Lecture 5 - Summary 1 Mass concentrations between us and a given object in the sky distort the image of that object on the sky, acting like magnifying lenses. Strong and weak lensing provide very accurate mass maps. Much more mass is detected than what is visible, strengthening the case for dark matter. The Hubble constant can be independently measured via lensing, as mass concentrations also stretch time, causing time-delays. Lensing can detect dark matter satellites as well, if any..
Lecture 5 - Summary 2 Most of the mass content in the Universe is unaccounted for: what is dark matter? If baryonic, it could be due to massive, compact halo objects (MACHOs). They dont seem to be enough. If non-baryonic (as likely), dark matter could be made of elusive, massive particles (WIMPs), or massive neutrinos. Searches have proved inconclusive so far. The possibility remains that Newtonian gravity does not apply everywhere..(next lecture).
To TA: Universe 24.47, 25.29, On your own: Universe > Assignments. Due Friday April 24, 4pm:
Lecture 6: Quasars, Active Galaxies and Gamma-Ray Bursters
Active galaxies are galaxies with exceptionally bright and compact nuclear regions, called Active Galactic Nuclei (AGN). The energy source of AGN is ultimately gravity, in the form of accretion of gas onto a super-massive black hole, one the most efficient engines in the Universe. Similarly, the collapse of a very massive star into a stellar mass black hole can give rise to the most violent cosmic explosions we know of: gamma-ray bursts. Lecture 6: Quasars, Active Galaxies and Gamma-Ray Bursters
THE AGN ZOO blazars quasarsQSOs Radio galaxies Seyferts
Q uasars very discovered (in the late 40s) as powerful radio sources, even though now we know that about 90% of them show little or no radio emission. They were star-looking objects whose optical spectra showed - unlike normal galaxies - emission lines. These emission lines were heavily red-shifted. If their redshifts were due to the Universe expansion, then quasars had to be 1000s times more luminous than the Milky Way. At that time, many astronomers very puzzled by this hypothesis. The nature of quasars was still highly debated. THE AGN ZOO: Quasars
In 1963 M. Schmidt realizes that the quasar 3C 273 has 4 hydrogen Balmer lines red- shifted by z= > v rec =44000 km/s, hence it must be outside the Milky Way. However, quasars could still be nearby objects ejected by nearby galaxies with high velocities (in which case though, some of them should be blue-shifted..). THE AGN ZOO: Quasars
The debate ended in the 70s, when quasars were increasingly being found in clusters, and their redshifts were consistent with those of cluster galaxies. Long-exposure observations in the 80s revealed that quasars were embedded in fuzzy halos, whose spectra showed absorption lines: the quasars host galaxies! THE AGN ZOO: Quasars
THE AGN ZOO: Quasars distribution Today we know more than 100,000 quasars, with redshifts up to 6. The number of quasars begun to decline sharply 10 billion years ago. This mirrors the star formation history of the Universe. Hint that star formation and AGN activity are connected?
THE AGN ZOO: Seyferts and Radio Galaxies M87 Seyferts are dim, radio-quiet quasars. Mostly in spirals. Radio-galaxies are dim, radio- loud quasars. All in ellipticals.
THE AGN ZOO: Seyferts and Radio Galaxies Radio galaxy M87 nucleus jetSeyferts are dim, radio-quiet quasars. Mostly in spirals.
AGN: Emission Mechanisms The nucleus emits thermal radiation (like infrared emission by human bodies), completely defined by one temperature. Jet and lobes emit non-thermal radiation (like cell phones). Specifically, jets emit synchrotron radiation. Requires relativistic particles in strong magnetic fields.
What is the energy source of AGN? The one characteristic that all AGN share is fast variability, from which astronomers infer the size of the central engine: No object can vary in brightness faster then light can travel across that object.
Since the brightness of an AGN can vary over time scales of days, this implies that it cannot be larger than a few light-days. Thus, a region as big as our solar system emits more energy per second than a thousands galaxies. What is the fuel? GRAVITY What is the energy source of AGN?
AGN shine by means of accretion onto a super- massive black hole Gas spirals in towards the black hole, forming an accretion disk. As the gas spirals in, it gets heated by friction up to K, releasing enormous amounts of optical, UV and X- ray light. Accretion can convert up to 40% of the rest mass of the in- falling gas into radiation (vs. <1% of nuclear fusion!). If you throw a marshmallow onto a black hole, youll get roughly the energy release of the Hiroshima bomb!
The magic of relativistic jets The kinetic energy (=motion) of the jets can be even greater then the energy released by the active nucleus in the form of radiation.
The magic of relativistic jets The kinetic energy (=motion) of the jets can be even greater then the energy released by the active nucleus in the form of radiation. The energy in the lobes of Cygnus A equals the complete annihilation (via E=mc 2 ) of kg… 1 kg equals a 100 Megaton H-bomb!
A Unified Model for AGN Are the different classes of AGN truly different beasts? In the Unified Model for AGN, the apparent differences are mainly due to inclination effects. The ingredients are: the hole, the disk, the jet, some orbiting clouds of gas, plus a dusty torus that surrounds the inner disk.
A Unified Model for AGN: observational confirmations
Are normal galaxies (inactive) AGN? Theres mounting evidence that ALL galaxies host a super-massive black hole. They are active only for short periods of time. The black hole appears to regulate the evolution of its host galaxy. The Milky Way itself hosts an extraordinarily dim, super-massive black hole at its center: Sagittarius A*
A super-massive black hole in our backyard: Sagittarius A*
The mass of the central black hole can be estimated from the motion of these stars: its million solar masses (Ghez et al 2008)!
GAMMA-RAY BURSTERS (GRBs) GRBs are super-energetic flashes of gamma-rays. They come in 2 types: long-duration (2-100 seconds) short-duration (1/100 to 2 seconds -less comon) Discovered in the 60s, GRBs are now being found at the rate of 1 per day.
GRBs distribution and optical afterglows The GRB distribution is isotropic, suggesting that they are not in the disk of the Milky Way. Their nature remained elusive until 1997, when the first optical afterglow was detected. Absorption lines were measured, with z=0.835: the GRB was 7 billion ly away!
GRBs host galaxies GRBs afterglows are usually found in the outskirts of galaxies, thus are likely to involve a cataclysmic event related to stars.
GRBs are powerful jets from dying stars The confirmation came in 2003, when a bright supernova was imaged at the position of a nearby (long- duration) GRB. Therefore, GRBs are related to the death of stars.
GRBs are powerful jets from dying stars The confirmation came in 2003, when a bright supernova was imaged at the position of a nearby (long-duration) GRB. Therefore, GRBs are related to the death of stars. However, GRBs are super- energetic supernovae, or hypernovae: it is thought that long- duration GRBs are generated when a very massive, rapidly rotating star collapses into a stellar mass black hole.
GRBs are powerful jets from dying stars During this event, calledcollapsar, a relativistic jet is formed, and drills its way out through the outer layers which havent reached the hole yet. Only if the jet is aimed towards us, can we observe a GRB (thus we miss most of them). The collapsar model does not explain short GRBs, probably due to the merging of 2 neutron stars.
GRBs as cosmological probes Ultimately we hope GRBs will help us investigate the star formation rate, and the intergalactic matter through the history of the Universe. The GRB-dedicated NASA satellite SWIFT is able to detect GBRs up to z=10! Its only a matter of time.. As of Aug 2008