The first systematic study of activity in galacic nuclei is that one of C. K. Seyfert (1943) who studied the emission lines in 6 emmission-line galaxies (EL galaxies): NGC 1068, 1275, 3516, 4051, 4151, and 7469; these galaxies became prototypes of a sub-class of the active extragalactic objects named Seyfert galaxies. In the radio waveband a number of radio sources were identified with galaxies or with stellar-like objects. One of the major breakthroughs in the investigations of nuclear activity took place in the beginning of 60's when M. Schmidt solved the problem with the unusual spectrum of the extragalactic radio source 3C 273. The spectral lines were identified as the Balmer ones but redshifted to z = 0.16. 3C 273 belongs to a class of active objects, named quasars or QSOs, that are the most powerful objects known to us. Up to now more than ten thousand active extragalactic objects have been identified (see Veron-Cetti & Veron catalogue).




Seyfert galaxies are mainly spiral, hosting bright nuclei, sorces of strong non-thermal radiation  (1039-1045 erg/s), with contimuum spectrum in the range 1012 - 1022 Hz. They were divided into two sub-classes, namely Seyfert 1 and Seyfert 2, depending on the width of the permitted emission lines. Quasars were divided into radio-loud and radio-quiet; a number of broad absorption lines (BAL) quasars were identified. Radio galaxies were also divided into two sub-classes: broad line and narrow line radio galaxies. They are strong radio sources. The nearest of them are the brightest members of galaxy clusters. Most of the radio galaxies are elliptical. BL Lac objects, violent variable extragalactic radio sources with featureless spectra, are another sub-class of active galactic nuclei (AGN). Lacertae are named after BL Lac. They are starlike objects with a nebular envelope. They are optically variable with an amplitude of 4m - 5m, radio-variable and their radiation is polarized. They have no emission lines in the optical range. Furthermore, BL Lac objects and flat-spectrum radio-loud quasars were joined and named blazars. Depending on the radio activity AGNs could be divided into radio-quiet and radio-loud: the former sub-class contains Seyfert galaxies and radio-quiet quasars, and the latter contains radio galaxies, radio-loud quasars and BL Lac objects. Host galaxies of the sub-classes are also different: radio-loud nuclei are hosted by elliptical galaxies and radio-quiet objects are hosted by spiral galaxies. In our days AGN activity could be studied throughout the whole electromagnetic spectrum - from radio-waves to TeV gamma-rays.

The basic AGN features are: enhanced radiation in the UV, IR, radio and X-ray region compared to normal galaxy nuclei, variable radiation, presence of emission lines, non-thermal radiation, the nucleus luminosity is a large part of the total galaxy luminosity. It is not necessary all above signs to be present in an AGN.

What is the nature of the activity in galactic nuclei? What are the processes that lead to the high energy release observed in AGNs? What is the mechanism that turns the nuclear activity on? What are the mechanisms that fuel the central engine?

The main constraints on the AGN energy source model are the huge energy output and the small size. The most popular model of the energy source of AGN is accretion onto a supermassive black hole, originally proposed by Salpeter (1964) and Zel'dovich (1964) for quasars. The basic idea behind the so called Unified Model of AGNs is that all AGNs which we observe have the same internal structure in their nucleus. All the differences that we see are caused by the angle of the system (i.e. the plane in which the accretion disc, the outer parts of the broad line region and the molecular torus all lie).




Active galactic nuclei are thought to contain a massive relativistic object (a black hole), surrounded by an accretion disk (Shields, 1986). Rees (1984) made a detailed review of models of active nuclei, containing a black hole. Subsidiary evidence in favour of that contention was provided by Davidson (1972) and Shields and Oke (1975), who explained the emission lines in the spectra of Seyfert galaxies and quasars as caused by UV radiation-induced photoionization, generally an extrapolation of the power-law spectrum. Pravdo and Marshall (1984) pointed to the accretion disk around the relativistic object as a source of soft X-ray emission in active nuclei. Ferland and Osterbrock (1985) proved that in 3C 192 and 3C 223, narrow emission line galaxies, photoionization is induced by an X-ray source.

Disk accretion onto a black hole with accretion rate Ma is known to cause luminosity L ~ 1046Ma erg.s-1, where M is the accretion rate in solar masses per year. Besides, Eddington luminosity of an object of mass Μ is LEdd ~ 1046M8 erg.s-1, where M8 is the object mass in 108 solar masses. These two estimations indicate that AGN luminosities can possibly be explained with accretion of matter onto a black hole of mass 108 MS (MS is the Sun mass) and accretion rate Ma ~ 1MS.yr-1.
(1)     The ratio of the thermal to nonthermal energy, released as a result of the accretion is not defined, but if both types of energy are comparable and nonthermal energy is the consequence of the power-law spectrum Ev = να, α = - 1.2, the thermal energy will be manifested as an excess in the near-ultraviolet, what is frequently observed.
The dynamic time R/Vk at distances 100.Rg (where R is the distance from the centre of the black hole, Vk, the Keplerian velocity for a given R, and Rg, the gravitational radius) is about 1 day. In consequence, we should observe a certain variability of 1 day time-scale.
As it is well known, higher velocities are observed closer to the central source.  Shuder (1982) indicated that the smaller the distance from the centre, the higher the velocity of Balmer and HeI lines. On the other hand, if the broad lines of low-ionization atoms are spectrally shifted in respect to the highly ionized, this is an indication of a radial motion of the emitted gas (Gaskell 1982). These facts are in good agreement with another couple of models - the disk accretion model and the one of gas clouds, revolving on strongly elongated orbits (see Kwan and Carrol 1982). Additional indications (for instance, Vk is about 15 000 km.s-1 at Teff ~ 104 K) help us to choose the first one.
Gaskell (1984) studied the possibility for a binary supermassive black hole to exist in AGN. Analogically to binary stars, known to be X-ray sources, the central peak of broad emission lines will be shifted in respect to the Z-system, defined by the narrow lines. A number of objects of asymmetric profiles of the broad lines have been observed, and this is suggestive of such a possibility. Peterson et al. (1987) found a double broad-line region in NGC 5548.
(5) Tennant and Mushotzky (1983) showed that X-ray variability is in good agreement with the model of the disk's atmosphere being heated to large radii. Pounds et al. (1986) found strong X-ray variability on a time-scale of 1-2 hours for Mrk 335. As a result of Compton heating a corona having Τ ~ 108 K is formed at distances, where Vk > 103.5 km.s-1, and thermal gas pressure-induced wind appears (Begelman et al. 1983). If the heated gas is optically thick in respect to the electron collisions polarization will be observed.
(6)  Begelman (1985) showed that when the energy source is a massive black hole, the density of the emitting gas will decrease along the radius in conformity with R-2. Peterson and Ferland (1986) detected accretion in the Seyfert galaxy NGC 5548.

The full text with the identification charts (Petrov 1988) could be find here in PDF format.






There is a hypothesis that many galaxies have massive black holes in their nuclei. These nuclei turn active only when refueled. A possible source is gas as a result of interactions that finally falls onto the black hole (Osterbrock 1993). There existed alternative models beforehead like photoionization by very hot stars rather than by a central massive object (Terlevich & Melnick 1985). In both cases we might expect to find an excess of AGN among galaxies having nearby companions, or among interacting or disturbed systems.          

There is a strong evidence that many Seyfert galaxies are distorted (Vorontsov-Velyaminov 1977; Adams 1977; Wehinger & Wyckoff 1978), and that many of them have bars (Heckman 1978; Simkin et al. 1980). However, Seyfert galaxies seem to avoid extremely distorted systems (Dahari 1985; Keel et aJ. 1985; Bushouse 1986). Several statistical studies have found an excess of Seyferts with nearby neighbours compared to nonactive galaxies (Petrosian 1982; Dahari 1985, MacKenty 1989). This seems to suggest some correlation between tidal interactions and nuclear activity triggering, but it is not clear whether this is the dominant mechanism. Indeed, the contrary result showing only a marginal excess of neighbours among Seyferts also was obtained (Fuentes-Williams & Stocke 1988).

The full text (Laurikainen et al. 1994) in PDF format could be found here.


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  1. A proposed programme for observations of extragalactic X-ray sources. The program was stimulated mainly by the paper of Shields (1986). 98 AGN were studied by after Aldrovandi’s (1981) method. According to it model the black hole mass and the accretion rate may be reckoned from Hβ line luminosity and the relative intensity of HeI λ4686 and Hβ IHeII/IΗβ. Out of the 98 objects studied, 47 may have as energy source accretion disk around a black hole (Petrov and Velichkova 1988). We propose a maximum and a minimum programme. The former includes 64 objects, both Seyfert galaxies and X-ray sources. 34 of them are Markarian galaxies. The minimum programme comprises 11 objects that furthermore satisfy Aldrovandi’s conditions for accretion disk arround a black hole.  The programmes take in investigations of both direct images and spectra of the selected galaxies.
  2. Environments of Seyfert Galaxies. In order to build a satisfactory picture of Seyferts related to normal galaxies we have started a series of papers first establishing some of their basic environmental properties, especially the difference in the number of close companions between Seyfert 1 and Seyfert 2 galaxies. Here we report sample selections and discuss selection effects involved for the samples of 104 Seyferts and 138 control galaxies. The results of the statistical analyses are presented by Laurikainen & Salo (1994, Paper II). The neighbouring galaxies are counted on the Palomar Sky Survey Plates to the limiting magnitude 19 mpg within the circles of 1.5 Mpc in diameter (H0 = 100 km.s-1 Mpc-1) , large measuring circles enabling good elimination of the background galaxies. Subsamples are selected in order to compare our results with those obtained by Dahari (1984) and by Fuentes-Williams & Stocke (1988). Elimination of background galaxies and problems related to comparison galaxy sample selection are discussed. The most important problem in our control galaxy sample selection is that the redshifts for most of the control galaxies are unknown. The size of this uncertainty is estimated by determining the redshifts by two ways: (1) by assuming that the selected comparison galaxy has the same redshift as the nearby Seyfert and (2) by Monte-Carlo simulations for Holmberg (1975) galaxy size distribution in space, taking into account the Malmquist bias. The mean redshifts of the comparison sample are estimated with these methods to be 0.028 and 0.029, respectively, compared with 0.026 for the Seyfert sample. Distribution of Seyferts in Zwicky's clusters is also addressed, and compared with previous studies. Taking into account optical projections, about 3/4 of the Seyferts are found to be field galaxies. Galaxies in clusters lie preferentially at the cluster borders.
  3. Johnson-Cousins UBVRI surface photometry of Seyfert galaxies. Our aim here is to decompose the galaxy surface brightness profiles into a nucleus, bulge, disk and bar.
  4. Spectroscopy of X-ray selected active galaxies. We are still in the phase of data collecting. We are using FoReRo and FoReRo-2 focal reducers with grisms installed.
  5. Photometric monitoring of AGNs. This is a standard photometric monitoring and it is performed as an addition to the main observing programme. The presence of secondary standards in the field of the programme objects makes the data obtained useful even in the nights of bad quality - bad seeing, non-photometric conditions, unability to get transformation coefficients. This monitoring was inspired by Dr. K.-J. Schramm from Hamburg, Germany. Our data points could be appended to the existing data sets in order to get a better temporal coverage of the light curves. We shall continue this work at least in the near future.




1.      PETROV G. T., Astrofizika, v. 15, 59-65, 1979 (in Russian)
”Dependence of emission line luminosities of type-1 Seyfert galaxies upon colour index”

2.      PETROV G. T., Pis'ma AJ, v. 5, 267-270, 1979 (in Russian)
”Physical conditions in the nuclei of galaxies with emission lines”

3.      PETROV G. T. , Astrofizika, v. 15, 383-392, 1979 (in Russian)
Physical conditions in the nuclei of Seyfert galaxies of type 1”

4.      PETROV G. T. , C. r. A. S. Armenia SSR, v. 69, 52-56, 1979 (in Russian)
”Contents of the ions and chemical abundances in the nuclei of type 1 Seyfert galaxies and broad lines radio galaxies”

5.      PETROV G. T., Yought Astrophysicists Conference, 2-5 oct.,1979, Bjurakan
”Abundances in the Radio- and Seyfert galaxies”

6.      GOLEV V. K., YANKULOVA I. M., PETROV G. T., Pis'ma AJ, v. 6, 554-558, 1980 (in Russian)
”Preliminary spectrophotometric investigation of the nucleus of the galaxy NGC 5929”

7.      PETROV G. T., C. r. A. S. Armenia SSR, v. 70, 46-49, 1980 (in Russian)
”Ion abundance and chemical composition in the nuclei of type 2 Seyfert galaxies and narrow lines radio galaxies”

8.      YANKULOVA I. M., GOLEV V. K., PETROV G. T., Pis'ma AJ, v. 6, 691-695, 1980 (in Russian)
Phisical conditions in the nucleus of the galaxy Mrk 534”

9.      YANKULOVA I. M., PETROV G. T., GOLEV V. K., C. r. Acad. Sci. Bulg., v. 33, 1297-1300, 1980
”Preliminary spectrophotometric investigation of the nucleus of the galaxy NGC 5929”

10.  GOLEV V. K., PETROV G. T., YANKULOVA I. M., C. r. Acad. Sci. Bulg., v. 33, 1033-1036, 1980 (in russian)
”Spectrophotometric investigation and phisical conditions in the nucleus of the galaxy Mrk 534”

11.  PETROV G. T., GOLEV V. K., YANKULOVA I. M., Astr. Tsirc.  No. 1143, 1-3, 1980 (in Russian)
”Spectrophotometry of the nuclei of the emission line galaxies. NGC 7463, Mrk 313, 531 and III Zw 103”

12.  PETROV G. T., GOLEV V. K., YANKULOVA I. M., C. r. Acad. Sci. Bulg., v. 34, 461-464, 1981
”Physical conditions in the double galaxies with emission lines. Mrk 171a, b”

13.  PETROV G. T., YANKULOVA I. M., GOLEV V. K., Astrofizika, v. 17, 43-51, 1981 (in Russian)
”Physical conditions in the nuclei of the emission line galaxies”

14.  YANKULOVA I. M., PETROV G. T., GOLEV V. K., Astr. Tsirc. No. 1169, 1-3, 1981 (in Russian)
”Some spectrophotometric data about the double galaxy NGC 3690 + IC 694”

15.  PETROV G. T., KOVACHEV B. J., MINEVA V. A., C. r. Acad. Sci. Bulg., v. 35, 725-728, 1982
”Physical conditions in the galaxy nuclei with emission lines. Mark 558”

16.  MINEVA V. A., PETROV G. T., KOVACHEV B. J., C. r. Acad. Sci. Bulg., v. 36, 713-716, 1983
”Physical conditions in the nucleus of the Seyfert galaxy NGC 7469. II. Spectrophotometric investigation. ”

17.  GOLEV V. K., YANKULOVA I. M., PETROV G. T., Adv. Space Res., v. 3, 235-237, 1984
”On the physical state in the narrow-line region of Classical Seyfert galaxy NGC 7469”

18.  PETROV G. T., YANKULOVA I. M., GOLEV V. K., C. r. Acad. Sci. Bulg., v. 37, 411-414, 1984
”Nuclear H II regions in galaxies with emission lines”

19.  GOLEV V. K., YANKULOVA I. M., PETROV G. T., C. r. Acad. Sci. Bulg., v. 37, 549-551, 1984
”On the physical state in the narrow-line region of classical Seyfert galaxy NGC 7469”

20.  PETROV G. T., MINEVA V. A., KYAZUMOV G. A., C. r. Acad. Sci. Bulg., v. 37, 1287-1289, 1984
”Gas component parameters in the nucleus of the galaxy NGC 5879”

21.  GOLEV V. K., TSVETANOV Z. I., PETROV G. T., Astr. Invest. (Bulg.AS), v. 4, 95-105,1985 (in Russian)
”Results of a spectroscopic investigation of some Arakelian galaxies”

22.  PETROV G. T., Astr. Tsirc. No. 1480, 3-4,1988 (in Russian)
Spectroscopy of the Seyfert galaxy Markarian 609”

23.  PETROV G.T., Ap & Spa. Sci., v. 148, 305-341, 1988
”A proposed program of observations of extragalactic X-ray sources”

24.  MINEVA V., PETROV G., C. r.Acad. Sci. Bulg., v. 43, No. 3,1990
”Masses and rotational momenta for 47 Seyfert X-ray galaxies”

25.  PETROV G., VELICHKOVA K., Astr. Invest. (Bulg.AS), v. 6, p. 26-31, 1991
”Gas accretion onto the black hole as an energy source in the nuclei of the active extragalactic objects”

26.  PETROV G., MINEVA V., C. r., v. 45, No. , 1992
”Dependences between some parameters of Active galaxies”

27.  LAURIKAINEN E., SALO H., TEERIKORPI P., PETROV G.T., Astron. Astrophys. Suppl. Ser., v. 108, 491-508, 1994
”Environment of Seyfert galaxies. I.Construcvtion of the sample and selection effects”

28.  PETROV G., MINEVA V., Astr. Invest.(Bulg. AS), v. 7
”Relations "relative momentum - mass" and "absolute magnitude – maximal rotational velocity" for some types of active galaxies”

29.  BACHEV R., PETROV G., C. r., v. 49, No. 7-8, 1996
On the Black hole masses, accretion rates and unification of Seyfert galaxies”

30.  SLAVCHEVA L., PETROV G., MIHOV B., Compt. rend. l'Acad. bulg. Sci., vol. 51, No 1-2, p. 5, 1998
”Spectral analysis of Sefert 1 Galaxies”

31.  PETROV G. T., BACHEV R., STRIGACHEV A. A., C. r., to be published
”On the nature of active galactic nuclei”

32.     SLAVCHEVA L., MIHOV B., PETROV G., BACHEV R., AGN and Related Phenomena, Proceedings of IAU Syposium 194, held 17-21 Aug. 1998, in Yerevan, Armenia. Edited by Y. Terzian, E. Khachikian, and D. Weedman, San Francisco: Astronomical Society of the Pacific, p. 87, 1999
Spectrophotometry of Selected AGN: Seyfert Galaxy AKN 564

33.  BACHEV R., PETROV G., SLAVCHEVA L., MIHOV B., AGN and Related Phenomena, Proceedings of IAU Syposium 194, held 17-21 Aug. 1998, in Yerevan, Armenia. Edited by Y. Terzian, E. Khachikian, and D. Weedman, San Francisco: Astronomical Society of the Pacific, p.311, 1999
Black Hole Masses and Unification of Seyferts

34.  PETROV G., SLAVCHEVA L., BACHEV R., MIHOV B., AGN and Related Phenomena, Proceedings of IAU Syposium 194, held 17-21 Aug. 1998, in Yerevan, Armenia. Edited by Y. Terzian, E. Khachikian, and D. Weedman, San Francisco: Astronomical Society of the Pacific, p. 84,1999  Surface Photometry of Barred AGN Arakelian 564

35.  SLAVCHEVA-MIHOVA L., PETROV G., MIHOV B., Aerospace Reserch in Bulgaria, 2005, 19
”Multicolour Surface Photometry Of Seyfert Galaxies: First Results”

36.  PETROV G., DETTMAR R.-J., Astron. & Astrophys. Trans., in preparation    “Surface photometry of barred AGN. NGC 6764         

37.  Photometric Monitoring of Selected Quasars: The Highly Luminous Quasar HS 1946+7658, Mihov, B. M., et al, 1999, AGN and Related Phenomena, Proceedings of IAU Syposium 194, held 17-21 Aug. 1998, in Yerevan, Armenia. Edited by Y. Terzian, E. Khachikian, and D. Weedman, San Francisco: Astronomical Society of the Pacific, p. 175, 1999

38.  Search for Optical Variability in Two Seyfert Galaxies, Bachev, R., Slavcheva-Mihova, L. 2000, Proceedings of the Second Serbian-Bulgarian Astronomical Meeting. Held June 23-26, 2000, in Zaječar, Serbia. Edited by Milan S. Dimitrijević, Luka Č. Popović, and Milcho Tsvetkov. Published by the Astronomical Observatory, Volgina 7, 11000 Belgrade, Yugoslavia, p.3



Created by L.Slavcheva-Mihova, G. T. Petrov