INTERPLANETARY SCINTILLATION OBSERVATIONS OF METERWAVELENGTH RADIO EMISSION FROM GALAXIES
V. S. Artyukh
Abstract. We present an explanation of the interplanetary scintillation method. Observations
of various types of galaxies indicated that scintillating sources with flat or inverted spectra
generally occur in the nuclei of giant elliptical galaxies, while the steep-spectrum sources are
found in spiral galaxies. This effect can be explained if the magnetic field is weaker and the
relativistic plasma density is higher in the nuclei spiral galaxies than in the nuclei of elliptical
galaxies. The measured integrated flux densities of M 31 and M 33 at 102 MHz indicated that
the density of supernova remnants is an order of magnitude higher in M 33 than in M 31. No
halo was observed around these galaxies.
1. Introduction
Observing the radio emission from galaxies is one of the most pressing problems of
current years. In recent years, a special interest has been taken in active galaxies (which show
excess emission above that of nearby normal galaxies in some region of the electromagnetic
spectrum). This excess emission frequently originates in very small regions located at the
center of the galaxy, which suggests that active galactic nuclei are the reason -behina the
activity of galaxies. Broad, strong emission lines of hydrogen and forbidden lines of several
other elements, strong infrared and X-ray emission (which is variable in many objects), and
optical and radio jets are typical signs of activity in galactic nuclei.
The first research on active galaxies dates back to 1943, when Seyfert's paper on
spectroscopic observations of 6 spiral galaxies with bright stellar nuclei [l] appeared.
Unusually broad emission lines of high excitation were observed in the spectra of these
objects. The linewidths correspond to gas velocities between 300 and 3000 km/sec. After the
war, the development of radio astronomy led us to observe galaxies with abnormally high
radio emission—radio galaxies [2]. Optical identifications of radio galaxies indicated that
some of them could be identified with N galaxies [3]. The development of a relay
interferometer with a 122-km baseline [4] enabled us to observe several compact sources with
angular sizes less than one arcsecond, such as 3C 48. The development of lunar occultation
methods [5] also led to an increase in resolving power and enabled us to determine the precise
coordinates for sources. In 1963, this method was used to detect a compact component in 3C
273 and measure its precise coordinates [б]. The optical identification revealed a 13'" star
with an unusual spectrum containing broad emission lines was located at this position. It was
shown in the same year that these lines could be identified with the hydrogen Balmer series
and Mg II if the object was assumed to have a redshift z = 0.158 [7]. Thus this quasi-stellar
radio source turned out to be an extremely bright extragalactic object — and so quasars were
discovered.
Many other compact blue objects with all the properties of quasars, but without any
radio emission were observed [8]. Radio-quiet quasars are sometimes called quasags or
simply quasistellar sources in the literature. Some quasistellar objects were later identified as
BL Lac objects [9]. These are highly variable objects whose continuous spectra bear a very
close resemblance to those of quasars, but the spectral lines are so weak that they can only be
observed when the source is at minimum brightness. The idea of an interferometer with
independent local oscillators [10] then enabled us to construct very-long-baseline
interferometers which were then used to discover superluminal motion of the radio emitting
clouds [11, 12].
The violent processes in Seyfert galaxies, radio galaxies, N galaxies, BL Lac objects
and the unusually high rate of energy production in quasars—these all indicate that the
physical conditions are unusual compared to those found in normal galaxies and are of special
interest to astrophysicists. These galaxy types are not very well-defined, so reports
periodically appear in the literature claiming that a certain galaxy that was originally
classified as a certain type should actually be classified as a different type. As the
observational material accumulates, proposals for expanding the existing classification
scheme to include classifications for galaxies with narrow emission lines, roarers, blazars, etc.
These classifications are not based on any physical theory, and it is unclear whether galaxies
in the different classes are actually different physical systems or the same objects at different
evolutionary stages.
Active galaxies are observed throughout the electromagnetic sp.ectrum. For example,
galaxies with ultraviolet continuum (Markaryan galaxies) have been observed [13, 14]. With
the advent of space-based platforms, observations have begun at infrared and X-ray
wavelengths, where galaxies with active nuclei are also strong sources of radiation. Detailed
reviews of this research are presented in [15, 16].
Highly effective observations are also being carried out at radio wavelengths, where
quasars, BL Lac objects, N galaxies, and radio galaxies have been observed. Most of these
objects are faint and of small angular size due to the fact that they are at great distances, so
that high sensitivity and resolution are required in order to study them. Almost all of the data
on galactic nuclei at radio wavelengths has been obtained at decimeter and centimeter wavelengths, where both the resolution and sensitivity are much higher than at meter wavelengths.
For example, the giant VLA array (which enables one to observe compact sources with flux
densities of order a few millijanskys) operates at centimeter wavelengths, and very-longbaseline interferometry has yielded a record resolution of ~ 0.0001". On the other hand, highresolution, high-sensitivity low-frequency observations are also required to determine the
physical conditions within galactic nuclei. Here, the most effective technique involves the
observation of interplanetary scintillations, which have a limiting resolution of 0.01".
Nearby normal galaxies are also of interest. Their radio luminosities are several orders
of magnitude less than those of radio galaxies, so we can only observe those objects which
are relatively close to us. Just as for the active galaxies, almost all of the research on radio
galaxies has been carried out at centimeter wavelengths, where the thermal component of the
radio emission makes a noticeable contribution in normal galaxies. A detailed review of this
work is contained in [17].
At meter wavelengths, the radio emission from galaxies can be assumed to be
completely nonthermal at meter wavelengths, and the generally accepted mechanism is
synchrotron radiation. Supernovae and pulsars are the most likely sources of relativistic
electrons, although young evolving stars and the cores of galactic nuclei may also provide
some relativistic particles. The question of whether supernovae can provide all of the
nonthermal emission in normal galaxies is still open, and it would be interesting to examine
the relationship between supernova remnants and the nonthermal emission of galaxies in this
connection.
Cosmic rays formed in the galactic disk will move away from the plane of the galaxy
by diffusion, and should form a galactic halo after leaving the disk [18]. The presence of such
a halo can only be detected by its nonthermal radio emission, and the search for radio halos
around normal galaxies is one of the problems currently under study by radio astronomers.
Another current problem in radio astronomy is the search for supernova remnants
(SNRs) in external galaxies. Until now, all SNRs in external galaxies have been found by
optical methods and the radio emission was observed later, unlike our Galaxy, where the
situation was the reverse. The radio emission from SNRs is nonthermal, while H II regions
(which are frequently confused with SNRs) have thermal radio emission. It is therefore
appropriate to search for SNRs at meter wavelengths; however, this requires high sensitivity
and high resolution. Interplanetary scintillations provide a quite suitable method for solving
this problem.
The present review paper describes the interplanetary scintillation observations of the
radio emission from galaxies at 102 MHz currently under way at the Pushchino Radio
Astronomy Station of the Lebedev Physics Institute (USSR Academy of Sciences).
2. Interplanetary Scintillation
The scintillation effect was first observed in 1964 [19], and has been widely used in
studying the properties of the interplanetary plasma (IPP). In particular, a great deal of the
research (both experimental and theoretical) on the properties of the interplanetary plasma has
been carried out at the Lebedev Physics Institute Radio Astronomy Laboratory. The results of
this research have been presented in review paper and book form [20, 21]. In addition
research on the IPP itself, scintillation observations have been used to obtain information on
the angular sizes of compact sources. This is precisely the aspect of scintillation that we will
be interested in.
Scintillations of radio sources are due to the diffraction of radio waves on
inhomogeneities in the IPP. This process is described by the theory of wave propagation in
random media. The modern theory [21-23] is based on a fairly complex set of mathematical
tools; this makes it difficult to explain. In the phase-screen approximation (which provides a
completely satisfactory description of the interplanetary scintillations at small elongations),
however, the theory is relatively straightforward. Indeed, since the density of the IPP
decreases with increasing distance from the Sun as 1/r2 [24], the distribution of electron density along the line of sight will be as shown in Fig. la, which indicates that one can identify a
region with thickness L much less than 1 AU at small elongations (e < 50°) where the density
of the IPP is largest. This is precisely the layer that introduces the largest perturbations in the
radio waves as they pass through the layer. Since only the phase of the wave is modulated
while the amplitude of the wave remains virtually unchanged within the layer we have
isolated, we can use a phase-screen model to describe the propagation of the radio waves in
the IPP.
Figure 1 Density distribution of the interplanetary plasma along the line of sight (a) and a
diagram illustrating the phase screen model (b).
Fig. 1b contains a diagram illustrating the passage of a plane wave through a phase
screen in the one-dimensional case. The variations in the index of refraction lead to variations
in the velocity of propagation. This is due to the fact that the path length for the wave is has
been lengthened due to the refraction of the direction of propagation of the wave front, and
means that upon exiting from the screen, the plane wave will suffer a random phase change
Ф(а;) relative to the unperturbed wave at the point x. This leads to fluctuations in the intensity
of the radiation in the far field, where the observer is located. We shall assume that all of the
random processes under discussion are completely described by their autocorrelation
functions. For example, the fluctuations in the intensity of the radiation can be characterized
by the autocorrelation function M(r) :
M ( r)  I ( x) I ( x  r)  1 ,
(1)
where M (0)  ( I  I ) 2 / I and is called the scintillation index. The Fourier transform of
2
this function, M ( q)   M ( r )e  qr dr , is called the spatial scintillation spectrum. The following
simple equation relates the spatial scintillation spectrum to the spectrum of electron density
fluctuations in the IPP and the spectrum of the source brightness distribution [25]:

2
2  q z  2  qz 

(2)

(
q
)
sin
 e
 2k  F  k dz ,

  

where A = 510–25 cm2,  is the wavelength of the radiation, k is the wavenumber, z is the
line-of-sight coordinate, q is the spatial frequency, Фe(q) is the spatial spectrum of the
fluctuations in the electron density of the IPP, and F(qz/k) is the spatial spectrum of the radio
source.
The inhomogeneities in the IPP are moving away from the Sun with a mean velocity
V  400 km/s (at the Earth). This phenomenon is called the solar wind. The diffraction
pattern created on the Earth by the inhomogeneities in the IPP moves with the same velocity,
M ( q)  4 A
2
and the radio telescope records the time variation in the intensity. The time spectrum of the
scintillations is of the following form [26]:
P( f )  4 A2 
2
dz
2f
2  q z  2  qz 

 F   q 
dq

(
q
)
sin

e


V ( z )
V ( z )
 2k   k 
(3)
where / is the time frequency, V^(z) the projection of the solar wind velocity onto the plane of
the sky at the point z, and qu the component of the spatial frequency parallel to the projection
of the solar wind velocity onto the plane of the sky.
Thus, the statistical characteristics of the fluctuations in the intensity of radiation are
determined by the statistical characteristics of the medium and the brightness distribution of
the source. If we know the characteristics of the medium, we can in principle obtain a strip
map of the brightness distribution across the source in the direction of motion of the solar
wind. This requires solving integral equation (3). The presence of measurement errors in the
estimated time spectrum of the scintillations turn this into an ill-posed problem and make the
problem more difficult to solve. Because of this, the solution is generally restricted to a single
parameter of the brightness distribution—the angular size of the source.
There are two methods used in determining the angular sizes of sources: 1) using the
behavior of the scintillation index for the sources as a function of elongation; and 2) using the
behavior of the time spectra of the scintillations. The latter method is the one used at the
Lebedev Physics Institute Radio Astronomy Station, where the angular diameters of several
hundred radio sources have been measured to date. Shishov and Shishova [27] have discussed
various scintillation modes and carried out numerical calculations of the scintillation time
spectra for sources of various angular sizes under the following assumptions:
1) The spectrum of the turbulence in the IPP is a power law:
 e ( q )   02 q  n ;
2) The turbulence decreases with distance from the Sun like
 02  1 / r 4 ;
3) The solar wind velocity is constant, radial, homogeneous, and spherically
symmetric; and
4) The brightness distribution of the source is a spherically symmetric Gaussian:
I ( )  exp(  2 /  02 ) ,
where 0 is the 1/e radius of the source.
These calculations were repeated in [28], but for a larger number of angular source
sizes and higher frequencies. The theoretical results from [28] are shown in Fig. 2; the
frequencies are given in units of f 0  V
1 / 2  1AU
(where V is the velocity of the solar
wind and 1 AU is the astronomical unit). The spectra have been superposed at low frequency
in order to emphasize the differences between the scintillation spectra for sources of various
angular sizes. The figure clearly indicates that the differences between the spectra are larger
at the higher frequencies. Thus, the larger the observed region of the spectrum accessible for
analysis, the more reliably the angular size can be determined by the method of interplanetary
scintillation. The resolution of the
method
depends
on
the
signal/noise ratio. The weaker the
signal, the lower the resolution.
Active galactic nuclei are
generally faint objects, so the
highest resolution obtained in the
present observations was 0.01".
Artyukh and Shishova [28]
showed that saturation is the main
effect that limits the resolution.
Artyukh and Shishova analyzed
the effect of various factors on
the accuracy with which the
scintillation spectra can be
measured, and showed that the
method has a limiting resolution
of 0.01" at 102 MHz . Note that
an interferometer of the same
resolution operating at the same
Figure 2 Theoretical scintillation spectra for sources
frequency must have a baseline
of various angular size (from [28]).
of 60 000 km, or much larger
than the size of the Earth! Thus, interplanetary scintillation has the highest resolution of any
of the ground-based methods of studying compact radio sources at meter wavelengths.
The observations whose results will be presented below were carried out using the
Lebedev Physics Institute VLPA antenna [29] at a frequency of 102 MHz. The antenna beam
is 1° x 0.5° sec z (where z is zenith distance). The antenna system has an effective area
20 000 m2, the bandwidth  = 1.5 MHz , the receiver time constant  = 0.5 s, and the
fluctuation sensitivity S = 0.14 Jy . Figure 3 shows a sample analog trace for the scintillating
source 3C 48, and Fig. 4 its estimated scintillation spectrum.
The method used to reduce the data has been described in [30, 31]. We shall not go
into detail on this subject, but merely note that the following parameters can be determined
from the observations: the coordinates and flux densities of sources, their scintillation indices,
and the angular sizes of the scintillating components.
3. Information Content of the Observations
We shall now discuss the information on the physical conditions in galactic nuclei that
can be provided by observations of scintillating sources at meter wavelengths. The fact that
radio sources have power-law spectra similar to the cosmic-ray power-law spectrum and
polarized emission is consistent with the hypothesis that the emission is due to the
synchrotron effect, which is also implied by various theoretical considerations {32]. We can
therefore assume that this is indeed the mechanism that is operating in the nuclei of active
galaxies. The spectra of many sources are observed to have low-frequency cutoffs. There are
several possible reasons for these cutoffs. We shall now describe them in brief:
1. A low-energy cutoff in the
electron energy spectrum. If the
relativistic
electron
energy
spectrum is of the form N{E) ==
NoE~^ for E > Ey and N(E) = 0
for E < EQ , the emission will
have spectral index cc = —1/3
(6' ~ ^-0!) below VQ , the critical
frequency corresponding to EQ
[33]. Other, less severe cutoffs in
the electron spectrum will
naturally lead to an even
shallower rate of decrease in the
emission at low frequencies.
However, since the vast majority
of compact radio sources have
steeper cutoffs, this mechanism
is not appropriate for explaining
them.
Figure 3 Analog recording of the scintillating source
2. An index of refraction
3C 48
different from unity. The
presence of thermal non-relatiyistic plasma in the region where the emission originates leads
to suppression of the radiation because of the fact that the coefficient of refraction of the
plasma is different from unity. This is the so-called Tsytovich-Razin effect [34, 35]. The
phase velocity of light in plasma c/n is greater than с , since n  1   02 /  2 , where 0 is the
plasma frequency. This means that at low frequencies, the electrons become nonrelativistic,
and the cone within which the emission is concentrated becomes larger, which leads to a
corresponding decrease in the intensity of emission. The frequency at which this occurs,
 p ( Hz )  20n e / H
(4)
where ne is the density of thermal electrons and H is the magnetic field strength in the source.
The emission coefficient shows the following dependence on frequency at frequencies
less than i/p:
   ~  3 / 2 exp  3.7 p / .
(5)
A sharp, exponential cutoff in the spectrum is a very rare phenomenon (which we did
not in fact encounter in our observations), so this mechanism is also illsuited to explaining the
cutoffs in the spectra of the compact sources observed at meter wavelengths.
3. Thermal absorption. Hot plasma within the radio source itself or along the line of
sight near the source leads to absorption of radiation in electron-ion collisions. The absorption
coefficient is given by the classical formula [18]
10 2 n 2
  3 / 2 e2
Te 
3/ 2

 Te  
17.7  ln    ,
   

(6)
where Те is the electron
temperature. The frequency at
which the absorption becomes
substantial,
 T (GHz )  Ln e2 Te3 / 2 
1/ 2
where L is the size of the source
along the line of sight (in
parsecs).
The compact radio
sources in Seyfert galaxies
(and, one might expect, the
nuclei of other galaxies as well)
have linear sizes between 1 and
10 pc [36]. The electron
temperature is of order 107 –
108 К,
since
appreciable
Figure 4
Scintillation time spectrum for 3C 48.
amounts of X-ray emission are
1) Noise spectrum; 2) source spectrum.
observed the majority of active
5
galaxy nuclei [15]. If we assume L = 100 pc and Т = 10 pc for purposes of estimation, the
density of -thermal electrons must be of order 105 cm"3 in order to obtain a cutoff in the
spectrum at ~ 1 GHz (as we observe). In our Galaxy, the optical depths indicate [37] that
n  2103 cm–3 within a 10-pc radius and n  104 cm–3 within a 1-pc radius [38]. This is less
than the required electron density; however, there is no reason to presume that such electron
densities cannot exist in the nuclei of active galaxies. One possible proof that this was in fact
the mechanism for the cutoff would be the low-frequency spectrum of the source. Once the
frequency decreases to the point where the optical depth becomes greater than 1, the
frequency should decrease according to the law   e  (where  is the optical depth) until the
main contribution to the emission comes from the thermal emission of the absorbing cloud
itself, with spectral index  = –2. But observation of this effect would require detailed lowfrequency observations of compact sources at several frequencies.
4. Synchrotron self-absorption. In the simplest case, the solution to the equation of
transport takes the following form:
I   
  

1  e  
  
where (v) is the emission coefficient, () is the absorption coefficient,  is the optical depth,
L
   dl , and L is the size of the source along the line of sight. From the theory of
0
synchrotron emission [39], we have the following for an ensemble of relativistic electrons
distributed according to a power law:
for <1:
I      L  C  N 0 H
for >1:
I    b H
1 / 2

  


2
C
1


(1 ) / 2

  


 2C1 
(1 ) / 2
L,
(7)
5/ 2
,
(8)
where H is the component of the magnetic field strength perpendicular to the line of sight,
C1 is a constant, and C() and b() are functions tabulated in [33].
At frequencies where the source is optically thick, it should have spectral index
  2.5 (8). If, however, the source is not homogeneous and different parts of the source
become optically thick at different frequencies, the low-frequency spectrum of the source
should be flatter. If  > 1, a maximum should be observed in the spectrum of the source.
From the condition (с) = 1, we obtain the following simple expression for с, which is very
close to the frequency at which the source spectrum reaches its maximum:
 c  2C1 N 02 /( 4) H ( 2) /( 4) C6  2 /( 4) L2 /( 4)
(9)
( С6() is also tabulated in [33]).
The observed parameters of the compact radio sources—their angular sizes, flux
densities, and expected magnetic field values are such that the frequency Vc. should fall at
radio wavelengths and synchrotron radiation should be observed. This implies that this
mechanism is the most likely explanation for the low-frequency cutoffs in the spectra of the
compact sources.
We shall now discuss what information can be extracted from the observations if we
are indeed observing synchrotron self-absorption. Slysh [40] has suggested that it is possible
to estimate the angular sizes of compact radio sources from their spectra. According to [40],
  4.3  1016 S 1 / 2 5 / 4 H 1 / 4 1  z 1 / 4 ,
(10)
where v is the frequency at which  > 1, S is the flux density of the source at this frequency,
and z is the redshift of the source.
The magnetic field was assumed to have a value H = 10–4 G. However, this same
formula can obviously be used to determine the strength of the magnetic field in the source
once the angular size of the source and its flux density have been measured. If the redshift z is
unknown, it can be assumed equal to unity. When the redshift of the galaxy containing the
compact radio source in question is embedded is known, the linear size L of the source can be
determined from its angular size, and, substituting the resulting H and L into (7), we can
obtain an estimate of the relativistic electron density N0 .
We shall now estimate the accuracy of this method of determining H and N0 . To do
so, we rewrite (9) in the following form:
H  2.3  10 17   5 4 S 2 1  z  .
4
1
(11)
The frequency v makes practically no contribution to the error in the estimated H
(even though it appears to the fifth power), since the frequency of observation is always
known to high accuracy (with an error equal to the receive bandwidth ~ 1%). At meter
wavelengths, the flux density can be measured to within 10-20%, which leads to a factor of
1.5 error in H. If the redshift is not known, and is assumed to be equal to 1, this can lead to an
error of no more than a factor of two in H , since z is probably between 0 and 3. The largest,
error is that due to the error in the measured angular diameter of the source. Interplanetary
scintillations yield an error of ~ 30% in the estimated angular diameters of sources for strong
sources and an error of as much as 100% for weak sources [28], depending on the
signal/noise ratio, i. e., the measured  may be in error by a factor of two. The corresponding
error in the estimated value of H is then a factor of 16. Thus, in the worst case, we can obtain
a value for the magnetic field strength which is accurate to an order of magnitude, or in the
best case, a factor of 2-3. Substituting L in terms of z into (7), we have
1  z 
h
N 0  C 1  
c
z
  7  / 2
 H
3
 1   / 2

  


 2C1 
 1 / 2
S,
(12)
where с is the speed of light, h is the Hubble constant, and  and S are the angular size and
flux density of the source, measured at frequency v . At high frequencies, interferometer
measurements of angular sizes lead to an error of ~ 10%, and the largest error in No is that
due to the error in H . Most radio sources have   3, with No ~ H–2, so that the error in N0 will
range from a factor of four to a factor of 100. The situation is better for flat-spectrum sources
( = 1), since N0 ~ H–1 and the error in the estimated N0 is the same as that in H.
Thus, observations of compact radio sources whose low-frequency spectrum is
determined by synchrotron self-absorption enable us to obtain information on the magnetic
fields and the density of relativistic plasma in the nuclei of the galaxies occupied by these
sources. Note that this method of measuring H and N0 does not require one to assume
equipartition of energy within the source.
4. Active Galaxies
We shall now list some of the results obtained on active galaxies at 102 MHz . Since
interplanetary scintillation only provides information on the compact components, the results
only refer to the nuclei of the galaxies.
1.
Quasars [41]. One hundred and one quasars from the Parkes survey [42] with
declinations between ±4° were studied. These are so-called high-frequency quasars—they
have low flux densities, and have been practically ignored at low frequencies. For example,
the most complete catalog available for scintillation sources at 81.5MHz [43] contains only
three of these 101 quasars. We observed radio emission from 53 sources, 40 of which have
scintillating components; in 13 cases, the complex pattern that was observed prevented us
from reliably identifying the radio emission from the quasars, and 35 sources had flux
densities below the sensitivity of the observations. Only those sources with flux densities
greater than 0.5 Jy were considered to be reliably detected scintillating sources, since the
median confusion limit for scintillating sources on the VLPA is ~ 0.14 Jy [44]. The mean
compactness (the ratio of the flux in the scintillating component to the integrated flux density)
is large, R = 0.5, meaning that the sources remain quasistellar at meter wavelengths, but only
11 of the quasars appear to be true point sources; the others are observed to have extended
components ( > 1).
These same sources have also been observed at high frequencies (962 and 1666 MHz)
using interferometers with resolutions of similar to ours [45] (approximately 0.1"). We used
the high-frequency observations to obtain spectra of the point components in the quasars
between 102 and 1666MHz. Fig. 5 shows the distribution of the point-source components in
the quasars with respect to spectral index. As the figure indicates, an overwhelming majority
of the sources have flat or inverted spectra. Only 9 sources have steep (a > 0.5) spectra
( S ~   ). Curiously, 9 of the 11 point sources have flat spectra.
2.
BL Lac objects [46]. Observations were carried out on 67 sources from
surveys [47-49]. It turned out that only 63 of the sources were still considered to be BL Lac
objects by the time observations had been completed. Radio emission was detected in 40 of
these 63 objects, 25 turned out to have scintillating components, and 6 turned out to be point
sources. A lack of high-frequency observations prevented us from constructing the spectra of
the point components. The mean spectral index for the integrated flux densities of the
scintillating sources between 102 and 408 MHz is  = 0.44 , which indicates that flat spectra
dominate. The distribution of the BL Lac objects with respect to spectral index is broader
than that for the quasars. The mean compactness R = 0.51 is just as high as for the quasars.
Thus, both classes of objects show high compactness and primarily flat or inverted
spectra at meter wavelengths.
Figure 5
Distribution of quasars with respect to spectral index from [41].
3. Seyfert galaxies [36]. Galaxies appearing on list [50] were observed. Eighty-eight
galaxies were observed, but only 73 of the objects were still considered Seyfert galaxies by
the time observations had been completed. Six of these have scintillating components
characterized by steep spectra (a. > 0.7). The linear sizes of these components, which range
from 5 to 70 pc , are similar to those of the milliarcsecond components in quasars. The mean
compactness R = 0.3 . Note that this is similar to the mean compactness R = 0.2 for randomly
chosen radio sources at the limiting flux accessible to observation with the VLPA [51].
4. Galaxies with UV continuum [52]. The observations of these galaxies (from lists
[13, 14, 53, 54]) were carried out at the same time as the Seyfert galaxy observations, and
some galaxies turned out to be common to both programs, since most galaxies with UV
continuum are Seyfert galaxies. Eighty-seven objects were studied, 72 of which were Seyfert
galaxies. Radio emission was observed in 12 objects, and scintillating components were
observed in 5. All of the scintillating components (except for Mrk 3) have steep spectra. The
mean compactness R = 0.2.
One problem in the observations described above was the low accuracy of the source
coordinates. The extremely large beam of the VLPA leads to the following errors in the
measured coordinates:  = 10' in right ascension and  = 5' in declination. Of course, there
is a chance that a scintillating source not associated with the target galaxy might fall within
this 10' x 5' error box. Numerical estimates of the probability for a random coincidence
between the observed scintillating sources and the target galaxies were made in [52, 55]. This
probability turned out to be negligibly small, and we concluded that the observed scintillating
sources are indeed associated with the galaxies being studied. However, it remains unclear
where these radio point sources are located within the galaxies. We assumed that they were
associated with the nuclei of the galaxies, since high-resolution observations at higher
frequencies indicate that only the galactic nuclei lie within the isophotes of the region
observed [16].
We shall now present a detailed discussion of our observational results for the active
galactic nuclei. We shall select the following two main results for discussion:
1. The vast majority of the scintillating components in quasars and BL Lac objects
have inverted spectra at meter wavelengths, but the Seyfert galaxies and galaxies with UV
continuum have steep spectra ( а > 0.7).
2. Quasars and BL Lac objects are more compact objects; they show a higher fraction
of the energy concentrated in the scintillating components than other active galaxies. Since
quasars and BL Lac objects are generally the nuclei of giant elliptical galaxies [47, 56-58],
and Seyfert galaxies are mainly spirals [59], this can be assumed to mean that most of the
compact scintillating sources in elliptical galaxies have flat and inverted spectra, while those
in spirals have steep spectra.
This relationship is statistical in nature, and it would be of interest to find out how
frequent the exceptions to this rule are. Fig. 5 indicates that 9 of the quasars have steep
spectra ( > 0.5) and are therefore exceptions. However, this may yet turn out not to be true,
since optical observations of the host galaxies of radio-quiet quasistellar objects indicate that
most of them are spiral galaxies [58]; thus, if some quasars turn do out to be the nuclei of
spiral galaxies, the radio point sources in them should have steep spectra.
As for the spiral galaxies, all the spiral galaxies we have observed to date have had
steep-spectrum scintillating components. The radio source associated with Mrk 3 may be an
exception—it has a relatively flat spectrum, but Jenkins [60] has shown that this galaxy is
probably an elliptical.
Observations of high-surface-brightness galaxies [55] from list [61] indicate that 7 of
the 44 galaxies observed have scintillating components. Two galaxies (numbers 448 and 451)
have pure scintillating components: All of the emission at meter wavelengths is confined to a
region < 0.1" in size; both sources have steep spectra. From the relationship between
morphological type of galaxy and type of spectrum for the compact components, one might
expect these objects to be spiral galaxies, even though they appear to be quasistellar on
photographic plates.
We now turn to a discussion of the physical conditions in the nuclei of active galaxies.
Since we do not observe cutoffs in the spectra of the compact sources in these galactic nuclei
(c < 102 MHz), and we only have upper estimates for their angular sizes, only an upper limit
to H can be determined from our observations alone. The results are presented in Table 1,
which is reproduced from [62]. We can obtain estimates for the magnetic fields in ellipticalgalaxy nuclei from observations of quasars which show a clear low-frequency cutoff in the
spectrum (Table 2). Most of the sources for which angular diameters have been measured
have flat spectra between 100 and 5000MHz and will, in our view, require special analysis.
Most of the quasars with inverted spectra have flux densities below the sensitivity of our
observations. Since they are not visible, we cannot estimate H using the method described
above; however, it is possible to obtain a rough estimate of the magnetic fields in these
sources.
Our upper limits on quasar flux density
enable us to state with confidence that their spectra
clearly do not have a low-frequency cutoff. Several
such spectra are shown in Fig. 6. The lack of highresolution observations between 102 and 962 MHz
does not enable us to make a precise determination
of the cutoff frequencies for these sources, but it is
clear from the figure that the spectra peak between
400 and 1000 MHz , i. e., they can be assumed to
be optically thick at frequencies below 400 MHz .
Table 1. Estmated magnetic field strengths in spiral
galaxy nuclei
Galaxy
S102,
H, mG
,

Name
Jy
arcsec
NGC 1068
0.2
5
>0.8
<15
NGC 4151
0.8
<37
 0.1
 0.8
Mrk 463
< 0.1
2.3
0.9
<4
Mrk 464
< 0.1
0.4
1.3
< 150
Figure 6 Spectra of the compact components in several quasars
Table 2. Estmated magnetic field strengths in elliptical galaxy nuclei
Galaxy
H, mG
, arcsec S102, Jy
Name
0056-00
4.5
 0.1
 0.5
0458-02
0.2
1.2
120
2003-025
0.1
1.5
6
The measurements in [45] indicate that the quasars under discussion are < 0.1" in size.
Very-long-baseline interferometer observations [63-66] indicate that the compact sources
have fine structure. Some consist of two or more components ~ 0.01" in angular size, and
there is generally one dominant component. Many have milliarcsecond-scale components.
For example, Preston, Morabito, and Jauncey [67] have studied the structure of Parkes
quasars with 0.002" resolution at 2.3 GHz . Thirty-two of their sources are in common with
those studied here. Of these, 23 have milliarcsecond components, while none were detected in
the 9 remaining sources, although four of these have scintillating components. Very weak
millarcsecond components have also been observed in the nuclei of spiral galaxies [68].
The regions within the source that are responsible for the high- and low-frequency
radio emission are not always identical in size. 3C 48 is one example of this. At centimeter
and meter wavelengths, it is a compact source with angular size 0.1". It shows strong
scintillation at 102 MHz (see Fig. 3), but does not scintillate at 25 MHz , which implies that
its angular size at decameter wavelengths is > 2". What angular size should we then adopt for
purposes of magnetic-field determination in quasars? Special high-frequency observations of
steep-spectrum sources showed a variations in size from 0.03" to 0.1" [63]. We should also
note that for the 32 sources in common between [67] and our observations [41], the coherent
component measured in [67] is quite small, i. e., most of these quasars were resolved, with
angular sizes greater than 0.002" and probably of order 0.01", which implies that most of the
compact sources under discussion are between 0.01" and 0.1" in angular size. A mean value
of  = 0.05" can be adopted for purposes of estimation.
The spectra of the compact components of the sources studied above have maximum
flux densities of approximately 1 Jy (see Fig. 6). The similarity of the flux densities can
probably be explained as a selection effect: Strong sources are rare, and we cannot detect
those with S < 0.5 Jy. So we can adopt values of S = 1 Jy and  = 0.05" for the "average
source" in our observations. The sources that became optically thick at frequencies less than
400 MHz will then have a mean magnetic field H  10–1 G, while the upper limits on H for
the sources with c < 100 MHz will be three orders of magnitude smaller. In spite of the crude
estimates, we can infer that the mean magnetic field in the nuclei of spiral galaxies is weaker
than that in the nuclei of elliptical galaxies.
We shall now discuss the density of relativistic electrons in radio sources with various
magnetic fields. Table 1 indicates that the compact sources in Seyfert galaxies have spectral
indices   1. According to [41], many quasars (with the exception of 20 flat-spectrum
sources) also have steep spectra between 962 and 1666 MHz. We shall assume on this basis
that the spectral indices of the sources  = 1 at high frequencies, where they are optically
thin; correspondingly,  = 3. We also note that both equation (7) and (8) can be used in the
vicinity of the cutoff in the spectrum where  = 1. We can then write the following
relationship for the spectra of two sources with equal intensity but different cutoff frequencies
L1 N 01 H 12 c11  L2 N 02 H 22 c21 ;
since H ~  c5 ,
9
 
L1 N 01   c 2  L2 N 02 .
  c1 
This clearly indicates that the density of relativistic electrons should be a quite strong
function of cutoff frequency, even for large variations in linear size. Sources with weaker
magnetic fields require a higher density of relativistic plasma to produce the same radio
emission.
Thus, the observed spectral characteristics of the scintillating components associated
with the nuclei of active galaxies may be explained by the different physical conditions that
occur in galaxies of various types: The magnetic field generally tends to be weaker (and the
relativistic plasma densities much higher) in spiral galaxy nuclei than in elliptical galaxy
nuclei.
5. Local Group Galaxies
The Andromeda Nebula (M 31) and the galaxy in Triangulum (М ЗЗ) – both members
of the Local Group – are the nearest spiral galaxies, and have very large angular sizes (of
order 1° ). Their nuclei also have large angular sizes (of order several arcminutes), and the
only possible scintillating sources in these galaxies are highly compact supernova remnants
(SNRs). The literature has been filled with discussions on the nature of the nonthermal radio
emission from normal galaxies – whether supernovae alone are capable of supplying this
emission or other, additional sources of relativistic electrons are needed. As for the
supernovae themselves, it would be of interest to know when supernova remnants begin to
show strong radio emission. In recent years, large, rapid increases in radio brightness have
successfully been detected in several galaxies with optical supernova outbursts using the
VLA [70]. A new term has even been coined – radio supernova. The observations indicated
that the strong radio emission dies out approximately one year after the supernova outburst,
and then becomes observable again in supernova remnants a few hundred years old. What is
the time between the supernova outburst and the appearance of radio emission in an SNR? In
particular, when will the radio emission from the supernova remnant in S Andromedae
(which exploded at the very center of M 31 in 1885) become visible? How many young
supernova remnants like Cas A are there in M 31 and M 33? And, finally, the problem of
galactic halos (which are also associated with SNRs and the cosmic rays formed in
supernovae). This set of problems can only be solved using observations at meter
wavelengths, where the radio emission is completely nonthermal.
The radio emission of M 31 and M 33 has been fairly well studied at the higher
frequencies. Aperture-synthesis maps of the Andromeda Nebula have been constructed at 408
and 1407MHz [71]. At 408 MHz, two hundred and thirteen discrete sources with flux density
greater than 12 mJy were detected in a 4° x 4° area around the nebula at 80 X 120" resolution.
Optical identifications revealed that only a very few of these sources were associated with the
nebula itself. In addition, we also observed the structure of the radio emission of M 31 was
obtained at 4' resolution, revealing a well-defined nucleus and spiral arms. Subsequent highfrequency observations [72-75] showed excellent agreement between the high-frequency
maps and the map obtained in [71].
Isophotes of M 31 with 37' resolution were obtained at 408MHz in [76]. Analysis of
this data indicated that if M 31 does have a radio halo, it is very weak [77]. A detailed review
of the optical and radio observations of M 31 is contained in [78].
M 33 is a relatively weak radio source, and the radio emission has likewise only been
studied at high frequencies [79-82]. A comparison of the optical and radio data is carried out
in [83].
6. Observational Results for M 31 and M 33
The
Andromeda
nebula was observed at 103
MHz
with
resolution
49' x 27'. The isophotes of
M 31 are shown in Fig. 7,
where the positions of the
observed
scintillating
sources are shown by
crosses. In all, 21 strong
and
somewhat
weak
mutually
interfering
sources
(regions
of
continuous
scintillation)
were detected in a 6° x 4°
area. The scintillating
sources closest to M 31
Figure 7 Isophotes of the Andromeda Nebula from [84]. The
were assigned numbers for
positions of the scintillating source are denoted by crosses
ease of description (see
Fig. 7). Their coordinates, flux densities, and angular sizes at 103 MHz and their spectral
indices between 103 and 408 MHz are given in [84]. Sources 2, 5, and 6 have the very steep
spectra characteristic of pulsars (a > 2.3), while the other scintillating sources are so far from
the galaxy that they are very unlikely to be associated with it.
Figure 8 shows the isophotes of M 33 obtained at 102 MHz with resolution 49' X 29'.
Seventeen scintillating sources were detected towards M 33 in the same 6 х 4° area, with
none being in the galaxy itself. The strong source 4C 29.03 lies 44' from the center of the
galaxy, and is probably not associated with the galaxy. The figure also shows the
identifications of the scintillating sources with sources in the В 2 catalog. If we assume that
the number of scintillating sources towards M 33 is identical to the mean density across the
sky, the excess number of sources towards M 31 is not statistically significant if the
distribution of sources over the sky is a Poisson distribution. However, the fact that three
steep-spectrum sources fall within this small area towards M 31 seems non-random to us.
Unfortunately, the broad beamwidth of the VLPA did not enable us to obtain detailed
brightness distributions for the galaxies, but it was possible to estimate their integrated flux
densities: For M 33, S = 15 ± 4Jy, while for M 31, integration over a circle 2° in radius
yielded S = 80 ± 25 Jy and integration of the flux from the galaxy alone led to S = 50 ± 15 Jy.
Figure 8 Isophotes of M 33 from [85]
7. Discussion of the Observational Results
If the youngest supernova remnant in our Galaxy, Cas A, were located in the
Andromeda Nebula, it would have a flux density of 0.5 Jy and an angular size of 0.6" at 103
MHz. A radio source of this size would scintillate, and it would definitely have been detected
by the VLPA. The same is true of M 33, whose distance is only slightly greater than that of
M 31. No young SNRs were observed in M 33, but two scintillating sources were detected in
M 31: Sources 1 and 2. The second source (if it indeed lies in the galaxy itself) is unlikely to
be a young SNR, since it has too steep spectrum ( = 2.8). This spectrum is typical of
pulsars. Source 1 is probably a young SNR. The reasons for this are outlined in detail in [84].
We shall briefly repeat them here. Since the angular size of Source 1 (0.3") is smaller than the
expected scattering angle within M 31 (1"), the source must be located either within the
galaxy or in front of it. The region where it lies is completely obscured by dust. No optical
objects are visible against the background of dust, so this means the source is located inside
the galaxy, behind the dust. Its spectral index between 103 and 1407MHz is 0.56, which is
typical of SNRs. This SNR has an estimated age of 150 yr, since it is a factor of two smaller
in linear size than Cas A. This makes it one of the youngest known SNRs. From the statistics
of supernovae in other galaxies, we would expect the total number of supernovae in M 31 to
be a factor of 8 higher than that in M 33 [86]. It is therefore not surprising that a young SNR
was detected in M 31 but not in M 33.
The radio emission from galaxies at meter wavelengths is almost completely
nonthermal, and it is currently believed to be due to synchrotron emission. The radio spectra
of these galaxies at 102 MHz indicate that they are still optically thin [85]. Using equation (7)
for the intensity of synchrotron radiation, we can write the ratio of the flux densities for the
two galaxies as
S1 N 01V1 H 1( 1 1) / 2 R22  1  2  / 2
.


S 2 N 02V2 H 2( 2 1) / 2 R12
By substituting the distances (R), volumes (V), magnetic fields (H) , and integrated
flux densities at 102 MHz of the galaxies into this equation, we can obtain the ratio of the
density ofrelativistic electrons in the two galaxies. A value of  0.05 was estimated for this
ratio in [85]. Assuming that the relativistic electrons are formed in supernova remnants, we
should expect this same ratio to exist in the space density of SNRs in these two galaxies.
However, this the value quoted for this ration in [85] was a factor of two too large, because of
the fact that the mean luminosity of the SNRs in M 33 is 1.7 times that in M 31 [87]. Note
that this ratio of SNR densities is also in good agreement with the expected frequency of
supernova outbursts on the basis of supernova statistics in other galaxies.
Finally, we shall discuss our attempts to observe radio halos in other galaxies.
Isophotes of M 31 at 408 MHz with 37' X 37' resolution were obtained in [76]. Examination
of these isophotes indicated that the integrated flux from the halo of M 31 was no greater than
the observational error [77]. The great similarity between the isophotes at 103 and 408MHz
indicates that the distribution of brightness over the nebula shows little variation as a function
of frequency, so one might expect the halo to no longer be visible at meter wavelengths.
Analysis of the observations at 103 MHz indicates that the flux density from the halo is < 27
Jy and that the emissivity of the radio halo in M 31 is an order of magnitude less than that in
our Galaxy. As was noted above, the density of SNRs in M 31 is an order of magnitude less
than that in M 33, and lower than in our Galaxy; moreover, M 31 does not have an active
nucleus, so there is simply no physical reason to expect a large halo around M 31.
As for M 33, the large beam of the VLPA and the presence of scintillating sources
near the galaxy did not enable us to detect a halo around M 33 from the form of the isophotes.
Although the Andromeda Nebula is viewed nearly edge-on, M 33 is nearly face-on. However,
it would still have been possible to detect a halo from a change in the slope of the spectrum at
meter wavelengths, as in other galaxies. The observations indicated that the spectrum of this
galaxy seems to decrease rather than increase at low frequencies [85]. Thus, if there is radio
emission from the halo, it is not substantial.
8. Conclusion
Our interplanetary scintillation observations of a large number of active galaxies at
meter wavelengths enabled us to determine the characteristic traits of the compact radio
sources in their nuclei. The scintillating components in the nuclei of elliptical galaxies
primarily have flat or inverted spectra, while those in spiral galaxies generally have steep
spectra. In our view, this is due to the weaker magnetic fields in the nuclei of spiral galaxies.
Correspondingly, the relativistic plasma density in them should be larger than in ellipticals (it
should also be noted that spiral galaxies also contain more cold gas than ellipticals).
Sources in which the spectrum peaks at meter and decameter wavelengths should have
a higher density of radio photons, which leads to large energy losses due to inverse Compton
scattering, and this makes the spectra steeper at high frequencies [88]. This probably explains
the steep spectra of the scintillation components in Seyfert galaxies.
What makes galaxies active? One possible explanation might be interactions between
galaxies. Meter-wavelength observations of binary galaxies aimed at detecting activity in
them were carried out in order to test this hypothesis [89]. Radio emission was detected in 5
out of the 95 galaxy pairs observed. This is a higher number than random coincidence with
field sources would indicate, but still insufficient to consider interaction between galaxies to
be the main reason for activity in galaxies. We must apparently look within the galaxies
themselves to find out why they are active.
In spite of the fact that faint scintillating sources with fluxes an order of magnitude
below the sensitivity of catalog [43] can be detected using the VLPA, more sensitive radio
telescopes are needed to obtain reliable data on the physical conditions in the nuclei of active
galaxies. Further progress in meter-wavelength observational astronomy will require the
construction of a larger antenna that will allow us to track sources [90]. Tracking sources will
enable us to observe such phenomena as occultations of scintillating sources by the Moon.
Such observations will provide information on the coordinates of the compact sources
accurate to within 1" , and these can be used for reliable optical identifications. The
construction of such an antenna will not only allow us to increase the number of observed
scintillating sources but also enable us to use completely new techniques for observing the
compact sources [91].
Observations at meter wavelengths with 0.01" resolution would enable us to detect the
interstellar scattering in our own Galaxy. This effect places limitations on our observations of
compact extragalactic sources, but would provide information on inhomogeneities in the
interstellar medium. Interplanetary scintillation measurements of the apparent sizes of
pulsars, when combined with observations of their interstellar scintillations, would enable us
to determine the distances of pulsars and their velocities in the plane of the sky [92].
As far as our work on the normal galaxies M 31 and M 33 is concerned, the
observational results we have obtained are quite consistent with the hypothesis that SNRs are
the sources for the relativistic electrons that give rise to the synchrotron emission from them.
However, it is also clear that activity in galaxies is mainly associated with active galactic
nuclei. For example, observations of 330 galaxies with Byurakan classifications showed that
galaxies with more well-defined nuclei are more likely to show radio emission and are also
more likely to show scintillating components, while the observed scintillating sources were
steep-spectrum sources [93, 94].
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