Ionizing Spotlight of Active Galactic Nucleus

The study of individual unique extended ionization cones shades light into the history of AGN triggering and fading of specific galaxies. The smaller and fainter analogue of HV was found in the post-merger galaxy NGC 7252 [57], whereas CFHT and VLT observations of the significantly more distant ($z=0.326$) galaxy J2240-0927 reveal ENLR ∼70 kpc in size [58]. This system of ionized gas clouds has [O iii] luminosity on two order larger than in HV and it is also considered as a result of AGN ionization echo. An EELR with properties resemble other voorwerpjes was found in the post-starburst galaxy PGC 043234 [59] with the MUSE integral-field spectrograph. A very interesting EELR was found in the multi-wavelength study of the interacting pair of galaxies SDSS J1354+1327 [60]. The authors argued that the ∼10 kpc ionization cone is a result of previous AGN activity, whereas after

The extensive search of EELR candidates by volunteers of the Galaxy Zoo project continues under supervision by Prof. William Keel (University of Alabama) including follow-up snapshot imaging at the HST in the frameworks of Gems of the Galaxy Zoos project [61]. In addition to SDSS, new GalZoo search is based on deeper large-field digital sky surveys like DESI Legacy Imaging Surveys (https://www.legacysurvey.org/, accessed on 1 December 2023). A follow-up spectral confirmation of new candidates is carried out at the SAO RAS 6m telescope with SCOPRIO-1/2 multi-mode spectrographs.

The approach based on an integral-field massive survey was recently illustrated by French et al. [54]. The authors gave evidences of 6 EELRs in the sample of 93 post-starburst galaxies selected in Mapping Nearby Galaxies at APO survey (MaNGA [62]). It was shown that 5/6 of found EELRs has ionized by AGN with fading luminosity. The full duty cycle of AGN in post-starburst systems was estimated as ∼0.1 Myr whereas the luminous phase continues during only about 5% of this time. However the typical radius of these [O iii] nebulae don’t exceed 10 kpc, that is smaller than in HV and many other systems considered in our review.

The second approach based on the medium-band filters survey is developed in the project TELPERION (Tracking Emission Lines to Probe Extended Regions Ionized by Once-active Nuclei [63,64]). The search of candidates was performed with 1m SARA robotic telescopes with a filter centered on the doublet [O iii]$\lambda \lambda 4959, 5007$ for galaxies spanning redshifts $z=0.009$–$0.029$. The detection limit of extended emission was about 1/10 of the Hanny’s Voorwerp surface brightness.

Strictly speaking, not only ionization cones, but also other type of sources could produce [O iii] emission in galaxy outskirt: H ii regions, galactic wind outflow driven by central star burst, etc. Optical spectroscopy of selected candidates allows to separate gas clouds with different mechanisms of ionization on the diagnostic diagrams plotting the characteristic flux ratios in the pair of emission lines close in wavelength ([O iii]$/\mathrm{H}\beta$, [N ii]/H$\alpha$, etc.), called ‘BPT-diagrams’ after the first authors of the paper [65]. Different areas on these diagrams correspond to the gas ionized by UB-radiation of OB-stars in H ii-regions, by different types of AGN (UV-radiation in Seyfert galaxies and shocks in LINERs) and by ‘composite’ ionization between AGN and HII areas (Figure 5). Such diagrams are often supplemented by grids of model calculations of photoionization or shock excitation, taking into account metallicity, shock velocity, magnetic field, etc. [66,67].

Follow-up spectroscopy of the TELPERION, candidates were conducted with several Russian telescopes equipped with high-efficient low-resolution spectrographs developed in SAO RAS under the supervision of Prof. Victor Afanasiev: SCORPIO-1 and SCORPIO-2 prime focal reducers at the SAO RAS 6m telescope spectrograph, ADAM at the 1.6 m telescope AZT33-IK of Sayan Observatory (The description of all listed devices can be found at this URL: https://www.sao.ru/hq/lsfvo/devices_eng.html, accessed on 1 December 2023). For the most interesting confirmed EELRs, deep [O iii] images were obtained at the 6m telescope and at the 2.5 m telescope of the SAI MSU using the tunable filter photometer MaNGaL (Mapper of Narrow Galaxy Lines (https://www.sao.ru/hq/lsfvo/devices/mangal, accessed on 1 December 2023)). The MaNGaL instrument uses scanning FPI as a narrow-band filter, about 1.3 nm wide, whose transmission can be fine-tuned to the wavelengths corresponding to both the emission line and the continuum, taking into account the redshift of a target galaxy.

With the above technique the first search for distant emission clouds in a luminosity-limited sample of nearby AGN was done [64]. The sample contained 111 galaxies—all known AGN brighter than $M=-{20}^m$ in the redshift range of $z=0.009$–$0.029$ and $\delta >-36°$. Among 15 emission clouds discovered in [O iii] images, 6 gaseous systems were confirmed as EELRs via follow-up spectroscopy. Two of them (in NGC 235 and NGC 5514) are distant clouds projected at $r=25-75$ kpc from the nucleus.

NGC 5514 (Figure 6) exhibits an almost classical illustration of the described above scenario of the Hanny’s Voorwerp formation. In merging pair of galaxies the interaction leads to the gas inflow to the central SMBH in one of the galaxies. The [O iii] emission reveals both an inner ionization cone ($r<12$ kpc) and more distant gas clouds in the tidal tail up to 75 kpc away in projection distance. Whereas in the circumnuclear region signs of AGN-induced outflow is observed, the EELR gas is kinematically quiescent, i.e., it is characterized the low gas velocity dispersion without significant non-circular gas motions. Also the detection He II$\lambda 4861$ emission line (see the spectrum in Figure 6) clearly indicates the ionization by hot source like AGN accretion disk. An alternative explanation is a strong shock excitation, but it the corresponded shock velocities (>$400$

) disagree with a dynamically cold gas. Also an emission-line ratios indicate the AGN-type photoionization of the distant clouds in NGC 5514 (Figure 5). An estimation of the energetic budget of AGN using the $L_{ion}/L_{FIR}$ criterion shows that activity in NGC 5514 has decreased by more than a factor of 3 in the last ∼0.25 Myr [64].

In addition to NGC 235 and NGC 5514, large (up to 10 kpc in size) ionization cones inside galactic disks have been found in three galaxies: ESO 362-G08, NGC 7679 (Figure 7) and IC 1481. Interestingly, the latter, as well as NGC 5514, contains a low activity AGN—LINER. Why nucleus with a relatively low ionizing flux produces such prominent cones is not yet very clear. Perhaps this is another indication of the transient activity phenomenon of galactic nuclei in combination with properties of surrounding interstellar medium (gas distribution, its density, metallicity, etc.). Interesting that the prototype voorwerpjes galaxy–IC 2497 (the Hanny’s Voorwerp) is also classified as a LINER.

Despite the fact that the statistics is not yet very large, the EELRs outside the galaxy stellar disks are detected among 2–5% percent of AGNs depending on selection of samples: bright AGN and Toomre merging sequence [64], AGN with H i external structures [63] or their combinations. Recent counting of the total TELPERION sample of 241 galaxies gives the EELRs incidence $1.7\pm 0.6$% [70]. This fraction in several tens times higher that in the previous Galaxy Zoo survey based on SDSS broad-band images with higher surface brightness threshold. Moreover the EELR detection rate is significantly higher in AGN hosted in interacting and merging galaxies (10–12%, [64,70]). The off-plane gas in tidal tails enters the cone of illuminating ‘spotlight’ with higher probability and on the larger distances than gas in a galaxy plane.

The first example of known geometry of the screen is a pair where at least one of the companion galaxies contains AGN and the other one is a gas-rich disk galaxy. In this case we can expect cross-ionization effect, when an EELR is created in the companion galaxy’s disk. Such ‘ionizing footprint from a neighbour’s even allow to find hidden AGN in an interacting system, as it was shown in Was 49 [71]. An inverse case is ShaSS 073 in the Shapley supercluster, where the EELR excited in the companion’s disk gave evidences of a dramatic fading of AGN luminosity (in 20 times in the past 30,000 yr) hosted in the second component of the pair at projected separation 21 kpc [72]. The most extreme example of cross-ionization among nearby galaxies was recently found in Mrk 783. The AGN in the main galaxy ionizes not only the surrounding tidal structure up to the distance 41 kpc, but also the external part of the disk of the companion galaxy SDSS J130257.20+162537.1 at the projected distance 99 kpc [73]. The preliminary calculation of the ionized balance indicates that there is no significant decreasing ionizing radiation during last 0.1–0.3 Myr. New in-depth studies of this and similar pairs with integral-field facilities on large telescopes will help to better restore the history of AGN radiation output.

In the close pairs of galaxies it is possible to estimate the solid angle at which the companion disk is visible from the nucleus (see the diagram in Figure 8). Therefore, the width of the ionization cone can be constrained from the statistics of the EELR occurrence in AGN pairs. It was done in the paper [74] considered the sample of pair with Seyfert nucleus in one or both companions selected in the SDSS DR8 according the following criteria: the distance between the nuclei $r<{15}^{″}$ (∼18 kpc for the median redshift $\langle z\rangle =0.06$), difference in radial velocities within $400$

implies a gravitational bound pairs. Out of 212 pairs, 32 were selected for spectral observations, distinguished by the smallest distance between centers and the largest ${\theta }_d$ angle. Figure 8 shows one of the most interesting system–UGC 6081. The nuclei of both companions are active and two systems of ionization cones are observed simultaneously.

A different value of ${\theta }_{cone}$ was obtained for the other type of a screen with known geometry: in the sample of AGN with extended systems of neutral hydrogen [63] mapped in 21 cm line. From 26 H i-rich systems only one EELR was found. It was the gas clouds projected in 12 kpc away from the nucleus of Seyfert galaxy Mrk 1 that has a common H i envelope with NGC 451 [63]. Calculation gives ${\theta }_{cone}<20°$ if the AGN are continuously bright for scales longer than the light-travel times to the external H i structures. The contradiction with previous estimation from cross-ionized pairs is explained if the AGN ionizing radiation undergoes strong variations at times of ${10}^4$–${10}^5$ years. This characteristic time is similar that was presented above for fading AGN.

The examples considered in this review are only the first attempts to estimate the characteristics of the AGN accretion machine from the statistics of the occurrence of its ionization trace. Increasing of the samples of such objects as well as more detailed study will make it possible to answer a number of relevant and interesting questions in AGN physics, related, for example, with the precession of the ionization cone or with evolution of dust torus collimating the AGN UV radiation. For this purpose, it is relevant to carry out a deep mapping of the environment of nearby galaxies in the main emission lines (H$\alpha$, [O iii], etc.). Even the first attempts in this direction are already yielding interesting results. Deep H$\alpha$ imaging of famous nearby interacting pair ‘The Whirlpool’ (NGC 5194/5) revealed an extended low-brightness cloud at 32 kpc away from the center of the main galaxy. The properties of gas excitation correspond to that observed in classical ionization cones [75,76].