milliarcsecond_structure_of_radio_galaxies_and

THE MILLIARCSECOND  STRUCTURE

OF

 RADIO GALAXIES

AND

 QUASARS 

A.C.S.

 Readhead

  and T.J.

 Pearson 

Owens Valley  Radio  Observatory  

California  Institute

of

  Technology  

Hybrid maps of the nuclei of radio galaxies and quasars show a  variety  

of morphologies.  Among compact sources, two structures are common:  an  

asymmetric, "core-jet" morphology  (eg, 3C 273), and an  "equal  double"  

morphology  with  two  separated, similar components  (eg, CTD 93).  The  

nuclei of extended, double radio galaxies  generally  have  a  core-jet  

morphology with the jet directed toward one of the outer components. 

1

.

   INTRODUCTION   

It

  is now

 generally  accepted  that

  one  can

  make  reliable  maps  with  

milliarcsecond   resolution

in

  VLBI

by

 means

of

 various  hybrid  mapping  

algorithms using  closure  phases

and

 closure  amplitudes

  [4,27,28].

Our

experience  with  hybrid  mapping

and

  model  fitting

has

 shown that

in

cases where  there

is

 only

a

 small   sample

of

  amplitudes

and

  closure  

phases,

 or no

 closure  phases

 at all,

 there

 may be

 significant  errors

in

the  derived  structure.

For

 this reason,

we

 shall  discuss   only   those   

objects  which  have  been  properly  mapped  according

  to the

 following 

criteria.

The

 observations must

(1)

be

 long,  continuous   tracks,

for

reliable   calibration

and

 good  (u,v)-coverage;

(2)

be

 made

at

 four

or

more  telescopes,

for

 good   (u,v)-coverage;

and

(3)

  include   closure   

phases. 

At

the

 time

 of the IAU

 Symposium

on

 Objects

of

 High  Redshift,

two

years

  ago,

  only

a

  dozen  objects

had

 been mapped

by

 VLBI

  [22].

The

number

  is now

 about

50

 (Table I).

  All of

 these objects vary,

and

  some  

have

now

  been  mapped

  at six

 epochs

 and six

 frequencies,

 so the

 total 

number

of

 hybrid maps

is

 about

  200.

In

 this review,

we

  shall   discuss   

the  morphology

of

 these very  compact  objects.  Table

I

 lists

the

 name 

of  each  object,

its

 optical  type  (Quasar,  Galaxy,

or

 Unidentified),

and

a brief  description

  of the

 milliarcsecond  structure, with references

to

the  original  observations.

We

 have been making

a

 VLBI   survey

  of  the  

structure

of

  northern   sources  stronger  than

1.3

 Jy at

5

GHz

 [15,16], 

and

a

 number

  of the

 results

we

  shall  present

are

  from   this  work.  

Unfortunately,

  as  the

  survey

  is  not yet

 complete,

 we do not

 have

a

279

D.

S.

Heeschen

and C. M.

Wade

(eds.),

Extragalactic Radio Sources,

279-288.

Copyright

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280

A. c. s. RI:ADHI:AD AND T.

J.

 PKARSON 

TABLE  I  : SOURCES MAPPED  BY  VLBI  

0055+300  NGC315     G

   0.0167

   Core-jet             [8]             

0133+476  0C457      Q            Slightly  resolved    [15]    

0212+735             Q            Core-jet?            [16]            

0316+162  CTA21      U            Slightly  resolved    [35]    

0316+413  3C84       G

   0.0177

   Complex              [14,15,34]              

0333+320  NRA0140    Q   1.258    Core-jet/superlum?   [10]   

0355+508  NRA0150    U            Core-jet             [12]             

0415+379  3C111      G

   0.0485

   Core-jet             [8]             

0428+205             G   0.219    Slightly  resolved    [20]    

0429+415  3C119      Q

   0.408

    Core-jet?            [17]            

0430+052  3C120      G

   0.032

    Core-jet/superlum    [2,26]    

0538+498  3C147      Q

   0.545

    Core-jet             [33,36]             

0710+439  01417      G            Triple               [16]               

0711+356  01318      Q   1.620    Double,Core-jet?     [16]     

0804+499  OJ508      Q            Slightly  resolved    [16]    

0814+425  0J425      Q            Slightly resolved    [16]    

0836+710             Q            Double,Core-jet?     [6,16]     

0850+581  4C58.17    Q   1.322    Slightly resolved    [16]    

0859+470  4C47.29    Q   1.462    Slightly resolved    [15]    

0906+430  3C216      Q

   0.670

    Slightly resolved    [16]    

0923+392  4C39.25    Q

   0.698

    Equal double        [15]        

0945+408  4C40.24    Q   1.252    Slightly resolved   [16]   

1003+351  3C236      G

   0.099

    Core-jet             [32]             

1226+023  3C273      Q   0.158    Core-jet/superlum    [2,18,26]    

1228+126  3C274      G

   0.004

    Core-jet             [30]             

1323+321  DA344      U            Equal double        [13]        

1328+307  3C286      Q

   0.846

    Core-jet             [17,33,35]             

1518+047             U            Equal double        [20]        

1607+268  CTD93      G?           Equal double         [19]         

1624+416  4C41.32    U            Slightly resolved    [16]    

1633+382  4C38.41    Q   1.814    Double,Core-jet?     [16]     

1637+574  0S562      Q

   0.745

    Slightly resolved    [16]    

1637+826  NGC6251    G

   0.023

    Core-jet             [3,23]             

1641+399  3C345      Q

   0.594

    Core-jet/superlum    [2,26]    

1642+690  4C69.21    Q            Slightly resolved    [16]    

1652+398  4C39.49    G

   0.0337

   Slightly  resolved    [16]    

1807+698  3C371      G

   0.050?

   Core-jet             [15]             

1823+568  4C56.27    Q?           Slightly resolved   [16]   

1828+487  3C380      Q   0.691    Core-jet?            [15]            

1845+797  3C390.3    G

   0.0561

   Core-jet             [8]             

1901+319  3C395      G

   0.635

    Double,Core-jet?     [6,19]     

1928+738  4C73.18    Q            Core-jet             [16]             

1954+513  OV591      Q   1.230    Slightly resolved   [16]   

1957+405  Cyg A      G

   0.0565

   Core-jet             [8]             

2021+614             Q            Double,Core-jet?     [16]     

2050+364             U            Equal double         [20]         

2200+420  BL Lac     Q

   0.069

    Core-jet/superlum?   [11,15]   

2251+158  3C454.3    Q

   0.860

    Core-jet             [5,17,35]             

2351+456  4C45.51    G            Slightly  resolved    [16]    

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THE MILLIARCSECOND STRUCTURE OF RADIO GALAXIES AND QUASARS 

281

well-defined,  complete  sample  of  objects, and   selection   effects   must   

bias   the  data   to   some degree.  Nevertheless,  there  are  already  some  

clear  trends  of  great  astrophysical  interest.  

It  is well  to remember  that,  as these  objects  are varying,   it   is   

in principle  possible  for  their morphology  to  change  quite  dramatically  

on  a time-scale  of months.  The  rapid  structural  changes  of  the  objects  

enable  us  to  study  the dynamical  evolution.  These variations  have  been  

described  in detail  at  this  Symposium  by Marshall  Cohen, Art Wolfe,  and  

others,

 and we  shall not  consider  them  further  here.  

2.

  COMPACT RADIO SOURCES 

One of the most interesting  facts to emerge from the  maps  is  that  a  

significant  fraction  (perhaps  as  many  as  50%)  of the objects are 

one-sided

  jets,

  with  a  flat-spectrum  core  at   one   end   of   a   

steep-spectrum  jet.   Good  examples are 3C 371 (Figure 1), 3C 273 and 

3C 345 (Cohen and Unwin, this

 volume).

  It is perhaps misleading  to use 

the term "jet" in this context, as the VLBI maps rarely have sufficient 

dynamic range to detect  low-brightness  structure,  but  the  term  is  

justified  in  cases like 3C 345 and NGC 6251 where  the milli-arcsecond 

jet appears to be continuous with  the  larger-scale  jet  detected  by  

conventional  interferometry.  

Observations of these one-sided jets over a range  of  frequencies  

show  that  the  cores  observed  at lower frequencies  can themselves be 

resolved  into one-sided  jets  at  higher  frequencies,  again  with  an  

optically  thick  core  at  one end  (eg, 3C 273

  [24]).

  Thus it appears 

that we are seeing continuous jets in which  the core  at  a  particular  

frequency  is simply the region where the jet becomes optically thick at 

that frequency.  It is therefore generally assumed  that  the  center  of  

activity  -  the  "central  engine"  - coincides approximately with the 

core.

  The asymmetric structure  itself  may  well  be  the  result  of  

relativistic  beaming.   This  has been discussed  in a number  of papers 

[1,7,24,31],

 and  in several contributions  to this Symposium. 

In some cases, objects appear at first  sight to be equal  doubles.  

However,  observations  at  other  frequencies  often show that the two 

components have  different  spectra,  so  that  the  source  is  really  

asymmetric.  For example, at  1.7 GHz 3C 380 looks like an equal double, 

but at higher frequencies  it looks more like a one-sided jet [15].  Two  

further  examples of objects that look like equal doubles at 5 GHz, but 

which  may  well  turn  out  to  be  one-sided

  jets,

   because   their   

high-frequency  spectra are flat, are 0836+710  [6,16] and 2021+614 [16]. 

It is now clear, however, that there  is a class of  objects  which  

have  two  equal  components  with  very similar spectra.  Phillips and 

Mutel  [21] have observed  a number of objects  with  spectra  that  peak  

near  1  GHz  (see  Figure  2) and have found that five out of six such 

objects are equal doubles  at  1.7  GHz.   Two  of  these  (CTD 93  and  

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282

A.

C.

S.

 READHEAD AND

T.

J.

 PEARSON 

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THE

 MILLIARCSECOND STRUCTURE

OF

 RADIO

 GALAXIES

AND

 QUASARS 

283

S.Jy

2050

+

364

CTD

93

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10

100

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1000     10,000     

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Figure 2. Radio spectra of CTD 93  [19] and 2050+364 [20]. 

2050+364) are shown in Figure 1.  3C 395 is rather  similar to 3C 380 in 

that the two components have very different  spectra;  but  the  spectra  

of  the  other  four  objects  are  consistent  with  two  very  similar  

homogeneous  synchrotron  components  (unlike  the  core-jet

  sources),  

suggesting  that  the  components  are  probably  nearly  equal  in flux 

density over a wide frequency range.  In all four cases, at  least  one  

of  the  components  is elongated along the source

 axis.

  In these equal 

double sources, there  is  no  obvious  candidate  for  the  center  of  

activity.   Phillips  and  Mutel  have  suggested  that  the components 

straddle an invisible nucleus.  They point out that the  high-frequency  

spectra  of  these  objects  are similar to those  in "classical double" 

sources,

 and they suggest that these compact doubles represent an early 

phase in the evolution of this class of object.  A difficulty with this 

interpretation  is  that  the  proportion  of  objects   showing   this   

morphology  is  higher  than  expected:   it  could  be as high as 15%, 

whereas the typical double source  is expected  to spend less than 1%  of  

its lifetime at separations < 1 kpc. 

A problem with present-day VLBI observations  is the  poor  dynamic  

range.

   This  is  illustrated  by 0710+439  (Figure 1).  Here again the 

source  is dominated by two components of almost equal  brightness,  one  

of  which is extended along the source

 axis;

  but in this case there is 

a third component with one-tenth the flux density of the other two.  If  

this third component had been a factor of two weaker we should not have 

detected it. 

Some objects are much more complex.  A unique example is  NGC 1275 

(3C 84),  described  at this Symposium by Jon Romney.  A

 5-GHz

 map made 

by Unwin et al. [34] is shown in Figure 1.  It consists of a number  of  

compact  regions  separated  by  less than a beam-width and embedded  in  

more extended structure.  The  structure  is  difficult  to  interpret,  

especially  at  low  frequencies where the resolution  is poor and where 

Figure

1

 (opposite).

  VLBI maps of (a)

 3C 371, 5 GHz [15];

(b)

 CTD 93, 

1.7  GHz   [19];

(c)

   2050+364,  1.7  GHz  [20];

(d)

  0710+439,

  5  GHz  

[16];

(e)

  3C  84  (NGC

  1275),

  5 GHz  [34].  

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