8.1.3 Absence of Superluminal Motion in Radio Galaxies
The subrelativistic kiloparsec-scale jet velocities in FR I galaxies, typically of the order of 1000-10,000 km s-1 (Bicknell et al. 1990), used to represent a problem for the FR I/BL Lac unification, which requires relativistic speeds at least on small scales. It was not clear that FR Is were relativistic even on the smallest scales, and if they were, it was not clear how they were decelerated or what the observational signature of that deceleration would be. Now, new observational (Venturi et al. 1993; Feretti et al. 1993; Giovannini et al. 1994) and theoretical results (Laing 1994; Bicknell 1994) supporting the presence of relativistic motion on parsec scales in these sources have changed our understanding of FR Is. Physically reasonable models have also been proposed for the required deceleration (Laing 1994; Bicknell 1994).
If FR Is are relativistic on small scales, they should show
superluminal motion, even near the plane of the sky. A jet with a
Lorentz factor
of 5 would have apparent transverse speeds 
a 
2.3 and 1
for viewing angles of 45 and 90 degrees, respectively
[Eq. (A4)]. The
few measurements of jet proper motions with VLBI, however, suggest that
presently studied FR I radio galaxies display relativistic but still
subluminal speeds. Using the data
compiled by
Vermeulen and Cohen
(1994),
we obtain an average value
< a >
~ 0.5 (converting to our adopted value of H0 = 50 km
s-1 Mpc-1)
for four FR Is (NGC 315, M87, Centaurus A, and NGC 6251). The speeds for
FR IIs seem to be no different: a 0.5-1.0 for Cygnus A
(Carilli et al. 1994).
These results may imply that radio galaxies have smaller
Lorentz factors than BL Lacs and radio quasars. It is important to remember,
however, that detection of VLBI components in radio galaxies is
hampered by relativistic deamplification and dilution by unbeamed
emission. For 
= 5, for
example, jets are deamplified for orientation
angles 
35°
(Appendix A), which
would include basically
all radio galaxies (see Table 3);
for larger Lorentz factors,
the angle is even smaller and the deamplification larger.
(The jet-to-counterjet ratio remains significantly larger than unity
even at large angles; Fig. 22.)
Similarly, at these angles the ratio of
transverse jet luminosity to unbeamed luminosity is 
10-2 for
= 5 and f = 0.01
[Eq. (C6)] and equals ~
10-4 at
90 ° [Eq. (C7)]. Thus the bulk of the emission may well appear
to be stationary even if a transverse relativistic jet is
present. Another consideration
is that significant beaming can be reconciled with subluminal motion of knots
if these are reverse shocks advected by the jet; their motion would then give
a misleading indication of the flow velocity
(Bicknell 1994).
|
| Figure 22. The jet to counter-jet ratio, J, versus angle
to the line of sight for p = 2. Different curves correspond to
different Lorentz factors: from the top
down, = 15, 10, 5, 2. Note
that the ratio is essentially independent
of at large angles.
|
In the end, either FR Is are intrinsically different from BL Lacs or they
will exhibit superluminal motion on small scales. Surveys of FR Is with the
Very Long Baseline Array (VLBA)
should help decide this question. It is extremely
interesting that in one nearby FR I galaxy, M87, which has been
extremely well mapped in the radio, superluminal motion has been detected
on kiloparsec scales, with a up to 2.5
(Biretta et al. 1995).
At least one FR I, then, must
have an appreciable bulk flow on both parsec and kiloparsec scales.