© sierra college astronomy department1 galaxy evolution

© Sierra College Astronomy Department 1

Galaxy EvolutionGalaxy Evolution



IntroductionIntroduction

• The Diversity of the Island UniversesThe Diversity of the Island Universes• SpiralsSpirals• EllipticalsEllipticals• IrregularsIrregulars

• Spectacular VariationsSpectacular Variations• Colliding and merging galaxiesColliding and merging galaxies• Massive star formation regionsMassive star formation regions• Supermassive black holes, accretion disks, and cosmic jetsSupermassive black holes, accretion disks, and cosmic jets

• As the 21As the 21stst century begins, a rudimentary understanding of century begins, a rudimentary understanding of galactic formation and evolution is being pieced together.galactic formation and evolution is being pieced together.



Looking Back Through TimeLooking Back Through Time

• Look-Back Time Revisited Look-Back Time Revisited • Finite speed of light means we observe objects as they Finite speed of light means we observe objects as they

were when the light left them and not as they are today.were when the light left them and not as they are today.• Farther we “look” into space, the farther back in time we see.Farther we “look” into space, the farther back in time we see.• The farthest galaxies seen are 13 billion light-years away – we The farthest galaxies seen are 13 billion light-years away – we

see them as they were when the Universe was only about 1 see them as they were when the Universe was only about 1 billion years old.billion years old.

• The most distant galaxies show The most distant galaxies show starlightstarlight..• Stars already forming 13 billion years ago – similar in age to Stars already forming 13 billion years ago – similar in age to

oldest Milky Way stars.oldest Milky Way stars.• Therefore, many or most galaxies must have started forming Therefore, many or most galaxies must have started forming

13 billion years ago.13 billion years ago.



Looking Back Through TimeLooking Back Through Time

• Look-Back Time Revisited Look-Back Time Revisited (continued)(continued) • By looking at galaxies at distances closer and closer to By looking at galaxies at distances closer and closer to

us, we can create an “album” of galaxies at various us, we can create an “album” of galaxies at various stages of development.stages of development.

• The general picture we obtain is one with The general picture we obtain is one with young young galaxiesgalaxies forming within the first 3 billion years of the forming within the first 3 billion years of the birth of the Universe, and then the three major galactic birth of the Universe, and then the three major galactic types appearing and evolving after that.types appearing and evolving after that.

• Because of the lack of starlight within the first 1 billion Because of the lack of starlight within the first 1 billion years, galactic formation and evolution relies on years, galactic formation and evolution relies on theoretical modeling.theoretical modeling.



Galaxy FormationGalaxy Formation

• Most Successful Models Assume: Most Successful Models Assume: • Hydrogen and helium filled all space within 1 million Hydrogen and helium filled all space within 1 million

years of the Universe’s birth.years of the Universe’s birth.• There were slight density variations in the H/He gas There were slight density variations in the H/He gas

distribution.distribution.• Initial Progression of FormationInitial Progression of Formation

• Denser regions initially expanded with Universe, but Denser regions initially expanded with Universe, but within 1 billion years, these regions began to contract within 1 billion years, these regions began to contract into into protogalactic cloudsprotogalactic clouds..

• Many galaxies form from the merger of several Many galaxies form from the merger of several protogalactic clouds. protogalactic clouds.



Galaxy FormationGalaxy Formation

• Initial Progression of Formation - Spirals Initial Progression of Formation - Spirals • Gas cools by radiating away thermal energyGas cools by radiating away thermal energy• First generation stars form in densest, coldest regions, First generation stars form in densest, coldest regions,

are very massive, and die within a few million yearsare very massive, and die within a few million years• Supernovae of first generation stars begin the Supernovae of first generation stars begin the

“polluting” of the interstellar gas with heavy elements.“polluting” of the interstellar gas with heavy elements.• Heating of initial crop of stars (the spheroidal Heating of initial crop of stars (the spheroidal

population) slows star formation process enough to population) slows star formation process enough to allow remaining gas to settle into rotating disk.allow remaining gas to settle into rotating disk.

• Disc population of stars are then formed.Disc population of stars are then formed.• But what about the ellipticals and irregulars?But what about the ellipticals and irregulars?



The Lives of GalaxiesThe Lives of Galaxies

• Different galaxy types appear to form through two distinct Different galaxy types appear to form through two distinct ways. ways. • Slight differences in protogalactic phaseSlight differences in protogalactic phase

• The higher the angular momentum and the lower the gas density in The higher the angular momentum and the lower the gas density in the protogalactic cloud, the more likely the protogalaxy will evolve the protogalactic cloud, the more likely the protogalaxy will evolve into a spiral; otherwise the protogalaxy will become an elliptical.into a spiral; otherwise the protogalaxy will become an elliptical.

• Evidence for this possibility comes from the fact that giant elliptical Evidence for this possibility comes from the fact that giant elliptical galaxies are seen in the early Universe.galaxies are seen in the early Universe.

• Interactions with other galaxies with similar initial conditionsInteractions with other galaxies with similar initial conditions• Elliptical galaxies dominate the central regions of rich clusters.Elliptical galaxies dominate the central regions of rich clusters.• Elliptical galaxy structural details can only be explained by collisions.Elliptical galaxy structural details can only be explained by collisions.• Central dominant galaxiesCentral dominant galaxies and and galactic cannibalismgalactic cannibalism..• Central regions of hot gas in rich clusters and lenticular galaxies.Central regions of hot gas in rich clusters and lenticular galaxies.• Irregulars may in part be explained as galaxies undergoing some sort Irregulars may in part be explained as galaxies undergoing some sort

of disruptive behavior.of disruptive behavior.



What are Starbursts?What are Starbursts?

• Typical Stellar Formation RatesTypical Stellar Formation Rates• Spiral galaxies from stars in disk at steady and slow rate.Spiral galaxies from stars in disk at steady and slow rate.• All stars in elliptical galaxies formed in the distant past.All stars in elliptical galaxies formed in the distant past.

• A Small Percentage of present-day galaxies are forming A Small Percentage of present-day galaxies are forming stars at a prodigious rate.stars at a prodigious rate.• These galaxies are called These galaxies are called starburst galaxiesstarburst galaxies..• Starburst activity must be a temporary condition since the rate of Starburst activity must be a temporary condition since the rate of

star formation can completely consume a galaxy’s interstellar gas star formation can completely consume a galaxy’s interstellar gas in just a few million years.in just a few million years.

• Many galaxies appear to have gone through a starburst phase more Many galaxies appear to have gone through a starburst phase more than once.than once.

• Starburst galaxies return to previous spiral, elliptical, or irregular Starburst galaxies return to previous spiral, elliptical, or irregular state.state.


Galaxy EvolutionGalaxy EvolutionMore on Starburst GalaxiesMore on Starburst Galaxies

• Some Starburst Galaxy Observations Some Starburst Galaxy Observations • Because of the presence of interstellar molecular clouds and dust, Because of the presence of interstellar molecular clouds and dust,

starburst galaxies are detected mainly in the infrared and not in the starburst galaxies are detected mainly in the infrared and not in the visible and UV.visible and UV.

• The luminosity in the far-infrared can exceed the luminosity in the The luminosity in the far-infrared can exceed the luminosity in the visible/near-infrared by as much as 100 times.visible/near-infrared by as much as 100 times.

• The large rate of star production also produces a large rate of The large rate of star production also produces a large rate of supernovae, which leads to the creation of supersonically supernovae, which leads to the creation of supersonically expanding expanding superbubblessuperbubbles (regions of hot gas) that burst into (regions of hot gas) that burst into intergalactic spaceintergalactic space creating a creating a galactic windgalactic wind..

• Galactic winds consist of low-density, but extremely hot gas (10-Galactic winds consist of low-density, but extremely hot gas (10-100 million K) which are detected via their x-ray emissions.100 million K) which are detected via their x-ray emissions.

• Small starburst galaxies can drive out much of their gas content, Small starburst galaxies can drive out much of their gas content, shutting down star production for billions of years.shutting down star production for billions of years.



Generating Starburst ActivityGenerating Starburst Activity

• The Most Luminous Starburst GalaxiesThe Most Luminous Starburst Galaxies• Many of these galaxies appear violently disturbed.Many of these galaxies appear violently disturbed.• Collisions between galaxies appears to initiate the starburst Collisions between galaxies appears to initiate the starburst

activity.activity.• Starbursts of this magnitude explain why elliptical galaxies lack Starbursts of this magnitude explain why elliptical galaxies lack

young stars and cool gas.young stars and cool gas.• Starburst uses up cool gas and galactic winds blow away the rest.Starburst uses up cool gas and galactic winds blow away the rest.• Massive stars die out quickly after starburst outbreak.Massive stars die out quickly after starburst outbreak.• When collision ends and the merger is complete, an elliptical galaxy When collision ends and the merger is complete, an elliptical galaxy

remains.remains.

• Cause of smaller-scale starbursts is unclearCause of smaller-scale starbursts is unclear• Some small irregular galaxies are undergoing starbursts in the Some small irregular galaxies are undergoing starbursts in the

absence of a collision.absence of a collision.• ““Close encounters” have been suggested as the cause.Close encounters” have been suggested as the cause.



Quasars & Other Active Galactic NucleiQuasars & Other Active Galactic Nuclei

• Galaxies with Highly Luminous Centers Galaxies with Highly Luminous Centers • The centers of galaxies which exhibit extreme The centers of galaxies which exhibit extreme

luminosities and sometimes very powerful jets are luminosities and sometimes very powerful jets are called called active galactic nucleiactive galactic nuclei..

• The very brightest active galactic nuclei are The very brightest active galactic nuclei are quasarsquasars, , the most powerful of which may emit more light than the most powerful of which may emit more light than 1,000 Milky Way galaxies.1,000 Milky Way galaxies.

• Quasars are found primarily at very far distances Quasars are found primarily at very far distances suggesting that quasars may represent an early stage in suggesting that quasars may represent an early stage in a galaxy’s evolution.a galaxy’s evolution.



What are Quasars?What are Quasars?

• A Brief History of QuasarsA Brief History of Quasars• In early 1960s, Maarten Schmidt discovered that the hydrogen In early 1960s, Maarten Schmidt discovered that the hydrogen

emission lines in the visible spectrum of the optical component of emission lines in the visible spectrum of the optical component of radio source 3C 273 was highly redshifted (about 80 nm), which radio source 3C 273 was highly redshifted (about 80 nm), which implied a recessional speed of 17% of the speed of light.implied a recessional speed of 17% of the speed of light.

• Assuming the redshift was cosmological (which was debatable for Assuming the redshift was cosmological (which was debatable for many years), the Hubble Law and the measured apparent many years), the Hubble Law and the measured apparent brightness gave a luminosity for 3C 273 of over ten trillion Suns - brightness gave a luminosity for 3C 273 of over ten trillion Suns - or more than 100 times more luminous than the entire Milky Way.or more than 100 times more luminous than the entire Milky Way.

• Since the first of these objects were strong radio sources and star-Since the first of these objects were strong radio sources and star-like in the visible, they were named quasars (like in the visible, they were named quasars (quasi-stellar radio quasi-stellar radio sourcessources).).



More Quasar CharacteristicsMore Quasar Characteristics

• 40 Years of Data and Analysis 40 Years of Data and Analysis • Most quasars lie more than halfway to the cosmological Most quasars lie more than halfway to the cosmological

horizon – light from most distant quasar emitted when horizon – light from most distant quasar emitted when Universe was less than 1 billion years old.Universe was less than 1 billion years old.

• Initially hard to determine, Quasars lie in the centers of Initially hard to determine, Quasars lie in the centers of galaxies.galaxies.

• Emit across an unusually broad range of the spectrum.Emit across an unusually broad range of the spectrum.• Power output about the same from infrared to gamma rays Power output about the same from infrared to gamma rays

implying a multi-temperature construct.implying a multi-temperature construct.

• Produce strong emission lines.Produce strong emission lines.



The Other Active Galactic NucleiThe Other Active Galactic Nuclei

• Nearby Active Galactic NucleiNearby Active Galactic Nuclei• About 1% of nearby galaxies (often called Seyfert About 1% of nearby galaxies (often called Seyfert

galaxies) have active galactic nuclei that look like galaxies) have active galactic nuclei that look like quasars with respect to their spectrum distribution, but quasars with respect to their spectrum distribution, but are much less luminous than quasars.are much less luminous than quasars.

• Due to their close range, the sizes of the active nuclei Due to their close range, the sizes of the active nuclei can be more readily determined.can be more readily determined.

• Visible images indicate a region of less than 100 ly.Visible images indicate a region of less than 100 ly.• Radio interferometry places the size at less than 3 ly.Radio interferometry places the size at less than 3 ly.• Rapid variations in luminosities lead to active nuclei sizes not Rapid variations in luminosities lead to active nuclei sizes not

much bigger than the Solar System – a size that is believed to much bigger than the Solar System – a size that is believed to be typical of all active galactic nuclei including quasars.be typical of all active galactic nuclei including quasars.



The Other Active Galactic NucleiThe Other Active Galactic Nuclei

• Radio Galaxies and JetsRadio Galaxies and Jets• Radio galaxiesRadio galaxies were discovered in the 1950s were discovered in the 1950s• Emit unusually strong radio waves from pairs of huge Emit unusually strong radio waves from pairs of huge radio lobesradio lobes, one , one

on either side of the galaxy, which is typically an elliptical galaxy.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 light-At the center of a radio galaxy is an active region only a few light-

years across.years across.• Plasma is seen shooting out of the active nucleus in Plasma is seen shooting out of the active nucleus in jetsjets and at speeds and at speeds

near that of light.near that of light.• At the end of the jets are the lobes, at distances as large as one million At the end of the jets are the lobes, at distances as large as one million

light-years from the center.light-years from the center.• The radio lobes are the result of the jets ramming into the intergalactic The radio lobes are the result of the jets ramming into the intergalactic

gas, heating, and then spreading out.gas, heating, and then spreading out.• Many quasars also have jets and radio lobes.Many quasars also have jets and radio lobes.• Many active nuclei of radio galaxies are obscured by donut-shaped Many active nuclei of radio galaxies are obscured by donut-shaped

rings of dark molecular clouds.rings of dark molecular clouds.



Active Galactic Nuclei Power SourceActive Galactic Nuclei Power Source

• Energy Source for Active Galactic NucleiEnergy Source for Active Galactic Nuclei• Only one possibility fills the job: Matter falling into a Only one possibility fills the job: Matter falling into a

supermassive black hole.supermassive black hole.• Gravitational potential energy of matter is converted into kinetic Gravitational potential energy of matter is converted into kinetic

energy which then heats the gas.energy which then heats the gas.• The heat then generates the emission of intense radiation.The heat then generates the emission of intense radiation.

• For a quasar to be as luminous as it is observed to be, the For a quasar to be as luminous as it is observed to be, the supermassive black hole at its center must consume one one-solar-supermassive black hole at its center must consume one one-solar-mass star per year.mass star per year.

• The broad spectrum of an active galactic nucleus is the result of The broad spectrum of an active galactic nucleus is the result of the varied temperature structure in and around the disc.the varied temperature structure in and around the disc.

• Hot gas in and above disc give x-rays and UV.Hot gas in and above disc give x-rays and UV.• Radiation ionizes surrounding interstellar gas leading to visible light.Radiation ionizes surrounding interstellar gas leading to visible light.• Dust grains encircling nucleus in molecular clouds emit infrared.Dust grains encircling nucleus in molecular clouds emit infrared.• Fast moving electrons give radio emissions.Fast moving electrons give radio emissions.



Active Galactic Nuclei Power SourceActive Galactic Nuclei Power Source

• Energy Source for Active Galactic Nuclei Energy Source for Active Galactic Nuclei (continued)(continued)

• Jets are believed to be created by magnetic fields Jets are believed to be created by magnetic fields embedded in ionized accretion disc.embedded in ionized accretion disc.

• These magnetic fields get twisted around the accretion disc These magnetic fields get twisted around the accretion disc spin axis.spin axis.

• Ionized gas then flows along magnetic field lines.Ionized gas then flows along magnetic field lines.

• Unsolved mysteriesUnsolved mysteries• Why do quasars run out of gas?Why do quasars run out of gas?

• How did supermassive black holes form in first place?How did supermassive black holes form in first place?



More on Supermassive Black HolesMore on Supermassive Black Holes

• Evidence for the Existence of Supermassive Black Evidence for the Existence of Supermassive Black HolesHoles• Most convincing evidence is the speed at which gas Most convincing evidence is the speed at which gas

rotates in a very small region in the center of the galaxy rotates in a very small region in the center of the galaxy (e.g., data from M87).(e.g., data from M87).

• Similar results from Similar results from masermaser emissions in NGC 4258 emissions in NGC 4258 indicate the existence of a 36 million solar mass object indicate the existence of a 36 million solar mass object inside a radius of 1 ly.inside a radius of 1 ly.

• The relationship between a galaxy’s central bulge mass The relationship between a galaxy’s central bulge mass (M(Mcbcb) and its central black hole mass (M) and its central black hole mass (Mbhbh) (M) (Mcbcb is about is about 500M500Mbhbh) for a large variety of galaxies, indicates that ) for a large variety of galaxies, indicates that the growth of the central black hole must be closely the growth of the central black hole must be closely linked to the process of galaxy formation.linked to the process of galaxy formation.



Quasars as an Investigative ToolQuasars as an Investigative Tool

• Using Quasar Spectra to Decipher Evolutionary Trends in Using Quasar Spectra to Decipher Evolutionary Trends in Intergalactic CloudsIntergalactic Clouds• Most distant quasars are at the fringes of the cosmological horizon.Most distant quasars are at the fringes of the cosmological horizon.• The light from these quasars will pass through several intergalactic The light from these quasars will pass through several intergalactic

clouds.clouds.• Due to each cloud’s different recessional speed, each cloud will Due to each cloud’s different recessional speed, each cloud will

etch its own unique spectral signature into the quasar’s spectrum.etch its own unique spectral signature into the quasar’s spectrum.• Studies of the absorption lines should allow us to determine what Studies of the absorption lines should allow us to determine what

happened in protogalactic clouds during each epoch of galaxy happened in protogalactic clouds during each epoch of galaxy formation. Early results:formation. Early results:

• More mass in gas phase in early Universe than now.More mass in gas phase in early Universe than now.• The percentage of heavy element content increases with the age of the The percentage of heavy element content increases with the age of the

Universe.Universe.

© sierra college astronomy department1 galaxy evolution

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