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Active Galactic Nuclei: from Central Engine to Host Galaxy
ASP Conference Series, Vol. xxx, 2003
S. Collin, F. Combes, and I. Shlosman
Hydrodynamic Simulations of Light Bipolar Large Scale
Jets
Martin Krause
Max Camenzind
Landessternwarte Heidelberg-Konigstuhl, 69117 Heidelberg, Germany
Abstract. We have carried out simulations involving both jets, remov-
ing the articial boundary condition at the symmetry plane. We use a low
density contrast (IGM/jet approximately 10000) and take into account
a decreasing density prole. We nd that the jet bow shock undergoes
two phases: First a nearly spherical one and second the well-known cigar-
shaped one. We propose Cygnus A to be in a transition phase, showing
currently clear signs from both phases. In a further simulation with ther-
mal cooling, three typical phases of such jets are found. This may be
relevant to high redshift radio galaxies.
1. Introduction
Recently, X-ray observations of nearby active cluster centers as well as emission
line studies of high redshift radio galaxies (HZRG) have been carried out with
the new generation of instruments (Chandra/XMM, VLT/FORS). These kinds
of objects are considered to belong to the same class at dierent evolutionary
stages [Carilli et al., 2001]. At least one low redshift object (Cygnus A, compare
Fig. 1) shares some of the properties of the high redshift relatives, namely low
jet density ( 10 4  environment) and high power (> 10 46 erg/s) [e.g. Krause,
2002a].
2. Setups & Results
We present two simulations with the code NIRVANA [Ziegler &Yorke 1997].
The rst one (RUN1) is a 3D hydrodynamic calculation adapted to literature
parameters of Cygnus A. The second simulation (RUN2) is a 2.5D simulation of
a very light jet with cooling.
Simulation results for RUN1 are displayed in Fig. 1. The bow shock is ini-
tially spherical (not shown here). Reaching the critical radius [Krause, 2002b],
which is in this case only  2r j
, the hot bubble dened by the bow shock slowly
starts to elongate in jet direction. At 0.3 million years (Ma), cigar shaped ex-
tensions to the bow shock form, and the axis ratio now grows rapidly. Cygnus A
shows such a hot bubble and just seems to enter the cigar phase [Smith et al.,
2002].
111

112 Krause & Camenzind
Figure 1. Density (left) and integrated X-ray emission (middle:
\edge on", right: pole on).
Figure 2. Three phases (density is shown) of cooling of the shocked
IGM: Adiabatic (left), dense shell (middle), disrupted shell (right).
The evolution of a jet with cooling of the shocked IGM can be described by
three phases. Before signicant cooling (phase 1, t=1.39 Ma), the situation
is very similar to the previous simulation. Then, the shocked gas cools all at
the same time, thereby loosing its pressure. It gets compressed into a smooth
neutral shell (phase 2, t=3 Ma). This shell is instable and forms condensations
of down to 10 6 solar masses in the simulation (phase 3, t=6.8 Ma). They are
likely to form stars. Since the typical Jeans mass is also about one million solar
masses, the prefered mode should be globular cluster formation [Krause 2002a].
The excess of globular cluster systems around the brightest cluster galaxies can
be explained by the globular clusters formed in the shocked shell.
3. Conclusions
Very light jets rst blow up a spherical bubble. Then the aspect ratio increases
and sometimes cigar shaped extensions develop. With cooling of the thermal
gas, the appearance is unchanged before the cooling time is reached. Then
the shocked IGM collapses to a thin mainly neutral shell, and nally this shell
disrupts by thermal instabilities and globular cluster formation.
Acknowledgments. This work was supported by the Deutsche Forschungs-
gemeinschaft (Sonderforschungsbereich 439)
References
Carilli, C.L. et al. 2001 in ASP Conf. Proc., 240, Gas and Galaxy Evolution,
eds. J.E. Hibbard et al., (San Francisco: ASP), 101
Krause, M. 2002a, A&A, 386, L1
Krause, M. 2002b, A&A, submitted
Smith et al. 2002, ApJ, 565, 195
Ziegler, U. & Yorke, H.W. 1997, Comp. Phys. Com., 101, 54