The X-ray view of the ionization cone in NGC5252
Matteo Guainazzi, Stefano Bianchi, Massimo Cappi, Mauro Dadina, Giuseppe Malaguti
aa r X i v : . [ a s t r o - ph ] O c t To appear in “Conference Title (2001)”
RevMexAA(SC)
THE X-RAY VIEW OF THE IONIZATION CONE IN NGC 5252
Matteo Guainazzi , Stefano Bianchi , Massimo Cappi , Mauro Dadina , Giuseppe Malaguti RESUMENABSTRACTWe present the results of a
Chandra soft X-ray observation of the spectacular ionization cone in the nearbySeyfert 2 galaxy NGC 5252. As almost invariably observed in obscured AGN, the soft X-ray emission exhibitsa remarkable morphological concidence with the cone ionized gas as traced by HST O[ iii ] images. Energy-resolved images and high-resolution spectroscopy suggest that the X-ray emitting gas is photoionized by theAGN, at least on scales as large as the innermost gas and stellar ring ( ≤ ∼ years. Key Words:
GALAXIES:ACTIVE – GALAXIES:NUCLEI – GALAXIES:SEYFERT – X-RAYS:GALAXIES – GALAXIES:INDIVIDUAL:NGC 5252
The Seyfert 2 galaxy NGC 5252 hosts oneof the most spectacular ionization cones ever ob-served in an Active Galactic Nucleus (AGN).Line emission exhibits a bi-conical morphology(Tadhunter & Tsvetanov 1989) extending out to ≃
20 kpc either side of the active nucleus. Opticalstudies have suggested that O[ iii ] emission is due tophotoionization by the active nucleus. On smallerscales ( ≃ ≃ ◦ ) stellar disk, andtwo gas disks associated with it and rotating in op-posite directions. It has been suggested that the gasof this inner structure is also photoionized by theAGN (Morse et al. 1998).Recent Chandra high-resolution observationshave revealed large scales (hundreds of parsecsto kilo-parsecs) soft X-ray emission in almost allnearby obscured AGN with Extended Narrow LineRegions (ELNRs) where this measurement is tech-nically possible (Bianchi et al. 2006). Soft X-raysshow a remarkable morphological coincidence withhigh-resolution O[ iii]
HST images. High-resolutionspectra taken with the Reflection Grating Spec-trometer on-board XMM-Newton confirm thatthe soft X-rays carry the unmistakable signaturesof photoionized gas: “narrow” ( δE ≃ European Space Astronomy Center of ESA,Apartado 50727, E-28080 Madrid, Spain; email:
[email protected] . Dipartimento di Fisica “E.Amaldi”, Universit`a di RomaTre, Via della Vasca Navale, I-00146, Roma, Italy. INAF-IASF Bologna, via Gobetti 101, 40129 Bologna,Italy. between the forbidden component of the He-likeH α triplets and the H-like Ly α (Sako et al. 2000;Sambruna et al. 2001; Kinkhabwala et al. 2002;Guainazzi & Bianchi 2007). The large intensity ofhigher-order transitions with respect to the K α areindicative of an important role played by resonantscattering.In this paper, we briefly discuss the soft X-rayproperties of NGC 5252. At its distance (92 Mpc)the spatial scale is: 1 ′′ = 450 pc.The soft X-ray (0.2-1 keV) Chandra /ACIS emis-sion in NGC 5252 is extended on scales as large as ≃
11 kpc (Fig. 1). The hard X-ray (1–10 kpc) im-age is instead unresolved. There is a good mor-phological coincidence between the X-rays and theO[ iii ] emission, taking into account the differentspatial resolutions. The agreement extends also tothe diffuse soft X-ray emission overlapping with thesmaller scale ( ≃ vi and by the Fe-L com-plex (Fig. 2). The measured flux ratio between thecounts in the O vii and O viii bands, once correctedfor the energy dependence of the ACIS effective area(2 . ± . U ) ∼ (see Fig. 3).In Fig. 4 we show the O[ iii ] to soft X-ray fluxratio as a function of the radius along the ionizationcone. The ratio is calculated at the position of opti-cally bright knots and filaments along the cone. On U is defined as Φ( H ) n ( H ) c , where Φ( H ) is the surface flux ofionizing photons, and n ( H ) is the total hydrogen density ACIS−S (contours) and HST/WFPC2 (grayscale)NGC5252
Fig. 1. O[ iii ] HST/WFC2 ( greyscale ) and 0.2-1 keV Chandra/ACIS ( smoothed contours ) images ofNGC 5252. The image is 35 ′′ aside.Fig. 2. Energy resolved Chandra /ACIS images ofNGC 5252. The coordinate reference system is cen-tered on the position of the X-ray active nucleus(Cappi et al. 1996). Fig. 3.
Solid line : distribution of the observed valuesof the flux logarithmic ratio between the O vii He α for-bidden component and the O viii Ly α ; dashed and dottedlines : Cloudy predictions as a function of the ionizationparameter U (y-axis) for different values of the gas col-umn density and ionizing continuum X-ray photon indexΓ. the same plot, we compare the predictions of simple,homogeneous and time-independent photoionizationmodels, following the method described in Bianchi etal. (2006). An almost constant ionization parameteralong the cone is required, implying a radial decreaseof the electronic density as: n ( r ) ∝ r − (1 . − . . Simi-lar trends had been observed in other obscured AGNwith ENLRs (Bianchi et al. 2006). Such a decreaseis steeper than required by optical diagnostics ofspace resolved NLRs (Bennert et al. 2006), and maysuggest that a local photoionizing source contributeto the soft X-ray emission. Shocks heating of the hotgas by stellar winds or interaction with a feeble jetare possible culprits.If AGN photoionization is still responsible for thebulk of the ionization equilibrium in the gas, onecan derive from the results in Fig. 4 and the knowngeometry of the cone knots and filaments the historyof the active nucleus responsible for the ionizationcone observed nowadays (Fig. 5). Its average levelof activity in the last ∼ Fig. 4. O[ iii ] to soft X-ray flux ratio along the ionizationcone (the x-axis is the distance from the optical positionof the active nucleus). The lines represent the predictionsof a
CLOUDY -based (Ferland et al. 1998) model in pho-toionization equilibrium, where the AGN is responsiblefor photoionizing the cone gas. The AGN Spectral En-ergy Distribution has: α ox = − . α U V = − .
50, and α X = 2 .
0. An X-ray to bolometric luminosity correctionof 1/0.03 is assumed (Elvis et al. 1994).