Introduction to Astronomy ! AST0111-3 (Astronomía) ! ! ! ! ! ! ! ! ! ! ! ! Semester 2014B Prof. Thomas H. Puzia So when we look out in the sky, what do we see???
! Where do we appear to live??? cosmic-ray electrons / “hot” ISM 180◦ 90◦ 0◦ 270◦ 180◦
warm (50K) gas
young star formation
cold (10K), dense gas
warm (50K) dust
hot (400K) dust
stars (unobscured)
stars (obscured)
hot (106 K) ISM (obscured)
cosmic rays + individual sources Models of the Milky Way over time Ok, we live in a disk. How do we know it has spiral structure? The Galaxy “Zoo” ! (http://www.galaxyzoo.org/)
“islands of stars” = galaxies all types of galaxies all orientations We can also map the atomic hydrogen 21 cm radio line in our disk. We find that: • gas clouds collect in spiral arms with distinct velocities Sun • gas close to the center moves fast • gas far away moves slower • can use these differential speeds and doppler effect to make a map through the disk of our galaxy. ! A bar?
The position of the Sun gives us a somewhat peculiar view of the galaxy. Is our galaxy symmetric or barred? In 1995 we confirmed the presence of a bar in the central region of the galaxy. The bar almost points directly toward the Sun, which made it more difficult to study. Dimensions?
The diameter of the galaxy disk is about 30 kpc (100,000 light years). However, it is difficult to measure the actual dimensions of the disk and the halo, because we are in the galaxy. In reality, we do not know exactly where the end our galaxy lies because there is no well-defined edge. Thus we do not know exactly how much total mass it contains. Within the halo of the Milky Way, we find several dwarf galaxies. The Galactic Halo + Nearby Galaxies
SGR
LMC
SMC A few galaxies do not appear to have haloes Pop II stars
Pop I stars Old clusters in our Galaxy
We find many globular clusters in the halo of the galaxy, some as far as 300000 light Recall: globular clusters formed stars in a single years away epoch, so can obtain distances to them via HR These globular clusters have elliptical orbits diagrams and standard candles. that make them pass very near the galactic center (Recall: Ecliptic + Oort cloud) There are ~160 known globular clusters in our Galaxy, although it is estimated that the original population could have been in the thousands. The majority have been destroyed by evaporation, collisions with the bulge, or passages through the disk.
Individual stars do same thing! How did our Galaxy form? Formation of the Milky Way
We do not know exactly how our galaxy formed some 12,000,000,000 years ago. Two debated theories: 1. Very rapid collapse of a giant cloud after the Big Bang 2. Accumulation of many small clouds that grew for a very long time. These protogalactic clouds would have eventually gathered and then shrank into the disk, where star formation could occur more or less continuously. Simulations of the formation of a spiral galaxy
http://www.youtube.com/watch?v=n0jRObc7_xo What is our Galaxy made of? 21 cm rotation curve of Milky Way
(Kepler's 3rd law) v2=GM/R The gravitational attraction should decrease very rapidly as we move away from the center of the galaxy (where most of the observable mass in stars and gas lies). But we do not see that! Ruh Roh! The missing mass problem: we see only 10% of the matter density. The remaining 90% is not seen directly. We call this missing mass “dark matter”. 90% of our galaxy is dark matter! Dark Matter in Other Galaxies too v2=GM/R Rotation Curve: Velocity vs. Distance FLAT means MR α R ! So there must be a large reservoir of dark matter in outskirts and haloes of galaxies
This is one of the biggest mysteries in astronomy today. Candidates for Dark Matter
Candidates for dark matter can be baryonic and non-baryonic. ! Non-baryonic (and weakly-interacting) dark matter would consist of elementary subatomic particles. For example: - neutrinos - WIMPs (weakly interacting massive particles) ! Baryonic dark matter would be made of normal matter, i.e. atoms and molecules. For example: - planets - brown dwarfs - white dwarfs - neutron stars - black holes Galactic Recycling
We are stardust.... ! The galaxy constantly reuses its gas and dust The Center of the Milky Way
The galactic center is obscured behind several clouds of dust and we cannot see anything directly in the optical (30 magnitudes of !optical extinction!!!!). But visible in IR! El Centro en IR
Deep IR images measure speed and acceleration of stars in the vicinity of center.! ! From them we require a black hole with a mass of
MBH = 2,500,000 M!! The Galactic Center
Neptune’s! Orbit http://www.youtube.com/watch?v=EvuV3GdVaY4&NR=1 The orbits of stars around the center of the galaxy indicate the presence of a massive object (BH). The Galactic Center in X-rays
BUT brighter in past!
CURRENTLY VERY WEAK (no “food”)
A black hole in the center of our Galaxy is eating material and emitting a powerful jet of radiation, which can have important effects on the environment nearby. In the Galactic center γ-rays are detected, as well as X-rays and radio waves. Black Hole in Andromeda High resolution images from the center of Andromeda taken by the HST reveal a dual core. One of the two nuclei is a black hole (flares in X-rays), the other is a star cluster. ! We think all galaxies host super massive black hole in center.
Black Hole in Andromeda