Chapter 17: Galaxies

Galaxy Types

Cosmic Distances

Cosmic Ages

Galaxy Evolution

Islands of Stars

• Hubble Deep Field: Tiny slice of the sky imaged during 10 days with HST.

• Galaxies of many sizes, colors and shapes.

• The observable universe contains > 8x1010 galaxies.

Galaxy Types

• Spirals have flat disks and a bulge. The disks are filled with cool gas and dust out of which new stars form. They usually display spiral arms.

• Ellipticals are redder and rounder than spirals. They contain very little cool gas and dust. They can be rather large (~1012 stars).

• Irregulars do not appear neither disklike nor rounded. They can be rather small (~108 stars).

Spiral Galaxies

• Similar structure to the Milky Way.

• Bulge+halo=Spheroidal component.

• Disk component contains ISM of gas and dust.

• Spirals with large bulges tend to have less gas and dust.

Peculiar Spiral Galaxies

Barred spiral galaxies have a straight bar of stars cutting across the center.

Lenticular (lens-shaped) galaxies have disks but do not appear to have spiral arms.

Groups of Galaxies

• ~75-85% of large galaxies are spiral or lenticular.

• Spirals are often found in loose groups.

• Our local group has two large spirals (Milky Way and M11).

• Lenticulars are common in clusters of galaxies which can contain up to few thousands of galaxies.

Elliptical Galaxies

• Ellipticals look like the bulge and halo of a spiral galaxy without a significant disk component.

• ISM in ellipticals is mainly hot, low-density, X-ray emitting gas. Little dust and cold gas.

• Large ellipticals make up ~50% of large galaxies in large cluster cores.

Dwarf Ellipticals

• Dwarf elliptical galaxies are small elliptical galaxies which have less than 109 stars and are often found near larger spiral galaxies.

• At least 10 dwarf elliptical galaxies belong to the Local Group.

Irregular Galaxies

• Miscellaneous class for galaxies that are neither elliptical nor spiral.

• Distant galaxies are more likely to be irregular than those nearby. Irregulars were more common when the universe was younger.

• The Magellanic Clouds are two small irregular galaxies that orbit the Milky Way.

Hubble’s Galaxy Classes

The galaxy classification system proposed by Edwin Hubble remains widely used. Letter E for elliptical with a number for the elongation. S for spirals. SB for barred spirals. SO for lenticulars. Lower case letters indicate the size of the relative sizes of bulge and disk.

Cosmic Distances

• Radar measurements of solar system size.

• Parallax measurements of the distances to nearby stars.

• Standard candles are objects for which we are likely to know the true luminosity. Some astronomical objects make good standard candles, but never perfect.

Main-Sequence Fitting

• Measure parallax to nearby star cluster (Hyades, Pleiades).

• Compare MS of distant cluster to that of a nearby one.

• Luminosity-distance formula (chapter 13): apparent brightness=L/4d2

Cepheid Variables

• Pulsating variable bright stars that follow a simple period-luminosity relation.

• The longer the time period between peaks in brightness, the greater the stellar luminosity.

• Cepheids are primary standard candles to determine distances in the Milky Way and other galaxies.

Hubble’s Law

• Edwin Hubble determined the distance to M11 (1924) and other spiral galaxies using Cepheids.

• In 1929, he announced that the more distant a galaxy is, the greater its redshift and hence the faster it is moving away from us.

• Hubble’s law: v=H0xd where v is the recession velocity, d stands for distance and H-naught is Hubble’s constant expressed in units of km/s/Mpc.

Using Hubble’s Law to Measure Distances

• We can use a galaxy’s recession velocity to determine its distance: d=v/H0

• However, galaxies may have peculiar motions that change their velocity, particularly in the Local Group and nearby galaxy clusters.

• The distances we find with Hubble’s law are only as accurate as our best knowledge of Hubble’s constant.

Measuring Hubble’s Constant

• One of the main missions of HST.

• Distant Cepheids can be measured with HST up to 30 Mpc (108 l.y.) reaching the nearest galaxy clusters such as the one in Virgo. However, this is not enough to calibrate Hubble’s constant.

White Dwarf Supernovae

• Cepheid distances are used to calibrate the distances to WD supernovae.

• HST has been used to determine the Cepheid distance to several historical WD supernovae.

• As expected WD supernovae are good standard candles.

Tully-Fisher Relation

• Both the luminosity and the rotation speed of a spiral galaxy depend on the mass, and hence they are connected with a simple relation.

• This relation allows to use large spiral galaxies as standard candles.

• As of 2002, H0=65 +/-10 km/s/Mpc.

The Distance Chain

• Radar ranging: Solar-system.

• Parallax: Solar-neighborhood.

• MS fitting: Milky Way.

• Cepheids: Galaxies up to 30 Mpc.

• WD supernovae and TF relation: Distant galaxies.

• Hubble’s law: Universe.

Universal Expansion

• The Universe is expanding as a whole.

• Gravity is working to slow the expansion rate and in the densest regions it wins and creates galaxies.

• Matter is evenly distributed on large scales (Cosmological Principle).

• No center, no edges.

Cosmic Ages

• Hubble constant=1/(age of the universe).

• It is called constant because it is the same at all locations in the universe.

• Age of Universe = 12-18 billion years.

• Cosmological redshift is the stretching out of photon wavelenghts due to the universal expansion.

Family Album of Galaxies

• Redshifts are used to estimate ages of galaxies.

• Each picture shows each galaxy at a single stage in its life.

• Younger galaxies appear smaller because they are more distant. They tend to be irregular in shape.

Cosmological Horizon

• It is a boundary in time, not in space. We cannot see back to a time before the universe began.

• Lookback time to the cosmological horizon is equal to the age of the universe.

Galaxy Evolution

• Our telescopes cannot yet see the details of what happened back in the time when galaxies formed the first stars.

• Theoretical models of galaxy formation assume that H and He gas filled all of space almost uniformly when the universe was very young. Certain regions were slightly denser than others.

• Within about 109 years after the BB, gravity overcame expansion in the denser regions and formed protogalactic clouds.

A Basic Model of Galaxy Formation

• Stars of the spheroidal population formed first from blobby clouds with little rotation.

• Stars of the disk population formed later after the cloud had been flattened by conservation of angular momentum.

Why Elliptical Galaxies have no Disks?

• Explanation A: If a protogalactic cloud has little angular momentum, its gas might not form a disk at all.

• Explanation B: If a protogalactic cloud is very dense, gravity could collapse clumps into stars before they have time to settle onto a disk.

• Distant giant galaxies have formed all their stars.

Galaxy Collisions

• The average distances between galaxies is not very much larger than their sizes.

• Collisions are inevitable. • The Antennae are a pair of

colliding spiral galaxies. • Distorted looking galaxies

were more common in the early universe when galaxies were closer together.

Simulations of Galaxy Collisions

• Computer models show that a collision between two spiral galaxies can create an elliptical galaxy.

• The central dominant galaxies found in dense clusters are usually giant elliptical galaxies that grew by consuming many other smaller galaxies.

Galactic Cannibalism

• The central dominant galaxy in the cluster Abell 3827 has multiple clumps that probably once were the centers of individual galaxies.

• Giant ellipticals can be over 10 times more massive than the Milky Way.

Starburst Galaxies

• The Milky Way produces an average of 1 star per year.

• Starburst galaxies form about 100 stars per year. It must be a short-lived phase.

• Superbubbles erupt out from the disk in a galactic wind.

• Starbursts result from tidal perturbations.

Active Galactic Nuclei and QSOs

• AGNs are unusually bright galactic centers, and quasars (QSOs) are the very brightest of them.

• They shine up to 1,000 times the Milky Way.

• QSOs are found primarily at great distances. They arouse when galaxies were young.

Seyfert Galaxies

• AGNs less powerful than QSOs that make up about 1% of the present-day galaxy population.

• Our best radio wave images show that AGNs must be smaller than 1 pc.

• Significant luminosity changes in ~hours imply a size similar to the Solar Sytem.

Radio Galaxies and Jets

• Radio galaxies are closely related to QSOs.

• Much of the radio emission comes from pairs of radio lobes produced by charged particles spiraling around magnetic fields at speeds near to c.

• Energy is transported by jets spurting from the nucleus.

The QSO 3C345

• Series of pictures show blobs of plasma moving outward from the QSO at close to c.

Supermassive Black Holes

• The energy in AGNs comes from matter falling into a supermassive black hole.

• Gravity converts the potential energy of the infalling matter into kinetic and thermal energy.

Evidence for supermassive BHs

• The relatively nearby AGN M87 features a bright nucleus and a jet.

• HST spectra shows a pattern of emission lines characteristic of gas orbiting around a BH with a mass of ~3x109 solar masses and a radius of up to 18 parsecs.

Loose Ends

• Where are the first stars?

• How did density enhancements first appeared in the universe?

• How giant black holes form?

• When do QSOs stop shining?

The Big Picture

• Chain of distance measurements culminating with Hubble’s law.

• Universal expansion implies a finite age of 12-16 billion years.

• Galaxies grown from protogalactic clouds of gas and collisions are important.

• Tremendous energy output of AGNs is probably powered by accretion onto a supermassive black hole, which still must remain in present-day galaxies.