Published on August 29, 2007
Galaxies And the Foundation of Modern Cosmology: Galaxies And the Foundation of Modern Cosmology What are the three major types of galaxies?: What are the three major types of galaxies? Slide3: Hubble Ultra Deep Field Slide4: Hubble Ultra Deep Field Slide5: Hubble Ultra Deep Field Spiral Galaxy Slide6: Hubble Ultra Deep Field Spiral Galaxy Slide7: Elliptical Galaxy Hubble Ultra Deep Field Elliptical Galaxy Spiral Galaxy Slide8: Hubble Ultra Deep Field Spiral Galaxy Elliptical Galaxy Elliptical Galaxy Slide9: Irregular Galaxies Hubble Ultra Deep Field Spiral Galaxy Elliptical Galaxy Elliptical Galaxy Slide10: Spiral Galaxy disk bulge halo Slide11: Spheroidal Component: bulge andamp; halo, old stars, few gas clouds Disk Component: stars of all ages, many gas clouds Type Sa Galaxy Sa Galaxies: Sa Galaxies: Dominant nuclear bulge Tightly wound spiral pattern Few (but some) newly formed stars, HII regions or other evidence of active star formation Sa Galaxies Sb Galaxies: Sb Galaxies Moderate nuclear bulge Intermediate spiral pattern Some evidence for massive young stars, HII regions, star formation Slide14: Blue-white color indicates ongoing star formation Red-yellow color indicates older star population Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge andamp; halo, old stars, few gas clouds Type Sc Galaxy Sc Galaxies: Sc Galaxies (Some classify Messier as as Type Sd) Small to nearly non-existent nuclear bulge Open spiral pattern Active star-formation Slide16: Blue-white color indicates ongoing star formation Red-yellow color indicates older star population Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge andamp; halo, old stars, few gas clouds Barred Spiral Galaxy: Has a bar of stars across the bulge Barred Spiral Galaxy Barred Spiral Types: SBa SBb SBc Barred Spiral Types S0 Lenticular Galaxy: Has a disk like a spiral galaxy but very little dust or gas (intermediate between spiral and elliptical) S0 Lenticular Galaxy S0 Edge-on: Note the clear presence of a disk, but absence of dust band in this S0 galaxy: NGC 3115 S0 Edge-on Elliptical Galaxy:: All spheroidal (bulge) component, no disk Elliptical Galaxy: Slide22: Elliptical Galaxy: All spheroidal component, virtually no disk component Red-yellow color indicates older star population Irregular Galaxies: Irregular I Galaxy Blue-white color indicates ongoing star formation Irregular Galaxies Irr II Galaxy - Messier 82: Irr II Galaxy - Messier 82 Hubble’s Galaxy Classes: Spheroid Dominates Disk Dominates Hubble’s Galaxy Classes Slide26: How are galaxies grouped together?: How are galaxies grouped together? Slide28: Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies) Our Galaxy & Andromeda belong to a small “Local Group”: Our Galaxy andamp; Andromeda belong to a small 'Local Group' of about 20 or so galaxies Slide30: Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies) How do we observe the life histories of galaxies?: How do we observe the life histories of galaxies? Slide32: Deep observations show us very distant galaxies as they were much earlier in time (Old light from young galaxies) Slide33: Slide34: Slide35: Denser regions contracted, forming protogalactic clouds H and He gases in these clouds formed the first stars Slide36: Supernova explosions from first stars kept much of the gas from forming stars Leftover gas settled into spinning disk Conservation of angular momentum Why do galaxies differ?: But why do some galaxies end up looking so different? M87 NGC 4414 Why do galaxies differ? Slide38: Slide39: Why don’t all galaxies have similar disks? Nature: Conditions in Protogalactic Cloud?: Spin: Initial angular momentum of protogalactic cloud could determine size of resulting disk Nature: Conditions in Protogalactic Cloud? Conditions in Protogalactic Cloud?: Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk Conditions in Protogalactic Cloud? Distant Red Ellipticals: Distant Red Ellipticals Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe Slide43: We must also consider the effects of collisions Slide44: Collisions were much more likely early in time, because galaxies were closer together Slide45: Many of the galaxies we see at great distances (and early times) indeed look violently disturbed Slide46: The collisions we observe nearby trigger bursts of star formation Slide47: Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical Slide48: Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical Slide49: Shells of stars observed around some elliptical galaxies are probably the remains of past collisions Slide50: Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together Slide51: Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies What is the evidence for dark matter in galaxies?: What is the evidence for dark matter in galaxies? Slide53: We measure the mass of the solar system using the orbits of planets Orb. Period Avg. Distance Or for circles: Orb. Velocity Orbital Radius Slide54: Rotation curve A plot of orbital velocity versus orbital radius Solar system’s rotation curve declines because Sun has almost all the mass Slide55: Rotation curve of merry-go-round rises with radius Slide56: Rotation curve of Milky Way stays flat with distance Mass must be more spread out than in solar system Slide57: Mass in Milky Way is spread out over a larger region than the stars Most of the Milky Way’s mass seems to be dark matter! Slide58: Mass within Sun’s orbit: 1.0 x 1011 MSun Total mass: ~1012 MSun Slide59: The visible portion of a galaxy lies deep in the heart of a large halo of dark matter Slide60: We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H Slide61: Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter Slide62: Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting These galaxies also have dark matter Clusters of Galaxies: We can measure the velocities of galaxies in a cluster from their Doppler shifts Clusters of Galaxies Slide64: The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars! Slide65: Clusters contain large amounts of X-ray emitting hot gas Temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% stars Slide66: Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass Slide67: Slide68: All three methods of measuring cluster mass indicate similar amounts of dark matter Does dark matter really exist?: Does dark matter really exist? Dark matter really exists, and we are observing the effects of its gravitational attraction Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter Either: Bottom Line:: Bottom Line: What is the evidence for dark matter in galaxies? Rotation curves of galaxies are flat, indicating that most of their matter lies outside their visible regions What is the evidence for dark matter in clusters of galaxies? Masses measured from galaxy motions, temperature of hot gas, and gravitational lensing all indicate that the vast majority of matter in clusters is dark Our Options: Our Options Dark matter really exists, and we are observing the effects of its gravitational attraction Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter Because gravity is so well tested, most astronomers prefer option #1 Two Basic Options: Baryonic (Ordinary) Dark Matter (MACHOS) Massive Compact Halo Objects: dead or failed stars in halos of galaxies Extraordinary Dark Matter (WIMPS) Weakly Interacting Massive Particles: mysterious neutrino-like particles Two Basic Options Slide73: MACHOs occasionally make other stars appear brighter through lensing Slide74: MACHOs occasionally make other stars appear brighter through lensing … but not enough lensing events to explain all the dark matter Why Believe in WIMPs?: There’s not enough ordinary matter WIMPs could be left over from Big Bang Models involving WIMPs explain how galaxy formation works Why Believe in WIMPs?