chapter 15 normal and active galaxies. m51 - whirlpool galaxy
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Edwin HubbleEdwin Hubble
• Mt. Wilson observatory
• 100” (2.5 m) telescope
• Categorized galaxies in 1924
Hubble classification scheme
Hubble classification scheme
• Spirals
• Barred spirals
• Ellipticals
• Irregulars
Spiral schemeSpiral scheme
• Based on size of central bulge
• Sa - largest bulge, tightly wrapped arms, least gas and dust
• Sb - more open arms
• Sc - smallest bulge, loose spiral structure, most gas and dust
Tilted viewTilted view
• Spiral structure can be hard to see on edge
• Presence of disk with dust, gas and newborn stars signifies spiral
Barred spiralsBarred spirals
• Elongated bar extending into disk
• Spiral arms project from ends of bar
• SBa, SBb, SBc
• Milky Way intermediate between spiral and barred spiral
Elliptical galaxiesElliptical galaxies
• No spiral arms
• No obvious disk
• Dense central nucleus
• E0 - most circular to
• E7 - most elliptical
• Classification depends on actual shape and orientation to earth
Elliptical galaxy sizesElliptical galaxy sizes
• Sizes range from• Dwarf ellipticals - 1 kpc diameter, million
stars • Giant ellipticals - few Mpc across,
trillions of stars• Dwarfs more common by 10 to 1• Most of mass in ellipticals is in giants
Other elliptical propertiesOther elliptical properties
• Little or no cool dust and gas• No young stars or star formation• Old reddish low mass stars• Large amounts of very hot interstellar
gas
Intermediate between E7 & Sa
Intermediate between E7 & Sa
• Thin disk and flattened bulge• No spiral structure• No gas and dust• S0 - no bar• SB0 - if bar present
Irregular galaxiesIrregular galaxies
• Rich in interstellar matter and young stars• Lack regular structure• Irr I - look like misshapen spirals• Irr II - often explosive or filamentary
appearance• Usually smaller than spirals, larger than dwarf
ellipticals• Between 108 and 1010 stars
Magellanic CloudsMagellanic Clouds
• Pair of Irr I galaxies• Orbit our galaxy• 50 kpc from center of our galaxy• Visible from southern hemisphere• Lots of dust, gas and blue stars• Also old stars and globulars
Dwarf irregularsDwarf irregulars
• Most common irregular• Dwarf ellipticals and irregulars most
common galaxies in universe• Often found close to a larger “parent”
galaxy
Measuring galactic distanceMeasuring galactic distance
• Cepheid variables to 25 Mpc• Need “standard candles” - astronomical
objects of known luminosity• Luminosity + apparent brightness
distance
Tully-Fisher relationTully-Fisher relation
• Correlation between rotation speed and luminosity of spiral galaxies
• Can be used out to about 200 Mpc• Other related correlations for ellipticals
Type I supernovaeType I supernovae
• Peak luminosity can be used as standard candle
• Can be used out to 1 Gpc
Local GroupLocal Group
• 45 galaxies in a local cluster• Gravitationally attracted• 3 spirals - Milky Way, Andromeda, M33• Remainder dwarf irregulars and
ellipticals
Universal RecessionUniversal Recession
• 1912 - Slipher found almost all spiral galaxies are redshifted
• All galaxies, except some local ones, are receding
• Motion of galaxies in a cluster is random• Clusters are receding, as are galaxies
not part of a cluster
Hubble’s LawHubble’s Law
• 1920’s - Hubble plotted recessional velocity versus distance for galaxies
• Hubble diagrams
• Discovered rate at which a galaxy recedes is directly proportional to its distance from us - Hubble’s Law
Hubble’s LawHubble’s Law
• Universal recession known as Hubble flow
• Distances separating clusters and superclusters is expanding
• Universe (space itself) is expanding
• Individual objects are not expanding
Hubble’s Constant - H0Hubble’s Constant - H0
• Recessional velocity = H0 X distance
• Some measurements give H0 between 70 - 80 km/s/Mpc
• Other types give 50 - 65 km/s/Mpc
• We will use H0 = 70 km/s/Mpc
Top of distance ladderTop of distance ladder
• Use Hubble’s law to find distances
• Redshift recessional velocity distance
RedshiftsRedshifts
• Largest redshifts greater than 6
• Means wavelengths shifted 7X
• UV spectral lines shifted to infrared
• Such objects 9000 Mpc away
Redshifts and look-back timeRedshifts and look-back time
• Redshift of 6.0 means galaxy is receding at 96% of the speed of light
• It is now 8420 Mpc = 27.5 billion light-years away
• It was 12.7 billion light-years away when it emitted the light we see today
• Its look-back time is 12.7 billion years• Light traveled 12.7 billion years to us
Active galaxiesActive galaxies
• More than 90% of all galaxies are normal• Few percent of all bright galaxies are active
galaxies• Overall active are brighter than normal, and at
more wavelengths• Normal galaxy - accumulated light of stars• Active galaxy - most of radiation nonstellar• Violent events in galactic nucleus
Seyfert galaxiesSeyfert galaxies
• Discovered in 1943 by Carl Seyfert• Resemble normal spiral galaxies except
Seyfert nucleus emits the most energy• Brightest Seyfert nuclei 10X brighter
than entire Milky Way
Seyfert spectrumSeyfert spectrum
• Some produce from infrared to X-ray• 75% emit most of their energy in infrared• Broad spectral lines - rapid internal motion• Varies in time within fraction of year - small
energy producing region
Radio galaxiesRadio galaxies
• Active galaxies emitting large amounts of energy at radio wavelengths
• Radio energy from huge radio lobes
Centaurus A radio galaxyCentaurus A radio galaxy
• Visible light E2 galaxy with band of dust• Perhaps merger of spiral and elliptical• 4 Mpc from earth• Radio lobes span half a Mpc• Lobes 10X size of Milky Way• Two lobed - roughly symmetrical
Radio galaxiesRadio galaxies
• Radio lobes emit roughly 10X energy of entire Milky Way
• Nucleus emits up to 100X energy of radio emission
• Total emission up to 1000X of Milky Way• Not all have radio lobes - depends on view
Common active featuresCommon active features
• Huge energy generation in compact nucleus
• Evidence of interacting galaxies• Many contain jets• If view jets end-on, see intense doppler
shifted radiation• This is a blazar - X or gamma rays
QuasarsQuasars
• In 1960, faint blue star like object located with radio source 3C48
• 1963 - 3C 273 - spectral lines found to be redshifted 16% - moving at 48,000 km/s
• 3C 48 redshifted 37%
QuasarsQuasars
• Large redshift - enormous distance• 3C 48 is 1400 Mpc away• 3C 273 has luminosity of 1040 W• Comparable to 20 trillion suns or 1000
Milky Way galaxies• Quasars range from 1038 W to 1042 W
QuasarsQuasars
• Quasars look star-like because of great distance
• Quasi-stellar radio sources, shortened to quasars
• Quasi-stellar object or QSO is more common today
• More than 30,000 quasars known• 250 Mpc to 9000 Mpc away
QuasarsQuasars
• Only seen at great distances• Means long ago in time • Perhaps early phase of galaxy formation• Variable over short periods (days or hours)• Jets• Bright cores of distant galaxies too faint to
see
Active galactic nuclei features
Active galactic nuclei features
• High luminosities, mostly nonstellar• Highly variable (small region)• Jets and explosive activity• Optical spectra with broad emission
lines
Active galaxy energy production
Active galaxy energy production
• Supermassive black hole - 106 - 109 M
• Accretion disk - infalling matter heated by friction
• Emits large amounts of radiation• Jets - ejected material in magnetic fields
along axis of accretion disk
Active galaxy energy production
Active galaxy energy production
• 10% to 20% of infalling mass-energy converts to radiation
• One M consumed per decade can power 1038 W active galaxy
Energy EmissionEnergy Emission
• Emitted energy at broad range of wavelengths
• Some radiation absorbed and reemitted by dust in surrounding disk
Synchrotron radiationSynchrotron radiation
• Ejected charged particles in jet spiral around magnetic field at high speeds
• This is nonthermal radiation • Jet slowed by intergalactic medium,
magnetic field tangled, forms radio lobes