6.3 Galaxy Evolution
When did galaxies form?
1. Big Bang, inflation, ……expansion and cooling …….
Protons and electrons form neutral atoms: recombination epoch,
the temperature was around 3000 K, the Universe was
approximately 379,000 years old.
Photon decoupling: relic radiation The Cosmic Microwave
Background, now seen at 2.7K.
During the Dark Ages, the Universe expanded and cooled. The
gas remained neutral……..
…….until stars started to form, followed by galaxies and
quasars, as larger objects collapsed
The number of irregular galaxies increases with redshift (e.g.,
the Hubble Deep Field)
Mergers: The rate of merging as a function of cosmic time
(redshift) can be estimated by counting the number of close pairs
(the merger fraction) in redshift surveys.
Parameterize merger fraction f(z) ~ (1+z)m and find values
for m ranging from 0 (no evolution) to 4 (lots of evolution)
Evolution of the star formation rate as a function of lookback time,
Pettini (2003) Springel & Hernquist (2003), Perez-Gonzales et al
2005. Star Formation tracers:
UV (but dust obscured), H-alpha / optical line, Far IR continuum,
Lyman-break galaxies, Gamma-Ray bursts.
As compared to their counterpart at z = 7, at z = 10: in the
Schecter function L* decreases by a factor of about 6.5 and
increases by a factor of 17–90: i.e. there is a large population of
low-mass galaxies at z = 10 which re-ionised the Universe, a
process complete by z = 6.
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Hopkins and Beacom 2006, ApJ
Kistler et al 2009, ApJ
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Galactic Nuclei: Many (all?) ellipticals (& bulges) have black
holes.
Can measure BH masses for galaxies without central disks via
their velocity dispersion
E.g. M32:
Currently there are observations of at least 40 BH masses in
nearby ellipticals and spiral bulges
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There is a strong correlation between black hole mass and
galaxy luminosity and velocity dispersion (Marconi & Hunt
2003):
Observations imply BH mass directly tied to the formation
of bulges and ellipticals
For Mbulge = 5 × 1010 Msun the median black hole mass
is 0.14% ± 0.04% of the bulge mass
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Either
All proto-galaxy clumps harboured an equal sized
(relative to total mass) BH, and BH merged as galaxy
formed
BH started out small and grew as galaxy formed – e.g.,
central BH is fed during process of formation and is the
seed of the formation process (all galaxies have BHs?)
Galaxy Formation: Monolithic? Hierarchical?
Downsizing?
Do massive galaxies form from scratch, or by chunking together smaller
galaxies?
Monolithic: This hypothesis posits that giant galaxies form all at
once, with the bulk of star formation happening at the same
time as the galaxy gains the bulk of its mass. Collapse and
dissipation of energy occurs together.
Ellipticals formed in a monolithic collapse, which induced violent
relaxation of the stars, stars have since reached an equilibrium
state.
In the monolithic theory, the gas is lost in the initial phases through
the burst of star formation.
Physical Origin of the Luminosity Function
(http://www.astro.virginia.edu/class/whittle/astr553/Topic04/Lecture_4.html )
Why does the galaxy luminosity function have the form that it
does? A complete understanding of this is not yet possible, but
here are the ingredients:
Making galaxies involves at least two things
•
dark matter halos must form (relatively straightforward)
•
baryons must fall in and make stars (complex physics)
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Large-scale simulations predict: too many huge and dwarf halos
without huge and dwarf galaxies:
To understand why, we need to look at what prevents baryons
from making stars within halos of different size (see figure).
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a Gas falling into huge halos is too hot to cool.
This becomes the intercluster medium in galaxy clusters.
b Gas falling into less massive halos is kept hot by AGN jets
c Gas falling into small halos can be easily blown out by
supernovae and star winds
d Gas cannot fall into tiny halos -- it is prevented by its own
pressure.
These processes are added to the cosmological dark matter
simulations using simple prescriptive formulae, to generate socalled: "semi-analytic models" (see figure).
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These nicely reproduce many galaxy demographic results,
including a galaxy mass function that is a much better match to the
observed galaxy luminosity function.
Hierarchical: Evidence is building for this theory. Using an
array of both ground-based and space telescopes, including ESO's
Very Large Telescope in Chile and the Hubble Space Telescope, a
team of astronomers recently observed groups of huge galaxies in
the process of merging, showing that large, established galaxies
can still grow bigger.
In this version, galaxies would form most of their stars early on
as small galaxies, but accumulate most of their mass later
through mergers.
A study, published in 2005 by Pieter van Dokkum of Yale
University, found a large number of established galaxies with old
stars that displayed signs of having recently merged with
other galaxies to add on to their mass.
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Evidence for mergers?
Red ellipticals: dominate the clusters
Blue spirals: dominate the voids
Questions remain
Though many astronomers agree that hierarchical formation seems to be occurring,
there are still some wrinkles to the theory.
For example, the very most massive galaxies don't seem to be growing at as high a
rate as middle-mass galaxies. When astronomers look at the brightest galaxies now
compared to the brightest galaxies at an earlier epoch, they don't seem to have
gained much mass.
It suggests that there might be an upper ceiling to how large a galaxy can grow.
Perhaps when a galaxy gets to be very large, its gravity is so strong that it rips up
smaller galaxies that pass nearby before they can join it.
Another question is why, if all galaxies are mergers of smaller ones, many of them
don't look it. Beautiful spiral galaxies, for instance, appear neat and symmetrical, not
as though they were formed from violent collisions of multiple smaller galaxies.
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Merging galaxies look like train wrecks. Maybe they only look like train wrecks for a
relatively short amount of time. Perhaps there are stabilizing forces, such as the
galaxies' angular momentum and the large halos of dark matter that surround them,
that help galaxies regain their orderly spiral structure after a merger.
Downsizing. A comprehensive survey of more than 4,000 elliptical
and lenticular galaxies in 93 nearby galaxy clusters has found a
curious case of galactic "downsizing."
Contrary to expectations, the largest, brightest galaxies in the
census consist almost exclusively of very old stars, with much of
their stellar populations having formed as long ago as 13 billion
years.
There appears to be very little recent star formation in these
galaxies, nor is there strong evidence for recent ingestion of
smaller, younger galaxies.
By contrast, the smaller, fainter galaxies are significantly younger
-- their stars were formed as little as four billion years ago.
The results of the survey contrast sharply with conventional
hierarchical model of galaxy formation, where large elliptical
galaxies in the nearby universe formed by swallowing smaller
galaxies with young stars; this theory predicts that, on average, the
stars in the largest elliptical galaxies should be no older than those
in the smallest ones.
.
The stars in the biggest, oldest galaxies formed early in the history
of the Universe. On average, the smaller galaxies have one-tenth
the mass of the larger ones, and are only about half their age.
The term 'downsizing' essentially means that when the Universe
was relatively young, the star formation activity occurred in
large galaxies, but as the Universe aged, the 'action' stopped in
the larger galaxies, even as it continued in smaller galaxies.
Formation of the Milky Way Galaxy
There are many theories on how the MW galaxy formed.
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Some believe that the halo formed first. Through collapse of one
overdense region which fragmented into many globular clusters.
In this theory, as gravity pulled the spherical halo material inward,
the material formed a disk to conserve its angular momentum.
During the collapse, stars in the halo continued to evolve,
producing metals (elements other than hydrogen and helium)
through the process of fusion. These metals were spewed into the
galactic medium through stellar winds and supernova explosions
and became part of the disc. This means that the stars in the disc
formed out of metal rich material. This process is known as the
"Outside-In" theory.
Starburst activity
Some galaxies, or their nuclei, show evidence of a recent
and transient increase in SFR by as much as a factor of 50.
Much of the star formation in starburst systems has been
found to occur in very luminous, compact star clusters (up to
108 solar luminosities, dimensions of a few parsecs), which
occur in bursting dwarfs, interacting galaxies, and mergers
Both direct mergers and more indirect interactions can
trigger star formation in galaxies
Caused by gas compression/accumulation, causing shocks
which trigger star formation
Gas which loses enough angular momentum will fall into the
centre (especially true if a bar is formed)
These can lead to strong nuclear starbursts
 M82 is currently forming a few M/year of stars (similar
to a large spiral) in a nuclear area only 100 pc across!
 Starburst phases are short.
Powerful starbursts surrounded by dust will be very bright in
the infrared
We observe numerous ultraluminous infrared galaxies
(ULIRGs), first discovered by the IRAS satellite, with L >
1012L
 These galaxies are merging too!
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