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ACTIVE GALACTIC NUCLEI
QUASARS,
EL GALAXIES, BL LAC, X-RAY SOURCES,
RADIO GALAXIES, AGN
ENVIRONMENTS
HISTORICAL BACKGROUND
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).
SCIENTIFIC BACKGROUND
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).
EXTRAGALACTIC X-RAY SOURCES
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
Ma
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.
(2) 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.
(3) 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.
(4) 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.
ENVIRONMENTS
OF SEYFERT GALAXIES
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.
Click here
for more details about Activity in Galactic Nuclei.
OUR
MAIN OUTGOING OR FINISHED RESEARCH SUBJECTS:
- 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.
- 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.
- 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.
- 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.
- 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.
PUBLISHED
PAPERS:
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
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