Galaxies and the Foundation of Modern Cosmology

Islands of Stars

Our goals for learning:• How are the lives of galaxies connected

with the history of the universe?• What are the three major types of galaxies?• How are galaxies grouped together?

Recall: The Sun is only one star among well over 100 billion stars in our own galaxy, the Milky Way, which has the form of a highly flattened “disk” of stars some 100,000 light-years in diameter. Galaxies are the “building blocks” of the Universe.

Andromeda Galaxy

• https://www.youtube.com/watch?v=udAL48P5NJU

How are the lives of galaxies connected with the history of the universe?

• Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away

This is the Hubble Deep Field, a small, apparently blank, piece of sky near the Big Dipper and well out of the Galactic Plane.

Big Dipper

Hubble Space Telescope image

Galaxies and Cosmology• A galaxy’s age, its

distance, and the age of the universe are all closely related

• The study of galaxies is thus intimately connected with cosmology—the study of the structure and evolution of the universe

Hubble Ultra Deep Field

What are the three major types of galaxies?

Hubble Ultra Deep Field

Hubble Ultra Deep Field

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SPIRAL GALAXY

Spheroidal Component:bulge & halo, old stars,few gas clouds

Disk Component:stars of all ages,many gas clouds

Another view of a spiral galaxy

Disk Component:stars of all ages,many gas clouds

Spheroidal Component:bulge & halo, old stars,few gas clouds

Thought Question

Why does ongoing star formation lead to a blue-white appearance?

A. There aren’t any red or yellow starsB. Short-lived blue stars outshine othersC. Gas in the disk scatters blue light

Thought Question

Why does ongoing star formation lead to a blue-white appearance?

A. There aren’t any red or yellow starsB. Short-lived blue stars outshine othersC. Gas in the disk scatters blue light

Barred Spiral Galaxy: Has a bar of stars across the bulge

Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical)

Elliptical Galaxy: All spheroidal component, virtually no disk component

Red-yellow color indicates older star population

Irregular Galaxy

Blue-white color indicates ongoing star formation

Edwin Hubble’s galaxy classes

SpheroidDominates

Disk Dominates

Spiral galaxies are often found in groups of galaxies

(up to a few dozen galaxies)

How are galaxies grouped together?

Elliptical galaxies are much more common in huge clusters of galaxies

(hundreds to thousands of galaxies)

What have we learned?

• How are the lives of galaxies connected with the history of the universe?– Galaxies generally formed when the universe

was young and have aged along with the universe

• What are the three major types of galaxies?– Spiral galaxies, elliptical galaxies, and

irregular galaxies– Spirals have both disk and spheroidal

components; ellipticals have no disk

What have we learned?

• How are galaxies grouped together?– Spiral galaxies tend to collect into groups of

up to a few dozen galaxies– Elliptical galaxies are more common in large

clusters containing hundreds to thousands of galaxies

Measuring Galactic Distances

Our goals for learning:• How do we measure the distances to

galaxies?

Recall:Brightness alone does not provide enough information to measure distance.

We must know the luminosity (power output) of the object.

Step 1

Determine size of solar system using radar.

Distances to astronomical objects can be determined using a series of over-lapping steps in what is called the DISTANCE LADDER.

Step 2

Determine distances of stars out to a few hundred light-years using parallax (trigonometry)

Distance in parsecs = 1/parallax angle in seconds of arc;1pc = 3.26 ly

p

dd = 1/p

Relationship between apparent brightness and luminosity depends on distance:

Luminosity Brightness = 4π (distance)2

We can determine a star’s distance if we know its luminosity and can measure its apparent brightness:

Luminosity Distance = 4π x Brightness

A standard candle is an object whose luminosity we can determine without measuring its distance

Step 3

The apparent brightness of a star cluster’s Main Sequence tells us its distance

Then, knowing a star cluster’s distance, we can determine the luminosity of each type of star within it.

Cepheid variable stars: are very luminous and can therefore be seen over great distances

These stars are excellent standard candles because of a relationship between their average luminosity and their period of variation, thus …

Step 4

Observe the variations in apparent magnitude of a Cepheid-type star and find its period. Read off the luminosity corresponding to this period.Calculate the distance.

Cepheid variable stars with longer periods have greater luminosities. Edwin Hubble found Cepheid’s beyond the Milky Way in nearby galaxies. The Hubble Space Telescope Key Project extended this to even more distant galaxies.

White-dwarf supernovae can also be used as standard candles.

Incredibly luminous!

Implies very large distances

These supernovae occur when a white dwarf in a binary system reaches the 1.4 MSun limit – same mass for all, same Luminosity.

Step 5

Apparent brightness of white-dwarf supernova tells us the distance to its host galaxy. This technique works for up to 10 billion light-years!

Tully-Fisher Relation

Entire galaxies can also be used as standard candles because galaxy luminosity is related to rotation speed

We measure galaxy distances using a chain of interdependent techniques

THE DISTANCE LADDER

What have we learned?

• How do we measure the distances to galaxies?– The distance-measurement chain begins with

parallax measurements that build on radar ranging in our solar system

– Using parallax and the relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles

– We can measure distances greater than 10 billion light years using white dwarf supernovae as standard candles

Hubble’s Law

Our goals for learning:• How did Hubble prove that galaxies lie far beyond

the Milky Way?• What is Hubble’s Law?• How do distance measurements tell us the age of

the universe?• How does the universe’s expansion affect our

distance measurements?

How did Hubble prove that galaxies lie far beyond the Milky Way?

The Puzzle of “Spiral Nebulae”

• Before Edwin Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way

• Hubble settled the debate by measuring the distance to the Andromeda Galaxy (M31) using Cepheid variables as standard candles; ~2.5 million light-years

The spectral features of all galaxies beyond the Local Group are redshifted they are all moving away from us!

Hubble’s Law:By measuring distances to galaxies, Edwin Hubble found that redshift (the amount by which spectral lines were shifted) and distance are related in a special way.

The redshift gives a velocity.

Hubble’s Law: velocity = H0 x distance

Using galaxies of known distance (from Cepheid Variables) one gets the slope of the line (H-naught) , known as Hubble’s constant.

For an unknown distance, the spectrum redshift of a galaxy tells us its distance through Hubble’s Law:

distance = velocity

H0

Inverting Hubble’s Law

Distances of farthest galaxies are measured from redshifts

How do distance measurements tell us the age of the universe?

The more distant a galaxy, the faster it is receding from us.

Hubble interpreted this as evidence for expansion of the universe.

Thought Question

Your friend leaves your house. She later calls you on her cell phone, saying that she’s been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?

A. 1 minuteB. 30 minutesC. 60 minutesD. 120 minutes

Thought Question

Your friend leaves your house. She later calls you on her cell phone, saying that she’s been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?

A. 1 minuteB. 30 minutesC. 60 minutesD. 120 minutes

Does the fact that all galaxies are receding from us mean that we are at the center of an explosion?

Hubble’s law tells us that the Universe is expanding and that it has no center and no edge (as far as we can tell).

Interpreting the Hubble Law

One example of something that expands but has no center or edge is the surface of a balloon.

ANALOGY FOR THE EXPANSION OF THE UNIVERSE

Fixed size dots on the surface of a balloon.

Dots are galaxies held together by gravity.

Cosmological Principle

The universe looks about the same no matter where you are within it

• Matter is evenly distributed on very large scales in the universe

• No center & no edges• Not proved but consistent with all observations to

date

Thought Question

Your observe a galaxy moving away from you at 0.1 light-years per year, and it is now 1.4 billion light-years away from you. How long has it taken to get there?

A. 1 million yearsB. 14 million yearsC. 10 billion yearsD. 14 billion years

Thought Question

Your observe a galaxy moving away from you at 0.1 light-years per year, and it is now 1.4 billion light-years away from you. How long has it taken to get there?

A. 1 million yearsB. 14 million yearsC. 10 billion yearsD. 14 billion years (t = D/V = 1.4 billion ly divided by 0.1 ly per yr)

Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies

Age =

~ 1 / H0

Distance

Velocity

distance?

Distances between faraway galaxies change while light travels

Astronomers think in terms of lookback time rather than distance

How does the universe’s expansion affect our distance measurements?

Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time

Cosmological Redshift

Cosmological HorizonMaximum lookback time of about 14 billion years limits how far we can “see”

What have we learned?

• How did Hubble prove that galaxies lie far beyond the Milky Way?– He measured the distance to the Andromeda

galaxy using Cepheid variable stars as standard candles

• What is Hubble’s Law?– The faster a galaxy is moving away from us,

the greater its distance:

velocity = H0 x distance

What have we learned?

• How do distance measurements tell us the age of the universe?– Measuring a galaxy’s distance and speed

allows us to figure out how long the galaxy took to reach its current distance

– Measuring Hubble’s constant tells us that amount of time: about 14 billion years

• How does the universe’s expansion affect our distance measurements?– Lookback time is easier to define than

distance for objects whose distances grow while their light travels to Earth

Looking Back Through Time

Our goals for learning:• How do we observe the life histories of

galaxies?• How did galaxies form?

Galaxy Evolution

Deep images (meaning a long exposure to reveal faint objects) show us very distant galaxies

We see those galaxies as they were much earlier in time!

Old light from young galaxies

Observing galaxies at different distances shows how they age; this works because of the finite speed of light and the enormous distances.

We still can’t directly observe the earliest galaxies

By looking back in time we can begin to see how galaxies form

Our best models for galaxy formation assume:

• Matter originally filled all of space almost uniformly

• Gravity of denser regions pulled in surrounding matter - clumps

Denser regions contracted, forming protogalactic clouds

H and He gases in these clouds formed the very first stars

Supernova explosions from first stars kept much of the gas from forming stars

Leftover gas settled into spinning disk - Conservation of angular momentum

But why do some galaxies end up looking so different?M87NGC 4414

Look back at the Hubble Space Telescope pictures and we notice that the youngest galaxies are more irregular and closer together – galaxies merge and change over time

What have we learned?

• How do we observe the life histories of galaxies?– Deep observations of the universe are

showing us the history of galaxies because we are seeing galaxies as they were at different ages

• How did galaxies form?– Our best models for galaxy formation assume

that gravity made galaxies out of regions of the early universe that were slightly denser than their surroundings

The Lives of Galaxies

Our goals for learning:• Why do galaxies differ?• What are starbursts?

Why don’t all galaxies have similar disks of stars?

Edwin Hubble’s Tuning Fork Diagram

Spin: Initial angular momentum of protogalactic cloud could determine size of resulting disk

Conditions in Protogalactic Cloud?

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

Conditions in Protogalactic Cloud?

Distant Red Ellipticals• Observations of

some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe

• All the stars in this galaxy are old (red) and any young blue stars have died

We must also consider the effects of collisions

Collisions were much more likely early in time, because galaxies were closer together–remember space is expanding

Many of the galaxies we see at great distances (and early times) indeed look violently disturbed–evidence of merging

The galaxy collisions that we observe in nearby systems clearly trigger bursts of star formation–hot blue stars

Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical

What is the end result when galaxies collide?

Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together, as in this large cluster of galaxies

Starburst galaxies are forming stars so quickly they would use up all their gas in less than a billion years

WHAT ARE STARBURSTS

These galaxies tend to be very bright in the infrared

The intensity of supernova explosions in starburst galaxies can drive galactic winds that expel material far above the galaxy

Intensity of supernova explosions in starburst galaxies can drive galactic winds

This X-ray image from the Chandra SpaceTelescope shows individual supernovaeand bubbles of hot gas.

Most of the gas can be driven out of the galaxy!

What have we learned?

• Why do galaxies differ?– Some of the differences between galaxies may

arise from the conditions in their protogalactic clouds

– Collisions can play a major role because they can transform two spiral galaxies into an elliptical galaxy

• What are starbursts?– A starburst galaxy is transforming its gas into

stars much more rapidly than a normal galaxy

Quasars and Other Active Galactic Nuclei

Our goals for learning:• What are quasars?• What is the power source for quasars and

other active galactic nuclei?• Do supermassive black holes really exist?• How do quasars let us study gas between

the galaxies?

If the center of a galaxy is unusually bright we call it an active galactic nucleus

Quasars are the most luminous examples

The word stands for quasi-stellar objects associated with radio sources …Active Nucleus in M87

Mid-1960s very intense, star-like radio sources discovered. Associated with faint stars but spectra with 200-inch (5m) Palomar telescope unlike any star. Maarten Schmidt (Caltech) realized the spectrum was highly redshifted hydrogen emission.

• The highly redshifted spectra of quasars indicate large distances according to Hubble’s Law

• From brightness and distance we find that luminosities of some quasars are >1012 LSun (10x the entire Milky Way)

Redshift & Variability• Redshift is defined by

z = change in wavelength divided by original wavelengthUsually, z = v/c and so z = 0.2 20% speed of lightBut quasars with z = 6 have been found. This does NOT mean a

velocity = 6x the speed of light!!!Why? Because the redshift is cosmological – due to expansion of

space – NOT due to the Doppler Effect, so z is not equal to v/cIn fact, 1/1+z is the size of the universe compared to now; z = 6

means that we see the object at a time when the universe was 1/7 of its present size

• Quasar light is variable over a time of a few hours. All the quasar energy must come from a region smaller than solar system.

Thought Question

What can you conclude from the fact that quasars usually have very large redshifts?

A. They are generally very distantB. They were more common early in timeC. Galaxy collisions might turn them onD. Nearby galaxies might hold dead quasars

Thought Question

All of the above!

What can you conclude from the fact that quasars usually have very large redshifts?

A. They are generally very distantB. They were more common early in timeC. Galaxy collisions might turn them onD. Nearby galaxies might hold dead quasars

Galaxies around quasars sometimes appear disturbed by collisions

Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperatures

Radio galaxies contain active nuclei shooting out vast jets of plasma that emits radio waves coming from electrons moving at near light speed. Note the scale of the radio “lobes” – over 100,000 light-years!

RADIO IMAGE

But the lobes of some radio galaxies can extend over hundreds of millions of light years! What drives such

enormous outflows of matter from a galaxy?

An active galactic nucleus can shoot out blobs of plasma (ionized gas) moving at nearly the speed of light!

Speed of ejection suggests that a black hole is present …False color images of an active galaxy

Quasars and radio galaxies are related:Radio galaxies don’t appear as quasars because dusty gas clouds block our view of the hot accretion disk, but we can see the jet and the radio lobes.

Characteristics of Active Galaxies

• Luminosity can be enormous (>1012 LSun)

• Luminosity can rapidly vary (comes from a space smaller than solar system)

• Emit energy over a wide range of wavelengths (contain matter with wide temperature range)

• Some drive jets of plasma at near light speed

What is the power source for quasars and other active galactic

nuclei?

A truly enormous black hole

Accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars

Quasars are super massive Black Holes at the centers of early galaxies.

• Gravitational potential energy of matter falling into black hole turns into kinetic energy – the speeds are very high

• Friction in the accretion disk turns kinetic energy into thermal energy (heat) – temperatures are very high (X-rays emitted)

• Heat produces thermal radiation (photons)• This process can convert 10-40% of the rest-mass

energy (E = mc2) into radiation – hence the high luminosity

Energy from a Black Hole

Jets are thought to come from twisting of magnetic fields in the inner part of the accretion disk

Do supermassive black holes really exist?

What is the evidence?

Recall:The orbits of stars at center of Milky Way indicate a black hole with a mass of 4 million MSun

Can we use motions to derive the mass of a quasar black hole?

Yes! The orbital speed and distance of gas orbiting the center of the galaxy M87 indicate a black hole with mass of 3 billion MSun – that is almost 1000x the mass of our black hole. This is enormous!

• Many nearby galaxies – perhaps all of them – have supermassive black holes at their centers

• These black holes seem to be dormant active galactic nuclei– The Milky Way’s black hole is very quiet (dormant)

• All galaxies may have passed through a quasar-like stage earlier in time – The earliest galaxies were bigger (more merging)– Their black holes are much bigger (quasars)

Black Holes in Galaxies

Galaxies and Black Holes• The mass of a

galaxy’s central black hole is closely related to the mass of its bulge of stars

• The larger the bulge mass the greater the black hole mass

The development of the central black hole must be somehow related to galaxy evolution.

Intergalactic gas clouds between a quasar and Earth absorb some of a quasar’s lightWe can learn about protogalactic clouds by studying the absorption lines they produce in quasar spectra

How do quasars let us study gas between the galaxies?

What have we learned?

• What are quasars?– Active galactic nuclei are very bright objects

seen in the centers of some galaxies, and quasars are the most luminous type

• What is the power source for quasars and other active galactic nuclei?– The only model that adequately explains the

observations holds that supermassive black holes are the power source

What have we learned?

• Do supermassive black holes really exist?– Observations of stars and gas clouds orbiting

at the centers of galaxies indicate that many galaxies, and perhaps all of them, have supermassive black holes

• How do quasars let us study gas between the galaxies?– Absorption lines in the spectra of quasars tell

us about intergalactic clouds between those quasars and Earth