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The Activity of the Neighbours of AGN and Starburst Galaxies: Towards an evolutionary sequence of AGN activity

We present a follow-up study of a series of papers concerning the role of close interactions as a possible triggering mechanism of the activity of AGN and starburst (SB) galaxies. We have already studied the close (<100 kpc) and the large scale (<1 Mpc) environment of Sy1, Sy2 and Bright IRAS galaxies and their respective control samples (Koulouridis et al.). The results led us to the conclusion that a close encounter appears capable of activating a sequence where a normal galaxy becomes first a starburst, then a Sy2 and finally a Sy1 galaxy. However since both galaxies of an interacting pair should be affected, we present here optical spectroscopy and X-ray imaging of the neighbouring galaxies around our Seyfert and BIRG galaxy samples. We find that more than 70% of all neighbouring galaxies exhibit thermal or/and nuclear activity (namely enhanced star formation, starbursting and/or AGN) and furthermore we discovered various trends regarding the type and strength of the neighbour's activity with respect to the activity of the central galaxy, the most important of which is that the neighbours of Sy2 are systematically more ionized, and their straburst is younger, than the neighbours of Sy1s. Our results not only strengthen the link between close galaxy interactions and activity but also provide more clues regarding the evolutionary sequence inferred by previous results.


Introduction
Since the discovery of Active Galactic Nuclei (AGN) significant effort has been put in the attempt to reveal their nature, but we still lack a full understanding of all the aspects of activity. We are almost certain of the existence of a massive black hole (MBH) in the centre of active and even non active (Ferrarese and Merrit 2000;Gebhardt 2000) galaxies (including our own; e.g. Bozza & Mancini 2009). In active galaxies accretion of material into this BH is responsible for the excess nuclear emission that we see in the galaxy spectrum. On the other hand, the triggering mechanism and the feeding of the black hole, the structure and the physics of the accretion disk and of the torus, which is predicted by the unified scheme (Antonucci et al. 1993), the connection with star formation and the role of the AGN feedback, the origin of the jets in radio loud AGN galaxies and their influence on the IGM and ICM and even the mechanism that produces the observed IR, X-ray, and gamma-ray emission, are only partially understood. The unification model itself has not been able to fully explain all the AGN phenomenology (in particular, the role of interactions Indeed, there are several studies that conclude that there is an evolutionary sequence from starburst to Seyfert galaxies (e.g. Storchi-Bergmann et al. 2001;). Furthermore, based on the number and proximity of close ( ∼ < 60 − 100 h −1 kpc) neighbours around the different type of active (Sy1, Sy2 and BIRG) galaxies ( e.g. Dultzin-Hacyan et al. 1999;Krongold et al. 2002;Koulouridis et al. 2006a,b) a very interesting evolutionary sequence has been suggested, starting with an interaction that triggers the formation of a nuclear starburst, which then evolves to a type 2 Seyfert, and finally to a Sy1. Such connection is likely independent of luminosity, as similar trends have been proposed for LINERs (Krongold et al. 2003) and ULIRGs and Quasars (Fiore et al. 2008 and references therein). This sequence can fully explain the excess of starbursts and type 2 AGN in interacting systems, as well as the lack of type 1 AGN in compact groups of galaxies (Martinez et al. 2008).
In addition, post starburst stellar populations have been observed around AGN (Dultzin-Hacyan & Benitez 1994;Maiolino & Rieke 1995;Nelson & Whittle 1996;Hunt et al. 1997;Maiolino et al. 1997;Cid Fernandes, Storchi-Bergmann & Schmitt 1998;Boisson et al. 2000Boisson et al. , 2004Cid Fernandes et al. 2001, 2004 and in close proximity to the core (∼50 pc), implying the continuity of these two states and a delay of 0.05-0.1 Gyr (Müller Sánchez et al. 2008) up to 0.5-0.7 Gyr (Kaviraj et al. 2008 between the onset of the starburst and the lighting up of the AGN. Davies et al.(2007), analyzing star formation in the nuclei of nine Seyfert galaxies found recent, but no longer active, starbursts which occurred 10 -300 Myr ago. Furthermore, most of these studies (e.g. Hunt et al. 1997;Maiolino et al. 1997, Gu et al. 2001) separate type I from type II objects implying that recent star-formation is only present in type II objects. Support for the interactions-activity relation was recently provided by HI observations of Tang et al. (2008), who found that 94% of Seyfert galaxies in their sample were disturbed in contrast to their control sample (where only 19% were disturbed). We point out that a great theoretical success of the starburst/AGN connection is the quenching of the star formation induced by the AGN feedback which can explain the formation of red and dead elliptical galaxies (e.g. Springel et al. 2005a;Di matteo et al. 2005;Khalatyan et al. 2008). Recent results have found that AGN ionized outflows may carry enough energy to cease star formation in the host galaxy (Krongold et al. 2007(Krongold et al. , 2009Chartas et al. 2008;Blustin et al. 2008). Furthermore, the correlation of morphologies of the host galaxies with the nuclear activity type can also lead us to a possible AGN evolutionary sequence (Martínez et al. 2008).
There is also evidence pointing in the opposite direction. Li et al. (2008), analyzing SDSS data, showed that there is an increase in the starformation rate but no AGN enhancement associated to the presence of a close companion. However, they left a window open for the possibility of a delay between the onset time of the two phenomena. In any case it appears very likely that a non axi-symmetric potential, such as a tidal interaction, provides a triggering mechanism of activity.
In addition, the quenching of star formation could be due to AGN feedback, activated with a possible delay of ∼0.5 Gyr, inducing an evolution from a Sy2 to a Sy1 state. Should this be true, the effect of interactions on the host galaxy would not be equally observable at the different phases of the AGN evolution. Of course, such an evolutionary scenario poses some problems to the simplest version of the unified scheme.
Observations of polarized broad emission lines from type 2 AGN galaxies gave strength to the unified picture, implying that this is scattered light by the torus. However, hidden broad lines are only observable in 50% of Seyfert 2s (Tran 2003) suggesting the possible existence of true type 2 objects, i.e. objects completely lacking a BLR. The lack of a BLR can be explained in terms of a very low accretion rate (Nicastro 2000, Elitzur 2008, and/or due to even greater obscuration (Shu et al. 2008). Supporting the low accretion model, we point out that unobscured type 2 AGN galaxies have also been observed by Brightman et al. (2008) and Bianchi et al. (2008).
We stress that the evolutionary scenario does not contradict the unification scheme. It implies that different types of AGN galaxies are in fact the same object (as the unification model proposes) but not necessarily at the same evolutionary phase. However, there could be a phase where only orientation defines the appearance of a Sy1 or a Sy2 state, which is the stage where the obscuring molecular clouds form a torus but have not yet been swept away, in agreement with the presence of polarized broad emission lines.
Taking into account the previous, when working with AGN samples, it is of crucial importance that the different types are analysed separately, in order to emphasize the similarities and the differences of each AGN type and not to smooth them by stacking them all together.

This work
This paper is the third in a series of 3-dimensional studies of the environment of active galaxies (Koulouridis et al. 2006a,b), based on previous 2D analyses (Dultzin et al. 1999, Krongold et al. 2002, and will attempt to shed some more light to the starburst/AGN connection and to the evolutionary scenario proposed in our previous papers. It is a follow-up spectroscopic study aiming at investigating the effects of interactions on the neighbours of our Seyfert and Bright IRAS galaxies.
We will discuss our galaxy samples in section §2. In §3 we present the observations and data reduction. The spectroscopic analysis and classification of the galaxies is presented in §4 and §5, while the analysis of the available X-ray observations is given in §6. Finally in sections §7 and §8 we will interpret our results and draw our conclusions. Due to the fact that all our samples are local, cosmological corrections of galaxy distances are negligible. Throughout our paper we use H • = 100 h −1 Mpc. We use the term "active" for all galaxies that exhibit emission lines in their spectra, their origin being nuclear activity or starformation, since we believe that these two phenomena are strongly related to interactions.

Sample Definition and Previous Results
Detailed presentation of the samples of active galaxies can be found in our previous studies (Koulouridis et al. 2006a,b). Here we only provide the basic properties of our samples and of our previous results. The original samples consisted of 72 Sy1 galaxies within redshifts 0.007 < z < 0.036, 69 Sy2 galaxies within redshifts 0.004 < z < 0.020 and 87 Bright IRAS galaxies within redshifts 0.008 < z < 0.018. We also used three control samples, compiled in such a manner as to reproduce the main host-galaxy characteristics.
Using the CfA2 and SSRS galaxy catalogues we search for physical neighbours and within a projected distance D ≤ 100 h −1 kpc and radial velocity separation δu ≤ 600km/sec, we found (see Figure 1) that: • The Sy1 galaxies and their control sample show a consistent (within 1σ Poisson uncertainty) fraction of objects having a close neighbour.
• There is a significantly higher fraction of Sy2 galaxies having a near neighbour, especially within D ≤ 75 h −1 kpc, with respect to both their control sample and the Sy1 galaxies.
• Results based on the BIRG sample, which includes mostly starburst and Sy2 galaxies, show that an even higher fraction has a close neighbour.
In order to investigate whether fainter neighbours, than those found in the relatively shallow CFA2 and SSRS catalogues, exist around our AGN/BIRG samples, we obtained our own spectroscopic observations of all neighbours with m B ∼ < 18.5 and D ≤ 100 h −1 kpc for a random subsample of 22 Sy1, 22 Sy2 and 24 BIRG galaxies. We found that the percentage of both Sy1 and Sy2 galaxies that have a close neighbour increases correspondingly by about 100% when we descent from m B ∼ < 15.5 to m B ∼ < 18.5, while the percentage of BIRGs rises only by ∼45% reaching the equivalent Sy2 levels. These results imply that the originally found difference between Sy1 and Sy2 persists even when adding fainter neighbours up to δm ∼ < 3, which correspond to a magnitude limit similar to the Large Magellanic Cloud.
For the purposes of the present study we compiled three samples consisting of all the neighbours around the aforementioned subsamples of Sy1, Sy2 and Bright IRAS galaxies, respectively. In Table 1, 2 and 3 we present the names, celestial coordinates, O maps magnitudes 1 and redshifts of those previously mentioned Sy1, Sy2 and BIRG galaxies which have at least one close neighbour (within δu < 600km/sec). Note that we have kept the original neighbours enumeration of the previous papers (for example, in table 3, NGC5990 has only neighbour 3, since 1 and 2 had δu > 600km/sec). We use O maps magnitudes because Zwicky magnitudes were not available for the fainter neighbours, and we needed a homogeneous magnitude system for all our objects.

Spectroscopic Observations
We have obtained spectroscopic data of all the neighbour galaxies in our samples in order to classify their type of activity. Optical spectra were taken with the Boller & Chivens spectrograph mounted on the 2.1 m telescope at the Observatorio Astronómico Nacional in San Pedro Mártir (OAN-SPM). Observations were carried out during photometric conditions. All spectra were obtained with a 2. ′′ 5 slit. The typical wavelength range was 4000-8000Å the spectral resolution R = 8Å. Spectrophotometric standard stars were observed every night.
The data reduction was carried out with the IRAF 2 package following a standard procedure. Spectra were bias-subtracted and corrected with dome flat-field frames.
Arc-lamp (CuHeNeAr) exposures were used for wavelength calibration. All spectra can be found at the end of the paper.

Analysis and Classification Method
In this section we present results of our spectroscopic observations of all the neighbours with D ≤ 100 h −1 kpc and m Omaps ∼ < 18.5 for all three samples of Sy1, Sy2 and Bright IRAS galaxies. We have also used SDSS spectra when available.
Our target was to measure six emission lines:  (Veilleux & Osterbrock, 1987). However, it was not possible to measure the H β and [OIII] emission lines for all of our galaxies and thus we classified many of our objects using only [NII] and H α .
Based on the above, we adopted the following classification scheme: galaxies with no emission lines are concidered to be normal, while those with emission lines are active, meaning that they exhibit nuclear or/and enhanced starforming activity. We choose to call all starforming galaxies with prominent emission lines SB galaxies. We do not attempt to divide the starforming galaxies into more subcategories since such a categorization appears to be highly subjective and depends on the applied methodology (eg., Knapen & James,

2009).
Although we can distinguish between a SB galaxy and an AGN using only the [NII]/H α ratio, we cannot distinguish between a low ionization (LINER) and a high ionization We can classify our objects in the following categories: • SB (starburst) galaxies: all the objects which are found below the line of Kaufmann et al.
• AGN galaxies: the objects which are found above the line of Kewley et al.
• TOs (transition objects): the ones that are found between the two lines and exhibit characteristics of both an active nucleus and a starburst.
We can further separate AGN galaxies in LINERs and Seyferts based on their ionization level, namely we define LINERs as those with: We also classify our objects using only the [NII]/H α ratio, as proposed by Stasińska et al. (2006), and find that it is accurate in most of the cases (see Table 4 Further classification of the Seyfert galaxies in type 1 and type 2 was obtained by direct visual examination of the spectra and the broadening of the emission lines. No Sy1 activity was discovered in any of the galaxies and therefore all neighbours classified as AGN should be considered of type 2. In Table 4 we list for all neighbours their line ratios and the two different classifications. Evidently, in most cases the classifications are absolutely consistent.
3. Results and analysis.
3.1. Activity of the neighbours.
In this section we discuss in more detail the results of our spectroscopy and classification. We can draw our first results for each sample separately inspecting Table 4.
From the analysed 15 neighbours of Sy1 only 4 are normal galaxies, while 7 of them are SB galaxies, 3 are classified as TOs and one is classified as AGN. Similar results hold for the neighbours of Sy2 galaxies. 4 out of 16 neighbours do not show activity, 7 are SB galaxies, -14of activity in their vicinity. For the Bright IRAS sample this percentage reaches almost 83% since only 4 out of 23 neighbours are inactive, 10 SB galaxies, 6 TOs and 2 AGNs.
However, we should stress out that we did not find any Seyfert galaxy in our samples and even those classified as AGN, by their NII/Hα ratio and the presense of the forbiden [OI] line, are objects of low luminosity or LINERs.
Note however that due to lack of detailed spectroscopic data for the majority of our BIRG's, it is not possible to correlate their activity type with that of their neighbours.
Only a crude classification was possible in Koulouridis et al. (2006b) by cross-identifying literature classifications while also using the available SDSS data, which showed that BIRGs exhibit various activity types. Therefore we will use in the remaining study, the neighbours of only those BIRGs which have SDSS spectra and which we classified as AGN. Accordingly, we plot in figure 2 the three neighbours of two Sy2 BIRGs. A more detailed analysis of the whole BIRG sample will be subject of a future work.
One of the most interesting results of our analysis is that the neighbours of Sy2 galaxies have systematically larger values of [OIII]/H β than the neighbours of Sy1 galaxies (see Figure 2). Since this ratio shows the ionization level and the age of the starburst importance to a possible evolutionary scenario and will be discussed further in §5. are the neighbours of Sy1 galaxies. Note that the neighbours which we have classified as AGN do not appear here due to lack of their spectral H β region, as we have already mentioned in the text.   (2006a) showed that the starforming members of close pairs, lie closer to the classification line than the starforming field galaxies.
Summarizing our main results of this section: • More than 70% of the neighbours of all AGN or BIRG samples exhibit activity.
• The neighbours of Sy2s are systematically more ionized, and their starburst younger, than the neighbours of Sy1s.
It would be interesting at this point to correlate the activity type of the neighbours to other properties of the host galaxy. To this end, we analyse in the following section the relation between the magnitude and the activity properties of each central active galaxy to those of its neighbour.

Magnitude analysis
Since we have already applied a homogeneous magnitude system to our samples, we can now study whether there is a correlation between the activity of the interacting galaxies and their size. On average, size and magnitude are correlated in small redshift intervals (as it is in our case) and therefore the activity-size comparison can be performed in two ways: Either by comparing their absolute magnitudes or alternatively by examining the apparent magnitude difference of the interacting pair.
While the latter is a good tracer of the strength of the interaction, the former can possibly point out differences between intrinsically large and small galaxies. We performed both type of analyses but we did not find significant differences between them, something which should probably be attributed to the fact that our galaxies reside in the local universe (z ∼ < 0.03). Therefore, we present results based on the pair relative apparent magnitude difference. We define the relative size of a neighbour using the apparent magnitude difference with its central AGN, ∆m, according to the following: 1. VL (Very Large): ∆m ≤ 0.5 2. L (Large): 0.5 ≤ ∆m ≤ 1.5 3. S (Small): ∆m ≥ 1.5 In Table 5 and figure 4 we present the results of this classification. Note that with M we mark a merging galaxy pair. Although this classification is rough and objects could lie near the limiting borders, we can still derive valuable information, as we will see below.
Examining Table 5 and fig.4 it becomes evident that: (a) small neighbours are preferentially SB galaxies, For simplicity we mark all galaxies that show AGN activity and TO's with red triangles.
For more details see Table 5. The horizontal lines denote the magnitude classification of the neighbours (see text), the vertical the limit ∼ > 30 above which we find the great majority of SB neighbours. From the above we can infer that when a small galaxy is in interaction with a much larger one, the encounter leads to a starburst in the smaller companion. Therefore, although a small neighbour appears to always exhibit enhanced star-formation, induced by the interaction, it does not show any AGN activity. This could be due to the absence in small galaxies of a massive black hole (Wang, Kauffmann 2007).
If this is correct, only galaxies which experienced a major merger can exhibit AGN activity and this could be the reason why AGN hosts are more frequently found in early type galaxies (e.g. Marquez & Moles 1994;Moles, Marquez, & Perez 1995;Ho et al. 1997;Knapen et al. 2000;Wake et al. 2004). This can also account for the big number of starburst galaxies in our samples. Nevertheless, the absence of small starburst galaxies within D ∼ < 30h −1 kpc, could imply that the interaction is so strong in such close distance that leads to the gas stripping of the small neighbour. Apparently, the interaction affects differently a large neighbour which can be found in any activity state or even remain unaffected and thus inactive. Finding relatively many massive galaxies in a normal state implies a probable delay of the outcome of the interactions or even that the interactions are not affecting those galaxies in a significant way. However, both scenarios are evenly probable since we also find many big neighbouring galaxies showing AGN activity. In the X-ray analysis section we also address this issue.
The morphology of the host galaxy, the mass of the central black hole or even more complicated issues, like the spin of the BH or the strength of the AGN feedback, could also play a role to the viability of the starburst phase and the feeding of the black hole.

The XMM-Newton observations
In addition we have explored whether the neighbours present nuclear activity, using X-ray observations from the XMM public archive. We find that 13 target fields have been In the same table we quote the 2XMM hardness ratios derived from the 1-2 keV and 2-4.5 keV bands (hardness ratio-3 according to the 2XMM catalogue notation). The upper limits, derived using the FLIX software, are estimated following the method of Carrera et al. (2007). This provides upper-limits to the X-ray flux at a given point in the sky covered by XMM pointings. The radius used for deriving the upper limit was 20 or 30 arcsec depending on the presence of contaminating nearby sources.
In Figure 5 we present the X-ray to optical flux diagram f X − f B (e.g. Stocke et al. 1991). The X-ray to optical flux ratio for any source is given by: where f X is the 0.2-12 keV flux and B is the Johnson B magnitude; we assumed B −V ≈ 0.8.
This diagram provides an idea on whether a galaxy may host an active nucleus. This is because AGN have enhanced X-ray emission at a given optical magnitude relative to normal galaxies. The space usually populated by AGN is shown between the lines. Note however, that Seyfert-2 galaxies with large amounts of absorption, as is the case of NGC7743 (Akylas & Georgantopoulos 2009), lie below the lines as their X-ray flux has not been corrected for X-ray absorption. One source which lies in the AGN regime (7682-N1) can be immediately seen. This has been classified as a TO galaxy in the optical spectroscopic analysis and is the only one of the 12 neighbours (with XMM observations) having an active nucleus based on spectroscopy. Additional information on the nature of our sources can be extracted from the hardness ratios. Two sources NGC526-N2 and N1358-N2 have hardness ratios suggesting absorption N H ≈ 10 22 cm −2 , consistent with the presence of a moderately obscured active nucleus. Both these galaxies present no optical emission and thus classified as normal, based on their optical spectra. However, as we saw in the previous section, these galaxies fall into the category of relatively massive neighbours, many of which present AGN activity.
In other words, the lack for optical emission lines from the nucleus of these objects could be a result of obscuration. In addition, we should mention here that in the group of objects with an X-ray observation, all Galaxies classified as "normal" through optical spectroscopy present an X-ray detection, indicating possibly the presence of nuclear activity. In contrast, all SB galaxies except one in the X-ray subsample do not show an X-ray detection. Our most important result is that the star forming neighbours of Sy2 galaxies appear to be always more ionized than the ones around Sy1 galaxies and their starburst is younger.
This is a direct indication that indeed the Sy1 stage follows the Sy2 stage.
If now the neighbouring galaxy is large enough to host a central super massive black hole, it would follow the same evolutionary path as the central AGN or BIRG, which is described in the next subsection.

Interacting pair: the central AGN or BIRG.
We now attempt to fit our results in an evolutionary scenario, partly presented in a previous paper of the series (Koulouridis et al 2006b). Something that is nowadays undisputed is the role of interactions in the creation of a starburst. Molecular clouds are being forced towards the galactic center, become compressed and light up the galaxy as a starburst.
Despite the fact that the exact mechanism is still unknown, in the local universe an -25accretion rate of ∼ 0.001 − 0.1M⊙/yr is needed to fuel the black hole. Theoretically this can only be achieved by means of a non axisymmetric perturbation which induces mass inflow. Such perturbations are provided either by bars or by interactions.
Whichever the mechanism may be, the result is the feeding of the black hole and the activation of the AGN phase, maybe ∼ 0.5Gyr after the initial interaction took place. An interaction certainly predicts such a delay, since after the material has pilled up around the inner Linblad resonance, producing the Starburst, it can be chanelled towards the nucleus by loosing significant amounts of angular momentum, a process which is not instantaneous.
Therefore, a delay exists between the SB and AGN phases, where active nucleus and circumnuclear starburst coexist. In this initial phase, the nucleus is heavily obscured by the still starforming molecular clouds and we can observe a transition stage of composite Sy2-starburst objects.
The most probable manner for the AGN to dominate is to eliminate the starburst, possibly by the AGN outflows or by radiation pressure. Outflows from the core have enough energy to dissipate the material around it and suffocate star formation (eg. Krongold et al. 2007, 2009, Hopkins & Elvis 2008.
This could also take some time and as the starburst fades, the Seyfert 2 state starts dominating to be followed, at the end, by a totally unobscured Sy1 state, possibly ∼ 1Gyr after the initial interaction.

From Sy1 back to normality
Here we will present a scenario which describes a possible avenue for a Sy1 galaxy to return to its formal normality.
Eventually, the radiation pressure can drive away not only the obscuring clouds but also the BLR and thus the AGN is driven to a totally unobscured Sy2 phase, followed later, and as accretion keeps diminishing because of the exhaustion of accreting material, by a LINER phase. However, we should note here that a pure Sy2 (without BLR) can also exist during the initial phase of the sequence, when accretion can also be very low, though in this stage might be obscured by the material of the surounding SB.
In the case where the BLR is still present, after all obscuration is gone, it is possible not to observe the "naked" Sy2 after the Sy1, but instead a type I LINER. On the other hand, if the BLR vanishes before the obscuring material, then an unobscured Sy2 appears immediately after the obscured one, omitting the Sy1 phase. Finally, the LINER turns into a low luminosity transition object (TO) which exhibits a combination of LINER and low ionization starforming characteristics and then shuts off. We repeat that not all TOs exhibit low luminosities and, as we have mentioned ealier in §3.1, some may be in transition from a pure SB to a pure Sy2 phase following the exact opposite evolutionary path.
This scenario proposes a circular process which drives a normal interacting galaxy through starbursting and different AGN states back to normality. It is clearly not a simple model but surely a unified one. It combines previously proposed evolutionary scenarios between starbursts and Seyferts or between different types of AGNs to a single route covering all the BPT diagram. Interestingly, it predicts two totally different populations of Sy2 galaxies, one previous to and one following the Sy1 phase. The former possesses HBLR while the latter does not. However, as we have commented before, pure Sy2 galaxies can also exist in the initial phase.
However, this is not the only possible scenario. Krongold et al. (2003) concluded that type II LINER and transition LINER galaxies are more often in interaction than type I LINER galaxies. Based on this result, an alternative scenario where all galaxies evolve from normal to AGN of any luminosity was proposed. In this case all objects would return to normal when the material driven to the center by the interaction exhausts.
From our point of view, in an ever evolving universe such schemes, as presented previously, are more probable than the original unification paradigm which proposes a rather static view of AGN. Of course, orientation could and should play a role between the obscured Sy2 and Sy1 phase, where the relaxing obscuring material forms a toroidal structure.

Conclusions.
We have found that the large majority (> 70%) of close neighbours of AGN (being Sy1 or Sy2) and of Bright IRAS galaxies show activity, mostly starbursting but AGN as well.
Furthermore, the close neighbours of Sy1 galaxies, being starbursts or AGN, are less ionised and thus seem to be a different, more evolved, population than those of Sy2s.
These results are in the same direction as those of our previous papers, ie., the significant excess of neighbours around Sy2s and BIRGs but not around Sy1s, and support an evolutionary sequence of galaxy activity, induced by interactions, the main path of which is from inactivity, starburst, Sy2, Sy1 and back to inactivity, since the "active" phase should cease with AGN feedback. Finally, such sequence could be repeated by a new encounter.
The time needed for type 1 activity to appear should be larger than the timescale for an unbound companion to escape from the close environment, or comparable to the timescale needed for an evolved merger (∼ 1Gyr, see Krongold et al. 2002). If our scenario is correct, then there should be two distinct populations of Sy2s: the obscured ones which pre-exist the Sy1 phase and should exhibit close companions and the "naked" ones which appear later and should be isolated.
The results of our analysis also provide valuable guidelines for future studies of the AGN environment, since they point out two very important issues: [b.] Starburst galaxies should not be handled under any circumstances as normal galaxies, since the starburst/AGN connection appears quite plausible and composite objects do exist anyway. In a nutshell, all different categories of active galaxies should be treated separately.
There are still many unresolved issues and caveats concerning these studies, since the evolutionary sequence is not unique and should also depend on the geometry, the density and other factors of the obscuring and the accreting material, as well as on the mass of the host galaxy and its black hole.