Galaxy Classification 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 Bulge Disk No Spiral Arms

Sa Central Bulge Disk Tight spiral arms

Sb Central Bulge Disk Spiral arms

Sc Central Bulge Disk Loose spiral arms

Sd Central Bulge Disk 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 Bar Disk No spiral arms

SBa Central Bar Disk Tight spiral arms

SBb Central Bar Disk Spiral arms Milky Way?

SBc Disk Central Bar Looser spiral arms

SBd Central Bar Disk 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 Gas, dust, young and old stars Like a galactic disk, no spirals, a mess Classified as Irr

Hubble’s Tuning Fork Spirals, Barred Spirals, and Ellipticals fit together in a pattern called the Tuning fork Diagram S0 - Sa - Sb - Sc - Sd E0-E1-E2-E3-E4-E5-E6-E7 SB0-SBa-SBb-SBc-SBd

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

Spiral Galaxy Structure Disk Bulge Nucleus Halo Globular Clusters

Elliptical Galaxy Structure Visible Part Nucleus Halo

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

Can we explain these differences? Differences - Spirals vs. Ellipticals 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 Can we explain these differences?

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

Near Miss Collisions Galaxy Collisions When two galaxies collide or nearly collide, they can affect each other Mostly through gravitational interactions Near Miss Collisions As they pass near each other, the gravity of 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

Actual Galaxy Collisions What 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 Why? Galaxies collided a lot in the past Many irregulars from recent collisions Galaxies got bigger from mergers

Galaxies Long Ago

Long, Long Ago