GALAXIES

WHAT IS A GALAXY?

A galaxy is a system of stars, gas, and dust bound together by their mutual gravity.

WHO DISCOVERED GALAXIES?

• In 1845, William Parsons the third Earl of Rosse in Ireland, built a telescope 72 inches in diameter.

• He sketched nebulae that was spiral shaped that later became known as spiral nebulae.

• 18th Centruy German philosopher Immanuel Kant proposed that the Universe was filled with great wheels of stats which he called island universes. Parsons later adopted the term island universes.

• Parsons later concluded that these spiral nebulae were spiral clouds of stars.

• Not everyone agreed that the spiral nebulae were clouds of stars lying beyond our own star system.

SHAPLEY-CURTIS DEBATE

• Debate over spiral nebulae being inside or outside of our galaxy

• In April 1920, two astronomers debated the issue at the National Academy of Sciences in Washington, D.C.

• Harlow Shapely of the Mt. Wilson Observatory argued that the spiral nebulae were just objects within the Milky Way star system.

• Heber Curtis of Lick Observatory argued that the spiral nebulae were island universes far outside the Milky Way star system.

• Historians mark the Shapely-Curtis Debate as a turning point in modern astronomy, but at the time it was inconclusive.

• This debate would later be resolved by a bigger telescope.

EDWIN HUBBLE

• In 1924, Mt. Wilson astronomer Edwin Hubble announced that he had taken photographic plates of a few bright spiral nebulae using the new 100 inch Hale telescope.

• He detected individual stars in the spiral nebulae and identified some of those as Cepheid Variable Stars.

• Cepheid Variable: variable star with a period of 1 to 60 days; the period of variation is related to luminosity (it goes from bright to faint to bright again).

• Hubble estimated they had an apparent magnitude of 18 but had a month long pulsation-period (variation period) which indicated they were supergiants.

• For supergiants to appear that faint, they had to be well outside of our Milky Way star system. In fact they were galaxies separate form ours, and rivaling ours in size.

HOW MANY GALAXIES ARE THERE?

• Only the largest telescopes can gather enough light to detect distant galaxies.

• Today telescopes are capable of detecting a few hundred billion galaxies at a wide range of wavelengths, and new, larger telescopes will reveal even more.

• Hubble Deep Field

GALAXY CLASSIFICATION

Spiral Galaxies Elliptical Galaxies Irregular Galaxies

SPIRAL GALAXIES• have flat disk, spiral arms, central bulge,

and a surrounding halo

• some have a “barred” bulge

• are fairly large (no dwarf spirals)

• have lots of gas and dust and younger stars in their arms, but older stars and little gas or dust in their halos and central bulges

halo

bulgedisk

Spiral Galaxy

LENTICULAR GALAXIES• have a disk but no arms

• have little or no excess gas and dust

ELLIPTICAL GALAXIES• range from spherical to

football shaped

• range from very small to giant

• have very little gas or dust

• mostly old stars

• similar to the central bulge of a spiral galaxy

• Have no disk or spiral arms

IRREGULAR GALAXIES• any galaxy that isn’t a Spiral,

Elliptical, or Lenticular

• usually have lots of gas and dust and young stars

• may have a distorted shape (shapeless) from interaction with another galaxy

VARIOUS GALAXIES (CAN YOU IDENTIFY TYPE?

HOW DO GALAXIES FORM?

HOW DO GALAXIES FORM?

HOW DO GALAXIES FORM?

• Denser regions contracted, forming protogalactic clouds

• H and He gases in these clouds formed the first stars….H and He formed protostars….then they became stars to be more specific

• Supernova explosions from first stars kept much of the gas from forming stars (Black Holes)

• Leftover gas settled into spinning disk

• Conservation of angular momentum

WHY DO GALAXIES DIFFER?

WHY DON’T ALL GALAXIES HAVE SIMILAR DISKS?

LOOK AT THIS IMAGE AND TELL ME WHY YOU THINK SOME GALAXIES END UP

LOOKING SO DIFFERENT?

NATURE: CONDITIONS IN THE PROTOGALACTIC CLOUD?

Spin: Initial angular momentum of protogalactic cloud could

determine size of resulting disk

NATURE: CONDITIONS IN THE 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 ELLIPTICALSObservations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe

EVOLUTION OF GALAXIESNASA Galaxy Evolution Explorer

• supports the long-held notion that many galaxies begin life as smaller spirals before transforming into larger, elliptical-shaped galaxies.

• This is based on the visible and ultraviolet wavelengths emitted by the galaxy.

• Young galaxies appear blue while older galaxies appear red.

ACTIVE GALAXIESAn ACTIVE GALAXY is a galaxy with an unusually large amount of energy emitted from the core.

ACTIVE GALAXIES

• Normally the core of a disk or elliptical galaxy is small, relatively faint, and composed of older, redder stars.

• However, in some galaxies the core is intensely bright, shinning with the power equivalent to trillions of suns, easily outshining the rest of the light of the galaxy combined.

• Active Galaxies are actually rare (only a few percent of galaxies are active), but so bright they can be seen clear across the visible universe.

ACTIVE GALAXIES: WHERE DOES THE ENERGY COME FROM?

• Modern observation of Active Galaxies shows that the energy comes from the active nuclei of the galaxies, which are now known as Active Galactic Nuclei (AGN)

• An Active Galactic Nuclei is the central energy source of an active galaxy.

ACTIVE GALAXIES

SEYFERT GALAXIES

• The first clue that the nuclei of galaxies could be active was found in 1943 by Mt. Wilson Astronomer Carl Seyfert, who published a comprehensive survey of spiral galaxies.

• Seyfert Galaxies are spiral galaxies with small, highly luminous nuclei.

• About 2% of spiral galaxies appear to be Seyfert Galaxies.

• Spectral lines of a Seyfert Galaxy contain broad emission lines, in comparison, normal galaxy spectral lines wash out the week spectral lines.

• The widths of Seyfert Galaxy spectral lines are understood to represent Doppler shifts produced by high velocities in the nuclei.

GALAXY CLUSTERS

• the Local Group

• includes the Milky Way, the Andromeda, and over 30 other smaller galaxies

• the Virgo Cluster

• hundreds to thousands of galaxies, 60 million light-years away

• giant elliptical at center, formed by galactic cannibalism

• the Local Group is “falling” toward the Virgo Cluster at 60 to 250 miles per second!

GALAXY CLUSTERS: THE COMA CLUSTER (AKA – ABELL 1656)

• One of the densest known galaxy clusters.

• 300 Million Light Years Away

• Contains thousands of galaxies.

• It’s spherical in shape and is 20 million light years in diameter.

GALAXY CLUSTERS• We can measure the

velocities of galaxies in a cluster from their Doppler shifts

• The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!

GALAXY CLUSTERS• 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

DARK MATTERDark Matter is nonluminous material that is detected only by its gravitational influence.

DARK MATTER

• Both observational evidence and mathematical models show that the extra mass lies in an extended halo, sometimes called a Galactic Corona, that may reach out ten times farther than the edge of the visible disk and could contain more than a trillion solar masses.

• Most of the mass is invisible, neither emitting nor absorbing light.

• The nature of dark matter is one of the most important unknowns in modern astronomy.

DARK MATTER: OBSERVATIONAL EVIDENCE

• Rotational Curve: variation in orbital velocity of stars/galaxies at different distances from the center.• Normally a solid disk would rotate such that the velocity increases linearly with

radius (velocity proportional to radius….like a marry-go-round)

• However, galaxies have a flat rotation curve which implies that the mass continues to increase linearly with radius.

• Therefore, the rotation curves of galaxies present strong evidence for dark matter.

• Spectral Analysis: the broadening of spectral lines in elliptical galaxies tells us how fast stars are orbiting….and elliptical galaxies have dark matter.

• Gravitational Lensing: the bending of light rays by gravity. This can tell us a Galaxy Clusters mass.

DARK MATTER

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

COLLIDING GALAXIES

We must also consider the effects of collisions

COLLIDING GALAXIES

• We now think that galaxies in groups and clusters often collide

• Stars don’t usually collide

• New orbits, gas piles up to form new stars

COLLIDING GALAXIES

Galaxies Interacting

COLLIDING GALAXIES

The occasional results of two galaxies colliding: ringed galaxies

COLLIDING GALAXIES

Collisions were much more likely early in time, because galaxies were closer together

COLLIDING GALAXIES

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

COLLIDING GALAXIES

The collisions we observe nearby trigger bursts of star formation

COLLIDING GALAXIES

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

COLLIDING GALAXIES

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

COLLIDING GALAXIES

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

COLLIDING GALAXIES

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

HOW FAR AWAY IS THE NEAREST GALAXY?

• At 2.3 million light-years, the Andromeda galaxy is the closest spiral galaxy to our Milky Way and the most distant thing you can see with your eye alone.

• The Milky Way is moving at 300,000 mph toward the Andromeda Galaxy

• They may collide in about 5 billion years….we’re okay for now!