Download the Local Group - Simon P Driver

Lecture 7: the Local Group
and nearby clusters
•  in this lecture we move up in scale, to explore
typical clusters of galaxies
–  the Local Group is an example of a not very
rich cluster
•  interesting topics include:
–  clusters and the structure of the Universe
–  the fate of galaxies: stable, destroyed or
cannibals?
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the Local Group
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Inner Solar System
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some Local Group galaxies, roughly to the same physical scale:
M31, Leo I
M32
LMC,
SMC
MW
M33
(images courtesy AAO)
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first impressions
•  there are some obvious properties of the Local Group:
–  it’s mostly empty, i.e. galaxies are quite distant from
each other
–  with some exceptions like satellite galaxies
–  the three spirals are easily the biggest
–  dwarf galaxies are on the outskirts of the group
•  how typical is this of other galaxy groups?
–  turns out that the Local group is not very rich in
galaxies
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groups and clusters
•  groups contain a smaller number of galaxies than clusters,
and are more compact in both space and velocity spread:
group:
cluster:
no. galaxies
~10+
>50
core radius
~300 kpc
~300 kpc
median radius
~1 Mpc
~ 3Mpc
v-dispersion
150 km/s
800 km/s
M/L
~200
~200
total mass
few
1013
Msolar
few 1015 Msolar
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classifying the Local Group
•  the Local Group has only about 10 significant galaxies
(L > 108 Lsolar), so does not qualify as a cluster
–  NB, dwarf spheroidals etc. are not detectable at
large distances, so don’t make up part of the total
galaxy count for the Local Group
•  about half of known galaxies are in groups and clusters
–  these are dense enough to halt cosmological
expansion locally, and so the galaxies remain bound
to each other
•  the other half of galaxies are loosely spread out in large
filaments and ‘walls’
–  part of the large-scale structure of the Universe; may
still be collapsing into clusters
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mapping the structure
•  to turn a map of the sky into a 3-D picture of the Local
Group, we need galaxy distances
–  Hubble’s law does not apply within the Group
because expansion has halted
•  need to remember that the uncertainty in the distance is
~10% even for bright galaxies
–  e.g. for the LMC, the range found for (m – M) is 18.1
to 18.8
–  from -5 log d formula, this makes a 40% difference in
the distance!
•  for dSph, distances could be uncertain by factor of ~2
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•  for the LMC,
some of the
methods just
aren’t very
reliable (not
enough stars of
a particular type,
for example)...
best estimates
constrain
absolute
magnitude to
~0.1, or 5% in
the distance
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galaxies in the Local Group
•  adding up those within 1 Mpc of the Milky Way:
–  4 spirals (MW, M31, M33, LMC)
–  1 elliptical (M32)
–  3 dwarf ellipticals (NGC 147, 185, 205)
–  3 irregulars (SMC, IC 10, NGC 6822)
–  25 dSph/dIrr (with L of 2 × 105 to 108 Lsolar)
•  this is rather different from typical large clusters
–  in core regions, proportions 10% / 40% / 50% in
spirals / ellipticals / lenticulars
–  in outer regions of a cluster, 80% / 10% / 10% in
spirals / ellipticals /lenticulars
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nearby clusters
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Richard Powell - http://www.anzwers.org/free/universe/
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supergalactic plane
•  there is a local ‘wall’
called the supergalactic
plane in which many of
the clusters within ~100
Mpc lie
–  top diagram is looking
down on the plane
–  bottom diagram is
looking along it
–  mesh regions have
>50% more than mean
density of galaxies
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Hudson (1993)
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Layout of the Local Group
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well known clusters
•  some of the
richest clusters
are the two in the
constellations of
Virgo (at 15-20
Mpc) and Coma
Berenices (at
~100 Mpc)
images: Astronomy
Picture of the Day
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velocities in a group
•  the mutual gravitaitonal pulls of the galaxies hold them
together as a group
–  the group must have assembled fairly early on, or the
expansion of the Universe would have spread
galaxies out more evenly
•  several kinds of motions are possible
–  stable orbits around the group
–  infall of two galaxies onto each other
–  destruction of a small galaxy due to tides induced by
a bigger one
•  our Galaxy and M31 are approaching each at ~120 km/s
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sub-structure
•  some galaxies in the LG are bound to the Milky Way,
some to Andromeda, and some neither:
–  companions of MW:
LMC (0.05 Mpc), SMC (0.06 Mpc),
various dSph (0.025 Mpc to 0.27 Mpc)
–  companions of M31 (0.77 Mpc from MW):
M32 (0.75 Mpc),
three dE (NGC 147, 185, 205 at 0.6 to 0.85 Mpc)
various dSph in Andromeda (0.6-0.8 Mpc)
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fates of galaxies
•  ‘near misses’ can perturb
galaxies, e.g. send density
waves through the gas
–  this leads to new bursts of
star formation, perhaps as
seen in the LMC irregular
spiral
•  strong interactions can
destroy galaxies as discrete
entitities
–  some originally in the
Local Group are now
gone, all we see now are
‘tidal streams’
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stability and dark matter
•  dark matter can help to hold a galaxy together, when the
mass in visible stars isn’t enough to explain why it has
survived
–  compare the Carina dSph to the globular cluster ω Cen
–  σ is 3x higher in ω Cen, but Rcore is 40x higher in Car
–  from the Virial Theorem, 2 KE + PE = 0, so approximately
σ2 = GM / Rcore, or (σCar / σCen )2 = (MCar/MCen)(RCen/RCar)
–  which gives MCar ≈ 4 MCen
–  but LCar = 2 x105 while LCen = 106 (!), so M/L is 20 times
larger in the dSph galaxy than in the globular cluster
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stability and potential wells
•  the effects that destroy a satellite galaxy are basically
tides
•  the maths for distortions of extended galaxies made up
of many stars are hard, but we can picture the potential
wells
–  start with the 5 Lagrangian points, which show stable
positions e.g. for satellites launched from the Earth
60°
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losing stars
•  for extended objects like galaxies, we can picture the
potential wells
–  consider where a star would be firmly attached to a big
galaxy of mass M or its smaller companion of m
–  at the Lagrange points the star’s situation is unstable,
and it may start to fall onto the other galaxy or out into
intergalactic space
L2
L1
m
L3
M
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LMC and SMC
•  it turns out that the L1 and L2 points are at
distances
xL = ± D [ m / 3M+m ]1/3
–  where D is the distance separating the galaxies
•  for the LMC, for example, its circular velocity
around our galaxy implies M(<50 kpc) ~ 5 × 1011
Msolar, while its own mass is ~ 1010 Msolar
–  so xL ~ ± 50 [ 1 / (3×10 + 1) ]1/3 ~ 11kpc
–  so as the galaxy radius is only 7 kpc, its stars
will stay bound
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fate of dSph
•  on the outer slopes of the potential well, stars will drift away
–  in fact, the SMC turns out to be unbound to the LMC so
is moving away
•  smaller galaxies can get torn apart
–  for example, the Sagittarius dSph is only 15 kpc from the
centre of the Milky Way
–  the MW rotation curve shows that ~1011 Msolar lies within
this radius
–  the Sgr galaxy would need 1010 Msolar to retain its own
stars, but has a luminosity of only 107 Lsolar
–  this means it will lose its stars (would need 1000 Msolar
of dark matter for every 1 Msolar of stars to be stabilised)
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merger of Milky Way and M31
•  the Milky Way and
M31 dominate the
Local Group but are
not in a mutually
stable orbit
–  a close pass
occurs in 5 Gyr
–  possibly an
actual collision
–  Sun could be
ejected into
intergalactic
space
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simulation, see movie at
http://www.cita.utoronto.ca/~dubinski/tflops/
(J Dubinksi)
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intergalactic stars
•  some evidence of intergalactic Planetary Nebulae in the
Virgo cluster
–  too far away to see individual stars like the Sun, but
they could be there
artist’s sketch, looking
back (NASA)
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