Lecture 19: Galaxies

Astronomy 111

Galaxies

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What is a galaxy?What is a galaxy?

• Large assembly of stars, gas and dust, held together by gravity

• Sizes:– Largest: ~1 Trillion stars (or more)g ( )– Smallest: ~10 Million stars– Milky Way & Andromeda: ~200 Billion starsy y

• [For comparison, Nabisco has baked ~350 Billion Oreos since 1913, or ~1 for each star…]

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Spiral galaxiesSpiral galaxies

• The Milky Way & Andromeda are examples of spiral galaxies.p p g

• All spirals share a common structure:– Thin disk of stars gas and dust– Thin disk of stars, gas, and dust.– Thick spheroid of stars with little gas or

dustdust.• All spirals have disks

S h id tl i i

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• Spheroids vary greatly in size.

Spheroid structureSpheroid structure

• Bulge: where inner spheroid & disk mergeg– Many RR Lyrae stars– A little gas & dustA little gas & dust

• Halo: sparse outer spheroidOld metal poor stars– Old metal-poor stars

– Globular clustersRR L St

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– RR Lyrae Stars

Sombrero galaxy (M104)Sombrero galaxy (M104)

Halo

Bulge

Disk

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Disk structureDisk structure

• Thick disk of stars (~1000 pc thick)– Open clusters & loose associations of starsOpen clusters & loose associations of stars– Mix of young & old stars– Cepheid stars in young clustersCepheid stars in young clusters

• Thin disk of gas & dust (~100 pc thick)Mostly cold atomic hydrogen gas– Mostly cold atomic hydrogen gas

– Dusty giant molecular hydrogen (H2) clouds

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clouds

Top View of a Spiral Galaxy

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St ll Di k

Dust & Gas Disk

Stellar Disk

Dust & Gas Disk

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Rotation of the diskRotation of the disk

• Measure using the Doppler Effect• Stars: Doppler shifts of stellar absorption

lineslines• Ionized gas: emission lines from HII

regionsregions• Atomic hydrogen (HI) gas:

– Cold H clouds emit a radio emission line at a fwavelength of 21-cm

– Can trace nearly the entire disk beyond where the stars have begun to thin out.

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g

Rotating diskRotating disk

R t ti A iRotation Axis

A hi SidApproaching SideBLUESHIFT

Receding SideREDSHIFT

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Typical spiral galaxy rotation curvecurve

)

00km/s

ec Differential Rotation

200S

peed

(

Solid-Body

100

otat

ion

S y

Rotation

0

0

255 10 15 20Radius from the Center (kpc)

Ro

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Radius from the Center (kpc)

Measuring masses of galaxiesMeasuring masses of galaxies

• Star or gas cloud is held in its orbit by the gravity of the mass interior to its g yorbit.

Newton’s gravity:RVRM rot

2

)(

Newton s gravity:

GRM )(

M(R) = mass interior to radius R

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M(R) mass interior to radius RVrot = rotation speed

Example: Milky WayExample: Milky Way

• Sun: R=8 kpc, Vrot=220 km/secR 8 kpc, Vrot 220 km/secGives: M = 91010 Msun inside R=8 kpc

• Gas Cloud in outer disk:• Gas Cloud in outer disk:R=16 kpc, Vrot=275 km/secGives: M=2 8 1011 M inside R=16 kpcGives: M=2.81011 Msun inside R=16 kpc

• A way to measure the masses of l i

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galaxies.

Hubble classification of galaxiesHubble classification of galaxies

• All bright galaxies fall into one of three broad classes according to their shape:g p– Spiral Galaxies (~75%)– Elliptical Galaxies (20%)Elliptical Galaxies (20%)– Irregular Galaxies (5%)

• Basic classification system developed• Basic classification system developed by Hubble (1936).

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Hubble “tuning fork” diagramHubble tuning fork diagram

S Sb ScSa Sb Sc

E0 E3 E7

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SBa SBb SBc

Type E: ellipticalsType E: ellipticals

• Show little internal structure:– Elliptical in shapeElliptical in shape– No disks, spiral arms, or dust lanes– Brightest stars are redBrightest stars are red

• Classified by the degree of apparent flatness:flatness:– E0 is circular

E7 i fl tt t (3 1 t ti )

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– E7 is flattest (3:1 aspect ratio)

Elliptical galaxiesElliptical galaxies

E1 E5

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Type S: ordinary spiralsType S: ordinary spirals

• Classified by relative strength of the central bulge & tightness of the spiral g g parms

• Types: Sa Sb and ScTypes: Sa, Sb, and Sc– Sa: strong bulge & tight, indistinct arms

Sb: intermediate type– Sb: intermediate type– Sc: small bulge & loose, well-defined arms

Th Milk W i b bl t Sb

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• The Milky Way is probably type Sb.

Ordinary spirals

SaSa

Sb

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Sc

Type SB: barred spiralsType SB: barred spirals

• Parallel group to the ordinary spirals:– About as many barred as ordinary spirals.About as many barred as ordinary spirals.

• Feature a strong central stellar bar:Bar rotates as a unit (solid body rotation)– Bar rotates as a unit (solid body rotation)

– Spiral arms emerge from the ends of the barbar

• Same subclasses: SB SBb d SB

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– SBa, SBb, and SBc

Barred Spirals

SBa

SBb

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Type I: irregularsType I: irregulars

• Show an irregular, often chaotic structure.

• Little evidence of systematic rotation.• Catch all class:• Catch-all class:

– Proposed systematic subclasses, but many irregulars defy classificationirregulars defy classification.

• Significant dwarf irregular population, l ifi d “dI”

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classified as “dI”

Irregular galaxiesIrregular galaxies

Large Magellanic Cloud Small Magellanic Cloud

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Spiral galaxiesSpiral galaxies

• Properties:– Mass: 109 1012 MsunMass: 10 10 Msun

– Diameter: 5 50 kpc– Luminosity: 108 1011 LLuminosity: 10 10 Lsun

• Structure & dynamics: Disk + spheroid– Disk + spheroid

– Supported by relatively rapid rotation, but spheroid is puffed up by random motions

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spheroid is puffed up by random motions.

Elliptical galaxiesElliptical galaxies

• Properties:– Mass: 105 1013 MsunMass: 10 10 Msun

– Diameter: 1 200 kpc– Luminosity: 106 1012 LLuminosity: 10 10 Lsun

• Structure & dynamics: Spheroid of old stars with little gas or dust– Spheroid of old stars with little gas or dust

– Supported by random motions of stars with some very slow rotation

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some very slow rotation

Irregular galaxiesIrregular galaxies

• Properties:– Mass: 106 1011 MsunMass: 10 10 Msun

– Diameter: 1 10 kpc– Luminosity: 106 few x 109 LLuminosity: 10 few x 10 Lsun

• Structure & dynamics: Chaotic structure lots of young blue stars– Chaotic structure, lots of young blue stars

– Moderate rotation in Irregulars, but very chaotic motions as well

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chaotic motions as well.

Relative stellar & gas contentRelative stellar & gas content

• Spirals:– Range is ~10-20% gas– On-going star formation in the disks– Mix of Pop I and Pop II stars

• Ellipticals:– Very little or no gas or dust– Star formation ended billions of years ago– See only old Pop II stars

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Relative stellar & gas contentRelative stellar & gas content

• Irregulars:– Can range up to 90% gas contentCan range up to 90% gas content– Often a great deal of on-going star

formation– Dominated by young Pop I stars

• Dwarf Irregulars:Dwarf Irregulars:– Very metal poor (<1% solar)

Forming stars for the first time only now

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– Forming stars for the first time only now.

Dwarf galaxiesDwarf galaxies

• Low-luminosity Ellipticals & Irregulars.– Significant number of dwarfsSignificant number of dwarfs

• There are no (convincing) Dwarf SpiralsSpirals.

• Possibilities:S ll i f th i l i– Small versions of their larger cousins

– Different population of objects with superficial similarities to larger E’s and Irr’s

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superficial similarities to larger E’s and Irr’s

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Groups & clusters of galaxiesGroups & clusters of galaxies

• Most galaxies are found in groups and clusters

• Basic properties:– Membership: Pairs to 1000’s of galaxies– Membership: Pairs to 1000 s of galaxies.– Sizes: 1 10 Mpc across

Mix of bright galaxies and dwarfs– Mix of bright galaxies and dwarfs• About 3000 clusters have been

t l d t d t

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cataloged to date.

The Local GroupThe Local Group

• Group of 30-odd galaxies centered on the Milky Way and Andromeda:y y– Size: ~1 Mpc– 4 Spirals4 Spirals– 15 Ellipticals (3 small Es & 12 dEs)– 13 Irregulars of various sizes13 Irregulars of various sizes– Total Mass ~5x1012 Msun

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The Local Group

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Virgo ClusterVirgo Cluster

• Nearest sizable cluster to the Local Group• Relatively loose cluster, centered on twoRelatively loose cluster, centered on two

bright Ellipticals: M87 and M84• Properties:Properties:

– Distances: ~18 Mpc– Size: ~ 2 Mpcp– 2500 galaxies (including many dwarfs)– Mass: ~1014 Msun

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Virgo Cluster

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Rich clustersRich clusters

• Contain 1000’s of galaxies:– Extend for 510 MpcExtend for 5 10 Mpc– Masses up to ~1015 Msun

– One or more giant Elliptical Galaxies atOne or more giant Elliptical Galaxies at center.

– Ellipticals found near the center.Ellipticals found near the center.– Spirals found at the outskirts.– 1020% of the mass is hot (1078K)

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10 20% of the mass is hot (10 K) intracluster gas seen in X-ray emission.

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Distant rich clustercluster

(HST Image)

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SuperclustersSuperclusters

• Clusters of Clusters• Properties:Properties:

– Sizes up to 50 MpcMasses of 1015 to 1016 M– Masses of 1015 to 1016 Msun

– 9095% empty space (voids)Long and filamentary in shape– Long and filamentary in shape

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Local superclusterLocal supercluster

• Roughly centered on the Virgo Cluster• Local Group is on the outskirtsLocal Group is on the outskirts• Properties:

Si 20 M– Size: ~20 Mpc– Mass: ~1015 Msun

l 5% f th l i d b– only ~5% of the volume occupied by galaxies

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Elbow roomElbow room

• Galaxies are large compared to the distances between them:– Most galaxies are separated by only ~20

times their diameters.– By comparison, most stars are separated

by ~107 times their diameters.• Galaxies are likely to encounter other

galaxies a few times over their histories.

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g

Tidal interactionsTidal interactions

• Galaxies interact via Gravitation.• Because of their large sizes, two galaxies

passing near each other raise mutualpassing near each other raise mutual tides.

• These tides distort the shapes of theThese tides distort the shapes of the galaxies

• Dramatic effects without direct collision.

• Most “peculiar galaxies” are interacting pairs.

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pairs.

Raising tidesRaising tides

• Tidal stretching along the encounter line

Far side Near sideTidal stretching along the encounter line.– Near side feels stronger gravitational pull from

the companionp– Far side feels weaker gravitational pull and

lags behind the near side.

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Whirlpool Galaxy (M51)Whirlpool Galaxy (M51)

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MergersMergers

• If two colliding galaxies can dissipate enough orbital energy:g gy– Wreckage merges into a single galaxy.– Gas clouds collide and form new stars.Gas clouds collide and form new stars.– Some portion of the old stars are ejected

from the system (carry off orbital energy).y ( y gy)• Mergers may play a pivotal role in the

formation of galaxies

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formation of galaxies

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Galactic nucleiGalactic nuclei

• Galaxy Nucleus: – Exact center of a galaxy and it immediate g y

surroundings.– If a spiral galaxy, it is the center of rotation.

• Normal Galaxies:– Dense central star cluster– A composite stellar absorption-line spectrum– May also show weak nebular emission lines.

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Active Galactic NucleiActive Galactic Nuclei

• About 1% of all galaxies have active nuclei.

• Bright, compact nucleus:– Sometimes brighter than the entire galaxy– Sometimes brighter than the entire galaxy.– Strong, broad emission lines from hot,

dense highly excited gasdense, highly excited gas.• Rapidly Variable:

Small: only a few light days across

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– Small: only a few light days across.

Bright A tiActive Galaxy (HST(HST Image)

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Discovery of Active GalaxiesDiscovery of Active Galaxies

• 1943: Seyfert GalaxiesCarl Seyfert identified 6 galaxies with strong,Carl Seyfert identified 6 galaxies with strong,

broad emission lines coming from a compact, bright galaxy nucleus.

• 1950s: Radio GalaxiesFirst radio telescopes found faint galaxies atFirst radio telescopes found faint galaxies at

the location of intense radio emission.Also show broad emission-lines in their

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spectra.

The riddle of the quasarsThe riddle of the quasars

• 1960s:– Radio astronomers found intense, point-Radio astronomers found intense, point

like sources of radio emission.– Photographs revealed slightly fuzzy or g p g y y

“quasi-stellar” objects at these locations.– The spectra were bizarre and full of broad,

unrecognized emission lines.• Quasars: “Quasi-Stellar Radio

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Sources”

The riddle of quasars solvedThe riddle of quasars solved

• 1963: Maarten Schmidt (Caltech)– Recognized that the lines in Quasars wereRecognized that the lines in Quasars were

normal Hydrogen lines with extreme redshifts

– Luminous objects located very far away.– The “fuzz” is the host galaxy lost in the g y

glare of an intense active nucleus.

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Cosmic beaconsCosmic beacons

• Quasars are the most luminous objects in the Universe:– Brightest are ~1014 Lsun

• Brightest Quasars:Brightest Quasars:– Among the most distant objects in the

UniverseUniverse– Most distant is ~4 Gpc

Probes of the Universe on very large

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– Probes of the Universe on very large scales.

Distant QuasarQuasar

(HST(HST Image)

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What powers AGNs?What powers AGNs?

• Properties that need to be explained:• Powerful:Powerful:

– Luminosities of Billions or Trillions of suns.Emit everything from Radio to Gamma rays– Emit everything from Radio to Gamma rays

• Compact: Vi ibl li ht i d ti l– Visible light varies on day timescales

– X-rays can vary on a few hour timescales

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The Black Hole paradigmThe Black Hole paradigm

• A plausible energy source: accretion of matter onto a supermassive Black Hole.p– Supermassive = 106 109 Msun

– Schwarzschild Radii: ~0.01 10 AUSchwarzschild Radii: 0.01 10 AU • Matter releases gravitational energy as

it falls init falls in.– Infalling gas settles into an accretion disk.

The hot inner parts of the disk shine

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– The hot inner parts of the disk shine brightly.

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The central engineThe central engine

• Black Hole accretion can be very efficient:– up to ~10% maximum efficiency– ~1 Msun/year of matter needed for bright

AGNs.Get “fuel” from surrounding gas and stars– Get fuel from surrounding gas and stars.

• Rapidly Spinning Black Hole:p y p g– Acts like a particle accelerator– Leads to the jets seen in radio-loud AGNs.

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Do most galaxies harbor a central black hole?black hole?

– Nearly all spirals show some level of “activity” if perhaps only very faintly.

– Dynamical evidence for massive black holes in many nearby “inactive” galaxies.

– Milky Way has a 3106 Msun Black hole, but lacks strong activity

– Recent evidence that almost all galaxies have large black holes in the center

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Summary:Summary:

• Three basic types of galaxies:– Spirals

• Disk and spheroid component• Rotation of disk allows measurement of galaxy massRotation of disk allows measurement of galaxy mass

– Ellipticals– Irregulars

• Differ in terms ofR l ti t t– Relative gas content

– Star formation History– Internal motions

• Galaxies tend to group into ClustersGalaxies tend to group into Clusters– Groups, clusters, and superclusters– Galaxies can collide and merge

• Some galaxies have “active” nucleiP d b l bl k h l i th t

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– Powered by large black holes in the center