Galaxies

Hubble’s Galaxy Classification• American astronomer Edwin Hubble was the first to categorize galaxies in a

comprehensive way.• His observations were performed on the 2.5 m optical telescope on Mount

Wilson in California in 1924.• Basic Hubble classification scheme: spirals, barred spirals, ellipticals, and

irregulars.

Spirals• Contain a flattened disk where spiral arms are found, a central galactic bulge with a

dense nucleus, and an extended halo of faint old stars.• Denoted by the letter S and classified as type a, b, or c according to the size of the

central bulge (Sa have the largest bulge, while Sc have the smallest).• Sa have the tightest spiral patterns, with Sc having the loosest spiral patterns and most

knots or clumps.• Bulges and halos contain large numbers of reddish old stars and globular clusters.• However, most of the light comes from the younger stars, which give spirals an overall

whitish color.• Disks are typically gas and dust rich, with Sc galaxies containing the most.• Spiral arms appear blue due to young, massive O and B type stars.

Variety of Spiral Arms

Flocculent spirals Grand-design spirals (highly organized)

Barred-Spirals• Contain an elongated “bar” of stellar and interstellar material passing

through the center and extending beyond the bulge, into the disk.• The spiral arms project from near the ends of the bar rather than from the

bulge (as they do in normal spirals).• Designated by the letters SB and subdivided into the a, b, and c groups.• Other than the presence of the bar, exactly the same as normal spirals.• Milky Way’s bulge has some central elongation, and probably contains a

bar.

Ellipticals• Have no spiral arms and no obvious disk.• Other than a dense central nucleus, often exhibit little internal structure.• As with spirals, stellar density increases sharply in the central nucleus.• Denoted by the letter E. Slightly flattened systems are denoted E0, and the most

elongated ellipticals are denoted E7.• Note that galaxy shape can be difficult to determine based on visual appearance alone.• Large range in size and number of stars of ellipticals.• Giant ellipticals range up to a few megaparsecs in diameter.• Dwarf ellipticals may be as small as one kiloparsec in diameter. Dwarf ellipticals

outnumber their larger counterparts 10 to 1, but make up a smaller percentage of total mass compared to their larger cousins. Dwarf ellipticals are not simply small ellipticals, but represent an entirely different class of galaxy.

Ellipticals• Some giant ellipticals contain disks of gas and

dust in which stars are forming. These are likely the results of mergers between gas-rich galaxies. Such mergers alter the appearance of galaxies.

• In between an E7 elliptical and Sa spiral are a class of galaxies known as S0 galaxies (no bar) and SB0 galaxies (bar present). Also called lenticular galaxies. Contain a stellar disk, but no gas or dust. S0s are likely closely related to ellipticals.

Irregulars

• Any galaxy that’s not an elliptical or spiral.• Tend to be rich in interstellar gas and young, blue stars.• Lack any regular structure.• Tend to be smaller than spirals and larger than dwarf ellipticals.• Most common irregulars are the dwarf irregulars.• Dwarf ellipticals and dwarf irregulars combined make up the vast majority of

galaxies and are often found near a larger “parent” galaxy.• Two subclasses: Irr I and Irr II.• Irr I look like misshapen spirals, such as the Large and Small Magellanic Clouds

(satellite galaxies of our Milky Way). Contain ongoing star formation.• Irr II galaxies are much more rare, and often have an “explosive” or

filamentary appearance. Probably the result of interaction between two more normal systems.

The Hubble Sequence• Hubble regarded the tuning fork as an evolutionary sequence from left to

right.• We still refer to ellipticals as early-type galaxies, and spirals as late-type

galaxies.• However, there is no direct evolution from one type to another for an

isolated system.• Galaxy collisions may however change the structure of a galaxy from one type

to another.

Hubble Tuning Fork

Galaxy Classification and Properties: Lecture Tutorial (p. 127)

1 2 3 4

5 6 7 8

Clusters of Galaxies

• Local Group - group of galaxies containing the Milky Way and Andromeda among others, a little over 1 Mpc in diameter. Contains 45 known galaxies.

• Galaxy cluster - a group of galaxies held together by their mutual gravitational attraction.

• Virgo Cluster - contains more than 2500 galaxies, extending about 3 Mpc across.

Active Galaxies

• 20 to 25 percent of all galaxies don’t fit well into Hubble’s classification scheme.

• Some of these galaxies are among the most energetic objects known.

• Their luminosities can be thousands of times that of the Milky Way.

• At visible wavelengths, such objects look like normal galaxies. At other wavelengths, they look drastically different.

Galactic Radiation• Normal galaxies emit most of their radiation at visible wavelengths (stellar

radiation).• Active galaxies emit most of their light at wavelengths both shorter and

longer than visible (nonstellar radiation).• The term active galaxy refers to a system where the abnormal activity is

related to violent events occurring in or near the galactic nucleus. Such systems are known as active galactic nuclei.

• Three types: Seyfert galaxies, radio galaxies, and quasars.

Seyfert Galaxies

• Named for Carl Seyfert, who first observed them in 1943 from Mount Wilson Observatory.

• Resemble normal spiral galaxies.• Most of the energy is emitted from the galactic nucleus.• Brightest Seyfert nuclei are 10 times more energetic than the entire Milky

Way.• 75% of Seyferts emit most of their radiation in the infrared (dust is re-

emitting higher energy radiation it has absorbed).• Energy emission often varies in time - by as much as 50% in under a year --

the source must be very compact (1 ly across since an object cannot flicker in less time than radiation takes to cross it).

Radio Galaxies

• Emit large amounts of energy in the radio portion of the electromagnetic spectrum.

• Centaurus A is one example.• The emission does not come from a compact nucleus, but from two huge

extended regions called radio lobes (which extend in size comparable to the entire Local Group).

• Radio lobes jet out from the center - possibly expelled from the nucleus due to a galaxy merger.

• Despite their name, they actually radiate more energy at short wavelengths.

Common Features of Active Galaxies

• Compact nucleus• Jets• Signs of interactions with other galaxies

Quasars• Brightest known objects in the universe.• Known as “quasi-stellar objects” (QSO or

quasars for short) due to star-like appearance. Look like stars since they are so far away from us.

• Radiation is nonstellar and may vary over time.• Some show jets and extended emission

features.• Most emit their energy in the infrared.• Believe they are the cores of distant active

galaxies (galaxies are too faint to be seen).

Common Properties of Active Galactic Nuclei

• They have high luminosities.• Their energy emission is mostly nonstellar.• Their energy output can be highly variable.• They often exhibit jets and other signs of explosive activity.• Their optical spectra may show broad emission lines, indicating rapid internal

motion within the energy-producing region.

Energy Production

• Supermassive black hole - large amounts of energy are produced as matter spirals down onto the central object. Matter emits large amounts of radiation as it is heated to high temperatures due to friction within the accretion disk.

• Instabilities in the accretion disk can cause varying energy output.

• Jets are a common feature of accretion flows, although not sure how they are formed (linked to strong magnetic fields).

Energy Emission• Emission spans infrared to X rays.• Much of the higher energy radiation is absorbed

by dust and then emitted in the infrared (reprocessing).

• Synchrotron radiation - produced as particles spiral around magnetic field lines. Not related to the temperature of the radiating object.