Diapositiva 1

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Granada, March 6, 2007 K.-H. Mack
Restarting Activity in Radio Galaxies
Karl-Heinz Mack Istituto Nazionale di
Astrofisica Istituto di Radioastronomia Bologna
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Granada, March 6, 2007 K.-H. Mack
A motivation
If we believe that the activity in radio-loud
active galactic nuclei is the result of accretion
onto a compact massive object, likely a black
hole, the life of a radio source would be
subordinated to the accretion rate. The vanishing
of accretion would lead a former radio source to
a dormant or hibernation phase. () However,
interaction and merging with neighboring galaxies
can trigger the activity, and eventually produce
a transition from a dormant to an active
phase.

Lara et al. (1999)
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Granada, March 6, 2007 K.-H. Mack
The Duty Cycle of Radio Galaxies
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Granada, March 6, 2007 K.-H. Mack
Found by chance
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Restarting activity in the giant radio galaxy
J1835620
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Granada, March 6, 2007 K.-H. Mack
Restarting activity in the giant radio galaxy
J1835620

• Narrow emission line galaxy at z0.518
• Total extent 1.12 Mpc
• Pair of double sources with common centre
• Inner source extended, edge-brightened
• Active hotspot in northern lobe
• Spectral ages 3.5 106 yr for outer lobes
• 2.3 106 yr for inner
  lobes
• expansion velocity 0.2 0.25 c
• Spectral ages are not consistent with
• an exhausted primary phase of activity
• Outer lobes are still supplied by an
• underlying jet
• Inner structure result of a dramatic
• increase of core activity

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Granada, March 6, 2007 K.-H. Mack
How do they work?
Kaiser et al. (2000) based on Kaiser Alexander
(1997) and Kaiser et al. (1997) Clarke Burns
(1991), Lara et al. (1999), Schoenmakers et al.
(2000) Old cocoon Standard evolution,
luminosity possibly decreased (if hotspots
switched off) New cocoon Alignment of old and
new jet axes ? inner structure expanding in old
cocoon material New jets end in hotspots ?
sufficient gas densities within old cocoon
symmetric hotspots ? simultaneous restart of jet
flow material within old cocoons has passed
through the jet shock density is insufficient for
the formation of hotspots at the end of the new
jets ? Old cocoons of DDRGs must be contaminated
from the outside
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Variation of the theme
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Granada, March 6, 2007 K.-H. Mack
B2 114435 - Variation of the theme
Giovannini et al. 1999

• Discontinuity between extended
• relaxed lobes and the high-brightness
• arcsecond core and jets
• Two different evolutionary phases
• relic emission 5 9 107 yr
• active emission lt 107 yr (for 0.02c)

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3C338
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Granada, March 6, 2007 K.-H. Mack
Multi-epoch VLBI studies of 3C338

• Multi-epoch morphological comparison
• at 8.4 GHz
• Assuming that central peak is true core
• Average velocity of new ejecta
• v gt 0.38 0.41c
• No visible connection between jets and
• diffuse features
• Extended emission is relic structure
• and not related to the present
• nuclear activity
• Relic structure embedded in strong
• cooling flow ? low adiabatic losses

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Granada, March 6, 2007 K.-H. Mack
X-shaped sources

• Possible scenarios
• Backflow from the active lobes into the wings
• (Leahy Williams 1984, Capetti et al. 2002)
• Slow conical precession of the jet axis
• (Parma, Ekers Fanti 1985, Mack et al. 1994)
• Reorientation of jet axis with continuing flow
• (Dennett-Thorpe et al. 2002)
• Reorientation of jet axis with stopped or
• reduced jets (Dennett-Thorpe et al. 2002,
• Klein et al. 1998)
• Reorientation of spin axis due to minor merger
• (Merritt Ekers 2002)
• Wings are relics of radio jets, active lobes
• are younger

Klein, priv. comm.
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Granada, March 6, 2007 K.-H. Mack
X-shaped sources

• But Some wings (e.g. 3C223.1) have flatter
  spectra (Dennett-Thorpe et al.
• 2002, Lal Rao 2005, Mack et al. 2005)
• Low-frequency spectral indices of 12 X-shaped
  sources (Lal Rao, 2007)
• (i) wings flatter spectra than active lobes 3
• (ii) wing spectra comparable to active lobes
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• (iii) wings steeper spectra than active lobes
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• No single model plausible to explain X-shaped
  sources as a class (Lal
• Rao 2007)
• X-shaped sources could consist of two pairs of
  jets which are associated with
• two unresolved AGN (Lal Rao 2007)
• No indication in HST images, high-resolution VLBI
  monitoring required

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Granada, March 6, 2007 K.-H. Mack
Centaurus A / Virgo A type sources
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Granada, March 6, 2007 K.-H. Mack
Virgo A / Centaurus A type sources
Similarity betwen Vir A and Cen A Structure in
three components abrupt changes in position
angle Vir A ejection of inhomogeneous jets by
a precessing nucleus or continuous ejection of
jets bent due to interaction with the
environment (Feigelson et al. 1981) Change of
orientation of the ejection during different
epochs (Klein 1998) Cen A bursting bubble
model Jet deflected during a jet-cloud inter-
action and restarted via de Laval nozzle
(Morganti et al. 1999)
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Granada, March 6, 2007 K.-H. Mack
Conclusions

• Recurrent activity and restarting sources exist
  and are observed
• DDRGs seem to be most obvious manifestation of
  the phenomenon
• open issue outer structures still replenished
  or full interruption?

• Close relatives
• new emission on smaller scales
• (e.g. B114435, 3C338)

• More distant relatives
• X-shaped sources
• Cen A / Vir A types

Caveat recurrent activity everywhere?