AnnouncementsAnnouncements

Starry NightSaint-Rémy: June, 1889

Van Gogh

Out in the sky the great dark clouds are massing; I look far out into the pregnant night …

Paul Laurence DunbarShips That Pass in the Night

Assigned studying: Chapter Assigned studying: Chapter 16.16.

Syllabus updates:Syllabus updates:

1) Details of Final Exam 1) Details of Final Exam posted.posted.

2) HW #8 will be posted on 2) HW #8 will be posted on Thursday 4; due on Thursday Thursday 4; due on Thursday 11.11.

GalaxiesGalaxies

Survey QuestionsSurvey Questions

1. What are the key characteristics of the four basic types of galaxies?

2. What is the the Hubble Tuning Fork Hubble Tuning Fork Scheme?Scheme?

3. Does the scheme really tell you a formation sequence of galaxies (NO)?

4. How do galaxies form?

Galaxies take one of four different Galaxies take one of four different morphologiesmorphologies

SPIRALSSPIRALS

Spirals have a disk with dust and gas: star formation

The tightness of a spiral The tightness of a spiral galaxy’s arms is correlated galaxy’s arms is correlated

to the size of its nuclear to the size of its nuclear bulgebulge

Type Sa Type Sb Type Sc

The tighter the arms, the bigger the bulge

Variety of Spiral ArmsVariety of Spiral Arms

Flocculent spirals (fleecy)

Grand-design spirals (highly organized)

We easily see these spiral arms We easily see these spiral arms because they contain numerous because they contain numerous

bright O and B stars which bright O and B stars which illuminate dust in the arms.illuminate dust in the arms.

However, stars in total are very evenly distributed throughout the disk.

BARRED SPIRALSBARRED SPIRALS

BARRED SPIRALSBARRED SPIRALS

The bar is not a spiral arm.It is made of old stars, likethose in the bulge

Bars of stars run through the Bars of stars run through the nuclear bulges of barred nuclear bulges of barred

spiral galaxiesspiral galaxies

Type SBa Type SBb Type SBc

ELLIPTICALSELLIPTICALS

Type E0 Type E3 Type E7

ELLIPTICALSELLIPTICALS

Ellipticals:have no dust and no cold gas, hence no star formation.Made mostly of old, metal-enriched stars.They like to cluster a lot

Discussion QuestionDiscussion QuestionGiven their color, do you expect elliptical Given their color, do you expect elliptical

galaxies to have more or less new star galaxies to have more or less new star formation than spirals? Why?formation than spirals? Why?

1) ellipticals have more new star 1) ellipticals have more new star formationformation

2) ellipticals have less new star 2) ellipticals have less new star formationformation

Elliptical galaxies display a Elliptical galaxies display a variety of sizes and variety of sizes and

massesmasses Giant elliptical Giant elliptical

galaxiesgalaxies can be 20 times can be 20 times larger than the Milky Waylarger than the Milky Way

Dwarf elliptical Dwarf elliptical galaxiesgalaxies can be as small can be as small as a big globular cluster. as a big globular cluster. They are extremely They are extremely common and can contain common and can contain as few as a million starsas few as a million stars

IRREGULARIRREGULAR

Irregulars mostlyhave gas, dust andstar formation.They contain both old and young stars

The classification scheme of galaxy morphology is The classification scheme of galaxy morphology is known as the known as the Hubble Tuning Fork SchemeHubble Tuning Fork Scheme

Galaxies like to cluster:Galaxies like to cluster:eg. M81 groupeg. M81 group

Optical Image

Radio Image

Hercules Hercules ClusterCluster

The ideas of galaxy The ideas of galaxy formationformation

Spiral galaxies form from the collapse of spinning gas Spiral galaxies form from the collapse of spinning gas cloud inside Dark Matter haloscloud inside Dark Matter halos

Elliptical galaxies form from the mergers of disk Elliptical galaxies form from the mergers of disk galaxies, or from clouds with low spinning (probably galaxies, or from clouds with low spinning (probably wrong for big galaxies; might work for early galaxies)wrong for big galaxies; might work for early galaxies)

Peculiar galaxies are formed through the interactions of Peculiar galaxies are formed through the interactions of galaxiesgalaxies

Most popular theory: CDM (Cold Dark Most popular theory: CDM (Cold Dark Matter)hierarchical cosmology. Structures form from Matter)hierarchical cosmology. Structures form from bottom to top: small ones first, bigger ones later by bottom to top: small ones first, bigger ones later by merging.merging.

CDM predicts hierarchical assembly of cosmic CDM predicts hierarchical assembly of cosmic structures. Many of its predictions are verified.structures. Many of its predictions are verified.

Billions of stars all tug on each other instead of just one planet tugged by the gravity of the Sun.

Galaxy Merging: evolution of morphology, trigger of star formation

How two spirals collide and How two spirals collide and merge, making tidal tails and, merge, making tidal tails and, at the end, an elliptical galaxyat the end, an elliptical galaxy

Survey QuestionsSurvey Questions

1. What are the key characteristics of the four basic types of galaxies?

2. What is the the Hubble Tuning Fork Hubble Tuning Fork Scheme?Scheme?

3. Does the scheme really tell you a formation sequence of galaxies? NO!!!!! It is just a morphological classification scheme

How did we find Dark Matter: How did we find Dark Matter: The mass of galaxies and rotation The mass of galaxies and rotation

curvescurves

M

v2 A A rotation curverotation curve is just a is just a plot of rotational plot of rotational velocity vs distance for velocity vs distance for objects in (roughly) objects in (roughly) circular orbits.circular orbits.

For a set of objects orbitinga common point, what isthe orbital velocity for different objects at different distances from the common point?

v1

If you think about it, this is just Kepler’s Third Law applied to galaxies

Rotation Curve for a rigid bodyRotation Curve for a rigid bodyno gravity involved, no gravity involved, no no

information on massinformation on mass

Straight because objects arerigidly held.

Rotation Curve for our Solar Rotation Curve for our Solar SystemSystem

gravity involved,gravity involved, information on information on massmass

Curve decreases because mass that generates the gravity is all at the center (Sun is much, much greater than all planets together.

Rotation curve of the MW Rotation curve of the MW diskdisk

(and of all other galaxies)(and of all other galaxies)It is essentially Kepler’s Third Law

One uses it to measure the mass of galaxies

Rotation Curves of GalaxiesRotation Curves of Galaxies

Observe frequency of spectral lines across a galaxy.

From blue / red shift of spectral lines across the galaxy

infer rotational velocity

Plot of rotational velocity vs. distance from the center of the galaxy:

Rotation Curve

Determining the Masses of GalaxiesDetermining the Masses of Galaxies

Based on rotation curves, use Kepler’s 3rd law to infer

masses of galaxies.

Evidence for Dark MatterEvidence for Dark Matter

Evidence #1 - The amount of mass Evidence #1 - The amount of mass that we can that we can ““seesee”” by counting the by counting the stars, the gas and the dust in a stars, the gas and the dust in a galaxy is not enough compared to galaxy is not enough compared to the observed gravitational mass the observed gravitational mass measured with the rotation curves.measured with the rotation curves.

Evidence for Dark MatterEvidence for Dark Matter

Evidence #2 - The amount of mass Evidence #2 - The amount of mass that we can that we can ““seesee”” in a cluster of in a cluster of galaxies does not account for the galaxies does not account for the observed gravitational pull on the observed gravitational pull on the galaxies in the cluster (as evidenced galaxies in the cluster (as evidenced by the large variation in velocities of by the large variation in velocities of the galaxies).the galaxies).

Hercules Hercules ClusterCluster

Galaxies in a cluster have more erratic velocities than we expect from the amountof matter we can “see” in the cluster

Evidence for Dark MatterEvidence for Dark Matter

Evidence #3 - The intracluster gas is too Evidence #3 - The intracluster gas is too hot to be retained by the gravity of the hot to be retained by the gravity of the visible matter in a cluster. visible matter in a cluster.

If all the mass there really were only that If all the mass there really were only that of visible matter, its gravity would not be of visible matter, its gravity would not be enough to retain the hot gas, which enough to retain the hot gas, which would evaporate rapidly. Since the gas is would evaporate rapidly. Since the gas is there, there must be more gravity, hence there, there must be more gravity, hence dark matter.dark matter.

Cluster of galaxies: Hydra A Cluster of galaxies: Hydra A

Optical X-ray

Evidence #4: Evidence #4: gravitational lensinggravitational lensing

Just as black holes and other massive objects curve space around them, galaxies and galaxy clusters curve space. When the alignment of a large mass and background objects is right, a dramatic effect is observed. This is known as a gravitational lens.

Lensing by a densecluster of galaxies.

Using Einstein’s laws of general relativity,we can estimate themass of the lens.

About 10 times moremass is present thanone would estimate bysumming the mass ofthe visible galaxies.

ForegroundCluster ofGalaxies

Background lensed galaxies

Our picture of the mass around Our picture of the mass around galaxies now looks something like galaxies now looks something like

thisthis

Dark matter outweighs Normal matter on average by 10 to 1

So what is this missing So what is this missing mass?mass?

The only way to explain The only way to explain the rotation curve of our the rotation curve of our galaxy is to say that galaxy is to say that there is lots and lots of there is lots and lots of mass that is not emitting mass that is not emitting light.light.

The halo of our galaxy The halo of our galaxy must be full of it. The must be full of it. The halo outweighs the disk halo outweighs the disk by a factor of 10.by a factor of 10.

As far as we can tell, this As far as we can tell, this mass doesnmass doesn’’t emit any t emit any light at any frequency.light at any frequency.

What is the form of the missing mass?What is the form of the missing mass?

Dark Matter PossibilitiesDark Matter Possibilities

Here is the first lists of candidate Here is the first lists of candidate materials for the dark matter that materials for the dark matter that dominates the mass in our galaxy.dominates the mass in our galaxy. Black HolesBlack Holes Black DwarfsBlack Dwarfs Brown DwarfsBrown Dwarfs Unknown particlesUnknown particles

Baryonic Matter (e, p, n … the same stuffthat we’re made of)

Non baryonic matter

Does dark matter matter?Does dark matter matter?

Yes! Dark Matter Rules the Yes! Dark Matter Rules the UniverseUniverse

Since dark matter outweighs other Since dark matter outweighs other matter by a factor of 10, nearly all the matter by a factor of 10, nearly all the large scale gravitational interactions large scale gravitational interactions in the Universe are dominated by the in the Universe are dominated by the dark matter:dark matter: Dark matter holds galaxies together – Dark matter holds galaxies together –

without it the outer stars would escapewithout it the outer stars would escape Dark matter holds clusters of galaxies Dark matter holds clusters of galaxies

together – without it, the clusters would together – without it, the clusters would disperse disperse

Dark matter causes these structures to Dark matter causes these structures to form in the first place! form in the first place!

Which of the following is NOT a good reason Which of the following is NOT a good reason for learning more about dark matter?for learning more about dark matter?

dark matter provides the gravitational force dark matter provides the gravitational force that holds galaxies togetherthat holds galaxies together

the total amount of dark matter determines the the total amount of dark matter determines the fate of our Universefate of our Universe

the Earth would be destroyed if it ever ran into a the Earth would be destroyed if it ever ran into a clump of cold dark matterclump of cold dark matter

dark matter is responsible for the formation of dark matter is responsible for the formation of structure in our Universestructure in our Universe

our understanding of dark matter may reveal a our understanding of dark matter may reveal a new, undiscovered form of matternew, undiscovered form of matter

What is the best evidence for dark What is the best evidence for dark matter in galaxies?matter in galaxies?

1) the orbital speeds of gas clouds and 1) the orbital speeds of gas clouds and

starsstars

2) the orbital distances of gas clouds and 2) the orbital distances of gas clouds and

starsstars

3) the low density of stars in spiral arms3) the low density of stars in spiral arms

4) the large orbital eccentricities of disk 4) the large orbital eccentricities of disk

objectsobjects

5) the lack of heavy elements in globular 5) the lack of heavy elements in globular

clustercluster

starsstars

Dark Matter SummaryDark Matter Summary 90% of the universe is made of matter that 90% of the universe is made of matter that

does not emit detectable radiation at any does not emit detectable radiation at any wavelength.wavelength.

Evidence to date suggests that only a very Evidence to date suggests that only a very small fraction of the dark matter is made of small fraction of the dark matter is made of familiar matter (baryons).familiar matter (baryons).

Dark matter is not stars or stellar remnants, Dark matter is not stars or stellar remnants, galaxies, dust clouds, or anything else made galaxies, dust clouds, or anything else made of protons, neutrons, or electrons – as far as of protons, neutrons, or electrons – as far as we can tell, it is an as of yet we can tell, it is an as of yet undiscoveredundiscovered form of matter.form of matter.

This is one of the biggest mysteries facing astronomers – What is our Universe made of?

Survey QuestionsSurvey Questions

1.1. What is a rotation curve of a galaxy?What is a rotation curve of a galaxy?

2.2. How can a rotation curve be used to How can a rotation curve be used to measure the mass of the galaxy?measure the mass of the galaxy?

3.3. Where is most of the gravitational Where is most of the gravitational mass in a galaxy?mass in a galaxy?

4.4. What are the lines of evidence for What are the lines of evidence for dark matter in the universe?dark matter in the universe?