25. High-Energy Celestial Objects• Quasars are like stars with huge redshifts

• Ultraluminous centers of distant galaxies

• Active galaxies span normal galaxies & quasars

• Quasars, blazars, Seyferts & radio galaxies

• Supermassive black holes power active galaxies

• A unified model of active galaxies

• Gamma ray bursters are extremely powerful

Quasars: “Stars” W/Huge Redshifts• Quasi-stellar radio sources

– An early discovery of radio telescope technology• Cygnus A Position firmly located in

1951– One of the strongest astronomical radio sources

Radio luminosity ~ 107 LSun– An unusual galaxy visible in the 200-inch Palomar telescope– Redshifted by 5.7% Very distant by the Hubble

law~ 220 Mpc (~ 720 Mly) from Earth

– Spectrum has emission lines Very unusual for a galaxy• Radio source 3C 48 Discovered in 1960

– Another unusual galaxy visible in the 200-inch Palomar telescope– Spectrum has emission lines Unidentified elements

• Radio source 3C 273 Discovered in 1962– Another unusual galaxy visible in the 200-inch Palomar telescope– Spectrum has emission lines A jet protruding from one

side

• Problems due to a preconceived notion– They are very close & have minimal redshifts

Breakthrough Measurements• Maarten Schmidt studies 3C 273 1963

– Four prominent emission lines are H Balmer lines• Redshifted by 15.8% Speed of ~ 45,000 km .

sec–1

Distance of ~ 600 Mpc (~ 2 Bly)• Greenstein & Matthews study 3C 48

– Four prominent emission lines are H Balmer lines• Redshifted by 36.7% Speed of ~ 0.3 . c

Distance of ~ 1,200 Mpc (~ 4 Bly)• Numerous additional discoveries

– Radiation properties• Some are “radio loud” Only ~ 10% of all

quasars• Others are “radio quiet”

– More than 200,000 quasars are now known• Redshifts range from 0.06 to ~ 7.0

Cygnus A & Its Radio Lobes

Quasar 3C 273’s High-Speed Jet

Redshifts In 3C 273’s Spectrum

Redshift From UV to the Visible

Quasar Density Since the Big Bang

z: The Relativistic Redshift

z approaches ∞ as v approaches c

Quasars: Distant Ultraluminous Galaxies• Basic observations

– Most quasars are at the center of unusual galaxies• Luminosities from 1038 to 1042 Watts

– Milky Way’s luminosity is 1037 Watts

– Brightest quasars are 10,000 times brighter than the Milky Way

• Quasar EMR properties– Dominated by X-rays & g-rays

• Emitting material must be > 100,000 K– Far too hot for any star surface

• High-speed gas clouds

~ 10,000 km . sec–1

The “Red Shift Debate”• Arp et al. discover anomalous relation 1960s

– Some quasars associated with low-redshift galaxies• Referred to as “discordant redshifts”

– Disputed by many astronomers• Argued that this was a line-of-sight phenomenon

• Resolution of the debate 1980s– Observations of quasars with remote galaxies

• Many quasars are in groups or clusters of galaxies– All have essentially identical redshifts

– Observations of “fuzz” around many quasars• These regions exhibit stellar absorption lines

– Telltale signs of quasars embedded in galaxies

Quasar PKS 0405-123’s Radiation

Seyfert Galaxies• Apparent anomaly

– Huge energy gap between galaxies & quasars• A factor of ~ 1,000

• Discovery of missing links– Carl Seyfert

1943• Analyzed spiral galaxies w/bright, compact nuclei

– Exhibit strong emission lines

• < 5% of most bright spiral galaxies are Seyfert galaxies

• > 700 Seyfert galaxies have been discovered

• Seyfert galaxies resemble dim, radio-quiet quasars

The Seyfert Galaxy NGC 7742

A Seyfert Galaxy at Different l’s

Visible light Visible light

X-rays

Quasars, Blazars, Seyferts, Radio…• Quasars ~ 1,000 LMilky Way

– About 10% of quasars are “radio loud”– Quasars emit some synchrotron radiation

• Blazars ~ 1,000 LMilky Way– Vary in brightness over hours to months– Emit intense synchrotron radiation + some radio

• Seyfert galaxies ~ 10 LMilky Way– Resemble low-luminosity radio-quiet quasars– Seyfert galaxies emit some synchrotron radiation

• Radio galaxies ~ 10 LMilky Way– Resemble low-luminosity radio-loud quasars– Elliptical galaxies midway between two radio lobes

Radio Galaxies• Basic observations

– First discovered as peculiar galaxies

1918• Short-exposure photograph of M87

– Compact nucleus w/non-polarized thermal blackbody radiation

– Long jet w/polarized non-thermal synchrotron radiation

– Often found embedded in rich galaxy clusters• High probability of galactic collisions

• Basic properties– Radio galaxies resemble dim, radio-loud quasars

• Normally exhibit two radio lobes in opposite directions– Largest at the point farthest from the nucleus

Non-Thermal & Thermal RadiationNon-thermal

Thermal

The Radio Galaxy M87

The entire galaxyThe center of the galaxy

The Radio Galaxy NGC 5128

Black Holes & Active Galaxies• Historical perspective

– Donald Lynden-Bell proposes “central engine”

1968• A supermassive black hole accreting gas & dust• Much smaller in diameter than the Solar System

– Expected minimum size of supermassive black hole• The Eddington limit of electromagnetic radiation

– Radiation pressure = Gravitational force– Quasar 3C 273 has a luminosity of ~ 3.0 . 1013 LSun

– M3C 273 > 109 MSun

• The Andromeda galaxy M31– Unusual rotation curve within 5” of the galactic core

• Probably stars rapidly orbiting the core• Highest speeds ~ 1.1” from the core

~ 110 km . sec–1

• Mass ~ 3.0 . 107 MSun

Rotation Curve of the Core of M31

Superluminal Motion• Basic physical processes

– Nothing can travel faster than EMR in vacuum• Some objects appear to be moving faster than c

• This is an optical illusion

• Basic observations– Quasar 3C 273

• Gas clouds are measured at speeds ~ 10 . c

• Smaller cloud is moving nearly along our line of sight– Shorter travel time to Earth Apparent superluminal motion⇒

Superluminal Motion in 3C 273

Superluminal Motion Explained

A Unified Model of Active Galaxies• Some common properties

– All have high luminosity & temperature– All have some radio, X-ray & synchrotron radiation

• A unified model of active galaxies– Quasars, blazars, Seyferts & radio galaxies same

• Active galaxies at different angles & different stages– The farther away, the earlier we see their evolutionary stage

– Three basic viewpoints• The accretion disk is oriented edge-on to us

– We see a double radio source• The accretion disk is oriented at a moderate angle

– We see either a quasar or a radio galaxy• One radio lobe is oriented directly toward us

– We see a blazar– Seyferts have consumed their accretion disks

A Unified Model of Active Galaxies

Active Galaxies: Physical Processes• Gravity near supermassive black holes

– Nearby stars, gas & dust drawn into the hole• Original spherical distribution becomes an accretion disk• Gravity & centrifugal effect balance outside event horizon• Matter piles up further away from this region

– Abundant radio, g-ray & X-ray l’s are produced• Pressure forces matter away to accretion disk⊥

– Gas is ionized & therefore constitutes an electric current– This electric current in turn produces a magnetic field– The magnetic field spirals outward due to rapid rotation– Relativistic electrons produce synchrotron radiation

• Recent supportive observations– Unusual appearance of the galaxy NGC 4261

• An accretion disk ~ 250 pc (~ 800 ly) in diameter• Two radio lobes ~ 2,000 pc (~ 6,400 ly) long

Flow Patterns Near an Accretion Disk

Supermassive Black Hole Jets

Jets & a Disk in NGC 4261

Gamma-Ray Bursters• Basic properties

– Discovered during the late 1960s by Vela satellites• Designed to detect g-rays from nuclear weapons tests

– Bursters randomly located in all regions of the sky• Either in the galactic halo or the Universe at large

– Last from ~ 10–2 sec to ~ 1 hour• They occur only once

• Basic observations– The Italian-Dutch BeppoSAX satellite

• Very good angular resolution of about 1 arcminute– 28 February 1997 BeppoSAX’s first burster

• A visible-light “afterglow” was observed– 8 May 1997 BeppoSAX’s second burster

• Afterglow had z = 0.835 They are extremely far away

– Most distant burster had z = 4.51

Source of Gamma-Ray Bursters• Recent observations

– They are not at the core of their host galaxies• Unlikely to be the flare-up of an active galaxy

– They are within their host galaxies• An extraordinary event

• Possible explanations– Energy is released in the formation of a black hole

• Two merging neutron stars

• A hypernova An exceptionally energetic

supernova

Table 25-1: Redshift & Distance

• Quasars– ~ 1,000 times brighter than a galaxy– Only 10% are actually “radio loud”

• Seyfert galaxies– Low-luminosity radio-quiet quasars– Emit some synchrotron radiation

• Radio galaxies– Low-luminosity radio-loud quasars– Elliptical galaxies between two lobes

• Blazars– Emit intense synchrotron radiation– Brightness varies in hours to months

• Explained by active galaxies– Three different perspectives

• Double radio sources• Quasars & radio galaxies• Blazars

– Seyfert galaxies may be old quasars

• Supermassive black holes– Probable source of intense radiation

• Accretion disk channels matter in• Centrifugal effect stops some matter• Plasma channeled away along axis

– Probable collision of galaxies• Gamma-ray bursters

– Brief one-time events• Not in the host galaxy core

– Possibly formation of black holes• Two neutron stars merge• A hypernova

Important Concepts