Chapter 21:Chapter 21:Galaxy EvolutionGalaxy Evolution

Galaxy Evolution…Galaxy Evolution………is the study of how galaxies form and how they change over time.is the study of how galaxies form and how they change over time.

As was the case with stars…As was the case with stars…• we can not observe an individual galaxy evolve we can not observe an individual galaxy evolve • but we can observe different galaxies at various stages of their life cyclesbut we can observe different galaxies at various stages of their life cycles

This is made easier by This is made easier by virtue of lookback virtue of lookback time.time.

We can plot a We can plot a ““family family albumalbum”” for each type for each type of galaxy.of galaxy.

The greater the redshift...The greater the redshift...• the younger the the younger the

galaxy!galaxy!

Modeling Galaxy FormationModeling Galaxy FormationWith our current telescope technology…With our current telescope technology…

• we are unable to see back to the time when galaxies first formedwe are unable to see back to the time when galaxies first formed• we must rely on theoretical (computer) models to describe how we must rely on theoretical (computer) models to describe how

galaxies formedgalaxies formed

The following assumptions are made when constructing The following assumptions are made when constructing these models:these models:1.1. the Universe was uniformly filled with Hydrogen & Helium gas for the Universe was uniformly filled with Hydrogen & Helium gas for

the first million years after the Big Bangthe first million years after the Big Bang

2.2. this uniformity was not quite perfect; some regions of the this uniformity was not quite perfect; some regions of the Universe were slightly denser than othersUniverse were slightly denser than others

All of the H & He gas expanded with the Universe at first.All of the H & He gas expanded with the Universe at first.• after about a billion years, the denser regions slowed down and after about a billion years, the denser regions slowed down and

began to collapse under self-gravitybegan to collapse under self-gravity• the collapsing gas became the collapsing gas became protogalactic cloudsprotogalactic clouds

How do we study galaxy formation?How do we study galaxy formation?

Modeling Galaxy FormationModeling Galaxy FormationAs a protogalactic cloud collapses, at first it radiates away its gravitational As a protogalactic cloud collapses, at first it radiates away its gravitational

potential energy.potential energy.• it gets colderit gets colder• stars begin to form in the coldest, molecular cloud coresstars begin to form in the coldest, molecular cloud cores• same physics as when ionized and atomic ISM condenses into molecular same physics as when ionized and atomic ISM condenses into molecular

clouds and forms star in the star-gas-star cycle of the Milky Wayclouds and forms star in the star-gas-star cycle of the Milky Way

Next clue comes from galaxy colorsNext clue comes from galaxy colors• the spheroidal component is the spheroidal component is

redred• the disk component is the disk component is

blue/whiteblue/white

Conservation of angular momentumConservation of angular momentum• caused remaining gas to rotate caused remaining gas to rotate

faster and flatten…star faster and flatten…star formation continues in diskformation continues in disk

• with no gas left in the spheroid, with no gas left in the spheroid, no new stars are formed and no new stars are formed and only old, red stars remainonly old, red stars remain

Modeling Galaxy FormationModeling Galaxy Formation

The Milky Way Provides CluesThe Milky Way Provides CluesStudying the halo stars in our own Milky Way provides data for our models.Studying the halo stars in our own Milky Way provides data for our models.

• random orientation of stellar obits implies that halo stars formed before the random orientation of stellar obits implies that halo stars formed before the protogalactic cloud collapsed into a diskprotogalactic cloud collapsed into a disk

stars which formed in the disk orbit on the same plane in the same directionstars which formed in the disk orbit on the same plane in the same direction• low content of heavy elements in halo stars implies that they formed before the star–low content of heavy elements in halo stars implies that they formed before the star–

gas– star cycle could significantly enrich the ISMgas– star cycle could significantly enrich the ISM

However, heavy-element content of halo stars does not simply decrease with However, heavy-element content of halo stars does not simply decrease with distance from the Galactic center.distance from the Galactic center.

• implies that halo star formation did implies that halo star formation did notnot begin in a single protogalactic cloud begin in a single protogalactic cloud

There must have been a few smaller There must have been a few smaller clouds where…clouds where…

• stars had already begun to formstars had already begun to form• they collided to form a larger they collided to form a larger

protogalactic cloudprotogalactic cloud

Remaining key questions:Remaining key questions:• where is the very first star generation?where is the very first star generation?• what caused the density enhancements what caused the density enhancements

in the early Universe? in the early Universe?

What Determines Galaxy Type? What Determines Galaxy Type? We can explore two options:We can explore two options:

• the initial conditions of the protogalactic cloud; the initial conditions of the protogalactic cloud; i.e. destined from i.e. destined from birthbirth

• later interactions with other galaxies; later interactions with other galaxies; i.e. a life-altering conversioni.e. a life-altering conversion

Two plausible explanations regarding the birth properties of the Two plausible explanations regarding the birth properties of the protogalactic cloud:protogalactic cloud:• Protogalactic spinProtogalactic spin…the initial angular momentum determines how …the initial angular momentum determines how

fast the cloud will form a disk before it is completely turned into stars fast the cloud will form a disk before it is completely turned into stars • Protogalactic coolingProtogalactic cooling…the initial density determines how fast the …the initial density determines how fast the

cloud can form stars before it collapses into a disk cloud can form stars before it collapses into a disk

What Determines Galaxy Type?What Determines Galaxy Type?This giant elliptical provides evidence for the This giant elliptical provides evidence for the

protogalactic cooling explanation. protogalactic cooling explanation. • it is very distant (young) and very red, even it is very distant (young) and very red, even

accounting for redshiftaccounting for redshift• white and blue stars are missingwhite and blue stars are missing• star formation has ceased very early in the star formation has ceased very early in the

galaxygalaxy’’s historys history• no gas will be left to form a diskno gas will be left to form a disk

Galaxy InteractionsGalaxy Interactions• when two spiral galaxies collidewhen two spiral galaxies collide• tidal forces randomize the orbits of starstidal forces randomize the orbits of stars• gas either falls to the center to form gas either falls to the center to form

starsstars• or it is stripped out of the galaxiesor it is stripped out of the galaxies• the disk is removedthe disk is removed

The galaxy becomes an elliptical.The galaxy becomes an elliptical.

Conditions in protogalactic cloudsConditions in protogalactic clouds

Spin:Spin: The initial angular momentum of the The initial angular momentum of the protogalactic cloud could determine the size of protogalactic cloud could determine the size of the resulting disk.the resulting disk.

Conditions in protogalactic cloudsConditions in protogalactic clouds

Density:Density: Elliptical galaxies could come from Elliptical galaxies could come from dense protogalactic clouds that were able to dense protogalactic clouds that were able to cool and form stars before gas settled into a cool and form stars before gas settled into a disk.disk.

We must also consider the effects of collisions.We must also consider the effects of collisions.

The Role of Galaxy ClustersThe Role of Galaxy ClustersGalaxy clusters provide evidence that some galaxies are shaped by Galaxy clusters provide evidence that some galaxies are shaped by

interactions:interactions:• elliptical galaxies are more common in cluster centerselliptical galaxies are more common in cluster centers• collisions will occur more often in crowded cluster centerscollisions will occur more often in crowded cluster centers• central dominant (CD) galaxiescentral dominant (CD) galaxies are gigantic ellipticals found in cluster centers are gigantic ellipticals found in cluster centers• they grow large by consuming other galaxies they grow large by consuming other galaxies

These CD galaxies often contain tightly These CD galaxies often contain tightly bound clumps of stars.bound clumps of stars.

They are probably the leftover cores of They are probably the leftover cores of galaxies which were galaxies which were cannibalizedcannibalized by the by the CD.CD.

Some CD galaxies are more than 10 times Some CD galaxies are more than 10 times as massive as the Milky Way.as massive as the Milky Way.

• making them the largest galaxies in the making them the largest galaxies in the Universe!Universe!

Starburst GalaxiesStarburst Galaxies

An average of 1 new star per year An average of 1 new star per year forms in the Milky Way.forms in the Milky Way.

We observe some galaxies with a We observe some galaxies with a star-forming rate of 100 per yr.star-forming rate of 100 per yr.

We call them We call them starburst galaxiesstarburst galaxies..• infrared image of Arp 220infrared image of Arp 220

They look normal in visible light (10They look normal in visible light (101010 L L like Milky Way). like Milky Way).• but they are 100 times brighter in infrared lightbut they are 100 times brighter in infrared light• molecular clouds block the visible/UV light from new starsmolecular clouds block the visible/UV light from new stars• dust in the clouds absorbs this light and reemits the energy as infrared lightdust in the clouds absorbs this light and reemits the energy as infrared light

With such a fast rate of star formation, the galaxy will use up its gas..With such a fast rate of star formation, the galaxy will use up its gas..• in only a few 100 million yearsin only a few 100 million years• starburst phase is temporary in light of fact that galaxy is billions of years oldstarburst phase is temporary in light of fact that galaxy is billions of years old

Starburst GalaxiesStarburst Galaxies

100 times star-forming rate also 100 times star-forming rate also means 100 times supernova rate.means 100 times supernova rate.• ISM is full of hot superbubblesISM is full of hot superbubbles• supernovae continue to pump energy supernovae continue to pump energy

into the superbubblesinto the superbubbles

The hot (10The hot (1077–10–1088 K) gas breaks out K) gas breaks out• and a and a galactic windgalactic wind streams from streams from

galaxygalaxy• NGC 1569 (X-ray–green; visible–red) NGC 1569 (X-ray–green; visible–red)

Starburst galaxies are irregular in type.Starburst galaxies are irregular in type.• lots of dusty molecular clouds and usually two distinct clumps of starslots of dusty molecular clouds and usually two distinct clumps of stars

This suggests that the starburst is caused by the collision of two This suggests that the starburst is caused by the collision of two spiral galaxies.spiral galaxies.• although a close encounter could trigger starburst, e.g. Large Magellanic although a close encounter could trigger starburst, e.g. Large Magellanic

CloudCloud

QuasarsQuasars• In the early 1960s, Maarten Schmidt identified the radio In the early 1960s, Maarten Schmidt identified the radio

source 3C 273 with a faint, blue star.source 3C 273 with a faint, blue star.• the the ““starstar’’ss”” spectrum displayed emission lines spectrum displayed emission lines • the wavelengths of these lines matched no know element the wavelengths of these lines matched no know element

• Schmidt realized that the emission lines belonged to Schmidt realized that the emission lines belonged to Hydrogen, but they were highly redshifted.Hydrogen, but they were highly redshifted.

• This object is very (> 10This object is very (> 101010 light years) far away. light years) far away.• other such objects were subsequently discoveredother such objects were subsequently discovered• they were called they were called quasi-stellar radio sourcesquasi-stellar radio sources or or quasarsquasars for short for short

• The farther away we look out in distance, the farther back The farther away we look out in distance, the farther back we look in we look in timetime!!

• Quasars exist only in the Quasars exist only in the earlyearly Universe! Universe!• LOOKBACK TIMELOOKBACK TIME

Quasar SpectraQuasar Spectra

Star-like objects which:Star-like objects which:• have spectra that look have spectra that look

nothing like a starnothing like a star• highly redshiftedhighly redshifted

• can be strong radio sourcescan be strong radio sources• turns out that 90% are turns out that 90% are notnot

• emit light at all wavelengthsemit light at all wavelengths

Quasars…Quasars…

• are extremely luminous. are extremely luminous. • 10104040 watts watts • 1,000 brighter than the entire Milky Way Galaxy1,000 brighter than the entire Milky Way Galaxy

• are extremely variable.are extremely variable.• luminosity changes < 1 hourluminosity changes < 1 hour• implies they have implies they have a very small sizea very small size

• have redshifted emission lines.have redshifted emission lines.• greatest is 6.8 times the rest wavelengthgreatest is 6.8 times the rest wavelength

• have absorption lines at lower redshifts.have absorption lines at lower redshifts.• from gas clouds & galaxies between us and the from gas clouds & galaxies between us and the

quasarquasar

Hubble ST shows us that quasars do live in Hubble ST shows us that quasars do live in galaxies…they are Active Galactic Nuclei!galaxies…they are Active Galactic Nuclei!

Active Galactic NucleiActive Galactic Nuclei

• Seyfert GalaxiesSeyfert Galaxies• spiral galaxies with an spiral galaxies with an

incredibly bright, star-like incredibly bright, star-like center (nucleus)center (nucleus)

• they are very bright in the they are very bright in the infraredinfrared

• their spectra show strong their spectra show strong emissionemission lines lines

The luminosity can vary by as much as the entire brightness of the Milky Way Galaxy!!

Circinus

Active Galactic NucleiActive Galactic NucleiRadio GalaxiesRadio Galaxies

• galaxies which emit large amounts of radio wavesgalaxies which emit large amounts of radio waves• the radio emission come from the radio emission come from lobeslobes on either side of on either side of

the galaxy; the galaxy; notnot the galaxy itself the galaxy itselfCygnus A

X-ray/Radio Image of Centaurus AX-ray/Radio Image of Centaurus A

X-ray is blue; radio is red

Radio Galaxy LobesRadio Galaxy Lobes

NGC 1265

These lobes are swept back because the galaxy is moving through an intergalactic medium.

Galaxy (which is actually

quite large)

Intergalactic gas jet

Giant Gas Clouds

(surrounding the galaxy)

Active Galactic NucleiActive Galactic Nuclei Jets of matter are shooting out from these galaxies and Jets of matter are shooting out from these galaxies and

emitting radio waves, but the matter is emitting radio waves, but the matter is notnot cold! cold! Synchotron emission --- non-thermal process where light Synchotron emission --- non-thermal process where light

is emitted by charged particles moving close to the speed is emitted by charged particles moving close to the speed of light around magnetic fields.of light around magnetic fields.

M 87

What powers these Active Galactic Nuclei?What powers these Active Galactic Nuclei?

Hubble Space Telescope gave us a clue

NGC 4261

Active Galactic NucleiActive Galactic Nuclei• The energy is generated from matter falling onto a The energy is generated from matter falling onto a

supermassive black holesupermassive black hole……• 1.2 x 101.2 x 1099 M M for NGC 4261 for NGC 4261

• 3 x 103 x 1099 M M for M87 for M87

• ……which is at the center (nucleus) of the galaxy.which is at the center (nucleus) of the galaxy.

Matter swirls through an accretion Matter swirls through an accretion disk before crossing over the disk before crossing over the event horizon.event horizon.

Gravitational pot. energy lostGravitational pot. energy lost• = mc= mc22 the mass energy the mass energy• 10 – 40% of this is radiated away10 – 40% of this is radiated away

Process is very efficient for Process is very efficient for generating energy.generating energy.

Jets of matter ejected from around a black hole Jets of matter ejected from around a black hole may explain quasars and active galaxiesmay explain quasars and active galaxies

Active Galactic NucleiActive Galactic NucleiFormation of the JetsFormation of the Jets

• magnetic fields in accretion disks are magnetic fields in accretion disks are twistedtwisted

• they pull charged particles out of the disk they pull charged particles out of the disk and accelerate them like a slingshotand accelerate them like a slingshot

• particles bound to magnetic field; particles bound to magnetic field; focused in a beam focused in a beam

Orientation of beam determines what we Orientation of beam determines what we see:see:

• if beams points at us, we see a quasarif beams points at us, we see a quasar• if not, the molecular clouds/dust of the if not, the molecular clouds/dust of the

galaxy block our view of the nucleusgalaxy block our view of the nucleus• so we see a radio galaxyso we see a radio galaxy• lobes are where jets impact intergalactic lobes are where jets impact intergalactic

mediummedium

Jets of matter ejected from around a black hole Jets of matter ejected from around a black hole may explain quasars and active galaxiesmay explain quasars and active galaxies

From where you observe it might make all the From where you observe it might make all the difference ...difference ...

Active Galactic Nucleus AnimationActive Galactic Nucleus Animation

Quasars are observed in the distant past (high redshift).Quasars are observed in the distant past (high redshift).• this implies that many galaxies had bright nuclei early in their histories, but this implies that many galaxies had bright nuclei early in their histories, but

those nuclei have since gone dormantthose nuclei have since gone dormant So many galaxies which look So many galaxies which look ““normalnormal”” today have supermassive black today have supermassive black

holes at their centers.holes at their centers.• such as such as AndromedaAndromeda and Milky Way? and Milky Way?

A A ““ForestForest”” of Absorption Lines of Absorption LinesAs light from a quasar travels toward Earth…As light from a quasar travels toward Earth…

• it passes through intergalactic Hydrogen clouds and galaxiesit passes through intergalactic Hydrogen clouds and galaxies• each cloud leaves absorption lines at a each cloud leaves absorption lines at a differentdifferent redshift on quasar spectrum redshift on quasar spectrum• this is the only way we can this is the only way we can ““observeobserve”” protogalactic clouds protogalactic clouds

Analysis so far has shown:Analysis so far has shown:• H lines at high redshift are H lines at high redshift are

broader than those at lowbroader than those at low• implies that the gas content implies that the gas content

of clouds/galaxies is higher of clouds/galaxies is higher in the early Universein the early Universe

• more heavy element lines more heavy element lines are seen at low redshiftare seen at low redshift

• supports element enrichment supports element enrichment of galaxies by supernovaeof galaxies by supernovae

These data support our models These data support our models of galaxy evolutionof galaxy evolution