Galaxies

1. Galaxies • Galaxies are clouds of millions to hundreds of billions of stars held together by their mutual gravity. • Often galaxies also contain enormous clouds of gas and dust from which new stars can form. • Galaxies can have many different shapes and sizes. • The distribution of galaxies across the Universe indicates that they generally appear in clusters with very large voids separating clusters from one another. The Andromeda Galaxy - nearest galaxy similar to our own. Only 2 million light years away!

2. Types of Galaxies • Galaxies are classified based on general characteristics and then further subdivided based on more specific characteristics. • Spiral galaxies • disk-shaped with spiral arms winding out from the center. • usually have clouds of gas and dust • usually have both young (Pop. I) and old (Pop. II) stars • some spirals have a rectangular-shaped bar through the central bulge and are referred to as Barred Spirals A Hubble Space Telescope view of the Whirlpool Galaxy M51

3. Types of Galaxies • Elliptical galaxies are always smooth in appearance. • can be spherical, egg-shaped or flattened in shape • usually have little or no gas and dust • usually only contain old stars (Pop. II) • Irregular galaxies have a random distribution of stars. • often have large clouds of gas and dust • often contain young stars (Pop. I)

4. Galaxy Collisions • galaxies were smaller in the past • collisions between galaxies appear to have been common in the early Universe • collisions can cause bursts of star formation as clouds of gas and dust collapse • galaxies may eventually merge together forming large elliptical galaxies A Hubble Space Telescope image of the Antennae Galaxies. Large streams of stars and gas are trailing off the galaxies while new stars are being formed near the center.

6. Active Galactic Nuclei (AGN) • Spiral (or disk) galaxies with nuclei that are more luminous than the rest of the stars in galaxy. • Spectrum of the nucleus is non-stellar • Luminosity of the nucleus may change over short (hours-months) • Some elliptical galaxies show radio-wave emitting jets on scales much larger than the visible light size of the galaxy

7. Quasars Originally detected in images as point-like (quasi-stellar) objects, although spectra are non-stellar Underlying faint host galaxies recently detected in some quasars by the Hubble Space Telescope Largest redshifts of any astronomical object Hubble law implies they are at great distances (as much as 10 billion light-years away) To be visible at those distances, they must be about 1000× more luminous than the Milky Way Based on output fluctuations, quasars resemble the AGNs of radio galaxies and Seyfert galaxies in that they are small (fractions of a light-year in some cases)

8. Measuring the Diameter of Astronomical Objects by Using Their Light Variability Technique makes three assumptions The rate at which light is emitted from an active region is the same everywhere in that region The emitting region completely defines the object of interest (there are no “dead” areas of significance) The speed of light is finite (a safe bet) The light variation then is just a measure of the time it takes light to travel across the active surface Multiplying this time by the speed of light gives the size of the emitting object

9. Measuring the Diameter of Astronomical Objects by Using Their Light Variability

10. Active Galactic Nuclei (AGN) • Large luminosity • Variable on short timescales • Implies small physical size • Can not fit enough luminous stars (or supernovae) into such a small region • A supermassive black hole is thought to be the source of energy for these AGNs. • The black hole likely formed initially as the remnant of a massive star supernova at the center of a galaxy • The black hole fed on the vast reservoir of gas in the galaxy nucleus • Eventually grows large enough to capture stars • Galaxy-galaxy interactions may cause gas infall within galaxy, supplying more fuel • The variable, luminous source is the accretion disk and associated gas clouds surrounding the Black Hole

11. Black Holes and Galaxy Formation • Black Holes may play a role in galaxy formation • Nearly all galaxy nuclei have a Black Hole (active, or inactive) • Black Hole mass is correlated with galaxy bulge mass 106 < M● < 109 solar-masses “Super-Massive!”

12. Distances to the Galaxies • Determining accurate distances to galaxies requires knowledge of the properties of stars. • From the luminosity of a star and its apparent brightness the star’s distance can be found. • Certain stars (called Cepheid variables) show regular patterns of variation in brightness. • The period of these variations are directly related to the stars luminosity. • So by measuring the time it takes these stars to vary in brightness and their apparent brightness their distance can be found. Cepheid stars are very bright and can be observed in nearby galaxies. Other methods must be used for more distant galaxies.

13. Recessional Velocity of Galaxies In 1911, it was discovered that all galaxies (with but a few exceptions) were moving away from the Milky Way Edwin Hubble found that these radial speeds, calculated by a Doppler shift analysis and called a recessional velocity, increased with a galaxy’s distance RPU Insert Figure 17.18a here

14. The Expansion of the Universe • Astronomers in the early 20th century found that Doppler shifts seen in the spectra of galaxies indicated that almost all galaxies are moving rapidly away from us. • Edwin Hubble, using careful determinations of galactic distances, showed that the farther a galaxy is from us the faster it appears to be moving away. • He showed that the velocity of the galaxy was simply equal to its distance times a constant. • This is known as Hubble’s Law and the constant is Hubble’s constant. • The recessional velocity is a consequence of the expansion of space. The exact value for Hubble’s constant has been one of the great problems in astronomy. It was one of the reasons for building the Hubble Space Telescope. Ho = 71 +/- 4 km/sec/Mpc

15. Galaxy Mass and Dark Matter • If you know the distance to a galaxy and measure its angular size, you can find its actual, linear size • Now measure the rotation curve: the velocity of stars as a function of distance from the center of the galaxy (x-symbols on plot) • Use Newton’s modification of Kepler’s third law to estimate a rotation curve for the light-emitting the mass in the galaxy (stars, gas, dust) (dot-symbols on the plot) • The observed rotation curve and the estimated curve do not match. • The mass derived from the observed rotation curve more than the estimated mass of the light emitting mass (stars, gas, dust) • Dark matter mass is estimated to be 10x mass of light emitting matter

16. Galaxy Clusters • Galaxies do not uniformly fill the universe but rather collect into clusters and superclusters spanning millions of light years. • The Milky Way is a member of a cluster called the Local Group.

17. Galaxy Clusters and Dark Matter:Galaxy Velocity • Astronomers find that galaxies often are moving in these clusters with very high speeds. The clusters should be flying apart but there appears to be enough mass to hold the cluster together. • This is more mass than can be accounted for just by gas, dust, and stars. • This invisible mass holding the cluster together may be 100x larger than all the visible mass and is another example of “dark matter”.

18. Galaxy Clusters and Dark Matter:Hot Gas in Clusters • The particle velocity of hot gas in galaxy clusters exceeds the escape velocity of the light-emitting matter • Infer that dark matter binds the observed hot gas to the cluster

19. Gravitational Lenses

20. Galaxy Clusters and Gravitational Lenses • Light can be bent as it passes close to massive objects. This is what happens in black holes and also happens in galaxy clusters. • Light from very distant galaxies can be bent around nearby galaxy clusters and appear as arcs or mirror images of the original galaxy. • Astronomers use these gravitational lenses to study galaxies that may have been too far to observe any other way. Hubble Space Telescope image of a gravitational lens formed by a galaxy cluster

21. Galaxy Clusters, Gravitational Lenses and Dark Matter • The lensing morphology depends on the mass distribution in the cluster • Lensing studies show that the mass of the cluster exceeds the mass of the light-emitting matter within the cluster  more evidence for dark matter in the universe

22. Dark Matter in Clusters • The figure shows two clusters that collided • The red regions shows the X-ray emitting shocked gas stripped out of the clusters by the collision • The blue regions show the estimated dark matter distribution of each cluster • Note that the dark matter seems to be disturbed less than the hot gas • Dark matter does not seem to interact strongly with itself, or has a faster relaxation time than normal matter

23. Dark Matter Candidates Dark matter cannot be: Ordinary dim stars because they would show up in infrared images Cold gas because this gas would be detectable at radio wavelengths Hot gas would be detectable in the optical, radio, and x-ray regions of the spectrum Objects that cannot be ruled out: Tiny planetesimal-sized bodies, extremely low-mass cool stars, dead white dwarfs, neutron stars, and black holes MACHOS: Massive Astrophysical Compact Halo Objects Subatomic particles like neutrinos Theoretically predicted, but not yet observed, particles WIMPS: Weakly Interacting Massive Particles

24. Large Scale Structure of the Universe • Two 2-D slices mapping angular position and distances of galaxies relative to the Earth • High redshift galaxy surveys show galaxies distributed in a pattern of filaments and voids

25. Large Scale Structure and Dark Matter • 3-D Snapshots of a large scale structure simulation as the universe evolves • Simulation input: expansion of the universe, mass of galaxies, gravity • Simulations that include dark matter result in large scale structure maps that show the observed filament + void morphology