Quasars and Active Galactic Nuclei (AGN) Astronomy Summer School in Mongolia National University of Mongolia, Ulaanbaatar July 21-26, 2008 Kaz Sekiguchi
Hubble Classification M94-Sa M81-Sb M101-Sc M87-E0 M110-E5 M86-S0 M95-SBa M91-SBb M61-SBc Images from SEDS Messier Database http://seds.org/messier/
Irregular galaxies come in two types: Irr I : which are having characteristics "beyond" those of class Sc - high gas content, dominant presence of a young population. Irr I galaxies may show bar-like structures and incipient spiral structure like the Large Magellanic Cloud. Irr II : which are galaxies which defy classification because of some form of disturbance. M82, shown below, is undergoing an intense period of star-formation. Irr I : LMC Irr II : M82
Summary ~1/3 of all spirals are barred spirals. There are "Field" and "Cluster" galaxies. Ellipticals are most common in clusters. Spirals are most common in the "field." The Local Group is just outside the Virgo Cluster Poor clusters have ~ 10 galaxies Rich clusters have ~10,000 galaxies Size of rich clusters: ~10 Mpc Millions of clusters in the universe Large scale structures containing many clusters have been found.
Other types of Galaxies Peculiar Exploding, Ring, Disrupted Seyfert Very Bright Nucleus N Extremely Bright Nucleus Interacting Tidal Effects, Tails (pairs) QSO Collapsed Nuclei?
Galactic Nuclei Galaxy Nucleus: Exact center of a galaxy and its immediate surroundings If a spiral galaxy, it is also the center of rotation. Normal Galaxies: Dense central star cluster Show a composite stellar absorption-line spectrum (May also show weak nebular emission lines).
Active Galactic Nuclei About 1% of all galaxies have bright active nuclei. Bright, compact nucleus: Sometimes brighter than the entire galaxy. Strong, broad emission lines from hot, dense, highly excited gas. Rapidly Variable: Means they are small: only a few light days across. In general, about 30-50% of spiral galaxies show some level of activity in their nuclei, but only about 1% are truly dominant.
Discovery of Active Galaxies In 1943, astronomer Carl Seyfert noticed that certain nearby spiral galaxies have very bright, pinpoint nuclei. Spectra of these galaxies, now named Seyfert galaxies, showed that they have unusual spectra with very strong, often broad, emission lines.
*Low-luminosity AGN at z=0.04
Seyfert Galaxies Spectral lines don't resemble normal stars. - Highly ionized heavy elements, e.g. iron! - Lines very "wide" => tremendously hot (>10 8 K) or rapidly rotating (~1000 km/s) Nearly all emission comes from the galactic nucleus (a small central region). - ~ 10 4 times brighter than the center of our galaxy Most energy emitted in infrared and radio parts of the spectrum. - Look very much the same as normal galaxies in the visual. Emitted energy varies with time! Compact source of energy => Emitting region < 1 lyr across Some emit more light than the Milky Way!
Discovery of Active Galaxies (2) 1950s: Radio Galaxies First radio telescopes found faint galaxies at the location of intense radio emission. Also show broad emission-lines in their spectra (sometimes). Radio galaxies are usually elliptical. They often exhibit jet structure from a compact nucleus. They typically exhibit two lobes of radio frequency emission that are often approximately aligned with the jets observed in the visible spectrum and that may extend for millions of light years.
Radio Galaxies Very bright in the radio Core-Halo radio galaxies Most emission from a very small core (< 1 parsec across) Extended (or Lobe) radio galaxies Emissions extend hundreds of kiloparsecs!
Discovery of Active Galaxies (3) 1960s: Radio astronomers found intense, point-like sources of radio emission. Photographs revealed slightly fuzzy or "quasi-stellar" objects at these locations. The spectra were bizarre and full of unrecognized broad emission lines. Named them Quasars, short for Quasi-Stellar Radio Sources. 3C 273 - The "First" Quasar
Astronomer Maarten Schmidt on the front page of TIME Magazine in 1963 The first spectrum of 3C273 was obtained by Caltech's Maarten Schmidt using the Palomar 200" telescope. Schmidt puzzled over the photographic spectrum for months before he recognized that the strong, broad emission lines in the star were the familiar hydrogen- Balmer series, but redshifted by 15%.
It was not the 15% redshift that had puzzled Schmidt, galaxies were already known with much larger redshifts, but rather the brightness of 3C273. 3C273 was a thousand times brighter than even a very luminous galaxy would appear at a distance of 2 billion light years, corresponding to a redshift of 15.8%. Great distances imply extreme luminosities for Quasars.
AAT/ 2dF Spectroscopy * QSO at z=4.5
Quasar Variability and its Size Variability of the Quasar 3C279 from Harvard Survey Plates by Eachus & Liller
Size of the Quasar Suppose the "quasar" above flashed in brightness like a photographic flash. Light would begin travelling from all three points at the same time, but light from the center of the quasar would always be a light week behind light emitted from the front. Light from the center would reach an observer a week after detection of light from the front, and light from the back of the quasar would be detected another week later. This "flash" would be observed as a rise and fall in the brightness over a two week period.
The variability of 3C273, 3C279 and other quasars requires that the quasar produce its luminosity greater than a thousand galaxies of billions of stars from a region smaller than our solar system!
Quasi-Stellar Objects (QSOs) Galaxies with extremely luminous sources Look like stars in photographs. Some evidence of faint "parent" galaxy Energy originates in a very small region. QSOs are variable Can see to great distances Most distant objects seen in the universe Quasar: quasi-stellar radio source a subset which is a strong radio emitter
Galaxy Luminosity (L MW *) Normal < 10 Seyfert 0.5-50 Radio 0.5-50 Quasar 100-5,000 where LMW = Luminosity of Milky Way = 2x10 10 Lsun
Properties that need to be explained: Powerful: - Luminosities of Billions to Trillions of Lsun - Emit wavelengths from from Radio to Gamma-rays Compact: - Visible light can vary on timescales of a few days - X-rays can vary on timescales of a few hours or less!
What Powers AGNs? We need to produce up to 5,000 times the luminosity of the Milky Way yet within a region ~1 pc in size!!!! Leading theory is a black hole with an accretion disk: Material gains energy as it falls towards the black hole. The gas heats up, and radiates energy.
The energy source of active galaxies is the steady accretion of matter onto a supermassive Black Hole. Supermassive = 10 6-10 9 M sun Schwarzschild Radii: ~0.01-10 AU Infalling matter releases gravitational binding energy Gas settles into an accretion disk. The hot inner parts of the disk shine brightly, especially at X-rays.
Unification of Quasars and other types of AGN Many astronomers believe that the different types of Active Galaxies are really all the same type of phenomenon simply seen from different viewing angles with respect to the molecular torus described above.
Accretion Disk Model
In this case Einstein's famous equation becomes: E ~ 0.1 mc 2 which is to say that material falling into a black hole near the nucleus of a galaxy may release up to about 10% of its rest energy in the form of gravitational potential energy transformed into x- rays, relativistically moving particles, etc. which produce the phenomena that we see. The release of gravitational energy by a massive black hole (about 100 million solar masses) "eating" one star per year would power a typical quasar.
The Central Engine Black Hole accretion is very efficient: up to ~10% efficiency ~1 M sun /year of matter needed to power bright active galaxies Get their fuel from surrounding gas and stars Rapidly Spinning Black Hole: Acts like a particle accelerator Leads to the jets seen in radio-loud AGNs.
Energetics Material needs to keep flowing onto the black hole to power the source. 1 M sun / 10years => ~ 10 L MW 10,000 L MW => 100 M sun / year!! Large black holes (10 8-10 9 M sun ) are needed to fit the models, otherwise the accretion disks blow themselves apart.
Radius for Black Hole of a Given Mass Object Mass Black Hole Radius Earth 5.98 x 10 27 g 0.9 cm Sun 1.989 x 10 33 g 2.9 km 5 Solar Mass Star 9.945 x 10 33 g 15 km Galactic Core 10 9 Solar Masses 3 x 10 9 km
Key Ideas Active Galactic Nuclei Powerful energy sources in the nuclei of some galaxies. Types of Active Galaxies: Quasars Seyfert Galaxies Radio Galaxies Power source: Accretion of matter by Supermassive Black Holes
XMM-Newton X-ray Observatory
SXDS Field 02h18m -5º00 1.3º
x-ray deg 400 ksec of EPIC data from XMM/ Newton: 100 ksec on central field 50 ksec on 6 flanking fields
X-ray image of the SXDF
X-ray cluster candidates red: High green: Possible blue: Not confirmed purple: not covered
Cluster location & redshift