Download PH607 – Galaxies

Spiral Galaxy Rotation Curve
The Milky Way rotation curve:

Although the enclosed mass, M(r), continues to grow apparently
without limit, the enclosed luminosity, L(r), tends to a finite
limit as we reach the edge of the luminous material in the galaxy.
There must therefore be significant amounts of dark matter which
continue to contribute to M(r) out to very large radii.
o Out to the furthest point measured, typical galaxies have a
luminosity of L ~ 1010 L , and a typical enclosed mass of M
~ 1011 M .
 The "mass-to-light ratio" M / L is hence ~ 10 solar
units.
 ~ 90% of the material in the galaxy is dark!
Example:
The Sun moves at about 220 km s-1 in a circular orbit around the centre of
the Galaxy, like almost all the stars near the Sun. We can assume that all
the matter is at the Galactic centre, a not too bad approximation.
Let the speed be V0 , the mass of the Galaxy be MG and the distance of
the Sun from the Galactic centre be R0. Then the centrifugal force due to
rotational speed must balance the gravitational force due to the mass of
the Galaxy.
GMG/R02 = V02/R0
where G is the gravitational constant.
Hence
MG = V02 R0/G
Substituting values of 8 kpc and 220 km s-1 for the Sun and G = 0.00430
M (km s-1)2 / pc, we get
MG = 1011 M
.
However, measurements of the rotation of the outer edge of the Milky
Way show that the stars out there also rotate at 220 km s-1, out to about
20 kpc.
Thus, within a radius of 20 kpc we get a mass of
MG = 2x1011 M
If the light distribution of the Galaxy were proportional to the mass
distribution, then the two mass estimates above would imply that the
amount of light emitted by the 8 kpc region would be the same as the
region from 8 - 20 kpc... whereas measurements show that the 8 kpc
region emits about 10 times more light than the 8 - 20 kpc region.
The major conclusion is that the distribution of emitted light is not
necessarily the same as the underlying distribution of matter.
Rotation curves of other galaxies
On the left, a spiral galaxy image, with spiral arms delineated by HII
regions. On the right, the light a narrow strip running along the major axis
of the galaxy has been spread into a spectrum, between about 6500 and
6800 Angstroms. The rotation of the galaxy is seen in the emission lines
from H alpha at 6563 Angstroms (the brightest line), as well as other
fainter lines in this region due to [NII]. HII regions appear reddish in this
image because of the prominence of the H alpha line in the red region of
the spectrum.
The Local Group
The biggest and brightest Local Group members are the Milky Way
Galaxy and the brightest Messier objects: M31 (Andromeda) and M33.
Next in line would be M32 and the two Magellanic Clouds (the LMC and
SMC). The Clouds are big and close, so we have good detailed studies of
them. The rest are smaller objects, either irregular galaxies or dwarf
ellipticals.
M31:
LMC:
SMC :
In fact, it seems that our own galaxy undergoes a gravitational interaction
with the LMC and SMC.
NGC 6822 (``Barnard's Galaxy'') and IC 1613: Two other more distant
and less luminous irregular galaxies that have been extensively studied
with large telescopes. They are providing new insights for both the
distance scale and the evolution of galaxies. Both have Cepheid variables,
still the best way of determining distances within the nearest 10 million
light-years, and both have current star-formation activity.
NGC 6822 (left) and IC 1613 – both are Irregular galaxies
The Local Group – dominated by the two giant spirals, Andromeda (M31) and our
own Milky Way. In addition to Messier 33, an intermediate mass Sc galaxy, there are
15 ellipticals and 13 irregular galaxies in the cluster, including the Magellanic
Clouds, our Galaxy's satellites, Messier 32 and NGC 205, satellites of Andromeda.
The group has a size of about 3 million ly., and has a total mass of 5 x
1012M
What is the criterion for inclusion in the Local Group? Proximity. If we
grant membership to all galaxies within 4 million light-years, we have
about 30 members.
This shows the 3-dimensional distribution of Local Group galaxies
Clusters
On the larger scale we have galaxy clusters such as the Virgo Cluster,
about 50 million light years away, that is the nearest regular cluster of
galaxies. Our Local Group is an outlying member of a "supercluster" of
galaxies of which the Virgo Cluster is the dominant member.
The Virgo Cluster
The Virgo Cluster with its some 2000 member galaxies dominates our
intergalactic neighbourhood, as it represents the physical centre of our
Local Supercluster (also called Virgo or Coma-Virgo Supercluster), and
influences all the galaxies and galaxy groups by the gravitational
attraction of its enormous mass.
The Virgo Cluster has slowed down the escape velocities (due to cosmic
expansion, the `Hubble effect') of all the galaxies and galaxy groups
around it, causing an effective matter flow towards itself (the so-called
Virgo-centric flow). Eventually many of these galaxies have fallen, or
will fall in the future, into this giant cluster which will increase in size
due to this effect. Our Local Group has experienced a speed-up of 100-400 km/sec towards the Virgo cluster.
Detail of Virgo cluster:
The Hercules Cluster (below), about 650 million light-years distant.
This cluster is loaded with gas and dust rich, star forming, spiral galaxies
but has relatively few elliptical galaxies, which lack gas and dust and the
associated newborn stars. Colours in the composite image show the star
forming galaxies with a blue tint and ellipticals with a slightly
yellowish cast. In this cosmic vista many galaxies seem to be colliding or
merging while others seem distorted - clear evidence that cluster galaxies
commonly interact. Over time, the galaxy interactions are likely to
affect the content of the cluster itself. Researchers believe that the
Hercules Cluster is significantly similar to young galaxy clusters in the
distant, early Universe and that exploring galaxy types and their
interactions in nearby Hercules will help unravel the threads of galaxy
and cluster evolution.
The Hercules Cluster
Distant Clusters. The Hubble Space Telescope has provided the first
opportunity to look back into the early universe at clusters. Billions of
years ago, clusters contained many more spiral galaxies than they do
today.
CL 0024+1654 is a large cluster of galaxies located 5 billion light-years
from Earth. It is distinctive because of its richness (large number of
member galaxies), and its magnificent gravitational lens. The blue loops
in the foreground are lensed images of a spiral galaxy located behind the
cluster.
The CL 0024+1654 Cluster – note the gravitational lensing
The rich galaxy cluster, Abell 2218, is a spectacular example of
gravitational lensing. The arc-like pattern spread across the picture like
a spider web is an illusion caused by the gravitational field of the cluster.
The cluster is so massive and compact that light rays passing through it
are deflected by its enormous gravitational field.
Hubble Deep Field: Probably the deepest image ever taken was by the
HST over about 150 consecutive orbits (about 10 days) from December
18 through 30, 1995 on a single piece of sky located at 12h 36m
49.4000s +62d 12' 58.000" (near the Big Dipper).
Distance to Galaxies – The Hubble Law
Cepheids., and went on to observe Cepheids in other galaxies as well and
so obtain distances to them.
Redshifts.
 In the 1920s, Hubble determined the distance to the Andromeda
galaxy by finding a Cepheid variable star there.
 Hubble and collaborators began a systematic study of nearby
galaxies which included measuring both their distance
(Cepheids etc) and radial velocity. They soon noticed a
remarkable trend: virtually all the galaxies they observed were
moving away from our galaxy (redshifted) and the recession
speed increased with distance.
 Hubble's first dataset included only a few dozen galaxies which were < 2
Mpc away but his basic conclusion has not changed as more and more
galaxies at larger distances have been observed. We live in an expanding
Universe. Due to the Big Bang, the universe is expanding.
 Hubble found that there was a direct linear relation between
distance and redshift: the further a galaxy was from us, the
faster its recession velocity. This relation, which has come to be
known as Hubble's Law, is written
v = Ho D
 where v is the recession velocity, D is the distance to
the galaxy, and Ho is the constant of proportionality
known as Hubble's
 Hubble found H0 ~ 500 km/s/Mpc !!
 Hubble’s original data:
Figure: A redshift versus magnitude plot reveals a linear relationship between recession velocity and
distance.
For relatively nearby objects, Hubble's law itself becomes a way to
determine distances. Suppose you had a galaxy in which you found an
emission line of sodium, which has a rest wavelength of 590 nm, shifted
to 620 nm. What is the distance to that galaxy using Hubble's Law?
First, compute the redshift:
z = ( - o) / o = (620 - 590) / 590 = 0.05
For speeds much less than the speed of light, z = v/c, hence this galaxy is
receding at a speed that is 5 percent the speed of light, or 0.05 x 3 x 105 =
1.5 x 104 km s-1. Using a value of the Hubble constant of 70 km s-1 Mpc-1
we can now solve for the distance in Megaparsecs:
1.5 x 104 / 70 = 214 Mpc.
The Doppler formula we have been using to relate the redshift z to the
velocity v is appropriate only for velocities much less than the speed of
light (i.e., nearby galaxies). The formula z = v / c implies that you can't
have redshifts greater than one because that would give you a velocity
greater than the speed of light, something not permitted by the laws of
physics. In fact, redshifts larger than 1 are possible, and are observed. For
example, if an object has a velocity near the speed of light we have to use
the "relativistic Doppler shift formula"
c  v

 z  1 
c  v
2
which is derived from special relativity. You can see by the presence of
the (c-v) in the denominator of the fraction that as v gets close to c the
redshift becomes increasingly large. (z = c would yield an infinite
redshift).
This means the Hubble law at high redshift becomes

Two galaxies have been moving apart for something like 13 Gyr
(assuming that they were one time very close together – such as at
the moment of the Big Bang). Another way of looking at this is to
see that:
and Ho is essentially an inverse Hubble time.
But there is sufficient mass in the Universe which slows down the
expansion - so our assumption that M87 and the Galaxy have been
moving apart at a constant speed since the expansion began is
probably false.
Galaxy Classification
External galaxies occur in a wide variety of shapes and sizes.
In the first systematic attempt to quantify their morphology, Hubble produced his
"tuning fork" diagram in the 1920s:

Galaxies on the left are designated "early type galaxies," and those toward the
right are called "late types." These labels arise because Hubble believed that
this diagram represents and evolutionary sequence. We now believe otherwise.
A detailed description of Galaxy classifications can be found at:
http://nedwww.ipac.caltech.edu/level5/Haynes/Haynes_contents.html
Four types of galaxies –
Hubble proposed a scheme for classifying galaxies in his 1936
book, The Realm of the Nebulae
 Ellipticals (E)
 Lenticulars (SO or SB0)
 S01, S02, S03 – strength of dust absorption, S01
has none
 SB01, SB02, SB03 – prominence of bar
 Spirals – normal (S) or barred (SB)
 Sa – Sc depending on bulge/disk ratio, tightness of
spiral arms, and gas content
 Irregulars (does not fit into above category)
Elliptical Galaxies
An elliptical galaxy shows no spiral structure and can vary from almost
round (what Hubble called E0) to almost cigar shaped (called E7). This
classification is based on our perspective from Earth and not on the actual
shape.
M89: E0
M59 E5

Elliptical galaxies are designated "E#," where # refers to their
apparent flattening:
o # = 10(1 - b/a)
o
o
o
o
o
Apparently round ellipticals are E0s
The flattest ellipticals observed are E7s.
Do not have perfect elliptical isophotes – typical deviations
of a few %
Deviations from ellipses can be classified as disky or
boxy(peanut)
Boxy galaxies tend to be more luminous, slower rotators
o
o
o
o
o
o
o
o
o
o
3-D shapes – are ellipticals predominantly:
Oblate: A=B>C (a flying saucer)
Prolate: A>B=C (a cigar)
Triaxial A>B>C (a football)
(A,B,C are intrinsic axis radii)
Want to derive intrinsic axial ratios from observed
Find that galaxies are mildly triaxial:
A:B:C ~ 1:0.95:0.65 (with some dispersion ~0.2)
Triaxiality is also supported by observations of isophotal
twists in some galaxies (would not see these if oblate or
prolate)
o
o
For normal bright ellipticals (Mv < -17) the surface
brightness falls off from the centre according to the
empirical formula
 cD’s are massive bright ellipticals at the centers of
galaxy clusters

In between the ellipticals and the spirals are the S0s which have
o very large bulges
o weak disks
o no spiral structure
Spiral Galaxies
 As a fiducial, the Milky Way
 Radial Scale Length of 3-4 kpc
 Blue Luminosity of ~ 1.5 x 1010 L
 Absolute blue magnitude, -20.7
 Total Mass of ~1011 – 1012 M 
 Depending on how much dark matter
there is
Spiral galaxies have outstretched, curving arms suggestive of a whirlpool
or pinwheel. Hubble distinguished different sub-classes according to the
tightness of the arms and the size of the nucleus. He called these Sa, Sb,
and Sc.
In terms of the arms, Sa is the tightest wound while Sc is the most open.

Normal spiral galaxies are designated S?. Barred spiral galaxies are
designated SB?. Definite spiral structures are seen in some 61% of
galaxies. These structures often extend throughout most of the
galaxy’s visible disk, which have scale lengths to 15 kpc or more.
Although individual galaxies often show irregularities in the light
distribution within the spiral patterns the underlying spiral
geometry is highly regular.
o The "?" is chosen from a, b or c, and was originally
classified on the basis of the pitch angle of the spiral arms:
o
Note that late-type spiral galaxies (Sc's) also tend to have:
 smaller bulges
 more "grand design" spiral structure
M65 Sa :
M33 Sc:
About ¾ of galaxies in the field are spirals
Most spirals are found in the field (in groups)
Spiral galaxy scale lengths run from ~1 kpc (dwarfs) to ~50 kpc
Absolute magnitudes ranging from –16 to –23, that’s a factor of
~1000 in luminosity!
Masses ranging from 109 to 1012 M
Barred Spirals
Barred spirals show the same spiral structure as normal spirals, and also a
prominent bar through the nucleus. The spiral arms emerge from the end
of the bar. The sub-classifications are the same as for normal spirals.
M58 SBb:
Dwarf galaxies
 Faint, M > -18,
 Dwarf Ellipticals, dwarf spheroidals, dwarf irregulars
 There are probably lots of these, in the Local Group
there are >30!
 Dwarf Spheroidal, Leo I :
 Leo I
Low Surface brightness galaxies (LSB)
 Very difficult to detect!
 Need dedicated surveys
 Recent automated CCD surveys suggest there may be more
LSB galaxies than all the other types of galaxy put together
Peculiar Galaxies
 In particular, interacting galaxies
 Many cataloged by Arp in 1966
NGC 4676 –
The Mice
Arp 188 The Tadpole:
Irregulars
Certain galaxies lack either an obvious spiral structure or
nuclear bulge, appearing instead as a random collection of
stars with no obvious order.
These are designated "Irr" for "irregular."
Make up a few % of the field galaxy population
Generally smaller, sizes of a few kpc
Absolute magnitudes of –13 to –20
Masses of 108 to 1010 M
o





.
Galaxy constituents:

Spiral galaxies contain:
o stars (population I and II) , gas, dust

Elliptical galaxies contain:
o stars (population II only – (i.e. old) stars)
Irregular galaxies are harder to classify. They usually contain:
o stars (population I (young stars) – in other words there are
significant amounts of gas in the galaxy which is being
transformed into young stars – with ages as short as a few
million years) and some population II) , star-forming regions
, gas (a higher proportion than in spirals)

The Large Magellanic Cloud at optical wavelengths
Mass M
Absolute mag
Luminosity L
M/L (M / L
=1)
Diameter (kpc)
Stellar
population
Ellipticals
105 - 1013
-9 -> -23
3 x 105 - 1011
100
Spirals
10 – 4 x 1011
-15 -> -21
108 – 2 x 1010
2 – 20
Irregulars
108 – 3 x 1010
-13 -> -18
107 - 109
1
1 – 200
II and old I
5 - 50
I in arms, II
and old I
overall
Yes
1 – 10
I, some II
83
4
Presence
of Almost none
dust
Total fraction
13
%
9
Yes
E
Colour
Red
S0
Sa
Red
Sb

Sc
Blue
Sd Irr
Blue
Stellar
Old Old +
Old +
Intermediate
Population
Intermediate Intermediate +
+ Young
Young
Star Form zero low
higher
high
Rate
HI (gas) Zero/ low
modest
high highest
low
dust
Zero/
Higher
highest
Lower
low
(less
metals)
Dynamics Bulge/halo Disk dominated, so
dom.
rotation
Lecture 3: Views of Galaxies
Our view of galaxies is greatly affected by the observing wavelength –
the infrared penetrating more deeply that optical radiation, and the HI
emission tracing atomic gas, which may be quite differently distributed to
the stellar populations:
NGC253 at optical and infrared wavelengths
M81 at optical wavelengths and using the 21cm wavelength HI tracer of
atomic hydrogen gas
Spiral Structure

The most obvious feature in a spiral galaxy is its spiral structure:
o Here, for example, are M101 and M100, with clear ‘grand
design’ spiral arms:
Flocculent spiral:

Structure is made up from young, bright stars
Over time, differential motions cause the spiral arms to wind up:

The stars in a spiral arm cannot always be the same stars:
o
Very rapidly, spiral structure will wind up very tightly.
 There are different types of spiral arms
o “Grand-Design” – two well-defined spiral arms (10%)
o Multiple-arm spirals (60%)
o Flocculent spirals – no well-defined arms at all, “ratty”
(30%)
 Are spiral arms leading or trailing?
 What is the nature of the arms?
So, open spiral structure cannot be maintained in this way.
o This problem is usually known as the winding dilemma
The solution to this dilemma was finally sorted out by Lin & Shu
in 1963.
o Their solution was to assume that:
 Stars follow slightly elliptical orbits
 The orientations of these orbits are correlated:
o

o
o
As is apparent from the above figure, this arrangement
produces a spiral density wave: spiral arms are caused by a
density perturbation that moves along at a speed different
from the speed of the objects within it. The density wave
resists the spiral’s tendency to wind up and causes a rigidly
rotating spiral pattern

material travels around undisturbed elliptical orbits,
but sometimes many orbits come close together, so the
density increases.
So, young bright stars should lie in front of the highest
density regions
High densities also compress the magnetic fields, which
produces a maximum in the radio continuum emission in
regions of highest density.
So, bright stars should appear "down stream" from the peak
in the radio continuum emission.
This effect is, indeed, observed, and so the density wave
theory is vindicated!
In the inner parts of disks, stars are moving faster than the
pattern speed and overtake the density wave.
In the outer parts, stars move more slowly than the pattern
speed, and the spiral arms over take the stars

o
o
o
o
o
o

The only remaining question is why orbits arrange themselves in
correlated ellipses.
o the answer is self organization:
o
This feedback loop can also generate the bars in SB galaxies
Such a runaway process is called a dynamical instability
o Note that this process only works if there is enough mass
in the disk for the perturbations to modify the gravitational
field
 In early-type spirals (Sa's) where most of the mass is
in the bulge not the disk, the instability will be partly
suppressed.
 This suppression explains the anti-correlation between
bulge size and strength of spiral structure.
Spiral arm pattern is amplified by resonances between the epicyclic
frequencies of the stars (deviations from circular orbits) and the
angular frequency of the spiral pattern
o Spiral waves can only grow between the inner and outer
Linblad resonances (p =  -/m ; p =  + /m ) where
=the epicyclic frequency and m is an integer (the # of spiral
arms)
o

o
Self propagating star-formation:
o Stars outside this region find that the periodic pull of the
spiral is faster than their epicyclic frequency, they don’t
respond to the spiral and the wave dies out
o Resonance can explain why 2 arm spirals are more
prominent
Note that density wave theory does not explain flocculent spirals.
Those can be explained by self-propagating star formation:
o Star forming regions produce supernovae, which shocks the
gas, which triggers more star formation, etc, etc, etc
o Differential rotation stretches out the regions of star
formation into trailing, fragmentary arms
o No global symmetry (as observed)
o

Barred Galaxies
e.g. NGC 1300:
Half of all disk galaxies show a central bar which contains up to
1/3 of the total light
Bars are almost as flat as surrounding disks.
S0 galaxies can have bars – a bar can persist in the absence of gas
Bar patterns are not static, they rotate with a pattern speed, but
unlike spiral arms they are not density waves. Stars in the bar stay
in the bar.
The asymmetric gravitational forces of a disk allow gas to lose
angular momentum (via shocks) compressing the gas along the
edge of the bar. The gas loses energy (dissipation) and moves
closer to the center of the galaxy.
Elliptical Galaxies

When you have seen one elliptical galaxy, you have pretty much
seen them all. So, here is a picture of one such system, the nearby
elliptical M32:


By studying absorption-line spectra of ellipticals, we can
investigate their internal dynamics and masses.
Many elliptical galaxies have a projected luminosity distribution
that follows a de Vaucouleurs (or R1/4) law, which states that the
surface brightness, I, of the bulge of the galaxy (measured in units
of L pc-2) shows a radial dependence according to:
1


4
 I (r ) 


r


log10 
  3.3307    1
I
r
 e 
 e 

where Ie and re represent the surface brightness at a radius Ie.
This is often referred to as a r1/4 law – and the distribution is
sometimes called a de Vaucouleurs profile. re has in the past
been used to represent the radius within which one half of the
bulge’s light is emitted.


So, a plot of surface brightness in magnitudes versus radius to
the quarter power for such a galaxy will appear as a straight
line.
Note that this law is a purely empirical fit with no physical
basis.
o However, any theory of elliptical galaxy formation must
reproduce it.
Dwarf Galaxies
Low-luminosity: 106 – 1010 L
Low-mass: 107 – 1010 M
Small in size, ~few kpc
Often low surface brightness, so they are hard to find!
Why are dwarf galaxies important??
Majority of galaxies are dwarfs!!
Dwarf galaxies may be remnants of galaxy formation
process: “proto-dwarf” gas clouds came together to
form larger galaxies (hierarchical formation)
Dwarf galaxies are currently being “absorbed” by larger
galaxies
Dwarf galaxies are relatively simple systems, not merger
products
Different types of dwarf galaxies
Dwarf ellipticals (dE): Note that these are structurally
very different from luminous E’s. Gas-poor, old stellar
population. Note that many dE’s have nuclei (dE,N).
Dwarf spheroidals (dSph): Gas-poor, diffuse systems.
Low luminosity (low surface brightness end of dE’s.
Dwarf irregulars (dIrr): Extreme end of late type spirals.
Active, on-going star-formation but low surface
brightness (like dSph’s). Gas-rich. Note that there are
no dwarf spirals!!
In the Local Group, we can study the resolved stellar
population (color magnitude diagrams) to determine the
star formation histories of dwarf galaxies
Dwarf ellipticals are generally old (stars formed > 10 Gyr
old), but some may have had more recent (a few Gyr ago)
weaker episodes of star formation
Dwarf irregulars tend to have quasi-continuous star
formation (perhaps interspersed with bursts). Lower
luminosity dIrr’s more likely to have a bursty history
Environmental effects may play a role (e.g., tidal
stripping removing gas from dSph’s)
No two galaxies have the same star formation history
Dwarfs do not contain dark matter.
Collisions and Mergers
Approximately 0.3% of galaxies are currently in the process of merging.
When two galaxies collide, they initially create long tidal tails and
plumes, but ultimately settle down to systems which look very like
normal elliptical galaxies. Could this be how the ellipticals formed?

Below is a sequence of images of various real galaxies which we
see at progressively later stages in the merger process

The system shown below, NGC7252, is a system at a very late
stage of merging – this is shown in successively deeper images of
the system – which appears with the short integrations, to be a
single galaxy.

NGC7252 above provides the "smoking gun" which shows that
mergers between galaxies can produce elliptical galaxies.
This HST Image below is of the ‘Cartwheel galaxy’, and is a
particularly impressive example of what can happen when two
galaxies collide face-on.


It is likely that the Milky Way will collide and merge with the
Andromeda Galaxy in about 3 billion years from now.
Clusters of Galaxies

Galaxies are not distributed at random in the sky - most are found
in clusters:
o Poor clusters contain ~ 10 - 100 galaxies of all types (spirals,
ellipticals & irregulars).
 See, for example the local group.
o Rich clusters contain ~ 100 - 1000 galaxies
 Mostly ellipticals
 Often dominated by 1 or 2 giant ellipticals (designated
"cD" galaxies) near their centres.
 Here, for example, is a picture of the centre of the
Coma cluster, the nearest rich cluster of galaxies,
which is dominated by two giant ellipticals:

Clusters are supported against collapse by the random motions of
the galaxies:
(c.f. the motions of stars within elliptical galaxies)
o
So, we can estimate their masses using the same formula as
for elliptical galaxies,
o
For a rich cluster, the sizescale r ~ 1 Mpc and typical
random velocities, v ~ 1000 km s-1, giving a mass estimate of
M ~ 2 x 1014 M .
A typical rich cluster contains ~ 1000 galaxies each with the
luminosity of the Milky Way (~ 1010 L ).
So the mass-to-light ratio of the whole cluster is M / L ~ 20
solar units.
 ~ 95% of the material in a cluster of galaxies is dark!
o
o
cD Galaxies and Cannibalism


How do the giant "cD" galaxies found at the centres of some
clusters form?
Perhaps by repeatedly merging with other cluster members.

Why do these mergers occur at the centre of the cluster?
o Because dynamical friction makes galaxies lose kinetic
energy:
 The motion of a galaxy creates an enhanced "wake" of
galaxies behind it
The excess gravitational pull of this wake slows the
motion of the galaxy --- it is a frictional force.
The net effect of this force is to make a galaxy slowly spiral
in toward the centre of the cluster (the point of lowest
energy).
Once there, it will merge with all the galaxies that have
proceeded it.
Evidence for this scenario comes from the large number of
"multiple nuclei" seen in cD galaxies:

o
o
o

These secondary condensations of light leftover
mergers – M31 is another example: