Chapter 19 Galaxies Hubble Ultra Deep Field: Each dot is a galaxy of stars. More distant, further into the past halo disk bulge Barred Spiral Galaxy: Has a bar of stars across the bulge Spiral Galaxy 1
Elliptical Galaxy: All spheroidal component, virtually no disk component Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical) What type? 1) M104 Irregular Galaxy: closest neighbor, Large Magellanic Cloud 2) NGC4881 3) NGC3370 2
4) NGC1365 5) NGC4214 Quiz Answers 1. M104 Spiral 2. NGC4881 Elliptical 3. NGC3370 Spiral 4. NGC1365 Barred Spiral 5. NGC4214 Irregular Good at this? www.galaxyzoo.org Distribution of Stellar Orbits Disk stars: nearly circular orbits around the galactic core (yellow orbits in the picture). Individual stars bob up and down due to gravity of the disk itself. This bobbing motion gives the disk its thickness. Distribution of Stellar Orbits Bulge stars: orbits are less organized (red lines in the picture). Orbits are elliptical, but randomly oriented. Halo stars: similar to stars in the bulge (green lines in picture) Orbits are actually a clue to how the galaxy formed! Distances to Galaxies Luminosity is spread over larger area 4π (radius) 2 with distance. If you know luminosity and brightness, you can estimate distance. A standard candle is an object whose luminosity is known; helpful if it is very luminous, seen to great distance. 3
Cepheid variable stars are very luminous. Luminosity related to pulsation period. Cepheid variable stars: measure period estimate luminosity compare with brightness get distance! Spiral nebulae: local clouds or Island universes? Nature of Spiral Nebulae? Edwin Hubble (1924) found Cepheid variable stars in M31 using the Mt Wilson 100-inch telescope: Distance = 700 kpc (23x Milky Way diameter) Standard Candles Cepheid and other variable stars Brightest stars Novae Supernovae (esp. white dwarf explosions) Mass luminosity relations for entire galaxies (Tully Fisher Relation) Milky Way just one of billions of galaxies in visible universe! 4
Hubble s Law Universal recession: Slipher (1912) and Hubble found that all galaxies seem to be moving away from us: the greater the distance, the higher the redshift Hubble s Law The relationship is set by Hubble s constant H 0 : The current value for Hubble s constant is Measure Doppler shift for velocity Hubble s law will then give distance. Best way to make large scale maps of Universe. Mass & Galactic Rotation Curves Orbital speed as a function of distance from the center: rotation_of_spiral_galaxy.htm Use Kepler s Third Law to get enclosed mass: mass_vs_dist_galaxy.htm Dark Matter in Milky Way: Rotation speed of stars at different distances from the center gives the total mass interior to orbit. Most of the Milky Way s mass seems to be dark matter! The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. Mass within Sun s orbit: 10 11 M Sun Total mass: 10 12 M Sun We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H 5
Thought Question What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk? Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter. Same is true for elliptical galaxies (from star orbits). A. Its mass is concentrated at the center B. It rotates like the solar system C. It s especially rich in dark matter D. It s just like the Milky Way Dark matter in galaxy clusters: 1. Mass from galaxy motions in a cluster is about 50 times larger than the mass in stars! 2. Clusters contain large amounts of X-ray emitting hot gas Find mass needed to keep gas from expanding away (so that the escape velocity is larger than the thermal velocity): 85% dark matter 13% hot gas 2% stars Thought Question What kind of measurement does not tell us the mass of a cluster of galaxies? 3. Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster s mass. A. Measure velocities of cluster galaxies B. Measure total mass of cluster s stars C. Measure temperature of its hot gas D. Measure distorted images of background galaxies 6
What is dark matter? Ordinary Dark Matter (MACHOS) Massive Compact Halo Objects: red dwarfs, brown dwarfs, big planets, and/or white dwarfs in halos of galaxies Extraordinary Dark Matter (WIMPS) Weakly Interacting Massive Particles: mysterious neutrino-like particles *** currently favored explanation *** WIMPs can t contract because they don t radiate away their energy. But their gravity helps regular matter condense in galaxies. http://vimeo.com/22956103 Active Nucleus in M87 If the center of a galaxy is unusually bright we call it an active galactic nucleus Quasars are the most luminous examples; large redshifts (Hubble s Law) Characteristics of Active Galaxies Luminosity can be enormous (>10 12 L Sun ) Luminosity can rapidly vary (comes from a space smaller than solar system) Emit energy over a wide range of wavelengths (contain matter with wide temperature range) Some drive jets of plasma at near light speed 7
Radio galaxies don t appear as quasars because dusty gas clouds block our view of accretion disk Radio galaxies contain active nuclei shooting out vast jets of plasma that emits radio waves coming from electrons moving at near light speed Blazar Quasar or Seyfert Galaxy Accreting, Supermassive Black Holes Radio Galaxy What we see depends on orientation Gravitational potential energy of matter falling into black hole turns into kinetic energy. Friction in an accretion disk turns kinetic energy into thermal energy (heat). Heat produces thermal radiation (photons). Active Galactic Nuclei and Galaxies AGN are always found in the centers of host galaxies. We know most galaxies have a supermassive black hole at their centers (1 million or more M sun). Conclusion: the supermassive black holes of AGN are actively accreting matter while those of quiescent galaxies (like our Milky Way) are not! Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion M Sun See Active Galaxy movie next. 8
Thought Question What can you conclude from the fact that quasars usually have very large redshifts? A. They are generally very distant B. They were more common early in time C. Galaxy collisions might turn them on D. Nearby galaxies might hold dead quasars E. All of the above 9