Enigmatic emission structure around Mrk 783: cross-ionization of a companion in 100 kpc away

Mrk 783 is a narrow-line Seyfert 1 galaxy that possesses a relatively large two-sided radio emission extending up to 14 kpc from the active nucleus possibly connected with a large-scale ionized gas emission. We obtained a deep [OIII] image that revealed an extended system of emission knots and diffuse ionized gas surrounding the main galaxy. The high-excited gas is related not only with the radio structure, but also with tidal features illuminated by the active nucleus radiation up to the projected distance 41 kpc as it follows from the emission lines intensities and kinematics derived from the long-slit spectroscopic data. Moreover the part of the disk of the companion galaxy SDSS J130257.20+162537.1 located at 99 kpc projected distances to the north of Mrk 783 also falls in the AGN ionizing cone. It is possible that Mrk 783 can be considered as `Hanny's Voorwerp precursor', i.e. a galaxy that demonstrates signs of sequential switching from radio-loud to radio-quiet nuclear activity, in the moment before falling of its ionization luminosity.


Introduction
Active galactic nucleus (AGN) feedback is important in the coevolution between AGN and its host galaxy.This fundamental physical processes has an impact on the interstellar medium and the intergalactic environment, but it has been under debate for a long time and is still not fully understood.Mrk 783 is an interesting example of a galaxy in which we can trace AGN radiation feedback on large spatial scales outside the host galaxy.
Mrk 783 was discovered by Markarian and Lipovetskii [1], its optical spectra was first define as Narrow Line Seyfert 1 galaxy (NLS1) by Osterbrock and Pogge [2].According to the current point of view, NLS1 is AGN with a relatively low mass of the central black hole (10 6 -10 8 M ⊙ ) in an early stage of AGN evolution [3].Mrk 783 black hole mass M BH = 4.3 • 10 7 M ⊙ is in the higher end of NLS1s distribution, its Eddington ratio is quite low (0.11) and its [O III] line is quite strong with respect to H β [4].
In the radio band Mrk 783 was extensively investigated by Congiu et al. [5,6].They have found in the center of galaxy a compact core with a pc-scale jet as well as two-sided extended component (up to 14 kpc from the nucleus).The authors stressed, that the galaxy 'is one of the few NLS1 showing such an extended emission at z < 0.1'.The small-scale jet and the large-scale radio emission are not aligned.Based on these facts and on the very steep spectral indexes Congiu et al. [5] concluded that the extended emission observed in Mrk 783 might be a relic and that the radio source might be in a quiescent period of its activity cycle.
No less interesting Mrk 783 is in the optical range.V-band image reveals low surface brightness extended structures on the both sides of the galaxy nucleus that looks like tidal tail observed in interacting galaxies.The isophotes of the internal part of the galaxy indicate the presence of a second point-like structure which might be the nucleus of the second galaxy involved in the proposed merging [6].Optical emission of the ionized gas is far more extended with respect to the radio emission mostly on the south-east side of the nucleus.The [O III], H β and H α lines were tracked up to ∼ 35 kpc from the nucleus in the spectrum obtained at the 6.5m Magellan telescope, that makes this EELR (Extended Emission Line Region) one of the most extended discovered so far [7].

Observations and data analysis
Images in the [O III] emission line were obtained in March 2022 at the 2.5m CMO SAI MSU telescope [8] with the tunable filter photometer MaNGaL (Mapper of Narrow Galaxy Lines).This instrument is using a scanning Fabry-Perot interferometer as a narrow-band filter (FW HM ≈ 13 Å).The filter was subsequently centered on the redshifted [O III]λ5007 emission line and on the continuum shifted in 80 Å from the line.The instrument descrip- tion and data reduction steps are described in Moiseev et al. [9], the log of observations is given in Tab. 1 where T exp , β and FW HM are a total exposure, seeing value and spectral resolution correspondingly.The field of view (FoV) was 5.6 ′ with the scale 0.33 ′′ /px.The images obtained in different nights were aligned using the astrometric calibration via the astrometry.netsoftware [10].The final emission line image after continuum subtraction is shown in Fig. 1.We have mapped [O III]λ5007 emission to the surface brightness ∼ 3.6 • 10 −17 erg s −1 arcsec −2 with the signal-to-noise ratio S/N ≥ 3.
The [O III] image reveals an extended system of emission knots and diffuse ionized gas surrounding the main galaxy.The brightest external features are (Fig. 2): (i) 'SE knot' at the projected distances r = 26 − 29 ′′ from the nucleus that is a part of emission 'tail'; (ii) 'E knot' -the bright region at r = 5 − 9 ′′ to the east that is root of the 'tail' and (iii) 'W knot' at r = 13 − 18 ′′ .Some of regions listed above are visible in DESI Legacy survey [11] as a faint blue structures (Fig. 1), SE knot was already found in the Magellan spectra [7].Also a significant [O III] emission was detected in the galaxy SDSS J130257.20+162537.1 located at ∼ 73 ′′ (99 kpc) projected distances to the north of Mrk 783 (hereafter SDSS J1302+1625 or 'the satellite').
In order to study in details faint stellar structures outskirts of Mrk 783 we performed deep imaging in the r-SDSS filter with the 1m Schmidt telescope of the BAO 2 .The updated 4K × 4K Apogee (USA) liquid-cooled CCD camera was used as a detector with the pixel scale 0.86 ′′ and FoV of ∼ 1 • .The detailed description of the telescope, the photometer and data reduction steps are given in Dodonov et al. [12].The r-band image was calibrated to the magnitudes by using the DESI Legacy Surveys DR10 on-line photometric catalogue3 of sources in the observed field.At the signal to noise value S/N = 3 we reached the surface brightness limit 25.3 mag arcsec −2 that is on ∼ 0.6 mag deeper than r-band DESI Legasy survey image (Fig. 2).
The spectral observations were carried out at the prime focus of the SAO RAS 6m telescope in the long-slit mode of the SCORPIO-2 focal reducer [13] providing the spatial scale 0.39 ′′ /px along the 1 ′′ × 6.4 ′ slit.Other parameters are listed in the Tab. 1.We put the slit along the most interesting features appearing in [O III] map: the slit with position angle PA = 34 • crossed the possible companion galaxy SDSS J1302+1625 and fainter red galaxy SDSS J130256.50+162521.8, the slit with PA = 95 • crossed the Mrk 783 nucleus together with W and E knots, whereas the slit PA = 137 • exposed the tail with E and SE knots (see.1).The spectrophotometric calibration was based on the observations of the standard star BD+75 325.Also the spectrum of this O-type star was used to correction of the galaxy spectra on the telluric absorption: O 2 B-band (686-698 nm) and H 2 O band at 710-730 nm.The last one affects the [S II]λλ6717, 6731 redshifted lines of Mrk 783.

Ionized gas properties
The integrated spectra of the emission knots mentioned above together with nuclei of the both galaxies are shown in Fig 3 .The nuclear spectrum demonstrates the same properties detected in previous studies, including SDSS archival data4 : a broad blue-shifted component of the Balmer emission lines and Sy-like flux ratio without bright iron emission.The both low-and high-excitation emission lines in external knots are narrow with a singlecomponent structure.Also we detected here high-excited He IIλ4686 emission with its relative intensity similar to the nuclear source: He II/Hβ≈ 0.25.Fig. 4 clearly demonstrates the extended emission in He II along the SE gaseous tail up to the projected distance r ≈ 30 ′′ (41 kpc).The spectrum of the satellite (Fig. 3

, bottom panel) corresponds to intermediate objects between AGN and starburst (see Sec. 4) with narrow Balmer, [S II],[N II] and [O III] lines without helium emission.
The parameters of the emission lines after subtraction of continuum interpolated by cubic spline (integrated flux, line-of-sight velocity and velocity dispersion corrected on the instrumental broadening) were estimated using a single-Gaussian fitting and shown in the Fig. 5.
In the inner few kpc (|r| ≤ 4 ′′ ) our long-slit data reveal features probably related with AGN outflow influence on the surrounding gas: a high velocity dispersion in the Hα line (including a spread of light from the broad line region), a significant difference between velocities in the forbidden and Balmer lines at (r ≈ −3 ′′ in PA = 95 • , see Fig. 5), a peak of negative velocities near the E-knot (r ≈ −2 ′′ in PA = 137 • ).In contrast with a circumnuclear region, the external gas in and around Mrk 783 is dynamically cold: the observed velocities in the forbidden and Balmer lines are in a good agreement within the errors; the typical velocity dispersion is σ < 100 − 150 km s −1 .The line-of-sight velocity curve along PA = 95 • at r > 3 ′′ seems like a typical flat rotation curve of a disk galaxy (the slit lies near its photometric major axis).In this case the falling velocities in the opposite (eastern) side of this curve should be correspond to rotation in the stellar tidal tail clearly visible in broad-band images which can be partly off-plane.The amplitude of observed velocities along PA = 137 • is about 100 km s −1 that is in an agreement with gas rotation on orbits slightly inclined to the main galaxy disk.
The [O III] to Hα flux ratio along PA = 137 • has a more or less constant value [O III]λ5007/Hα≈ 2 in a very large distant range r = −30... − 5 ′′ implying the same source of the gas excitation along the SE tail.Fig. 6 shows the diagnostic emission-line flux ratio diagrams [BPT, after Baldwin et al. 16] for the Mrk 783 EELR.In contrast with the previous spectroscopic observations [7] where only upper limits for the [N II]/Hα ratio were presented for the most distant SE-knot, we able to detect in this region both relatively faint [N II] and [S II] doublets of emission lines (Fig. 4).
On the diagram [O III]/Hβ vs. [S II]/Hα (Fig. 6, right) all points occupied an area corresponded to the AGN-type ionization, whereas in the case of [O III]/Hβ vs. [N II]/Hα plot (Fig. 6, left) the most distant regions locate near the border between the H II and composite excitation (W knot) or even in the H II area (SE knot).On the other hand, the high value of He II/Hβ lines ratio in these regions suggest (as well as in a bulk of EELR) the photoionization by hard UV continuum from AGN than by young OB stars.In this case, a decreasing of [N II]/Hα ratio for the external regions could be caused by a relatively low gas chemical abundance.Indeed, numerical calculations by Bennert et al. [17] for the   typical NLR exhibit the similar behaviour of points on the BPT-diagramms, if we accept a metal abundance values Z = 0.1 − 0.3Z ⊙ for the W-knot and Z ≈ 0.05 for the NW-knot.
Therefore the observed emission line ratios (He II/Hβ and BPT diagrams) suggest that AGN radiation is dominated source of the gas ionization up to projected distance 41 kpc from the nucleus.

The galaxy environment and the satellite
As we have already noted in Sec. 1, Mrk 783 was considered as an interacting galaxy having tidal tail and a possible secondary nucleus.The SCORPIO-2 spectrum along PA = 34 • reveals that the brightest and nearest candidate to a possible companion -the galaxy SDSS J1302+1625 ('the satellite') has a systemic velocity V sys = 20038 ± 15 km s −1 that deviates only on 72 km s −1 from the velocity of Mrk 783 nucleus according our estimations along PA = 34 • (20100 ± 20 km s −1 , it is a mean value in the Hα,[N II]and [O III] lines) or on 165 km s −1 if we accepted the velocity of Mrk 783 nucleus according SDSS data in NED.These small velocity difference implies that Mrk 783 and SDSS J130257.20+162537.1 is a gravitationally bound pair.The slit PA = 34 • also crossed a smaller galaxy in 20" from the satellite -J130256.50+162521.8, its spectrum corresponds to a distant galaxy redshifted at z ≈ 0.21.It is marked in Fig. 2 as a 'background galaxy'.We also found no other Mrk 783 companions by looking at the list of spectral and photometric redshifts for all NED objects up to projected distance 500 kpc (6.2 ′ ).
Can the satellite SDSS J1302+1625 create the observed peculiar morphology of Mrk 783?The most prominent tidal tail expands in E and NE directions up to projected distance 26 ′′ (35 kpc) from the Mrk 783 center.The both DESI Legacy survey and deep BAO images (Fig. 2) demonstrate an absence of any faint tidal structures between the main galaxy and the satellite at least up to surface brightness 25.3 mag arcsec −2 in the r-band (Sec.2).The comparison of SDSS DR18 red magnitudes of Mrk 783 (m i = 15.34,m z = 15.67) with those for the satellite (m i = 17.67, m z = 17.46) gives the ratio in their luminosity and hence stellar mass 1/5-1/9.This low ratio corresponds to the case of minor merging without significant perturbation of the main galaxy.Moreover in contrast with the main galaxy, the satellite seems unperturbed in both morphology and internal kinematics: it has a symmetrical rotation curve of the ionized gas (Fig. 5).All the facts listed above together with a possible sign of the secondary nucleus [6] suggest that the observed peculiar morphology of Mrk 783 was caused by previous external event (merging with a companion) rather than with the low-massive satellite in ∼ 100 kpc away.
The ionized gas properties of the satellite are intriguing.As we already mentioned in Sec. 3 its nuclear spectrum corresponds to a starburst galaxy.The emission line ratios correspond to the border between AGN and HII regions in BPT diagrams (open black star in Fig. 6, bottom panels).However the excitation of [O III] emission increase dramatically to the galaxy outskirts -the ratio [O III]/Hα reaches ≈ 2 that is similar with the Mrk 783 EELR (Fig. 5, bottom).The corresponded points on the BPT-diagramms moves upward in the AGN area, in the region occupied by Mrk 785 EELR (the filled black star in Fig. 6, bottom).This fact implies that the outer part of the satellite disk is also ionized by the Mrk 783 AGN.
The second argument in favor of the external origin of the gas ionization in the satellite disk comes from the [O III] emission distribution according MaNGaL data (Fig. 2).It is clearly seen that highly ionized gas in the satellite placed asymmetrically relative its nucleus -we observed it mostly on the side nearest to the Mrk 783.
We tried to estimate the electron density n e from the density-sensitive [S II] doublet flux ratio R = F([S II]λ6717)/F([S II]λ6731) using the diagnostic equations from [18] for T e = 10 4 K.The values of R derived from the integrated spectra of W-knot, SE-knot and of the outer part of SDSS J1302+1625 (in the range r = 3.5-9 ′′ ) are 1.41 ± 0.06, 1.03 ± 0.09 and 0.86 ± 0.19 that corresponds to n e = 20 ± 50, 400 ± 140 and 770 ± 500 cm −3 with the 1σ level.Preliminary we can conclude that the ionized gas density is significantly higher in the SDSS J1302+1625 disk than in Mrk 783 EELR, but the uncertainty in n e estimation is too great for more detailed analysis.New deeper spatially-resolved spectroscopic data are needed to better understand the ionization properties of the SDSS J1302+1625 outskirts.

The energetic budget
External EELRs are considered as a good probe to study the history of AGN radiative output on the time scale 10 4 -10 5 yr (it corresponds to light-travel time to the gaseous clouds).The well-known prototype is Hanny's Voorwerp, a cloud of highly ionized gas near the spiral galaxy IC 2497.The detailed comparison of the ionized gas properties and AGN luminosity clearly demonstrates that the radiation associated with a nuclear activity significantly fall at least two orders in the last ≈ 10 5 yr [19][20][21].New examples of 'fading' AGNs were discovered in follow-up spectroscopic observations of EELR candidates found in SDSS broad-band images by volunteers of the Galaxy Zoo citizen-science project [22,23], as well as in the data collected in surveys based on narrow-band [O III] imaging [24] or integral-field spectroscopy [25].The ionized gas clouds around Mrk 783 including the disk of the companion galaxy allows as to use the same technique for estimation of its ionization budget.
We calculated the ionizing luminosity (L ion ) required to creation the distant emission knots with the current bolometric luminosity of AGN (L AGN ) using the approach proposed in the paper cited above and briefly described below.The upper limit of the AGN flux absorbed by dust was estimated as a sum of fluxes in the far infrared (FIR, the wavelength range 42-122 µm) according the Infrared Astronomical Satellite (IRAS) point-source catalogue data [26] and mid-infrared (MIR, the wavelength range 3.4-42 µm) data from the Widefiled Infrared Survey Expoler [WISE,27].We evaluated the IRAS luminosity (L FIR ) using a standard linear combinations of the flux in the 60 and 100 µm bands in the similar way as described in Keel et al. [22].For the WISE luminosity (L MIR ) we use a power-low approximation of the flux values obtained from the NED in the four bands 3.4, 4.6, 11.6 and 22.08µm in the same manner as in Keel et al. [23].For the unobscured AGN luminosity we use an empirical equation from [24]: L unobs = 340 L OI I I , where L OI I I is a nuclear luminosity in the [O III]λ5007 emission line based on the nuclear flux derived from 2D Moffat fitting of the MaNGaL image.In this case: The lower limit of the nuclear source luminosity required for the ionization of the gas region having the Hβ flux F(Hβ) occupying an angle α was estimated according the equation proposed in [22]: where z is a galaxy redshift and α is a projection of a solid angle under which the considered region is viewed from the nucleus.Here r -is a projected distance from the nucleus, whereas W is a radius of this region.
The quantities related to the energy balance based on the eq.( 1)-( 2) are listed in the Table 2.For the SE-knot we integrated the Hβ flux from the SCORPIO-2 spectra along PA = 137 • in the range r = 24 − 31 ′′ , W = 0.5 ′′ that is a half of the slit width.The eq. ( 1) is written for the slit oriented radially to the AGN, that is true for PA = 137 • .But it is not valid for the slit orientation PA = 34 • , passing through the satellite.To account for this, we used the same technique as described in Keel et al. [24] for the similar observations of the external gas cloud near NGC 5514: i.e. we multiply the square of the emission edge of the satellite disk according [O III] MaNGaL map (Fig. 2, left) on the mean surface brightness of the external part of the disk at 4 − 9 ′′ from the satellite nucleus according spectra along PA = 34 • .In this case, W = 7 ′′ (the radius of the emission region) and r = 72 ′′ (the distance from the AGN).
The ratio a ion = L ion /L AGN could be considered as an indicator of a long-term fading of AGN radiation or a difference of dust obscuration of this radiation between the direction to an observer and to EELR.Usually a fading AGN has a ion ≥ 3 [22,24].However in the Mrk 783 this value is significantly smaller: a ion ≈ 1 in the SE-knot and even 0.12 in the outer disk of the companion galaxy.Of course, the real value a ion could be higher, because the eq.( 1) gives an upper bound of the active nucleus infrared luminosity and includes a fraction related with a dust heated by a star formation in the galaxy.Whereas, the eq.( 2) gives a low bound of the ionizing flux absorbed by the clouds, because it depends on spatial resolution and optical thickness in the Lyman continuum [see the discussion in 21].A real spatial geometry of the system is unknown.Nevertheless we have no arguments in favour of significant fall AGN radiation in last 0.1-0.3Myr, that corresponds to a projection of travel-light time from the Mrk 783 nucleus to the SE-knot and to the disk of SDSS J1302+1625.

Discussion
It is generally accepted that large-scale EELRs around AGN can be formed in two main ways: via nuclear outflow driven by the kinetic power of radio jet and/or superwind in radio-loud AGN [28,29] or as a result of ionization of the pre-existing gas surrounding radio-quiet Seyfert galaxies [22,24].The combination of the both cases is also possible, including a relic structures from the previous activity episodes [30].The radio to optical luminosity ratio puts the Mrk 783 between radio-quiet and radio-loud AGN, the galaxy contains both an inner jet and extended diffuse radio structure [5].What is an origin of this EELR?
Our spectroscopic data clearly manifest that the possible sign of jet outflow in the ionized gas kinematics appears only in the circumnuclear region (r < 5 kpc, Sec. 3).Whereas the more external gas exhibits properties similar with the external off-plane gas and tidal debris ionized by AGN in other galaxies [21,24, and references therin]:

•
The flux ratio of the most indicative emission lines (BPT-diagrams, high He II/Hβ) corresponds to the ionization by UV-continuum of AGN rather than by shocks related with jet or outflow.

•
The quiet kinematics of gas clouds (rotation on circular orbits, relatively low velocity dispersion) also indicates tidal induced motions or an external gas accretion.
In their paper based on the Magellan spectroscopy Congiu et al. [7] assumed that the ionized gas excitation in the Mrk 783 EELR is related with the extended radio structure.However the comparison of their synchrotron radio isocontours with MaNGaL [O III] image manifests that only SE and E knots are more or less aligned with a direction of central radio structure, wheres the W-knot and the surrounding gas are not coincide with the radio contours (Fig. 7).On the other hand, the gas excitation and kinematics are similar in all considered structures that implies a common source of its ionization.On Fig. 7 we draw a location of possible bi-symmetric ionization cones with axis aligned PA = 131 • according to a large-scale jet orientation [5].The most of EELR structure lies inside the cones if we accepted the projected value of cone's open angle 90 • .This value is in agreement with the mean value of 70 • for other ionization cones [31] if the projection effect will be taken into account.
The proposed orientation of the AGN cones allows to explain the ionization of the external parts of gaseous disk in the satellite galaxy (see the arrow in Fig. 7).As minimum a half of its disk that is close to the Mrk 783 is also ionized by the AGN.The effect of cross-ionization by companion's AGN have been already found in several galactic pairs, however the characteristic separation was about 15-20 kpc [31][32][33].In the case of Mrk 783 system the projected distance between companion's nuclei is about 100 kpc that is perhaps the largest known today.
In the sec.4 we presented arguments that the disturbed morphology of Mrk 783 and its tidal structures is not related with a distant companion galaxy.Most likely we observe the result of merging with a gas-rich dwarf galaxy.In this case the most of external gas in EELR came from a tidal destroyed low-metallicity companion (0.05 − 0.1Z ⊙ according the low [N II]/Hα ratio, sec.3).The stellar tidal structure is aligned with gaseous one on the west from the galaxy, but NE emission tail has no stellar counterpart.This stellar-gaseous spatial misalignment is also observed in some AGN interacting galaxies in which EELRs are not spatial coincide with stellar tidal structures (for instance: NGC 5278/9 [31] or a spectacular local example -NGC 5194/95 [34]).It is not surprising because in [O III] we have detected only high-ionized fraction of a whole gaseous structure, whereas a distribution of cold H I can be revealed only by radio observations.
Interesting to note that many properties of Mrk 783 are similar to those observed in IC 2497 with Hanny's Voorwerp nebula (Sec.5): both are post-interacting disk galaxies having detached AGN-ionized EELR at the spatial scale of tens kpc and a relic structure traces the previous phase of radio jet activity.It means that we possible caught both galaxies after the switching between different types of activity [30,35]: radio-loud (i.e kinetic) mode and radio-quiet (i.e.radiation) mode.However in IC 2497 the radio outburst occurred 100 Myr ago [36], whereas a significant shortfall of AGN ionizing radiation traced in Hanny's Voorwerp nebula is dated as ∼ 0.1-0.2Myr ago [19,21].In Mrk 783 the age of the relic radio structure was not evaluated, whereas our calculations of the energetic budget (Sec.5) presented no evidences for a significant decreasing of AGN radiation in last 0.1-0.3Myr.Comparing these time scales we can speculate that the Mrk 783 AGN ionizing radiation will be turned off in the nearest future.Other words it can be considered as 'Hanny's Voorwerp precursor'.However, a more in-depth study of the ionization balance in the disk of Mrk783 companion is needed to prove this conclusion, because in the present work we operated only with integrated spectrum of the SDSS J1302+16 outskirts having a low surface brightness.Now we have performed only the first attempt to estimation of L ion in the EELR and the satellite disk, whereas the spatial resolution is crucial for using of eq.( 2), see Keel et al. [22,23].Moreover, Congiu et al. [6] found in Mrk 783 an inner pc-scale radio jet that is significantly misalignment with a kpc-scale structure.For the explanation they proposed two scenarios: a jet precession, or reactivation after a period of inactivity.In this case the composition of recent activity episodes in Mrk 783 may be even more complex.

Conclusion
Based on new optical spectral and imaging observations we have studied the distribution, kinematics and excitation of the ionized gas in the giant EELR of Mrk 783 galaxy as well as the properties of its environment.The deepest to date spectra of this area allows us to consider the gas ionization conditions up to 41 kpc from the galactic nucleus.Moreover, its ionization trace was found in the disk of the satellite galaxy.The main results are following: • Mrk 783 forms a gravitationally bound pair with SDSS J130257.20+162537.1 (the projected distance between their nucleus is ∼ 99 kpc).However the disturbed morphology and tidal structures are most likely caused by merging with other pre-existing companion -a gas-rich dwarf galaxy.• Most of the gaseous structures detected in the emission lines are ionized by the AGN radiation, but not by the radio jet.

•
Part of the gas illuminated by the cone belongs to the stellar tidal structure, but the most distant SE-knot is a part of external gaseous structure without stellar counterpart.Gas in this region has a low metallicity (0.05 − 0.1Z ⊙ according the low [N II]/Hα ratio).

•
External regions of the satellite gaseous disk at the nearest side to Mrk 783 falls into the ionizing cone from the main galaxy active nucleus.This fact makes the Mrk 783 system perhaps the most extreme example among nearby AGN galaxies of the cross-ionization a galactic disk by a companion.• A comparison of the ionizing luminosity required to create the most distant emission knots (including the satellite's disk) with the current bolometric luminosity of the nucleus indicates that there is no significant decreasing ionizing radiation during last 0.1-0.3Myr.
• Mrk 783 can be considered as a 'Hanny's Voorwerp precursor', i.e. a galaxy that demonstrates signs of sequential switching from kinematics (radio-dominated) to radiation (ionization-dominated) AGN modes, in the moment before falling of its ionization luminosity.
We hope that new multi-wavelength observations, first of all H I mapping in a radio domain, and deep optical integral-field mapping of the both galaxies in the pair allow us to better understand the spatial structure and evolution of this galactic system.

Figure 1 .
Figure 1.DESI Legacy survey image of Mrk 783 (the combination of images in grz filters) together with MaNGaL [O III] map.The SCORPIO-2 slit positions are shown by grey and red lines.

Figure 2 .
Figure 2. Mrk 783 with the satellite SDSS J130257.20+162537.1.From left to right: the [O III] emission line map with contours of MaNGaL image in the continuum, the main emission knots are labeled; the DESI Legacy survey r-band image; deep r-band image from 1m BAO telescope.

Figure 3 .
Figure 3. SCORPIO-2 spectra of the Mrk 783 nucleus, W, E and SE emission knots and the satellite.All spectra are integrated in 6 ′′ aperture.The main emission lines are labeled.

Figure 4 .
Figure 4. Fragments of the 2D spectrum along PA = 137 • in the blue and red ranges binned to a scale of 0.8 ′′ /px.

Figure 5 .
Figure 5.The changes of the main emission lines parameters along PA = 95 • (left) and PA = 137 • (center) and PA = 34 • (right).From top to bottom : the surface brightness, line-of-sight velocities (the systemic velocity 20100 km s −1 and 20038 km s −1 were subtracted for the Mrk 738 and SDSS J1302+1625 correspondingly), velocity dispersion corrected on the instrumental width and flux ratio relative to the Hα.The largest symbols in the central panel correspond to the integrated values in the SE knot.The point r = 0 corresponds to the maximal emission in the stellar continuum.

Figure 6 .
Figure 6.Line ratio (BPT) diagrams for the binned spectra along PA = 95 • (top) and PA = 137 • (bottom).Dividing lines between HII regions, AGN and composite nucleus are taken from Kewley et al. [14], Kauffmann et al. [15].Different colours correspond to radial distances along the slit according to the scale box.Large red and blue points correspond to the integrated values for the W and SE knots.The open and filled black stars in the bottom panels show line ratios for the nuclear and external regions of the satellite.

Figure 7 .
Figure 7.The [O III] emission line image with contours of radio continuum at 5 GHz according Congiu et al. [5].Proposed orientation of ionization cone is shown.