Galaxies

Galaxy Classification

• Spiral (S)• Elliptical (E)• Irregulars (Irr)

Normal Spirals

• Sa, Sb, Sc• From a to c

– Bulge becomes smaller– Arms less tightly wound

Barred Spirals

• SBa, SBb, SBc• A to b to c similar to normal spirals.• Arms at end of a bar passing though bulge.

Elliptical Galaxies

• E0, E1, E2, E3, E4, E5, E6, E7• From E0 to E7

– Get more elliptical in shape– Get fuzzy

Elliptical Galaxies

Dwarf elliptical galaxies orbiting M31

Giant elliptical galaxy at core of a galaxy cluster.Two types of elliptical galaxies,

dwarfs and giants.

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Irregular Galaxies

• Small size• No defined

shape• fuzzy

Irregular Galaxies

SMC and LMC are irregular galaxies orbiting the Milky Way.

LMCSMC

Hubble Tuning Fork Diagram

There is no observational evidence that this diagram shows galaxy evolution.

NOTE: S0’s are spirals without arms.

S0 Class

• Look like E0’s• Contain dust

and gas like spiral galaxies.

• Therefore, they were named as S0’s.

Galaxy Clusters

Galaxies tend to congregateinto groups called galaxy clusters.

These clusters can be smallcontaining tens of galaxies or large containing hundreds or thousands of galaxies.

Galaxy ClustersThe Local Group

• Small cluster to which the Milky Way belongs.

• ~ 1,000,000 pc in diameter.

• Contains ~45 members.

• Andromeda Galaxy ~2.2 million LY away.

Galaxy ClustersThe Virgo Cluster

• 18 Mpc• 2500 galaxies• M87, the central

galaxy (giant elliptical)• Local Group may be

like an appendage to the Virgo Cluster.

• Super Clusters

Active Galactic Nuclei (AGN’s)

• ~20-25% of bright galaxies are not normal (do not fit Hubble classifications.)

• Among the most energetic object in the universe.

• Have bright nucleus

NGC 7742 looks like a fried egg.

Types of AGN’s

• Seyfert Galaxies

• Radio Galaxies

• Quasars

Seyfert Galaxies

• Very bright central nucleus

• Faint spiral arms• Variable in brightness

Seyfert Galaxies

• Nucleus changes brightness

• Time scales of weeks, months, and years.

PEGA at GSU has observedAGN microvariblility on time scales of minutes.

Radio Galaxies

• Large radio emitting lobes• Lobes extend out 10X larger than Milky Way or

~ size of The Local Group.

Radio Galaxies

M87

Cygnus AColliding Galaxies

M86

• Also near center of Virgo Cluster

• Core-dominated radio galaxy

Quasars

• Tiny blue stellar looking objects with large radio emission similar to radio galaxies.

• Highly red shifted spectra.

• ~ 50,000 km/s or more.

Quasars

• Quasar means quasi-stellar object.

• Distances from 240 Mpc to 9000 Mpc

• They are the brightest thing in universe but appear faint

Quasars

• They share characteristics of Seyfert galaxies and radio galaxies.

• Over 30,000 known

AGN Model

• Super massive black hole at center.

• Accretion disk• Intense magneitc field• Jets of high speed

particles.

AGN Model

AGN Model

• Viewing angle determines type of energy seen.

• Viewing angle determines the type of AGN seen, Seyfert, radio galaxy, etc.

Possible Galaxy Evolution

Resources

Chaisson & McMillan, (2002, 2004). Astronomy Today (5th Ed.)