Galaxy Formation and Evolution, Mo, van den Bosch &
White, 2010
Galactic Dynamics, Binney & Tremaine 2008
• Galaxy interactions are frequent in the
hierarchical scenario of galaxy formation.
• They play important in the evolution of the star
formation and morphology of galaxies.
• How to investigate them?
Galaxy interactions
 High-speed encounters, tidal heating
 Tidal stripping
 Dynamical friction
 Galaxy merging
 Galaxies in clusters
• Harassment
• Cannibalism
• Ram-pressure stripping
• Strangulation
Energy change of a particle q:
Energy change of the system S:
This energy is transferred to potential energy, which
becomes less negative, implying that the system S is
tidally heated and expands.
 Tidal radius: The radius where the tidal forces
exceed the binding forces.
 For a sub system of mass m moving on an orbit r in
an extended host system of mass M:
(Gan et al. 2010)
 The unbound mass outside rt are stripped gradually.
Tidal stream and tail
 The stripped stars move on
roughly the same orbit as the
satellite galaxy, forming an either
trail or lead stream (minor
 All stars along the stream have
similar orbital energy, it can be
used to constrain the
gravitational potential of their
host system (Milky Way).
 ‘tidal tail’ is usually refered to
the structures formed by tidally
stripped stars in major mergers.
Sagittarius stream
Tidal tail formed by NGC 4038
and NGC 4039
As an object M moves through a sea of particles, the particles
passing by are accelerated towards the object. As a result, the
particle number density behind the object is higher than that in
front of it, and the net effect is a drag force (dynamical friction)
on the object.
The satellite halos sink to the
host system center due to
dynamical friction within a
merging timescale (Colpi et
al., 1999; Boylan-Kolchin et
al., 2008; Jiang et al., 2008).
Mass loss due to tidal
stripping weaken the effect
of dynamical friction.
Evolution of the orbital angular
momentum of a satellite halo
(Gan et al. 2010). A: tidal
stripping efficiency
 The merging processes typically are treated with simulation.
 The violent relaxation plays an important role during the
relaxation of the merger remnant.
 The remnant typically has little resemblance to its
progenitors in major merger, while not in minor merger.
 Disks that accrete small satellites typically survive but can
undergo considerable thickening (disk heated).
 Major mergers that involve one or more disk galaxies tend to
create tidal tails.
 The gas-rich merger (wet merger), in general, triggers new
starburst and AGN activity.
 High density in mass
and number
 High speed
 Hot gas (ICM)
• Harassment
• Cannibalism
• Ram pressure stripping
• Strangulation
Phoenix clusters simulation
 The cluster galaxies have typically high velocity and suffer
frequent encounters.
 The cumulative effect of multiple high-speed impulsive
encounters is generally referred to as galaxy harassment.
 The fragile disks of late-type (Sc-Sd) spiral galaxies can be
almost entirely destroyed by harassment. The disks lose very
substantial amounts of mass.
 The bound stars are also heated, which transforms the disk
into spheroidal component. Dwarf ellipticals are ubiquitous
in clusters.
 For more compact early-type (Sa-Sb) disk galaxies, they can
be significantly heated and become more easily to be
stripped by tides or ram pressure.
 A galaxy will sink to the cluster center if the dynamical
friction time is sufficiently short.
 This galaxy merged with the central galaxy and this
process is called galactic cannibalism.
 Cannibalism causes a mass increase of the central galaxy,
and a depletion of massive satellite galaxies.
 Hence, it causes an increase of the magnitude difference
between the first and second brightest galaxies in cluster:
 The magnitude gap ΔM12 can be used as a measure for the
dynamical age of the cluster.
 The Tdf is determined by various factors.
 A ram pressure is just like one feels wind drag when cycling.
 Gas stripping occurs where the pressure exceeds the
binding force per unit area.
(Font et al. 2008)
 Ram-pressure stripping may be efficient and quench star
formation in satellite galaxy. But there are still debate.
 Only the gas at relatively large galactocentric radii is being
 The remaining non-stripped gas may actually be compressed
by the ram pressure, giving rise to enhanced star formation.
 The stripped gas may remain bound to the galaxy, fall back
and induce a later starburst.
 The outer parts of a satellite’s gas are stripped. Star formation
may continue at inner parts but the gas consumption time scale
is short (1-5Gyr).
 A gas reservoir surround the satellite: hot gas just falling; gas
shocked to high temperature; reheated and expelled gas by
 The gas reservoir is hot and loosely bound to the satellite, so it
is fairly easily stripped off, either by tides or by ram pressure.
 If a large fraction of gas is stripped off from a satellite after it
is accreted into the cluster, its star formation rate will decline
gradually and this is called strangulation.
 The strangulation is believed to be responsible for the
morphology-density relation.
 The motion of satellite after accretion is determined by various
The morphology–distance
relation in Abell cluster.
(Park & Hwang 2009)
C3: −20.5 > Mr > −22.5
C2: −19.0 > Mr > −20.5
C1: −17.0 > Mr > −19.0
 Galaxy interactions are of diversity and complicated,
especially in clusters.
 The relative effects of various interactions need to be
analyzed statistically based a large sample of evolving
 It may be a way out to examine the magnitude gap and
morphology-density relation, following the hierarchical
growth of cluster.

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