Draft version June 30, 2021 Typeset using LATEX twocolumn style in AASTeX61 SERENDIPITOUS DISCOVERY OF AN OPTICAL EMISSION LINE JET IN NGC 232 C. Lopez{Cob´ a,´ 1 S. F. Sanchez,´ 1 I. Cruz{Gonzalez,´ 1 L. Binette,1 L. Galbany,2 T. Kruhler,¨ 3 L. F. Rodr´ıguez,4 J. K. Barrera{Ballesteros,5 L. Sanchez{Menguiano,´ 6, 7 C. J. Walcher,8 E. Aquino-Ort´ız,1 and J. P. Anderson9 1Instituto de Astronom´ıa, Universidad Nacional Aut´onomade M´exico, A. P. 70-264, C.P. 04510, M´exico, D.F., Mexico 2PITT PACC, Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA 3Max-Planck-Institut f¨urextraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany 4Instituto de Radioastronom´ıay Astrof´ısica, Universidad Nacional Aut´onomade M´exico, C.P. 58190, Morelia, Mexico 5Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA 6Departamento de F´ısica Te´orica y del Cosmos, Universidad de Granada, Campus de Fuentenueva, 18071 Granada, Spain 7Instituto de Astrof´ısica de Andaluc´ıa(CSIC), Glorieta de la Astronom´ıas/n, 3004, 18080 Granada, Spain 8Leibniz-Institut f¨urAstrophysik (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany 9European Southern Observatory, Alonso de C´ordova 3107, Vitacura, Casilla 190001, Santiago, Chile (Received July 1, 2016; Revised September 27, 2016; Accepted June 30, 2021) Submitted to ApJL ABSTRACT We report the detection of a highly collimated linear emission-line structure in the spiral galaxy NGC 232 through the use of integral field spectroscopy data from the All-weather MUse Supernova Integral field Nearby Galaxies (AMUSING) survey. This jet{like feature extends radially from the nucleus and is primarily detected in [O III]λ5007 without clear evidence of an optical continuum counterpart. The length of the radial structure projected on sky reaches ∼ 3 kpc, which makes NGC 232 the second longest emission-line jet reported. The ionized gas presents extreme [O III]/Hβ and [N II]/Hα line ratios, increasing along the jet-like structure. We discuss three possible scenarios to explain the observed structure: (i) direct ionization of in-falling material from the intergalactic medium by the AGN; (ii) photo-ionization by an un-detected optical counter-part of the radio jet and (iii) fast shocks ionization due to the lateral expansion of the radio jet across the ISM. Our analysis favors in-situ ionization. Keywords: galaxies: individual (NGC 232) | galaxies: ISM | galaxies: jets | galaxies: spiral | ISM: jets and outflows | radio continuum: galaxies arXiv:1711.02785v1 [astro-ph.GA] 8 Nov 2017 Corresponding author: C. L´opez–Cob´a [email protected] 2 Lopez{Cob´ a´ et al. 1. INTRODUCTION the Multi Unit Spectroscopic Explorer (MUSE, Bacon In active galactic nuclei (AGN), the primary source of et al. 2010) on the ESOs Very Large Telescope UT4 ionizing photons responsible for exciting the extended (Yepun). MUSE is an integral-field spectrograph (IFS) 2 emission-line regions (EELR) comes from the accretion with a large field-of-view (FoV) of 1 arcmin and a high 00 −1 disk in the nucleus. However, in the case of colli- spatial sampling of 0.2 spaxel . It covers the wave- ˚ mated radio jets with enhanced optical emission lines, length range between 4750 and 9300 A, with a constant ˚ the source of the ionization is still an open question. It spectral sampling of 1.25 A and a spectral resolution of ˚ has been proposed that the ionizing continuum in this FWHM ∼ 2:6 A. A total integration time of ∼ 2000 s case is generated in situ (e.g., Binette et al. 1993). Two was performed on source. Data reduction is described possibilities have been suggested for generating such lo- in Galbany et al.(2016)& Kr¨uhleret al.(2017). The cally produced continua: 1) by lateral fast shocks along final dataset comprises one cube of ∼ 100; 000 individ- the relativistic radio jet as it propagates in the ISM (e.g., ual spectra with a spatial resolution determined by the 00 Axon et al. 1989; Fraix-Burnet 1992; Sutherland et al. average seeing during the observation ∼ 0: 67 (∼ 300 pc 1993), and 2) synchrotron emission from the radio jet at the distance of NGC 232, using a standard cosmol- −1 −1 itself (e.g., Jarvis 1990). Nevertheless, direct ionization ogy of Ho =70 km s Mpc ,Ωm =0.27, ΩΛ =0.73). by the AGN cannot be excluded. In addition we use radio maps extracted from the Very The EELR of Seyfert galaxies is usually studied in Large Array (VLA) archives, observed at a frequency of [O III]λ5007, a feature that shows a wide of shapes. 1.4 GHz (VLA AM384 project). Its synthesized beam 00 00 ◦ These are typically broadly conical or clumpy and ra- is 3: 22 × 1: 66, with a PA = -19.7 . Thus, the VLA im- dially elongated, ranging in lengths from hundreds to age has a coarse spatial resolution compared to the IFS thousands of parsecs. EELRs are frequently, but not al- data. ways, aligned with radio jets (e.g., Capetti et al. 1995b; 3. ANALYSIS Husemann et al. 2008) when such a jet is present. Gen- The data were analyzed using the Pipe3D pipeline erally, the emission line gas associated with the jets is (e.g., S´anchez et al. 2016), a code created to deter- observed as outflowing material from the core of galax- mine the stellar content and ionized gas properties of ies, but in some cases it can be observed as inflowing IFS data. In summary, the continuum is modeled as a gas (e.g., Jarvis 1990). In a small number of cases, such combination of synthetic stellar populations of different as 3C 120, the host galaxy, AGN and the jet have been ages and metallicities. Once the best stellar model for observed in many different bands from radio to X{rays, each spaxel has been determined, it is subtracted from including integral field spectroscopy (e.g., Axon et al. the original data cube to obtain a pure emission-line gas 1989; Hjorth et al. 1995; S´anchez et al. 2004; Garc´ıa- cube. Each emission-line profile is then modeled using a Lorenzo et al. 2005), providing a better understanding single Gaussian function to determine the corresponding of the physical processes involved. fluxes and errors, together with its velocity and velocity In this Letter we present the detection of a highly col- dispersion. The result of the fitting procedure is a set limated jet-like structure in NGC 232 observed in optical of bi{dimensional maps comprising the spatial distribu- emission-lines. The possible driving sources of the ob- tion of the flux intensities for each analyzed emission served structure and its correspondence to a radio emis- line including Hα, the [N II]λλ6548,6584 doublet, Hβ, sion are discussed. [O III]λλ4959,5007 and other weak emission lines. 2. DATA 4. IONIZED GAS AND KINEMATICS NGC 232 is an Sa galaxy located at z = 0.022, at a In Fig.1 we present a color-coded composite im- distance of 91.3 Mpc (e.g., Amanullah et al. 2010), and age of NGC 232. The high spectral and spatial reso- is part of a group of galaxies that also includes NGC 235. lution of MUSE allows us to clearly identify the spi- It has been classified as a luminous infrared galaxy (e.g., ral arms together with the clumpy structures associated Sanders et al. 2003) and as a radio{quiet source (e.g., with H II regions. Noticeably, an extended structure Condon et al. 1998; Hill et al. 2001). The luminosity in [O III]λ5007 emission (hereafter [O III]) is present 22 −1 at 1.4 GHz is L1:4 = 5:96 × 10 W Hz (e.g., Corbett (displayed in blue). This shows a collimated structure et al. 2002). that resembles that found in radio galaxies. This jet{ NGC 232 was observed on 2015 June 26th during the like structure is stronger in [O III], but it is also de- first semester of observations (095.D-0091; PI Anderson) tected in Hα, [N II], [S II] and at a lower signal to noise of the All-weather MUse Supernova Integral field Nearby (S=N) in Hβ. The elongated emission structure is char- ◦ ◦ Galaxies (AMUSING, Galbany et al. 2016) survey with acterized by a PA[OIII] of 163 , which differs by ∼ 20 Optical emission line jet in NGC 232 3 30 6 200 200 10 3 kpc velocity [OIII] 4 5 2 ) c e 20 s 0 c r a 0 ( 2 C E D 4 5 2 0 2 10 1.4 1.2 1.0 0.8 0.6 0.4 0.20.0 log Flux [OIII] 10 5 0 5 RA (arcsec) 0 200 2 100 10 0 1 Flux 100 velocity (km/s) 0 200 20 4850 4900 4950 5000 Wavelength (Å) 5.0 2.5 0.0 2.5 5.0 30 20 10 0 10 20 30 r (arcsec) Figure 1. Main panel: RGB color image of NGC 232: Hα intensity in red, V-band intensity in green and [O III]λ5007 intensity in blue. White contours trace the Hα velocity map in steps of 50 km s−1, with the solid contours displaying receding velocities and the dashed countours the approaching velocities. The black solid line indicates the location of zero velocity. The bottom-left inset shows the co-added spectra (within a resolution element of ∼ 000: 67) for three different positions located at (i) the nucleus (red), (ii) the disk (blue), and (iii) the elongated emission-line structure (orange).