Galaxies

(From Seeds Ch. 12-14)

The Milky Way

Almost everything we see in the night sky belongs to the Milky Way.

We see most of the Milky Way as a faint band of light across the sky.

From outside, our Milky Way might very much look like our cosmic

neighbor, the Andromeda Galaxy.

First Studies of the Galaxy

First attempt to unveil the structure of the galaxy by

William Herschel (1785), based on optical observations.

The shape of the Milky Way was believed to resemble a grindstone,

with the sun close to the center

Strategies to explore the structure of our Milky Way

I. Select bright objects that you can see throughout the Milky Way and trace

their directions and distances.

II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration),

and catalog their directions and distances.

III. Trace the orbital velocities of objects in different directions relative to our position.

Determining the Structure of the Milky Way

Galactic Plane

Galactic Center

The structure of our Milky Way is hard to determine because:

1) We are inside.

2) Distance measurements are difficult.

3) Our view towards the center is obscured by gas and dust.

Measuring Distances:The Cepheid Method

Instability Strip

The more luminous a

Cepheid variable, the

longer its pulsation period.

Observing the period yields a measure of its luminosity and thus its

distance!

The Cepheid Method

Allows us to measure the

distances to star clusters

throughout the Milky Way

Exploring the Galaxy Using Clusters of Stars

Two types of clusters of stars:

1) Open clusters = young clusters of recently formed stars; within the disk of the Galaxy

2) Globular clusters = old, centrally concentrated clusters of stars; mostly in a halo around the galaxy

Globular Cluster M 19

Open clusters h and Persei

Globular Clusters

• Dense clusters of 50,000 – a million stars

• Approx. 200 globular clusters in our Milky Way

• Old (~ 11 billion years), lower-main-sequence stars

Globular Cluster M80

Locating the Center of the Milky Way

Distribution of globular

clusters is not centered on the sun,

but on a location which is heavily obscured from

direct (visual) observation.

The Structure of the Milky Way75,000 light years

Disk

Nuclear Bulge

HaloSun

Globular Clusters

Open Clusters, O/B Associations

Infrared View of the Milky Way

Interstellar dust (absorbing optical light) emits mostly infrared.

Near-infrared image

Infrared emission is not strongly absorbed and provides a clear view

throughout the Milky Way

Nuclear bulge

Galactic plane

Far-infrared image

Orbital Motions in the Milky Way (I)

Disk stars:

Nearly circular orbits in the disk

of the galaxy

Halo stars:

Highly elliptical orbits; randomly

oriented

Orbital Motions in the Milky Way (II)

Differential RotationSun orbits around galactic center at

220 km/s

1 orbit takes approx. 240 million years.

Stars closer to the galactic center

orbit faster.

Stars farther out orbit more slowly.

Mass determination from orbital velocity:

The more mass there is inside the orbit, the faster

the sun has to orbit around the galactic

center.

Combined mass:

M = 4 billion MsunM = 11 billion MsunM = 25 billion MsunM = 100 billion MsunM = 400 billion Msun

The Mass of the Milky WayIf all mass was concentrated in the center, Rotation curve would follow a modified version of Kepler’s 3rd law.

Rotation Curve = orbital velocity as function of radius.

The Mass of the Milky Way (II)

Total mass in the disk of the Milky Way:

Approx. 200 billion solar masses

Additional mass in an extended halo:

Total: Approx. 1 trillion solar masses

Most of the mass is not emitting any radiation:

dark matter!

The History of the Milky Way

The traditional theory:

Quasi-spherical gas cloud fragments into smaller

pieces, forming the first, metal-poor stars (pop. II);

Rotating cloud collapses into a disk-like structure

Later populations of stars (pop. I) are restricted to the disk of the galaxy

Modifications of the Traditional Theory

Ages of stellar population may pose a problem to the

traditional theory of the history of the Milky Way.

Possible solution: Later accumulation of gas,

possibly due to mergers with smaller galaxies.

Recently discovered ring of stars around the Milky Way

may be the remnant of such a merger.

Exploring the structure of the Milky Way with O/B Associations

O/B Associations

Distances to O/B Associations determined using Cepheid variables

O/B Associations trace out 3 spiral arms near the sun.

Sagittarius arm

Orion-Cygnus arm

Perseus arm

Sun

Radio Observations21-cm radio observations reveal the distribution

of neutral hydrogen throughout the galaxy.

Distances to hydrogen clouds determined

through radial-velocity measurements (Doppler effect!)

Galactic center

Sun

Neutral hydrogen

concentrated in spiral arms

The Structure of the Milky Way Revealed

Distribution of dust

Sun

RingBar

Distribution of stars and neutral hydrogen

Star Formation in Spiral Arms (I)Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts

forming spiral arms trigger star formation.

Spiral arms are stationary shock waves, initiating star

formation.

Star Formation in Spiral Arms (II)

Spiral arms are basically stationary shock waves.

Stars and gas clouds orbit around the galactic center

and cross spiral arms.

Shocks initiate star formation.

Star formation self-sustaining through O/B

ionization fronts and supernova shock waves.

Self-Sustained Star Formation in Spiral Arms

Star forming regions get elongated due to differential rotation.

Star formation is self-sustaining due to ionization fronts and supernova shocks.

Grand-Design Spiral Galaxies

Grand-design spirals have two dominant spiral arms.

M 100

Flocculent (woolly) galaxies also have spiral patterns, but no dominant

pair of spiral arms.

NGC 300

The Whirlpool Galaxy

Grand-design galaxy M 51

(Whirlpool Galaxy):

Self-sustaining star forming

regions along spiral arm

patterns are clearly visible.

The Galactic Center (I)

Wide-angle optical view of the GC region

galactic center

Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust:

Extinction by 30 magnitudes

Only 1 out of 1012 optical photons makes its way from the GC towards Earth!

Radio View of the Galactic Center Many supernova remnants;

shells and filaments

Sgr A

Arc

Sgr A*: The center of our galaxy

The galactic center contains a supermassive black hole of approx. 2.6 million solar masses.

Sgr A

Measuring the Mass of the Black Hole in the Center of the Milky Way

By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to be ~ 2.6 million

solar masses.

www.mpe.mpg.de/www_ir/GC

Chandra X ray image of Sgr A*

Supermassive black hole in the galactic center is unusually faint in X rays,

compared to those in other galaxies.

Galactic center region contains many black-hole and neutron-star X-ray binaries.

X-Ray View of the Galactic Center

Distance Measurements to Other Galaxies (I)

a) Cepheid method: Using period – Luminosity relation for classical Cepheids:

Measure Cepheid’s period Find its luminosity Compare to apparent magnitude Find its distance

b) Type Ia supernovae (collapse of an accreting white dwarf in a binary system):

Type Ia supernovae have well known standard luminosities Compare to apparent magnitudes Find its distances

Both are “Standard-candle” methods:

Know absolute magnitude (luminosity) compare to apparent magnitude find distance.

Cepheid Distance Measurement

Repeated brightness

measurements of a Cepheid

allow the determination of the period and thus the

absolute magnitude.

distance

Distance Measurements to Other Galaxies (II): The Hubble Law

E. Hubble (1913):

Distant galaxies are moving away from our Milky Way, with

a recession velocity, vr, proportional to their distance d:

vr = H0*d

H0 ≈ 70 km/s/Mpc is the Hubble constant.

=> Measure vr through the Doppler effect infer the distance.

The Extragalactic Distance Scale

Many galaxies are typically millions or billions of parsecs from our galaxy.

Typical distance units:

Mpc = megaparsec = 1 million parsecs

Gpc = gigaparsec = 1 billion parsecs

Distances of Mpc or even Gpc The light we see has left the galaxy millions or billions of years ago!!

“Look-back times” of millions or billions of years

Galaxy Sizes and Luminosities

Vastly different sizes and luminosities:

From small, low-luminosity irregular

galaxies (much smaller and less

luminous than the Milky Way) to giant ellipticals and large spirals, a few times

the Milky Way’s size and luminosity

Rotation Curves of Galaxies

Observe frequency of spectral lines across a galaxy.

From blue / red shift of spectral lines across the galaxy

infer rotational velocity

Plot of rotational velocity vs. distance from the center of the

galaxy:

rotation curve

Determining the Masses of Galaxies

Based on rotation curves, can use Newtonian Gravity

to determine the

masses of galaxies

Supermassive Black Holes

From the measurement of

stellar velocities near the center of a galaxy:

Infer mass in the very center Central black

holes!

Several million, up to more than a billion solar masses!

Supermassive black holes

Dark MatterAdding “visible” mass in

stars,

interstellar gas,

dust,

etc., we find that most of the mass is “invisible”!

The nature of this “dark matter” is not understood at this time.

Some ideas:

Brown dwarfs, small black holes, exotic elementary particles.

Clusters of GalaxiesGalaxies do not generally exist in isolation,

but form larger clusters of galaxies.

Rich clusters:

1,000 or more galaxies, diameter of ~ 3 Mpc,

condensed around a large, central galaxy

Poor clusters:

Less than 1,000 galaxies (often just a few),

diameter of a few Mpc, generally not condensed

towards the center

Hot Gas in Clusters of Galaxies

Coma Cluster of Galaxies

Visible light X rays

Space between galaxies is not empty, but filled with hot gas (observable in X rays)

That this gas remains gravitationally bound, provides further evidence for dark matter.

Gravitational Lensing

The huge mass of gas in a cluster of galaxies can bend the light from a more distant galaxy.

Image of the galaxy is strongly distorted into arcs.

Our Galaxy Cluster: The Local Group

Milky Way Andromeda Galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Starburst Galaxies

Ultraluminous infrared galaxies

Starburst galaxies are often very rich in gas and dust; bright in infrared:

Interacting GalaxiesCartwheel Galaxy Particularly in rich

clusters, galaxies can collide and interact.

Galaxy collisions can produce

ring galaxies and

tidal tails.

Often triggering active star formation:

Starburst galaxies

NGC 4038/4039

Tidal Tails

Example for galaxy interaction with tidal tails:

The Mice

Computer simulations produce similar structures.

Simulations of Galaxy

Interactions

Numerical simulations of

galaxy interactions have been very

successful in reproducing tidal interactions like

bridges, tidal tails, and rings.

Galactic Interaction Simulations• Joshua Barnes: http://

www.ifa.hawaii.edu/faculty/barnes/transform.html

• John Dubinksi:• http://www.cita.utoronto.ca/~dubinski/nbody/

• Chris Mihos:• http://burro.astr.cwru.edu/models/models.html

• Bob Berrington (Wyoming):

• http://physics.uwyo.edu/~rberring/

• There are others…

Mergers of Galaxies

NGC 7252: Probably result of

merger of two galaxies, ~ a billion years

ago:

Small galaxy remnant in the

center is rotating backwards!

Radio image of M64: Central regions rotating backwards!

Multiple nuclei in giant elliptical

galaxies

Active Galaxies

Galaxies with extremely violent energy release in their nuclei (pl. of nucleus).

“active galactic nuclei” (= AGN)

Up to many thousand times more luminous than the entire Milky Way;

energy released within a region approx. the size of our solar system!

Line Spectra of Galaxies

Taking a spectrum of the light from a normal galaxy:

The light from the galaxy should be mostly star light, and should thus contain many absorption

lines from the individual stellar spectra.

Seyfert Galaxies

NGC 1566

Circinus Galaxy

Unusual spiral galaxies:

• Very bright cores• Emission line spectra.• Variability: ~ 50 % in a few months

Most likely power source:

Accretion onto a supermassive black hole (~107 – 108 Msun)

NGC 7742

Interacting GalaxiesSeyfert galaxy NGC 7674

Active galaxies are often associated with interacting galaxies, possibly result of

recent galaxy mergers.

Seyfert galaxy 3C219

Often: gas outflowing at high velocities, in opposite directions

Cosmic Jets and Radio LobesMany active galaxies show powerful radio jets

Radio image of Cygnus A

Material in the jets moves with almost the speed of light (“relativistic jets”).

Hot spots:

Energy in the jets is released in interaction

with surrounding material

Radio Galaxies

Radio image superposed on optical image

Centaurus A (“Cen A” = NGC 5128): the closest AGN to us.

Jet visible in radio and X-rays; show bright spots in

similar locations.

Infrared image reveals warm gas near the nucleus.

Radio Galaxies (II)

Evidence for the galaxy

moving through

intergalactic material

Radio image of 3C75

3C75: Evidence for two nuclei recent galaxy

merger

Visual + radio image of 3C31

Radio image of NGC 1265

Formation of Radio Jets

Jets are powered by accretion of matter onto a supermassive black hole.

Black Hole

Accretion Disk

Twisted magnetic fields help to confine the material in the jet and to produce synchrotron

radiation.

The Jets of M87

M87 = Central, giant elliptical galaxy in the Virgo cluster of galaxies

Optical and radio observations detect a jet with velocities up to ~ 1/2 c.

Jet:

~ 2.5

kpc l

ong

The Dust Torus in NGC4261

Dust torus is directly visible with Hubble Space Telescope

Model for Seyfert Galaxies

Supermassive black holeAccretion disk

dense dust torus

Gas clouds

Seyfert I:Seyfert I:

Strong, broad emission Strong, broad emission lines from rapidly lines from rapidly

moving gas clouds near moving gas clouds near the black holethe black hole

Seyfert II:Seyfert II:

Weaker, narrow Weaker, narrow emission lines emission lines

from more slowly from more slowly moving gas clouds moving gas clouds far from the black far from the black

holehole

UV, X-rays

Emission lines

Radio Galaxy:

Powerful “radio lobes” at the end points of the jets, where power in the

jets is dissipated.

Cyg A (radio emission)

Other Types of AGN and AGN Unification

Observing direction

Emission from the jet pointing towards us is enhanced

(“Doppler boosting”) compared to the jet moving in the other

direction (“counter jet”).

Other Types of AGN and AGN Unification

Quasar or BL Lac object (properties very similar to

quasars, but no emission lines)

Observing direction

The Origin of Supermassive Black HolesMost galaxies seem to

harbor supermassive black holes in their centers.

Fed and fueled by stars and gas from the near-

central environment

Galaxy interactions may enhance the flow of matter

onto central black holes

Quasars

Active nuclei in elliptical galaxies with even more powerful central sources than

Seyfert galaxies.

Also show very strong, broad emission lines in

their spectra.

Also show strong variability over time

scales of a few months.

The Spectra of QuasarsThe Quasar 3C273

Spectral lines show a large redshift of

z = = 0.158

Quasar Red Shifts

z = 0

z = 0.178

z = 0.240

z = 0.302

z = 0.389

Quasars have been detected at the highest

redshifts, up to

z ~ 6

z = /

Our old formula

/= vr/c

is only valid in the limit of low speed,

vr << c

Studying QuasarsThe study of high-redshift quasars allows astronomers to investigate questions of

1) Large scale structure of the universe

2) Early history of the universe

3) Galaxy evolution

4) Dark matter

Observing quasars at high redshifts

distances of several Gpc

Look-back times of many billions of years

Universe was only a few billion years old!

Probing Dark Matter with High-z Quasars:

Gravitational Lensing

Light from a quasar behind a galaxy cluster is bent by the mass in the cluster.

Use to probe the distribution of matter in the cluster.

Light from a distant quasar is bent around a foreground galaxy

two images of the same quasar!

Gravitational Lensing of Quasars

Gallery of Quasar Host GalaxiesElliptical galaxies; often merging / interacting galaxies