MNRAS 457, 3629–3636 (2016) doi:10.1093/mnras/stw230 Advance Access publication 2016 January 29 Extended X-ray emission in the IC 2497 – Hanny’s Voorwerp system: energy injection in the gas around a fading AGN Downloaded from https://academic.oup.com/mnras/article-abstract/457/4/3629/2589023 by Pontificia Universidad Catolica de Chile user on 11 June 2019 Lia F. Sartori,1‹ Kevin Schawinski,1‹ Michael Koss,1 Ezequiel Treister,2 W. Peter Maksym,3 William C. Keel,4 C. Megan Urry,5 Chris J. Lintott6 and O. Ivy Wong7 1Institute for Astronomy, Department of Physics, ETH Zurich,¨ Wolfgang-Pauli-Strasse 27, CH-8093 Zurich,¨ Switzerland 2Departamento de Astronom´ıa, Universidad de Concepcion,´ Casilla 160-C, Concepcion,´ Chile 3Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 4Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA 5Yale Center for Astronomy & Astrophysics, Physics Department, PO Box 208120, New Haven, CT 06520, USA 6Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 7ICRAR, The University of Western Australia M468, 35 Stirling Highway, Crawley, WA 6009, Australia Accepted 2016 January 25. Received 2016 January 25; in original form 2015 November 27 ABSTRACT We present deep Chandra X-ray observations of the core of IC 2497, the galaxy associated with Hanny’s Voorwerpand hosting a fading AGN. We find extended soft X-ray emission from 42 44 −1 hot gas around the low intrinsic luminosity (unobscured) AGN (Lbol ∼ 10 –10 erg s ). The temperature structure in the hot gas suggests the presence of a bubble or cavity around the 54 55 fading AGN (Ebub ∼ 10 –10 erg). A possible scenario is that this bubble is inflated by the fading AGN, which after changing accretion state is now in a kinetic mode. Other possibilities 46 −1 are that the bubble has been inflated by the past luminous quasar (Lbol ∼ 10 erg s ), or that the temperature gradient is an indication of a shock front from a superwind driven by the AGN. We discuss the possible scenarios and the implications for the AGN–host galaxy interaction, as well as an analogy between AGN and X-ray binaries lifecycles. We conclude that the AGN could inject mechanical energy into the host galaxy at the end of its lifecycle, and thus provide a source for mechanical feedback, in a similar way as observed for X-ray binaries. Key words: galaxies: active – quasars: general – quasars: individual: IC 2497 – X-rays: galaxies. and low electron temperature (T = 13500 ± 1300 K) exclude the 1 INTRODUCTION e possibility of ionization from shocks (Lintott et al. 2009). IC 2497 is a massive (M ∼ 1010 M), nearby (z = 0.05) spiral Large-scale radio observations of the system revealed an extended 10 galaxy associated with Hanny’s Voorwerp (HV), an extended emis- H I structure (M = 8.5 ± 2.1 × 10 M) with irregular kinematics sion line region (11 × 16 kpc in projected extent) located at ∼20 kpc around the southern part of IC 2497, which suggests a tidal origin in projection from the core of the galaxy and closely matching its (Jozsa´ et al. 2009). These observations also show a kpc-scale struc- redshift. HV was discovered by Hanny van Arkel,1 a citizen scientist ture which may be a jet coming from the AGN. HV lies where the participating in the Galaxy Zoo project (Lintott et al. 2008, 2009). jet meets the H I reservoir and corresponds to a local decrement of The optical spectrum of HV is dominated by [O III] λλ4959, 5007 Å H I column density which may be due to photoionization. At smaller emission, and the presence of lines of high ionization species such scales, Rampadarath et al. (2010) found a radio AGN in the core of as [Ne V] λλ3346, 3426 and He IIλ4616 suggests that the cloud is IC 2497, and a second nuclear source which may be a jet hotspot in photoionized by the hard continuum of an active galactic nucleus the large-scale jet reported by Jozsa´ et al. (2009). (AGN) in IC 2497 rather than star formation (Lintott et al. 2009). In order to produce sufficient ionizing photons to power the ob- −1 Moreover, the relatively quiet kinematics (line widths <100 km s ) served [O III] emission in HV, the quasar in IC 2497 has to have a 46 −1 bolometric luminosity of at least Lbol = 10 erg s .However,the optical nuclear spectrum shows only very weak emission lines, so E-mail: [email protected] (LFS); [email protected] that the ionizing source is classified as LINER or low-luminosity (KS) Seyfert galaxy (Lintott et al. 2009; Keel et al. 2012a). The two 1 Hanny’s Voorwerp means Hanny’s object in Dutch. possible explanations to reconcile observed and expected emission C 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 3630 L. F. Sartori et al. are: (1) the quasar is obscured only along our line of sight but not (PIX_ADJ=EDSER; Tsunemi et al. 2001;Lietal.2003, 2004), and in the direction of HV, or (2) IC 2497 hosts a faded AGN. This flagged the background events most likely associated with cosmic second option would mean that the quasar dropped in luminosity rays by running the task ACIS_PROCESS_EVENTS (CHECK_VF_PHA=YES within the last ∼200 kyr, the travel time needed from the photons in CHANDRA_REPRO). We analysed the background light curves with to reach the cloud (Keel et al. 2012a), but HV still remains lit up the CIAO routine lc_sigma_clip() and found no interval of un- because of this travel time. Support for the fading scenario was usually strong background flaring. Because of the low count rate Downloaded from https://academic.oup.com/mnras/article-abstract/457/4/3629/2589023 by Pontificia Universidad Catolica de Chile user on 11 June 2019 given by Schawinski et al. (2010) who analysed the IR data from the observations show no evidence of pileup. IRAS and X-ray data from XMM and Suzaku obtained for IC 2497. In order to correct for the offset in the relative astrometry of the First, as already reported in Lintott et al. (2009) the IR data show no two observations, we first rebinned the reprocessed event files to evidence of the reprocessed luminosity expected from an obscured 1/4 pixel resolution (0.125 arcsec) using the CIAO DMCOPY routine strong quasar. In addition, the XMM spectrum is best fitted with an with binning factor 0.25. We then ran the CIAO DMSTAT routine on the unobscured power law with photon index = 2.5 ± 0.7, as expected rebinned images to get the centroid of the emission from IC 2497. from an unobscured AGN, and a component for the hot gas in the In the following we will consider the centroid as the centre of the galaxy. The quality of the fit is not improved if additional absorp- emission in the respective observation. tion and obscuration are taken into account. This model implies a We extracted the spectra and generated the response files using 42 −1 bolometric luminosity of the AGN Lbol = 4.2 × 10 erg s ,which the CIAO tool SPECEXTRACT (see Section 3 for more details about the is ∼4 orders of magnitude lower than that expected from HV. In extraction regions). For the background estimation we considered addition, Suzaku observations show only a marginal detection with three source-free circular regions with a 5 arcsec radius ∼20 arc- the 15–30 keV PIN camera (hard X-ray detector). If these counts sec away from the source. We then grouped each spectrum with a are real, this would mean that the AGN is strongly obscured and minimum of three counts per bin using the HEASOFT tool GRPPHA. we are observing the reemission instead of the AGN power law, We simulated the PSF images needed for the analysis using MARX but the obtained luminosity would still be ∼2 orders of magnitude 5.1 (Davis et al. 2012). First, we created a model of the emis- below that expected from HV. Also the HST observations presented sion using XSPEC and SHERPA (see Section 3 for details about the by Keel et al. (2012a) argue against the hypothesis of a luminous, assumed models). We then used MARX to run the RAYTRACE sim- strongly obscured AGN. WFC3 images show a complex dust struc- ulation and project the ray-tracings on to the ACIS-S detector. ture near the nucleus of the galaxy, but the view of the nucleus is not Since we are interested in the sub-pixel regime we ran the sim- hindered by absorbing features, so that obscuration cannot be the ulations with pixadj=EDSER and AspectBlur=0.19 arcsec explanation of the lack of strong AGN features. Moreover, there is (telescope pointing uncertainty). We also included the telescope no high-ionization gas near the nucleus, suggesting that the current dithering (DitherModel=INTERNAL, DitherAmp_Ra=8, radiative output from the AGN is low. DitherAmp_Dec=8) and corrected for SIM drift and offset.2 The observations described above suggest that the quasar in IC 2497 dropped in luminosity by at least 2 orders of magnitude ∼ 5 in the last 10 yr. The system composed by HV and its galaxy is 3ANALYSIS therefore a great laboratory to study AGN variability on previously inaccessible time-scales. The analysis of the galaxy allows us to 3.1 Radial profile and source extension study the present state of the AGN and the AGN-host galaxy inter- action in this ‘post quasar’ phase, while HV provides information We compute the 0.5–6.0 keV background-subtracted surface bright- about the past state of the AGN.