• Classifying galaxies is hard, and different people do it different ways• Except for nearby, we can only see bright galaxies• I will ignore dwarf spheroidal (dSph) and dwarf elliptical (dE) classifications

• Galaxies come in four broad categories, based on their appearance• Spiral

• Central roundish bulge plus disk• Barred Spiral

• Central elongated bulge plus disk•Elliptical

• Elongated bulge, no disk• Irregular

• No discernible shape

Spiral galaxies• Pinwheel-like• Central Bulge, spiral arms• Spiral arms, etc., signs of rotation

• Young and old stars, gas, dust• 80% of large galaxies are Spirals• Classified by amount of arms

• S0 - no spiral arms• Sa, Sb, Sc, Sd - more spiral arms

S0

Central BulgeDisk•No Spiral Arms

SaCentral BulgeDisk•Tight spiral arms

SbCentral BulgeDisk•Spiral arms

ScCentral BulgeDisk•Loose spiral arms

SdCentral BulgeDisk•Very loose spiral arms

Barred Spirals•Like Spirals, but bulge is oblong•Central Bulge, spiral arms•Spiral arms, etc., signs of rotation

•Young and old stars, gas, dust•Milky Way is probably barred spiral SBb•Classified by amount of arms•SB0 - no spiral arms•SBa, SBb, SBc, SBd - more spiral arms

SB0

Central BarDisk•No spiral arms

SBa

Central BarDisk•Tight spiral arms

SBb

Central BarDisk•Spiral arms

Milky Way?

SBcCentral Bar

Disk•Looser spiral arms

SBdCentral BarDisk•Very loose spiral arms

Elliptical Galaxies•Look like a sphere or a flattened sphere•Little gas and dust•Mostly old stars•Classified by how round they look•E0 looks circular•E7 is very elongated

E0

E1

E2

E3

E4

E5

E6

E7

Elliptical Galaxy Shapes

•Appearance may depend on angle of view•Amount of flattening probably has to do with rotation

Irregular Galaxies

•Classified as Irr

•Gas, dust, young and old stars•Like a galactic disk, no spirals, a mess

E0-E1-E2-E3-E4-E5-E6-E7S0 - Sa - Sb - Sc - Sd

SB0-SBa-SBb-SBc-SBd

Hubble’s Tuning Fork

•Spirals, Barred Spirals, and Ellipticals fit together in a pattern called the Tuning fork Diagram

What determines a galaxy’s classification?• It’s not size or mass•Rotation!•Rotation measured by Doppler shift

•Most mass is dark matter, again

To Observer

•Disk•Bulge•Nucleus•Halo

•Globular Clusters

Spiral Galaxy Structure

•Visible Part•Nucleus•Halo

Elliptical Galaxy Structure

Elliptical Halos•Elliptical galaxies don’t have thick clouds, but they do have diffuse, hot gas•These gasses emit X-rays•Gravity vs. pressure – they expand to make a giant sphere

•Amount of gravity tells us 90% of the mass of the galaxy is dark matter in the halo

Differences - Spirals vs. Ellipticals

Can we explain these differences?

•Spirals appear to have more spin•Spirals have disks and spiral structure•Spirals have dust/gas/young stars in the disk•Ellipticals have hot gas spread out through a large halo

The Effects of Spin•Spin causes flattening – formation of a disk•And disks naturally form spiral structure!

•Rotation prevents the gas and dust from all falling to the center

• In an elliptical galaxy, any loose gas and dust falls into the dense center•Star formation gets very fast•Lots of supernovas•Loose gas gets swept out to halo•No more star formation

When two galaxies collide or nearly collide, they can affect each other• Mostly through gravitational interactions

• As they pass near each other, the gravityof each distorts the other• The slower they pass, the bigger the effect• If unequal in size, smaller galaxy is affected most

• Tidal heating – energy is transferred from net motion to internal motion of stars• Star motions get more randomized• Energy comes from kinetic energy of orbit – orbit loses energy and galaxies

move closer together• Over time, the two galaxies will move closer and closer with each pass• Eventually, a true collision will occur

Near Miss Collisions

Actual Galaxy CollisionsWhat happens depends on relative size of the two galaxies

Big + Small:• Small galaxy is completely disrupted• Stars enter large galaxy• Over time, they get absorbed• This is currently happening to our own galaxy• Sagittarius Dwarf and Canis Major Dwarf – currently being disrupted• Virgo Stellar Stream – a dead galaxy whose stars are being absorbed

Two Equal sized galaxies:• Resultant galaxy will be irregular, initially• Based on total spin, resultant galaxy eventually settles down to spiral or

elliptical

Colliding Galaxies

Giant Elliptical Galaxies• Sometimes many galaxies collide and merge• If large numbers collide, spin cancels out• Giant galaxy becomes an elliptical

Giant Elliptical

Looking Out = Looking Back• Light travels at about 0.3 pc per year• The farther away you are looking, the longer ago you are seeing

• 1 kpc 3.3 ky• 1 Mpc 3.3 My• 1 Gpc 3.3 Gyr

• You can see back almost to the beginning of the Universe!

Galaxies in the past: how do they differ?• Galaxies long ago• Smaller than modern galaxies• Irregulars are more common

• Galaxies collided a lot in the past• Many irregulars from recent collisions• Galaxies got bigger from mergers

Why?

Galaxies Long Ago

Long, Long Ago