Galaxies

Chapter 13:

Galaxies

• Contain a few thousand to tens of billions of stars,

• Large variety of shapes and sizes

• Star systems like our Milky Way

as well as varying amounts of gas and dust

Spiral galaxy

Bulge and Halo:

old stars, few gas clouds

Disk Component:

stars of all ages,

many gas clouds

M31: Andromeda Galaxy

Pinwheel Galaxy

M81

NGC 7742

M104: Sombrero Galaxy

M51: Whirlpool Galaxy

Elliptical

Galaxy:

All spheroidal

component,

virtually no

disk

component

Elliptical

Galaxy:

All spheroidal

component,

virtually no

disk

component

Red-yellow

color

indicates

older star

population.

M49

M87

Irregular Galaxy: Neither spiral nor elliptical.

Blue-white color indicates ongoing star formation.

NGC 1427

M82

Barred Spiral Galaxy: Has a bar of stars

across the bulge.

NGC 1672

NGC 1097

Hubble’s galaxy classes Spheroid

dominates

Disk

dominates

Galaxy Classification

Sa

Sb

Sc

Elliptical Galaxies Spiral Galaxies

E0 =

Spherical

Small nucleus;

loosely wound

arms

E1

E6

E0, …, E7

Large nucleus;

tightly wound

arms

E7 =

Highly

elliptical

Barred Spirals

Sequence:

SBa, …, SBc,

analogous to

regular spirals.

Gas and Dust in Galaxies

Spirals are rich in

gas and dust.

Ellipticals are almost

devoid of gas and dust.

Galaxies with disk and bulge,

but no dust are termed S0

Clusters of Galaxies

Galaxies do generally not exist isolated,

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

Our Galaxy Cluster:

The Local Group

Milky Way Andromeda galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Some galaxies of our local group are difficult to

observe because they are located behind the

center of our Milky Way, from our view point.

The Local Group

The Canis Major Galaxy is

being ripped apart by the

Milky Way’s tidal forces.

Galaxies that get too close to each

other can join together. Systems in the

process of joining together are called

galaxy mergers.

Interacting Galaxies Cartwheel 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

Simulations of

Galaxy Interactions

Numerical simulations of

galaxy interactions have been

very successful in reproducing

tidal interactions like bridges,

tidal tails, and rings.

Tadpole Galaxy

NGC 4676: The Mice

Antennae Galaxies

Antennae Galaxies (zoomed out)

Rose Galaxy

Irregular Galaxies Often: result of galaxy

collisions / mergers

Often: Very active star formation

(“Starburst galaxies”)

Some: Small (“Dwarf galaxies”)

satellites of larger galaxies

(e.g., Magellanic Clouds)

Large

Magellanic

Cloud NGC 4038/4039

The Cocoon Galaxy

Starburst Galaxies

Ultraluminous

Infrared Galaxies

Starburst galaxies are often very rich in gas

and dust; bright in infrared:

The Puzzle of “Spiral Nebulae”

• Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, whereas other scientists maintained they were smaller collections of stars within the Milky Way.

• The debate remained unsettled until someone finally measured the distances of spiral nebulae.

Hubble settled the debate by measuring the distance

to the Andromeda Galaxy using Cepheid variables

as standard candles.

Cepheid Variable Stars

The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50-day period.

Cepheid variable stars with longer periods have

greater luminosities.

Distance Measurements

to Other Galaxies

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):

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

Hubble also knew that the spectral features of virtually

all galaxies are redshifted they’re all moving

away from us.

By measuring

distances to

galaxies,

Hubble found

that redshift

and distance

are related in a

special way.

Discovering Hubble's Law

Hubble’s law: velocity = H0 distance

The Extragalactic Distance Scale

Many galaxies are typically millions or billions

of parsecs from our Galaxy.

The light we see has left the Galaxy

millions or billions of years ago!!

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

The Furthest Galaxies

The most distant galaxies visible by HST are seen at a

time when the Universe was only ~ 1 billion years old.

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,

use Kepler’s 3rd law to infer

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 Matter

Adding “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

Gravitational Lensing

The huge mass of gas in a

cluster of galaxies can bend the

light from a more distant galaxy.

The galaxy’s image is strongly distorted into arcs.

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;

the energy is released within a region

approx. the size of our solar system!

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 Galaxies

Seyfert galaxy NGC 7674

Active galaxies are

often associated with

interacting galaxies,

possibly result of

recent galaxy mergers.

NGC 1275, in the center of the

Perseus galaxy cluster

Often: gas outflowing at high velocities, in opposite directions

Cosmic Jets and Radio Lobes

Many 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

Centaurus A (“Cen A” = NGC 5128): the nearest AGN

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 Quasars

The Quasar 3C 273

Spectral lines show

a large red shift of

z = Dl / l0 = 0.158

Model for AGNs

Accretion disk

Dense dust torus

Gas clouds

UV, X-rays

Emission lines

Supermassive

black hole

Seyfert I:

Strong, broad emission lines from

rapidly moving gas clouds near the BH

Seyfert II:

Weaker,

narrow

emission

lines from

more slowly

moving gas

clouds far

from the BH

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.

Radio Galaxy:

Powerful “radio lobes”

at the end points of the

jets, where power in the

jets is dissipated

Cyg A (radio emission)

AGN Unification

Emission from the jet pointing

towards us is enhanced

(“Doppler boosting”) compared

to the jet moving in the other

direction (“counter jet”).

AGN Unification

Quasars: