Galaxy Evolution
Majority of galaxies belong to clusters and groups of galaxies.
Density of galaxies in clusters is roughly 100x greater than that of stars in galaxies.
There is a higher probability of interactions and/or mergers between galaxies in
regions of higher galaxy number density.
Interactions tend to increase the velocity dispersion, and probably destroy gaseous
disks in late-type galaxies, causing their stars to become elliptical-like.
Interactions also apply tidal effects on gas and stars. Causes gas to compress and
form stars, often in immense bursts of star formation which can exceed the
“quiescent” star formation rate by 10-100 x !
NGC4038/4039,
Antennae Galaxies
NGC4038/4039,
Antennae Galaxies
NGC4038/4039,
Antennae Galaxies Z. Wang et al.
Galaxy Evolution
What happens when galaxies collide ?
Stars are very far apart and pass right through each other. Effects are gravitational.
As a galaxy of mass M passes through another, if feels the gravity and produces a
“wake” of higher density (because stars in the other galaxy have been compressed
along the path of the moving galaxy.
This is dynamical friction and is a net gravitational force on M that opposes
the galaxy’s motion. Kinetic energy was transferred from M to the surrounding
material as M’s speed is reduced.
Dynamical Friction
M
Contours show the
density enhancement of
V
stars due to the motion
of a mass M in the
positive z direction.
A high-density “wake” trailing M
causes a net gravitational force
on M opposing its motion
Mulder 1983, A&A, 117, 9
Dynamical Friction
Derivation of Force of dynamical fraction is complicated. It depends on the
speed, vM, the density of the surrounding material, ρ, and mass, M, squared.
One can estimate the timescale for dynamical friction to halt two colliding
objects. Consider the dynamical friction force on Globular clusters within
the Milky Way. The dark matter distribution is
Insert this into the dynamical friction force equation above, which gives
Dynamical Friction
Assume globular clusters’ orbits are circular, then the angular momentum is just
L=M vM r. And, the torque is τ = r fd = (dL / dt).
For a flat rotation curve, vM is constant at large radii. Thus, the derivative of L is
(dL/dt) = MvM (dr/dt). This gives:
Integrating this equation gives an expression describing the time required for the
globular cluster to spiral into the center of the host galaxy from an initial radius, r1.
Or:
Or, you can calculate the maximum
distance a globular cluster could
have traveled in a time tmax:
Dynamical Friction
Example:
Consider a globular cluster that orbits in the Andromeda galaxy (M31).
Assume the cluster’s mass is 5 x 106 solar masses with vM = 250 km s-1.
Age of M31 is approximately ~13 Gyr.
The maximum radius at which a GC could have spiraled into the center of
M31 is rmax = 3.7 kpc, whereas the “halo” of M31 is more like 100 kpc.
Note that rmax ~ M1/2. Clusters with greater masses have higher maximum
radii, so this likely explains the lack of massive GCs in M31 today.
NOTE ! Dynamical friction affects globular clusters, and satellite galaxies
around larger galaxies, and other galaxies in clusters of galaxies.
Dynamical Friction
Artists rendition of the Sagittarius dwarf galaxy merging with the Milky Way.
NOTE ! Dynamical friction affects globular clusters, and satellite galaxies
around larger galaxies, and other galaxies in clusters of galaxies.
Rapid encounters
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How do Galaxies Form ?
I. Monolithic Collapse. A galaxy forms from a single gas cloud that
collapse, fragment and form stars. First serious model proposed by Olin
Eggen, Donald Lynden-Bell, and Allan Sandage in 1962 (Eggen, Lynden-
Bell, & Sandage - ELS model).
ELS noticed that metal-rich stars lie in the Galactic plane with circular
rotations. Metal-poor stars have more eccentric orbits, some with highly
elliptical orbits in and out of the plane of the Galactic disk
Hypothesis: Milky Way (and other galaxies) formed from a rapid collapse
of a large proto-Galactic nebula.
Oldest stars formed early in the collapse, which locked in their orbits.
These would be metal-poor (Population II) since the gas had had little
enrichment.
As proto-Galactic cloud continued to fall inward, gas collisions are more
frequent, so more stars form, with higher metallicity.
Gas settles into disk with higher metal-mass fraction, and continues to
form stars (such as our Sun). These would be Population I stars.
How do Galaxies Form ?
I. Homologous Collapse - ELS model.
One can calculate the free-fall time for a gas cloud with the mass of the
Milky Way to collapse. Assume a Mass of 5 x 1011 solar masses with a
nearly spherical volume of radius 50 kpc (dark matter halo, and size of
stellar halo). Assume mass was initially distributed uniformly.
The initial density is ρ0 = 3M/4π r3 = 8 x 10-23 kg m-3.
Ch. 12 works out that the “free-fall” time, tff, for collapse is
tff = [(3π/32) (1/Gρ0) ]1/2 = 200 Myr.
If the inner regions of the Galaxy were more centrally condensed (as the
NFW dark-matter distribution is), then the inner parts would collapse
faster and the outer parts slower.
This could explain the existence of old stellar populations in the bulge.
How do Galaxies Form ?
I. Homologous Collapse - ELS model.
Problems with ELS model
1. Given initial rotation of the proto-Galactic cloud, all halo stars and
globular clusters should orbit in the same net direction. Observations
show that the net rotation in the halo is 0 km/s.
2. The age spread in globular clusters is about 2 Gyr. An order of
magnitude larger than the ELS free-fall time.
3. ELS model does not explain multiple disk components (thin and thick
disks).
4. Globular clusters located nearest the Galaxy center are generally the
most metal-rich and oldest, while clusters in the outer halo have a wide
range of metallicity and tend to be younger. This is not naturally explained
in the ELS model.
How do Galaxies Form ?
2. Hierarchical Merger Model
Galaxies probably have some formation similar to ELS, but their formation
also has a bottom-up hierarchical process of mergers.
Realization in the 1970s and 80s that the Big Bang would have left small
matter fluctuations that would grow through gravity and merge to form
larger mass objects. Our understanding is that there are many, many
more low-mass fluctuations than large-mass fluctuations.
Density fluctuations of 106 - 108 solar masses were much more common
than those of 1012 solar masses.
Consider that the Milky Way grew from fragments of mass 106 to 108 solar
masses. Initially these fragments are in isolation, forming stars and
globular clusters. They have their own metal enrichment and star-
formation histories. They then merge.
In this model, the inner regions of the growing spheroid where the density
of matter was greater, evolution would be fastest. This produces things
like metal-rich, old bulges.
Aq-A-2-evolv.mp4
Courtesy V. Springel
How do Galaxies Form ?
2. Hierarchical Merger Model
Collisions and tidal interactions between merging fragments would disrupt
some globular clusters (remember dynamical friction?) and left some
intact. In this model, the disrupted systems would have led to the present
distribution of field halo stars, while leaving intact globular clusters
distributed throughout the spheroid.
Through many random mergers, there should be no net rotation of
objects in the halo.
Model also predicts that some proto-Galactic fragments should still be out
there. This explains significant number of small galaxies orbiting the Milky
Way and nearby Andromeda. These are surviving proto-Galactic
fragments. Some are merging, like the Sagittarius dwarf.
Andromeda galaxy
and dwarfs
Tidal streams of
previously merged bits
Andromeda galaxy
and dwarfs
Galaxy Interactions
Simulation by Chris Mihos (ca. 1996)
Simulation by Volker Springel (ca. 2006)
T = 0 Myr 150 Myr 300 Myr
Elliptical Galaxies can form
from gas-rich spiral galaxy
750 Myr mergers.
These are simulations of a
merger of 2 spiral galaxies
with 16,384 particles in each
450 Myr 600 Myr
disk and 4096 particles in
900 Myr 1 Gyr 1.2 Gyr each bulge.
The result is an elliptical
galaxy.
See Hernquist 1993, ApJ,
1.4 Gyr 1.5 Gyr 1.7 Gyr 409, 548.
Elliptical Galaxies can also grow
from mergers of elliptical galaxies.
van Dokkum 2005, AJ, 130, 2647
Elliptical Galaxy Formation
Elliptical Galaxy Formation
Rings may be left
over remnants of
merged galaxies
NGC 3923
Malin & Carter, Nature, 285, 643, 1980
Ring galaxies are the result of
high speed collisions of galaxies
in which the smaller one passes
through another galaxy almost
perpendicular to its disk.
How do Galaxies Form ?
Remember that galaxies with high redshifts are very far away:
cz ~ v = H0 d (for z 6 !
Many of these come from the Sloan Digital Sky Survey (SDSS).
You book quotes that there are 520 quasars with z > 4. At z = 4 the
recessional velocity is 0.92 c !
To determine distances at such large redshifts requires geometrical
considerations (more on this when we do cosmology).
Effectively, the fractional change in wavelength due to the redshift is
the same as the fraction change in the size of the Universe (recall
the Universe is expanding!)
z = (λobs - λrest) / λrest = (Robs - Remitted)/Remitted
Where Robs is the size of the Universe when the photon is observed
and Remitted is the size of the Universe when the photon was emitted.
Quasar Red Shifts
Quasars have been
z=0 detected at the highest
red shifts, up to
z = 0.178 z~6
z = 0.240 z = Δλ/λ0
z = 0.302 Our old formula
Δλ/λ0 = vr/c
z = 0.389
is only valid in the
limit of low speed,
vr 1 ??
First, relativistic Doppler effect is described by a different formula:
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However, cosmological redshift is not a Doppler effect!
The redshift is due to the expansion of the Universe:
as a light wave travels through space, the universe expands
and the light wave gets stretched and therefore redshifted.
R(tobs )
z= −1
Rem )
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t = tem
Slide p.308
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t = tobs
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Two galaxies permanently located at positions (x1 , y1 , z1 ) and ( x2 , y2 ,
z2 ) at one time find themselves one billion light years apart. Then a few
billion years later while located at the same coordinates, they find
themselves 3 billion light years apart. The galaxies have not 'moved',
nevertheless, their separations have increased.
R(tobs )
z= −1
Rem )
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Another evidence of cosmological distance to quasars:
gravitational lensing
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Active Galactic Nuclei
Quasars Luminosities
Calculate the luminosity of 3C 273. The apparent magnitude is
V=12.8 mag. The modern day distance for its redshift is 620 Mpc.
MV = V - 5 log10(d / 10 pc) = -26.2 mag.
Using MSun = +4.82 for the absolute magnitude, we can estimate
3C273’s visual luminosity:
LV = 100(Msun-MV)/5 L⊙ = 2.6 x 1012 L⊙ = 1039 W.
Bolometric luminosities of Quasars range from 1038 to 1039 W, this is
more than 100 times the output of a galaxy like the Milky Way !
Quasar sizes
Quasars are point sources even in HST images, implying the regions
emitting the intense luminosity are 100 than the Milky Way, this is an
incredibly small size !
Recall that there is a maximum luminosity before an object will blow
itself apart due to radiation pressure. This the Eddington Limit.
L
3.3 x 108 M⊙.
Finding such a large mass in such a small space is clear evidence for a
supermassive black hole. The mass for an object with a
Schwarzschild radius =7.2 AU (above) is M = 3.7 x 108 M⊙.
Jets and host galaxies have been resolved for “nearby” quasars
3C273
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Bahcall 1995, ApJ, 479, 642
Active Galactic Nuclei
Superluminal Motion
Individual radio knots in quasar jets:
Sometimes apparently moving
faster than speed of light!
Light-travel
time effect:
Material in
the jet is
almost
catching up
with the light
it emits
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Evidence for Quasars in Distant
Quasar 0351+026 at the
same red shift as a galaxy
→ evidence for quasar
activity due to galaxy
interaction
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Galaxies Associated with Quasars
Two images of the same
quasar, 1059+730
New source probably a
supernova in the host galaxy
of the quasar
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Host Galaxies of Quasars
Host galaxies
of most
quasars can
not be seen
directly
because they
are outshined
by the bright
emission from
the AGN.
Blocking out the light from the center of the quasar 3C
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273, HST can detect the star light from its host galaxy.
Gallery of Quasar Host Galaxies
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Elliptical galaxies; often merging / interacting galaxies
Quasars
1) Spectra contain strongly redshifted lines indicating large cosmological
distances to the objects
Gravitational lensing also indicates huge distances
This means that quasars are most luminous objects in the Universe!
L ~ 1012 – 1014 Lsun
2) Broad emission line as in Seyferts, indicating rapid motion
3) Jets, intense radiation from radio waves to gamma-rays observed
4) Host galaxies are found around nearby quasars
5) Rapid variability on the scale of days is observed
1)-5) indicate that quasars sit in the centers of galaxies, are extremely
compact and super-luminous.
They are similar to AGN!
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Quasars were much more numerous in the early Universe than now
Galaxy collisions were more frequent; they supplied more
stars and gas to the central black holes
Collisions and mergers play crucial role in the AGN activity
Modern galaxies with central black holes are sleeping quasars?!
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