| Reminders/Assignments | |
| Quasars and Active Galactic Nuclei |
| FIRST/NVSS and 2MASS reports | ||
| Discussion/comments/questions regarding the Physics Today Articles? | ||
| Research Homework Update | ||
| Dan finished phase 1, will start phase 2 | ||
| Chris, Sey, and Cassandra primary now | ||
| Presentation topics | ||
| Read Shields “A Brief History of AGN” astro-ph/9903401 | ||
| Skip astro-ph Friday -- evaluations | ||
| Last week of class, 2 Wed., 4 Fri. WHO WHEN? | ||
| Plan for 20 minutes, plus questions. | ||
| Other astronomy faculty invited. | ||
| Topics: | ||
| Gunn-Peterson Test and Reionization | ||
| Supernovae and the Accelerating Universe | ||
| Weighing Supermassive Black Holes | ||
| The Hubble Deep Fields | ||
| Sub-millimeter (SCUBA) Galaxies | ||
| X-ray Background | ||
| Background in papers/proposals | ||
| Tasks | ||
| Identify from the SDSS EDR spectra (two levels) – lots of effort now vs. later | ||
| Morphology from SDSS images (two levels) | ||
| Bruzual & Charlot ISB modeling | ||
| Quasar measurements, derived properties | ||
| Compiling statistics, correlation analyses (lots of effort later vs. now) | ||
| Oversight, science, figures/tables, etc. | ||
| Will require individual meetings | ||
Quasars and Active Galactic Nuclei (AGNs)
The (slightly) active nucleus of our galaxy
| Probable Black hole | ||
| High velocities | ||
| Large energy generation | ||
| At a=275 AU P=2.8 yr Þ 2.7 million solar masses | ||
| Radio image of Sgr A* about 3 pc across, with model of surrounding disk |
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| A small fraction of galaxies have extremely bright “unresolved” star-like cores (active nuclei) | |
| Shown here is an HST image of NGC 7742, a so-called “Seyfert galaxy” after Carl Seyfert who did pioneering work in the 1940s |
NGC4151 with a range of exposures
Spectra of Stars, Spectra of AGNs
| Small fraction of galaxies have
extremely bright “unresolved” star-like nuclei |
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| Very large energy generation | |||
| Brightness often varies quickly | |||
| Implies small size (changes not smeared out by light-travel time) | |||
| High velocities often seen (> 10,000 km/s in lines) | |||
| Emission all over the electro-magnetic spectrum | |||
| Jets seen emerging from galaxies | |||
Many Views of Radio Galaxy Centaurus A
The Central Engine of Centaurus A
| http://imgsrc.stsci.edu/op/pubinfo/pr/1998/14/content/centauf.mov |
| Black hole is “active” only if gas is present to spiral into it | ||
| Isolated stars just orbit black hole same as they would any other mass | ||
| Gas collides, tries to slow due to friction, and so spirals in (and heats up) | ||
| Conservation of angular momentum causes gas to form a disk as it spirals in | ||
Different Views of the Accretion Disk
| The torus of gas and dust can block part of our view | ||
| Seyfert 2 galaxies: Edge on view Only gas well above and below disk is visible See only “slow” gas Þ narrow emission lines |
||
| Seyfert 1 galaxies: Slightly tilted
view Hot high velocity gas close to black hole is visible High velocities Þ broad emission lines |
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| BL Lac objects: Pole on view Looking right down the jet at central region Extremely bright – vary on time scales of hours |
||
| Quasars: Very active AGN at large
distances Can barely make out the galaxy surrounding them Were apparently more common in distant past |
||
Different Views of the Accretion Disk
| The torus of gas and dust can block part of our view | ||
| Seyfert 2 galaxies: Edge on view Only gas well above and below disk is visible See only “slow” gas Þ narrow emission lines |
||
| Seyfert 1 galaxies: Slightly tilted
view Hot high velocity gas close to black hole is visible High velocities Þ broad emission lines |
||
| BL Lac objects: Pole on view Looking right down the jet at central region Extremely bright – vary on time scales of hours |
||
| Quasars: Very active AGN at large
distances Can barely make out the galaxy surrounding them Were apparently more common in distant past |
||
Different Views of the Accretion Disk
| The torus of gas and dust can block part of our view | ||
| Seyfert 2 galaxies: Edge on view Only gas well above and below disk is visible See only “slow” gas Þ narrow emission lines |
||
| Seyfert 1 galaxies: Slightly tilted
view Hot high velocity gas close to black hole is visible High velocities Þ broad emission lines |
||
| BL Lac objects: Pole on view Looking right down the jet at central region Extremely bright – vary on time scales of hours |
||
| Quasars: Very active AGN at large
distances Can barely make out the galaxy surrounding them Were more common in distant past |
||
Quasar Images III: “Starburst-Quasar”
| Need a supply of gas to feed to the black hole | ||
| (Black holes from 1 million to >1 billion solar masses! | ||
| Scales as a few percent of galaxy bulge mass.) | ||
| Collisions disturb regular orbits of stars and gas clouds | ||
| Could feed more gas to the central region | ||
| Galactic orbits were less organized as galaxies were forming, also recall the “hierarchical” galaxy formation | ||
| Expect more gas to flow to central region when galaxies are young => Quasars (“quasar epoch” around z=2 to z=3) | ||
| Most galaxies may have massive black holes in them | ||
| They are just less active now because gas supply is less | ||