A Universe of Galaxies

Transcription

1 A Universe of Galaxies Today s Lecture: Other Galaxies (Chapter 16, pages ) Types of Galaxies Habitats of Galaxies Dark Matter

2 Other Galaxies Originally called spiral nebulae because of their shape. A big controversy in the early 1900s was: Are these nebulae gas clouds in Milky Way or other universes? Edwin Hubble observed very faint Cepheid variable stars in some of them. Cepheids are evolved supergiant stars that brighten and fade periodically as their size oscillates. Remember, these are standard candles. If Cepheids appear faint, then they must be way outside the Milky Way Galaxy. From the distance and angular size of spiral nebulae, Hubble deduced that they are HUGE stellar system, island universes

3 Two Main Hubble types 1) Spiral Galaxies (like Milky Way Galaxy): new stars forming in spiral arms. Old stars also in the disk and dominate halo. 2) Elliptical Galaxies: no disk, no arms, very little gas and dust. Consist of mainly old stars. Biggest have M sun.

4 Other types of Galaxies Irregular: Galaxies with no particular shape, like the Large Magellanic Cloud. Peculiar: Looks like a spiral or elliptical galaxy, but with strange features (many are now known to be merging galaxies) S0 (Lenticular): These have the shape of a spiral galaxy, but no arms and little gas and dust, like the ellipticals.

5 Galaxy groups and clusters Most galaxies are in binaries, small groups, or clusters. For example, the Magellanic Clouds are gravitationally bound to the Milky Way. Andromeda also has bound dwarf galaxies. Local Group : 25 to 30 galaxies Milky Way and M31 (Andromeda) are the dominant galaxies; also includes M33 and two Magellanic Clouds. Diameter of ~ 10 6 parsec = 1 Mpc (megaparsec)

6 Galaxy groups, clusters (cont.) Vergo Cluster: over 1000 galaxies Covers 15º of the sky! Distance of 20 Mpc. Coma Cluster: 10,000 galaxies, mostly elliptical Dense environments preferentially have elliptical galaxies. Merging spiral galaxies probably makes elliptical galaxies. (But not all ellipticals formed this way) Superclusters: clusters of clusters. These are seen as bubbles with diameters of 30 Mpc or more.

7 Sloan Legacy Survey

8 The rotation curve of the Milky Way tells us Mass enclosed within a circle of radius R from the center of our Milky Way Galaxy is M = V 2 R/G (from Kepler s third law) For R = 8000 pc (Sun s distance from the center), we find M = M sun! If a typical star is 0.5M sun, then there are about 200 billion stars within the Sun s orbit.

9 Our Galaxy s Rotation Curve is Weird!

10 Why the flat rotation curve? The flat rotation curve implies that mass of a galaxy grows linearly with distance beyond the Sun s orbit yet we don t see enough stars to account for this. If must be mostly invisible, dark matter! Dark matter also found in binary galaxies, groups, and large clusters of galaxies. Sometimes called missing mass -- but it s not really missing What is this mysterious dark matter?

11 Gravitational Lensing (Einstein Ring) Light from a distant object is bent by gravity of an intervening galaxy or cluster (warped spacetime!) A compact sphere lensed by a perfectly aligned, point-like lens, creates a ringed image: Einstein Ring When not as well aligned, you get multiple discrete images (sometimes called Einstein cross) The amount of lensing that occurs depends on how much mass is in the lens: more evidence for dark matter! About 90% of the total mass is dark matter!

12 Possible dark matter candidates MACHOs (massive compact halo objects): Black holes, neutron stars, brown dwarfs, white dwarfs, massive planets Golf balls Small, rock-like objects -- hard to detect and not clear how they would form. WIMPs (weakly interacting massive particles) Neutrinos? (recently found to have mass) Exotic subatomic particles that haven t been discovered yet?

13 Conclusion: Not enough MACHOs to account for all the dark matter. WIMPs are the current best bet! Contents of the Universe (Mass+Energy) Hot gas (10 6 K) may be about 3% of the mass/energy content of the Universe Stars and visible gas: about 0.5% MACHOs: perhaps 0.5% Total ( normal matter made of protons and neutrons): 4% The rest: not normal matter dark matter 26% (WIMPs?) and dark energy 70% (we ll discuss this later)