Lecture 29 The Milky Way Galaxy Disk, Bulge, Halo Rotation Curve Galactic Center Apr 3, 2006 Astro 100 Lecture 29 1 Our Galaxy: "Milky Way" Milky, diffuse band of light around sky known to ancients. Galileo shows to be combined light from large number of faint stars. Knowledge of structure is based on indirect distance methods for distant objects: For stars close enough for heliocentric parallax (< 200 pc): Diskshaped, stars extend beyond observable region For stars with observable spectral types: disk shaped, numbers appear to fall off with distance from sun. Now known to be only apparent due to interstellar extinction by dust of about 1 mag/kpc in disk (1 kpc (kiloparsec) = 1000 pc). Most reliable: distances based on stellar "standard candles" like RR Lyrae stars (post Helium flash stars: pulsation period 1/2 day, luminosity 50 L sun ). Seen in globular clusters. Shapley (1915) finds largest numbers of globular clusters centered in region towards Sagittarius. Suggests is true center of Galaxy Modern distance to the center of the Galaxy: 8 kpc. Apr 3, 2006 Astro 100 Lecture 29 2
Milky Way Regions Large number of studies of distances to stars and interstellar material in Galaxy: four superposed regions. In all regions the density of stars drops off with distance from center (no exact boundaries): From inside out: Nucleus: Compact cluster of stars - few 100 parsecs in diameter. Interstellar gas/dust. Unknown energy source at very center. Central Bulge: Spherical distribution of stars - 5 kpc in diameter. Disk: Stars and gas in disk-shaped volume - 25 kpc in diameter. Solar System located out toward edge in disk. Halo: Spherical distribution of stars and globular clusters. Goes out beyond disk. Apr 3, 2006 Astro 100 Lecture 29 3 Milky Way Facts Total number of stars: over 10 11 (100 billion!) Plane ("Milky Way") is highly tilted (62 deg) to Earth's equatorial plane. "North Galactic Pole" is just between Leo and Ursa Major. Galactic positions are sometimes given in "galactic latitude and longitude", with longitude 0 in the direction of the center. Center (longitude 0) between Sagittarius and Scorpius Apr 3, 2006 Astro 100 Lecture 29 4
Galactic Rotation Galactic material must be in some kind of orbit around the center, or gravity would pull everything into the center! Rotation of Galaxy is real tough to measure (can't actually see rotation in our lifetime), but get a rough idea as follows, mainly from doppler radial velocities: 1) Assume stars in the flattened part of galaxy (the disk) are orbiting together, and stars in the spherical part (the bulge) are just boiling about (orbits are in all directions). Then can measure rotation of disk stars with respect to the bulge stars: Solar orbital motion - 250 km/sec in direction of Cygnus (perpendicular to the direction of the center in Sagittarius) Apr 3, 2006 Astro 100 Lecture 29 5 Motions in the Disk 2) Measure motion relative to the Sun of nearby stars/gas from doppler effect (radial motion) and "proper motion" (small angular motions accumulated over years): Nearby objects are orbiting at very close to same rate as Sun (no more than 20 km/sec relative motion) Objects in disk that are closer to the Galactic Center than we are are moving slightly faster, objects farther out are moving slower. Orbital velocity plotted vs distance from center is the rotation curve Interpreted as (nearly) circular motion, with stars closer in having shorter orbital periods than those farther out ("differential rotation"). Apr 3, 2006 Astro 100 Lecture 29 6
Galactic Orbits and the Mass of the Galaxy How long will we have to wait to verify circular orbit assumption? Solar Period = 2 π 8000 pc / 250 km/sec = 48000 3 10 13 km / 250 km/sec = 6x10 15 sec = 200,000,000 years! If true, the sun is only 25 "galactic years" old. How much mass inside sun's orbit is required to bend it around into a circle? Use Kepler's Third Law! Orbital Period = 200,000,000 years Orbital Size = 8000 pc = 1,600,000,000 AU Central Mass / M sun = A 3 /P 2 = 10 11! This is about right if you estimate the number of stars in there and multiply by an estimated mass/star. Apr 3, 2006 Astro 100 Lecture 29 7 The Dark Matter Halo However, there is a serious, difficult to explain problem if you try to do a similar estimate using the orbits of stars more distant from the Galactic Center: Find stars at great distance have orbital speeds just as large or larger than Sun's => estimate large mass for Galaxy out there, which cannot be accounted for by known stars. Current explanation: Galactic halo includes mass that is not stars, and not observable in light Dark matter (later for this) Apr 3, 2006 Astro 100 Lecture 29 8
The Galactic Center The nature of the center of the Galaxy is poorly known. Most evidence is from infrared and radio (some strong γ-rays, too). (interstellar dust blocks visible): About 1 kpc from center, expanding gas 100 km/sec => some explosive event about 10 million years ago About 100 pc across: radio source Sagittarius A (strongest radio source in sky). Synchrotron radiation (light from accelerated subatomic particles). Radio images show effects of strong magnetic field. About 10 pc across: pinwheel-like arrangement of gas < 1 pc across: "Sagittarius A*" almost point-like radio/infrared/x-ray source. Orbits of nearby stars => 3 x 10 6 M sun. Nucleus: Supermassive Black Hole? Most believe that this much mass must mean a black hole: IR, X-rays from Sag A* varies with t(var) ~ 1 minute => size < 1 AU 10 6 M sun is far above the limit for a neutron star. Apr 3, 2006 Astro 100 Lecture 29 9 The Milky Way Apr 3, 2006 Astro 100 Lecture 29 10
Milky Way Structure Apr 3, 2006 Astro 100 Lecture 29 11 Stellar Orbits in Milky Way Disk Orbits: Circular in Plane Bulge Orbits: Elongated, Random Apr 3, 2006 Astro 100 Lecture 29 12
Milky Way Rotation Curve Position of Sun Expected from number of stars Apr 3, 2006 Astro 100 Lecture 29 13 Galactic Center X-rays Apr 3, 2006 Astro 100 Lecture 29 14
Magnetic Arches Apr 3, 2006 Astro 100 Lecture 29 15 Galactic Center Stellar Orbits Black Hole? + Apr 3, 2006 Astro 100 Lecture 29 16
Galactic Center Black Hole Apr 3, 2006 Astro 100 Lecture 29 17