisation state of the NLRs as well. Experimental support to this hypothesis has so far been presented in one case only (Axon et al. 1998). In this context, X-rays have recently opened a comple- mentary window on NLR physics. Whenever (extended) NLRs in nearby Seyfert 2 galaxies are larger than the angular resolution of the Chandra optics (≃0.5′′), X- ray extended emission on scales from a few hundred to about 2 kpc has been discovered (Wilson et al. 2000, Young et al. 2001, Bianchi et al. 2006). X-rays bear a re- markable morphological similarity to images in the [[OIII] band. Simple photoionisation models applied to the optical to X-ray surface-brightness ratio indi- cates that solutions exist in terms of a single phase AGN-photoionised medium, where the ionisation pa- rameter remains constant across the whole ENLR (Bianchi et al.

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