ASTR 1514 1st Edition Lecture 34Galaxies – Black HolesChapter 14.4 and 15.4Black Holes in the Milky Way- The Galactic Center- There is a lot of dustin our line of sight to the galactic center- Only one optical photon per every trillion can pass through to us- We cannot see the galactic center in optical light - The Galactic Center is in the constellation Sagittarius - Radio observations show that the center is very small – less than 1 AU in radius- Only a black hole could be that small and dense- Observations in infrared show that the stars near the center of the galaxy move very fast- However stars orbiting the small center are very close to it and move very fast- It must be a massive black hole.- There is a 4 million solar mass black hole in the center of the Milky Way- X-Rays can also penetrate gas and dust- X-ray observations of the center of the galaxy show a faint X-ray source that has properties similar to other black holes. Black Holes in Other Galaxies- Doppler Shifts tell us how fast gas clouds are moving- Newton’s Laws tell us the size of the massive object in the center. - Astronomers find that the black hole size is proportional to the galaxy size- Larger galaxies have larger black holes. - Black hole size is proportional to the mass of the galaxy bulge Black Hole/ Galaxy Bulge Relationship- Black hole sphere of influence (the region that it will pull matter in from) is very small.- Current thinking is that the black hole and galaxy grew together since their formation.Black Holes – Active Galaxies and Quasars - In a small fraction galaxies, the central region is very bright – can be brighter than all fo the stars in the galaxy - These are active galaxies - Active galaxies and quasars are the most luminous persistently-emitting objects in the universe- The origin of their immense power is thought to be the conversion of gravitational energyto thermal energy (and other forms of energy) as gas falls into a super massive black hole.- We know these objects are small because their X-Ray emissions can change very dramatically in a short amount of time.- So an active nucleus can only vary by a large factor in a small amount of time if it is very compact- A black holes event horizon is small – about the size of our solar system. Immense Power- Luminosity of AGN and quasars ranges from 250 million to 2.5 trillion times the suns luminosity Black Hole Accretion - Immense power is produced through the conversion of gravitational energy to thermal energy - If a black hole is present, more gravitational energy is present the closer you get - You can get a lot of energy out of a black hole accretion - The amount of energy emitted depends on how much gas is falling in. Jets- When too much gas falls into a quasar the quasar has to expel some energy- It does this through jets and radio lobes- Formation of jets is thought to be caused by the spinning of black holes and magnetic field lines. Jets produce Radio Lobes- After emerging from the quasar the jet keeps going, spewing energy into the intergalactic medium- They produce radio lobes, large structures of radio-emitting gas- These radio lobes can be up to 1 million light years long- Black Hole accretion Powers the jet and radio lobes. Black holes in normal galaxies like the Milky Way have little gas falling into them, these black holes are quiescent Interacting Galaxies- Average separation of galaxies is only 20 times their diameter, while average distance between stars is about 10 million times their diameter. - So collisions of galaxies may be inevitable - Collisions: Stars don’t collide but gas and dust collide. - Irregular galaxies are often the consequence of collisionsCollisions in Clusters- Collisions may be very common in clusters of galaxies- Often clusters have a giant elliptical galaxy in their center that may have been formed from collisions Effects of Interactions- Distortions: Tidal tails and shells of stars- Can destroy Spiral Structure: Elliptical galaxy may result - Can Stimulate spiral structure - Ignite Star Formation: shocks cause compression of gas clouds, and star formation results.- May produce irregular galaxies. - Our own Galaxy is interacting with others in the Local Group- The Andromeda galaxy is predicted to collide with the Milky Way galaxy in 4 billion yearsInteractions and AGN Fueling- Some gas can be funneled down into the black hole of a galaxy, causing the quiescent galaxy to become active - Some studies have found a higher incidence of AGN in clusters of galaxies- The MilkyWay is not undergoing any strenuous interactions right now – just swallowing some smaller galaxies. - It is possible that the Black Hole in the Milky Way was active in the past- The Fermi Satellite discovered giant gamma-ray bubbles perpendicular to the