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Galaxy Evolution
© Sierra College Astronomy Department
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Galaxy Evolution
Introduction
• The Diversity of the Island Universes
• Spirals
• Ellipticals
• Irregulars
• Spectacular Variations
• Colliding and merging galaxies
• Massive star formation regions
• Supermassive black holes, accretion disks, and cosmic jets
• As the 21st century begins, a rudimentary understanding of
galactic formation and evolution is being pieced together.
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Galaxy Evolution
Looking Back Through Time
• Look-Back Time Revisited
• Finite speed of light means we observe objects as they
were when the light left them and not as they are today.
• Farther we “look” into space, the farther back in time we see.
• The farthest galaxies seen are 13 billion light-years away – we
see them as they were when the Universe was only about 1
billion years old.
• The most distant galaxies show starlight.
• Stars already forming 13 billion years ago – similar in age to
oldest Milky Way stars.
• Therefore, many or most galaxies must have started forming 13
billion years ago.
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Galaxy Evolution
Looking Back Through Time
• Look-Back Time Revisited (continued)
• By looking at galaxies at distances closer and closer to
us, we can create an “album” of galaxies at various
stages of development.
• The general picture we obtain is one with young
galaxies forming within the first 3 billion years of the
birth of the Universe, and then the three major galactic
types appearing and evolving after that.
• Because of the lack of starlight within the first 1 billion
years, galactic formation and evolution relies on
theoretical modeling.
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Galaxy Evolution
Galaxy Formation
• Most Successful Models Assume:
• Hydrogen and helium filled all space within 1 million
years of the Universe’s birth.
• There were slight density variations in the H/He gas
distribution.
• Initial Progression of Formation
• Denser regions initially expanded with Universe, but
within 1 billion years, these regions began to contract
into protogalactic clouds.
• Many galaxies form from the merger of several
protogalactic clouds.
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Galaxy Evolution
Galaxy Formation
• Initial Progression of Formation - Spirals
• Gas cools by radiating away thermal energy
• First generation stars form in densest, coldest regions,
are very massive, and die within a few million years
• Supernovae of first generation stars begin the
“polluting” of the interstellar gas with heavy elements.
• Heating of initial crop of stars (the spheroidal
population) slows star formation process enough to
allow remaining gas to settle into rotating disk.
• Disc population of stars are then formed.
• But what about the ellipticals and irregulars?
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Galaxy Evolution
The Lives of Galaxies
• Different galaxy types appear to form through two distinct
ways.
• Slight differences in protogalactic phase
• The higher the angular momentum and the lower the gas density in
the protogalactic cloud, the more likely the protogalaxy will evolve
into a spiral; otherwise the protogalaxy will become an elliptical.
• Evidence for this possibility comes from the fact that giant elliptical
galaxies are seen in the early Universe.
• Interactions with other galaxies with similar initial conditions
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Elliptical galaxies dominate the central regions of rich clusters.
Elliptical galaxy structural details can only be explained by collisions.
Central dominant galaxies and galactic cannibalism.
Central regions of hot gas in rich clusters and lenticular galaxies.
Irregulars may in part be explained as galaxies undergoing some sort
of disruptive behavior.
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Galaxy Evolution
What are Starbursts?
• Typical Stellar Formation Rates
• Spiral galaxies from stars in disk at steady and slow rate.
• All stars in elliptical galaxies formed in the distant past.
• A Small Percentage of present-day galaxies are forming
stars at a prodigious rate.
• These galaxies are called starburst galaxies.
• Starburst activity must be a temporary condition since the rate of
star formation can completely consume a galaxy’s interstellar gas
in just a few million years.
• Many galaxies appear to have gone through a starburst phase more
than once.
• Starburst galaxies return to previous spiral, elliptical, or irregular
state.
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Galaxy Evolution
More on Starburst Galaxies
• Some Starburst Galaxy Observations
• Because of the presence of interstellar molecular clouds and dust,
starburst galaxies are detected mainly in the infrared and not in the
visible and UV.
• The luminosity in the far-infrared can exceed the luminosity in the
visible/near-infrared by as much as 100 times.
• The large rate of star production also produces a large rate of
supernovae, which leads to the creation of supersonically
expanding superbubbles (regions of hot gas) that burst into
intergalactic space creating a galactic wind.
• Galactic winds consist of low-density, but extremely hot gas (10100 million K) which are detected via their x-ray emissions.
• Small starburst galaxies can drive out much of their gas content,
shutting down star production for billions of years.
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Galaxy Evolution
Generating Starburst Activity
• The Most Luminous Starburst Galaxies
• Many of these galaxies appear violently disturbed.
• Collisions between galaxies appears to initiate the starburst
activity.
• Starbursts of this magnitude explain why elliptical galaxies lack
young stars and cool gas.
• Starburst uses up cool gas and galactic winds blow away the rest.
• Massive stars die out quickly after starburst outbreak.
• When collision ends and the merger is complete, an elliptical galaxy
remains.
• Cause of smaller-scale starbursts is unclear
• Some small irregular galaxies are undergoing starbursts in the
absence of a collision.
• “Close encounters” have been suggested as the cause.
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Galaxy Evolution
Quasars & Other Active Galactic Nuclei
• Galaxies with Highly Luminous Centers
• The centers of galaxies which exhibit extreme
luminosities and sometimes very powerful jets are
called active galactic nuclei.
• The very brightest active galactic nuclei are quasars, the
most powerful of which may emit more light than 1,000
Milky Way galaxies.
• Quasars are found primarily at very far distances
suggesting that quasars may represent an early stage in
a galaxy’s evolution.
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Galaxy Evolution
What are Quasars?
• A Brief History of Quasars
• In early 1960s, Maarten Schmidt discovered that the hydrogen
emission lines in the visible spectrum of the optical component of
radio source 3C 273 was highly redshifted (about 80 nm), which
implied a recessional speed of 17% of the speed of light.
• Assuming the redshift was cosmological (which was debatable for
many years), the Hubble Law and the measured apparent
brightness gave a luminosity for 3C 273 of over ten trillion Suns or more than 100 times more luminous than the entire Milky Way.
• Since the first of these objects were strong radio sources and starlike in the visible, they were named quasars (quasi-stellar radio
sources).
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Galaxy Evolution
More Quasar Characteristics
• 40 Years of Data and Analysis
• Most quasars lie more than halfway to the cosmological
horizon – light from most distant quasar emitted when
Universe was less than 1 billion years old.
• Initially hard to determine, Quasars lie in the centers of
galaxies.
• Emit across an unusually broad range of the spectrum.
• Power output about the same from infrared to gamma rays
implying a multi-temperature construct.
• Produce strong emission lines.
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Galaxy Evolution
The Other Active Galactic Nuclei
• Nearby Active Galactic Nuclei
• About 1% of nearby galaxies (often called Seyfert
galaxies) have active galactic nuclei that look like
quasars with respect to their spectrum distribution, but
are much less luminous than quasars.
• Due to their close range, the sizes of the active nuclei
can be more readily determined.
• Visible images indicate a region of less than 100 ly.
• Radio interferometry places the size at less than 3 ly.
• Rapid variations in luminosities lead to active nuclei sizes not
much bigger than the Solar System – a size that is believed to
be typical of all active galactic nuclei including quasars.
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Galaxy Evolution
The Other Active Galactic Nuclei
• Radio Galaxies and Jets
• Radio galaxies were discovered in the 1950s
• Emit unusually strong radio waves from pairs of huge radio lobes, one
on either side of the galaxy, which is typically an elliptical galaxy.
• At the center of a radio galaxy is an active region only a few lightyears across.
• Plasma is seen shooting out of the active nucleus in jets and at speeds
near that of light.
• At the end of the jets are the lobes, at distances as large as one million
light-years from the center.
• The radio lobes are the result of the jets ramming into the intergalactic
gas, heating, and then spreading out.
• Many quasars also have jets and radio lobes.
• Many active nuclei of radio galaxies are obscured by donut-shaped
rings of dark molecular clouds.
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Galaxy Evolution
Active Galactic Nuclei Power Source
• Energy Source for Active Galactic Nuclei
• Only one possibility fills the job: Matter falling into a
supermassive black hole.
• Gravitational potential energy of matter is converted into kinetic
energy which then heats the gas.
• The heat then generates the emission of intense radiation.
• For a quasar to be as luminous as it is observed to be, the
supermassive black hole at its center must consume one one-solarmass star per year.
• The broad spectrum of an active galactic nucleus is the result of the
varied temperature structure in and around the disc.
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Hot gas in and above disc give x-rays and UV.
Radiation ionizes surrounding interstellar gas leading to visible light.
Dust grains encircling nucleus in molecular clouds emit infrared.
Fast moving electrons give radio emissions.
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Galaxy Evolution
Active Galactic Nuclei Power Source
• Energy Source for Active Galactic Nuclei (continued)
• Jets are believed to be created by magnetic fields
embedded in ionized accretion disc.
• These magnetic fields get twisted around the accretion disc
spin axis.
• Ionized gas then flows along magnetic field lines.
• Unsolved mysteries
• Why do quasars run out of gas?
• How did supermassive black holes form in first place?
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Galaxy Evolution
More on Supermassive Black Holes
• Evidence for the Existence of Supermassive Black
Holes
• Most convincing evidence is the speed at which gas
rotates in a very small region in the center of the galaxy
(e.g., data from M87).
• Similar results from maser emissions in NGC 4258
indicate the existence of a 36 million solar mass object
inside a radius of 1 ly.
• The relationship between a galaxy’s central bulge mass
(Mcb) and its central black hole mass (Mbh) (Mcb is
about 500Mbh) for a large variety of galaxies, indicates
that the growth of the central black hole must be closely
linked to the process of galaxy formation.
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Galaxy Evolution
Quasars as an Investigative Tool
• Using Quasar Spectra to Decipher Evolutionary Trends in
Intergalactic Clouds
• Most distant quasars are at the fringes of the cosmological horizon.
• The light from these quasars will pass through several intergalactic
clouds.
• Due to each cloud’s different recessional speed, each cloud will
etch its own unique spectral signature into the quasar’s spectrum.
• Studies of the absorption lines should allow us to determine what
happened in protogalactic clouds during each epoch of galaxy
formation. Early results:
• More mass in gas phase in early Universe than now.
• The percentage of heavy element content increases with the age of the
Universe.
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