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Galaxies And the Foundation of Modern Cosmology. What are the three major types of galaxies?. Hubble Ultra Deep Field. Hubble Ultra Deep Field. Hubble Ultra Deep Field. Spiral Galaxy. Hubble Ultra Deep Field. Spiral Galaxy. Hubble Ultra Deep Field. Elliptical Galaxy. Elliptical Galaxy.

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slide7

Hubble Ultra Deep Field

Elliptical Galaxy

EllipticalGalaxy

Spiral Galaxy

slide8

Hubble Ultra Deep Field

Elliptical Galaxy

EllipticalGalaxy

Spiral Galaxy

slide9

Hubble Ultra Deep Field

Elliptical Galaxy

EllipticalGalaxy

Irregular Galaxies

Spiral Galaxy

slide10

halo

disk

bulge

Spiral Galaxy

slide11

Disk Component:

stars of all ages,

many gas clouds

Type Sa Galaxy

Spheroidal Component:

bulge & halo, old stars,

few gas clouds

sa galaxies
Sa Galaxies
  • Sa Galaxies:
    • Dominant nuclear bulge
    • Tightly wound spiral pattern
    • Few (but some) newly formed stars, HII regions or other evidence of active star formation
sb galaxies
Sb Galaxies
  • Moderate nuclear bulge
  • Intermediate spiral pattern
  • Some evidence for massive young stars, HII regions, star formation
slide14

Type Sc Galaxy

Blue-white color indicates ongoing star formation

Disk Component:

stars of all ages,

many gas clouds

Spheroidal Component:

bulge & halo, old stars,

few gas clouds

Red-yellow color indicates older star population

sc galaxies
Sc Galaxies

(Some classify Messier as as Type Sd)

  • Small to nearly non-existent nuclear bulge
  • Open spiral pattern
  • Active star-formation
slide16

Disk Component:

stars of all ages,

many gas clouds

Blue-white color indicates ongoing star formation

Spheroidal Component:

bulge & halo, old stars,

few gas clouds

Red-yellow color indicates older star population

barred spiral galaxy
Barred Spiral Galaxy

Has a bar of stars across the bulge

s0 lenticular galaxy
S0 Lenticular Galaxy

Has a disk like a spiral galaxy but very little dust or gas (intermediate between spiral and elliptical)

s0 edge on
S0 Edge-on

Note the clear presence of a disk, but absence of dust band in this S0 galaxy: NGC 3115

elliptical galaxy
Elliptical Galaxy:

All spheroidal (bulge) component, no disk

slide22

Elliptical Galaxy:

All spheroidal component, virtually no disk component

Red-yellow color indicates older star population

irregular galaxies
Irregular Galaxies

Irregular I Galaxy

Blue-white color indicates ongoing star formation

hubble s galaxy classes
Hubble’s Galaxy Classes

Spheroid

Dominates

Disk Dominates

slide30

Elliptical galaxies are much more common in huge clusters of galaxies

(hundreds to thousands of galaxies)

slide32

Deep observations show us very distant galaxies as they were much earlier in time

(Old light from young galaxies)

slide35

Denser regions contracted, forming protogalactic clouds

H and He gases in these clouds formed the first stars

slide36

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

Leftover gas settled into spinning disk

Conservation of angular momentum

why do galaxies differ
Why do galaxies differ?

M87

NGC 4414

But why do some galaxies end up looking so different?

nature conditions in protogalactic cloud
Nature: Conditions in Protogalactic Cloud?

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

conditions in protogalactic cloud
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

distant red ellipticals
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
slide45

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

slide47

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

slide48

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

slide49

Shells of stars observed around some elliptical galaxies are probably the remains of past collisions

slide50

Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together

slide51

Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies

slide53

We measure the mass of the solar system using the orbits of planets

  • Orb. Period
  • Avg. Distance
  • Or for circles:
  • Orb. Velocity
  • Orbital Radius
slide54

Rotation curve

A plot of orbital velocity versus orbital radius

Solar system’s rotation curve declines because Sun has almost all the mass

slide56

Rotation curve of Milky Way stays flat with distance

Mass must be more spread out than in solar system

slide57

Mass in Milky Way is spread out over a larger region than the stars

Most of the Milky Way’s mass seems to be dark matter!

slide58

Mass within Sun’s orbit:

1.0 x 1011MSun

Total mass:

~1012MSun

slide60

We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H

slide61

Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

slide62

Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting

These galaxies also have dark matter

clusters of galaxies
Clusters of Galaxies

We can measure the velocities of galaxies in a cluster from their Doppler shifts

slide65

Clusters contain large amounts of X-ray emitting hot gas

Temperature of hot gas (particle motions) tells us cluster mass:

85% dark matter

13% hot gas

2% stars

slide66

Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

does dark matter really exist
Does dark matter really exist?

Either:

Dark matter really exists, and we are observing the effects of its gravitational attraction

Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

bottom line
Bottom Line:
  • What is the evidence for dark matter in galaxies?
    • Rotation curves of galaxies are flat, indicating that most of their matter lies outside their visible regions
  • What is the evidence for dark matter in clusters of galaxies?
    • Masses measured from galaxy motions, temperature of hot gas, and gravitational lensing all indicate that the vast majority of matter in clusters is dark
our options
Our Options
  • Dark matter really exists, and we are observing the effects of its gravitational attraction
  • Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

Because gravity is so well tested, most astronomers prefer option #1

two basic options
Two Basic Options
  • Baryonic (Ordinary) Dark Matter (MACHOS)
    • Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

  • Extraordinary Dark Matter (WIMPS)
    • Weakly Interacting Massive Particles:

mysterious neutrino-like particles

slide74

MACHOs occasionally make other stars appear brighter through lensing

… but not enough lensing events to explain all the dark matter

why believe in wimps
Why Believe in WIMPs?
  • There’s not enough ordinary matter
  • WIMPs could be left over from Big Bang
  • Models involving WIMPs explain how galaxy formation works