Galaxies: Their Structure and Evolution

Physics 360/Geology 360AstronomyJ. Swez

Slides which are important are marked by

Hubble Deep Field

Island Universes or Nebula?

• Galaxies appear as either nearby gaseous nebula or else ‘island universes’ with stars like our galaxy

• Finding their distances was required to tell the difference

• Edwin Hubble found the distance to the Andromeda galaxy with a Cepheid Variable

• Nearest Galaxy Large Magellanic Cloud, 150,000 ly from the Sun

When did the study of galaxies begin?

*Charles Messier discovered galaxies as early as the 28th century. He began the Messier Catalog (e.g. M31)

*Sir William Hershel (19th century) began the classification system known today as NGC (New General Catalog). Sir Hershel was also known for his advances in photography.

*Edwin Hubble (1899-1853) classified galaxies in the 20th century in terms of Spiral, Elliptical and Irregular.

*The key to galactic classification was the measurement of distances.

*The key to the measurement of distances was Cephid Varibles

How we found distances for remote stars. Remember this only works up to a 100 parsecs (or 250 parsecs from apace). Certainly not galactic distances!

Review Slide. This you should know from Chapter 12.

Variable Star Light Curves

Remember! Cephids are classed as Type I and Type II depending on the shape of their period curve.

Pulsating Star HR Diagram

Review Slide! You should already know about HR diagrams.

Cepheid Period-Luminosity Relationship

• Cepheid variables are luminous variable stars

• The period of their variation is regular• The period corresponds to luminosity

(Period Luminosity Law) Cepheids are good distance indicators

(standard candles) More Slowly they Pulsate—More Luminous RR Lyrae 40 L, Cephids up to 106 L (an L is a

solar mass unit)

Distance From Cepheids

Hubble Tuning Fork Diagram

An organizational tool, not necessarily tied to structure or evolution of galaxies.

From Thomas Arny (text) page 485

The Hubble “Tuning Fork” Further Explanation

1. This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

Spiral Galaxies

Barred Spiral Galaxies

The giant elliptical galaxy M87, also called Virgo A, is one of the most remarkable objects in the sky. It is perhaps the dominant galaxy in the closest big cluster to us, the famous Virgo Cluster of galaxies

M87's diameter of apparently about 7' corresponds to a linear extension of 120,000 light years, more than the diameter of our Milky Way's disk. However, as M87 is of type E1 or E0, it fills a much larger volume, and thus contains much more stars (and mass) than our galaxy, certainly several trillion (10^12) solar masses (J.C. Brandt and R.G. Roosen have estimated 2.7 trillion). This galaxy is also of extreme luminosity, with an absolute magnitude of about -22.

•This is a 2048x2048 CCD image of the Coma Cluster. This cluster has a recessional velocity of about 7000 km/s and is the densest cluster in our local region of the Universe. In contrast to the Hercules cluster, Coma has almost no spiral galaxies in its central regions. The cluster is strongly virialized and has a hot intracluster medium which generates strong X-ray emission. This image shows the central few 100 kpc of the Coma cluster. At a slightly larger radius , images reveal that Coma is still rather devoid of spiral galaxies. It is generally believed that the cold hydrogen gas in the disks of spiral galaxies is swept out of them as they orbit through the intracluster medium of Coma.

An example of clusters of galaxies

Elliptical Galaxies

•Malin 1 is an excellent example of a very Low Surface Brightness Spiral Galaxy. This galaxy was only recently discovered and can barely be seen in this 30 minute exposure using a 100-inch telescope in Chile. The galaxy disk covers about half of the image Frame yet is barely visible. There are lots of these out there. http://zebu.uoregon.edu/images/malin1.gif

M77 Spiral Galaxy M77 (NGC 1068), type Sb, in Cetus Discovered 1780 by Pierre Méchain.

This magnificient galaxy is one of the biggest galaxies in Messier's catalog, its bright part measuring about 120,000 light years, but its faint extensions (which are well visible e.g. in the DSSM image) going perhaps out to nearly 170,000 light years. Its appearance is that of a magnificient spiral with broad structured arms, which in the inner region show a quite young stellar population, but more away from the center, are dominated by a smooth yellowish older stellar population.

SO Galaxy

Irregular Galaxies

•M101 is a High Surface Brightness Galaxy: It has lots of active star formation and many H II regions. This exposure was through a blue filter with an exposure time of 30 seconds with a 52-inch telescope using 4.85:1 re-imaging optics in front of the CCD

http://zebu.uoregon.edu/images/m101big.gif

Can you classify M101 according to the Hubble Classification?

•This is a 2048x2048 CCD image of the Hercules cluster. This cluster has a recessional velocity of 11,000 km/s. It is noteworthy in that it contains a number of spiral galaxies, many of which are intereacting. This is a sure sign that the cluster still has substructure and is not fully virialized yet.

Andromeda Galaxy M31 “a color picture” Spiral Galaxy

M31 (NGC

224), type Sb,

Andromeda

Galaxy among the brightest

of the Messier Galaxies

Elliptical Galaxy M32 (NGC 221), type E2, in Andromeda A Satellite of the

Andromeda Galaxy, M31

M32 is the small yet bright companion of the Great Andromeda Galaxy, M31, and as such a member of the Local Group of galaxies. It can be easily found when observing the Andromeda Galaxy, as it is situated 22 arc minutes exactly south of M31's central region, overlaid over the outskirts of the spiral arms.

•M31+N205 from McDonald Observatory 10/27/95

How would you classify this galaxy (lower left of picture)?

M51 (NGC 5194), type Sc, in Canes Venatici The distance to this galaxy is thought to be 37 Mly (million light years) but a recent (2001) STScI Press Release gave 31 million light years. According to our present understanding, the pronounced spiral structure is a result of M51's current encounter with its neighbor, NGC 5195 (the fainter one in Messier's description).

Discovered in 1773 by Charles Messier as the famous Whirlpool Galaxy. Its companion, NGC 5195, was discovered in 1781 by his friend, Pierre Méchain

http://www.seds.org/messier/galaxy.html

Galaxy TypesDwarf

EllipticalGiant

EllipticalLenticular

(S0)Spiral

(Sa and SBa)Spiral

(Sb and SBb)Spiral

(Sc and SBc)Irregular (Ir)

Percent of allGalaxies(probable)

~60 <1 … ~30 ~10

Percent of allGalaxies(observed)

~10 <1 <1 ~80 ~10

Dust None None Some Some SomePercent of mass asHydrogen clouds

0 0 0 1 3 9 20

Stellar Populations II II II I and II I and II I and II IDominant Color

Red Red RedRed (Halo, Core)

Blue (arms)Red (Halo, Core)

Blue (arms)Red (Halo, Core)

Blue (arms)Blue

Spectrum ofCenters

K K K K-G G F F-A

Luminosity (insolar luminosities)

105 1011 108-1010 107-109

Diameter (kilo-light years, kly)

>1 500 10-150 1-50

Mass (in solarmasses)

106-109 109-1012 ~5x1011 ~3x1011 ~1011 109

Mass/LuminosityRatio

1-5 5-80 50 20 10 5 3

Galactic Evolution

• The age of the oldest stars in each galaxy is about the sameAll galaxies are about the same age

• Galaxies are distinguished by the amount of star-forming material

• Different galaxy types correspond to:• different star formation rates, or• large scale changes in gas/dust content

Galaxy Collision - core

Galaxy Collision

Galaxy Collisions

Ring Galaxies

A result of galactic collisions. The central gas is disrupted.

Galactic Collisions / Mergers/Cannibals

• Galaxies may collide• Collisions occur between gas, not stars• Collisions -> rapid star formation / gas

loss• This produces ring galaxies and

ellipticals• A few mergers -> giant galaxyGalactic cannibals and giant ellipticals

Galaxy Clusters

• Poor Clusters• 10-100 galaxies• Spirals, Irregulars and lastly dwarf

ellipticals• Local Group - our galaxy cluster

• Rich Clusters• Thousand + galaxies• Ellipticals, SO, with spirals at edges only• Giant Ellipticals at the center

• Our ‘poor’ cluster of galaxies• Dominated by Milky Way and Andromeda • Probable additional large galaxy near

Milky Way

Local Group

Rich Galaxy Clusters and Local Supergroup

• Rich clusters have thousands of galaxies

• Hercules cluster

• Galaxy clusters organize into groups

• Local Supergroup

Large Scale Structure

Large scales show a bubble and void structure

Active Galaxies / Radio Galaxies/Quasars

• Read about them in the news!• Active Galaxies

• Galaxies that have bright, varying cores. Abnormally lare radiation from a tiny region in the galactic core

• Radio Galaxies• Galaxies with massive radio emissions from jets

• Quasars• Distant active/radio galaxies

• Seyfert Galaxies a spiral galaxy with abnormally luminous core from a region less than a ly year across

• Jets in Quasars sometimes move with superluminal motion• Most distant Quasar about 10 billion ly

Radio Galaxies

Some galaxies show massive, radio emissionsThese are tied to massive jets in the core

Galactic Accretion Disk

Core!

Jet from core of M87

Accretion Disk and Jet Formation

In 1914 Vesto Slipher (lived 1870--1963) announced his results from the spectra of over 40 spiral galaxies (at his time people thought the ``spiral nebulae'' were inside the Milky Way). He found that over 90% of the spectra showed redshifts which meant that they were moving away from us. Edwin Hubble and Milton Humason found distances to the spiral nebulae. When Hubble plotted the redshift vs. the distance of the galaxies, he found a surprising relation: more distant galaxies are moving faster away from us. Hubble and Humason announced their result in 1931: the recession speed = H × distance, where H is a number now called the Hubble constant. This relation is called the Hubble Law and the Hubble constant is the slope of the line.

The Hubble Law

The Redshift and the Hubble Law

In 1920’s astronomers discovered galaxies are moving away from one another

The spectrum of a galaxy is the spectrum of all its component stars added together

If a galaxy is moving toward or away from us its spectral lines will be Doppler-Shifted-e.g. motion away from us lengthens the wavelength

Nearly all galaxies are moving away

Hubble discovered in 1920 that V = HD where H is the Hubble Constant = 65 km/s per Mpc

Thus by measuring the red shift we can find the galactic distance (See Problem 16.2)

Left: Red Shift for a galaxy nearer; Right Red Shift for a more distant Galaxy

The Hubble Law as applied to the recession of galaxies

Gravitational Lenses

How do they Work?Einstein—matter bends light, see Fig. 16.26

Why do we think they ExistMultiple quasar images but idential spectral history

Existence of Black Holes and Dark Matter verified?

Why is there believed Dark Matter to Exist

• The line with the dots is the theoretical curve for a galaxy consisting only of stars

• The observed line is with the x’s

• Only the material between the star’s orbit and the galaxy’s center contributes to the gravity force

• Something must keep the outer stars from flying out

• Answer: Dark Matter

Figure from: Arny, an Introduction to Astronomy p 495