Chapter 15Normal and Active Galaxies

Chapter 15Normal and Active Galaxies

M51 - Whirlpool Galaxy

Edwin HubbleEdwin Hubble

• Mt. Wilson observatory

• 100” (2.5 m) telescope

• Categorized galaxies in 1924

Figure 15.1Coma Cluster

Hubble classification scheme

Hubble classification scheme

• Spirals

• Barred spirals

• Ellipticals

• Irregulars

Spiral galaxiesSpiral galaxies

• Example: Milky Way

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

Figure 15.2Spiral Galaxy Shapes

Tilted viewTilted view

• Spiral structure can be hard to see on edge

• Presence of disk with dust, gas and newborn stars signifies spiral

Figure 15.3Sombrero Galaxy

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

Figure 15.4Barred-Spiral Galaxy Shapes

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

Figure 15.5Elliptical Galaxy Shapes

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

Figure 15.6S0 Galaxies

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

Figure 15.7Magellanic Clouds

Figure 15.8Irregular Galaxy Shapes

Dwarf irregularsDwarf irregulars

• Most common irregular• Dwarf ellipticals and irregulars most

common galaxies in universe• Often found close to a larger “parent”

galaxy

Figure 15.9Galactic “Tuning Fork”

Table 15.1Basic galaxy Properties by Type

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

Figure 15.10Galaxy Rotation

Type I supernovaeType I supernovae

• Peak luminosity can be used as standard candle

• Can be used out to 1 Gpc

Figure 15.11Extragalactic Distance Ladder

Local GroupLocal Group

• 45 galaxies in a local cluster• Gravitationally attracted• 3 spirals - Milky Way, Andromeda, M33• Remainder dwarf irregulars and

ellipticals

Figure 15.12Local Group

Virgo clusterVirgo cluster

• 17-18 Mpc from Milky Way• 2500 galaxies• 3 Mpc across

Figure 15.13Virgo Cluster

Figure 15.14Distant Galaxy Cluster - 2 Gpc distant

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

Analogy 15-1Like fireflies in a moving jar

Figure 15.15Galaxy Spectra

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

Figure 15.16Hubble’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

Figure 15.17Cosmic Distance Ladder

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

Table 15-2Redshift, Distance, and Look-Back Time

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

Figure 15.18Galaxy Energy Distribution

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

Figure 15.19Seyfert Galaxy

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

Figure 15.20Seyfert Time Variability

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

Figure 15.21Centaurus A Radio Lobes

Figure 15.22Centaurus A,

Close Up

Figure 15.23Cygnus A

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

Figure 15.24Core-Dominated Radio Galaxy

Figure 15.25Radio Galaxy - view determines lobe visibility

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

Figure 15.26M87 Jet

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%

Figure 15.27Quasar 3C 273

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

Figure 15.28Quasar Spectrum

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

Figure 15.29Quasar Jets

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

Figure 15.30Active Galactic Nucleus

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

Figure 15.31Giant Elliptical Galaxy

Figure 15.32M87 Disk

Energy EmissionEnergy Emission

• Emitted energy at broad range of wavelengths

• Some radiation absorbed and reemitted by dust in surrounding disk

Figure 15.33Dusty Donut

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

Figure 15.34Nonthermal Radiation