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MUSE Observations of a Changing-Look AGN–I. The MNRAS 000, 000–000 (0000) Preprint 27 March 2019 Compiled using MNRAS LATEX style file v3.0 MUSE observations of a changing-look AGN I: The re-appearance of the broad emission lines S. I. Raimundo1?, M. Vestergaard1,2, J. Y. Koay3, D. Lawther1, V. Casasola4, B. M. Peterson5,6,7 1 DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen, Denmark 2 Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA 3 Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan 4 INAF-Instituto di Radioastronomia, Via Piero Gobetti 101, 40129, Bologna, Italy 5 Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210,USA 6 Center for Cosmology and AstroParticle Physics, The Ohio State University, 191 West Woodruff Avenue, Columbus, OH 43210, USA 7 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Accepted 2019 March 15. Received 2019 March 8; in original form 2019 January 18 ABSTRACT Optical changing-look Active Galactic Nuclei (AGN) are a class of sources that change type within a short timescale of years or decades. This change is characterised by the appearance or disappearance of broad emission lines, often associated with dramatic AGN continuum flux changes that are orders of magnitude larger than those expected from typical AGN variability. In this work we study for the first time the host galaxy of a changing-look AGN, Mrk 590, using high spatial resolution optical and near-infrared observations. We discover that after ∼ 10 yr absence, the optical broad emission lines of Mrk 590 have reappeared. The AGN optical continuum flux however, is still ∼ 10 times lower than that observed during the most luminous state in the 1990s. The host galaxy shows a 4.5 kpc radius star-forming ring with knots of ionised and cold molecular gas emission. Extended ionised and warm molecular gas emission are detected in the nucleus, indicating that there is a reservoir of gas as close as 60 pc from the black hole. We observe a nuclear gas spiral between radii r ∼ 0.5 − 2 kpc, which has been suggested as a dynamical mechanism able to drive the necessary gas to fuel AGN. We also discover blue-shifted and high velocity dispersion [O III] emission out to a radius of 1 kpc, tracing a nuclear gas outflow. The gas dynamics in Mrk 590 suggest a complex balance between gas inflow and outflow in the nucleus of the galaxy. Key words: galaxies: active – galaxies: individual: Mrk 590 – galaxies: kinematics and dynamics – galaxies: nuclei – galaxies: Seyfert 1 INTRODUCTION sight towards the AGN was put forward as an explanation for this dichotomy (Lawrence & Elvis 1982, Antonucci 1984, Two of the most easily recognisable features of Active Galac- Antonucci 1993). Due to the presence of an equatorial dust tic Nuclei (AGN) are optical-ultraviolet continuum emission and gas torus-like structure surrounding the accretion disc from the accretion disc and broad emission lines with a typ- and broad line region, broad emission lines would only be ob- arXiv:1903.08696v2 [astro-ph.GA] 26 Mar 2019 ical velocity broadening of v ∼ 1000 - 10000 km s−1 FWHM served for viewing angles close to the polar direction. AGN (Peterson 1997). However, broad emission lines are not ubiq- for which the viewing angle is along the equatorial plane uitous in AGN. A subset of AGN have narrow emission lines would be classified as type 2 due to the broad emission lines (v 500 km s−1) but lack broad lines in their spectra. FWHM . being obscured and therefore absent from the spectra. This prompted a historical classification into AGN types: type 1 to identify AGN with broad (and narrow) emission The simple paradigm described above has been chal- lines and type 2 to identify AGN without broad emission lenged by observations of AGN that defy this classification. lines (Khachikian & Weedman 1974). The observer’s line-of- For example the so-called ‘naked AGN’ (Hawkins 2004) or true type 2 AGN (Panessa et al. 2009, Bianchi et al. 2017), AGN in which broad emission lines are not observed even ? E-mail: [email protected] though there is no evidence for obscuration along the line- c 0000 The Authors 2 S. I. Raimundo of-sight. It has been suggested that these non-conforming based on a disc-wind scenario for the AGN broad line re- AGN are in a transition phase. Possibly due to low black gion. They suggest that as the AGN continuum luminosity hole mass accretion rates (e.g. Bianchi et al. 2017) these decreases, the distance from the BLR clouds to the accretion AGN are not able to produce broad emission lines. Notably, disc will decrease, reducing the fraction of continuum pho- there is another class of AGN that have been observed be- tons intercepted by the BLR clouds. In case of a decrease fore and after a transition between types occurred, the so in AGN continuum luminosity, the model of Elitzur et al. called ‘changing-look’ AGN. These AGN were discovered (2014) predicts a decrease in the broad line strength. This due to having significantly different spectral features when transition is controlled by changes in Lbol with the vari- 2/3 observed at two or more different epochs. ‘Changing-look’ able Lbol/MBH controlling the transition between spectral AGN was a term first used to classify AGN that showed classes (such as type 1, type 2 and intermediate types). an extreme variation in X-ray detected obscuration, tran- If a change in black hole mass accretion rate is respon- sitioning from what would be interpreted as heavily ob- sible for the change in AGN continuum flux, such change scured to unobscured and vice-versa, in a short period of needs to occur within a timescale of years or decades. This time (Matt et al. 2003, Bianchi et al. 2005). This definition timescale is much shorter than the characteristic timescale has been extended to include optical ‘changing-look’ AGN for significant changes in the surface mass density of a stan- and quasars, to refer to AGN of low and high luminosity dard thin accretion disc (Shakura & Sunyaev 1973), i.e. the that transition between type 1 and type 2 or vice-versa. viscous timescale ∼ 105 yr (e.g. Noda & Done 2018). Vari- There are now a few tens of known changing-look AGN, in- ations on shorter timescales have recently been put forward cluding: Mrk 590 (Denney et al. 2014), NGC 2617 (Shappee to explain the mass accretion rate variation timescales ob- et al. 2014), NGC 7603 (Tohline & Osterbrock 1976), Mrk served in changing-look AGN. These include for example, 1018 (Cohen et al. 1986, McElroy et al. 2016, Husemann the propagation of a cold front in an inflated accretion disc et al. 2016), NGC 1097 (Storchi-Bergmann et al. 1993), NGC (Ross et al. 2018, Stern et al. 2018) or the propagation of a 3065 (Eracleous & Halpern 2001) and NGC 7582 (Aretxaga cold front and a state transition (as seen in black hole bi- et al. 1999). Transition between intermediate Seyfert types: naries) in an accretion disc dominated by radiation pressure HE 1136-2304 (Zetzl et al. 2018, Kollatschny et al. 2018) (Noda & Done 2018). and transitions in quasars e.g.: SDSS J015957.64+003310.5 Additionally, for AGN where a single transition from (LaMassa et al. 2015), SDSS J012032.19-005501.9 (Li et al. type 1 to type 2 and a dramatic decrease in AGN continuum 2015), SDSS J155440.25+362952.0 (Gezari et al. 2017) and flux are observed, it is possible that the AGN will not return SDSS J141324.27+530527.0 (Wang et al. 2018), have also to a type 1. Such a transition could be associated with the been observed. While most of the first changing-look AGN turn-off of the AGN, as it is not guaranteed that the AGN were discovered serendipitously, systematic searches have re- will turn on again in the future. While it is not clear what cently increased the number of known changing-look AGN. controls the black hole mass accretion rate, a shortage of gas A study by Runco et al.(2016) of 102 low redshift Seyfert supply from the galaxy or lack of gas transport mechanisms galaxies finds that only 3 Seyferts (3% of the sample) show may limit the gas available for AGN fuelling. a disappearance of broad emission lines, while 38% show a In this work we study in detail and for the first time the transition between intermediate types. Systematic searches host galaxy of Mrk 590, a changing-look AGN, using optical at the higher luminosity range found a few tens of new and near-infrared integral field spectroscopy. We study the changing-look AGN in optical (Ruan et al. 2016, MacLeod past history of the host galaxy, determine its gas content et al. 2018, Yang et al. 2018) and infrared (Ross et al. 2018, and distribution and search for dynamical mechanisms that Stern et al. 2018) quasar catalogues. To identify AGN that may drive gas to and from the black hole. The goal of the transitioned between types, two or more epochs of observa- study presented here is to obtain clues on the host nuclear tions are needed. The transitions in the literature have been environment and physics of a changing-look AGN. observed to occur within a timescale of years up to a few decades, but the current timescale upper limit is likely due to the limited time span covered by historical monitoring 1.1 Mrk 590 data.
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