Galaxies

Galaxy Classification

Distances to Galaxies

Galaxy Mass

Galaxy Clusters

Interacting Galaxies

Large Scale Structure of the Universe

Galaxies

Star systems, some like our Milky Way

They contain a few thousand to tens of billions of stars

They contain varying amounts of gas and dust

They come in a large variety of shapes and sizes

The Family of Galaxies

Even seemingly empty

regions of the sky contain

thousands of very faint,

very distant galaxies

Galaxy

morphologies:

Spirals

Ellipticals

Irregular (some

interacting)

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

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

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.

Barred Spirals

Some spirals show a

pronounced bar

structure in the center.

They are termed barred

spirals:

Sequence:

SBa, …, SBc,

analogous to regular

spirals.

Is the Milky Way a Barred

Spiral?

Distribution of dust

Sun

Ring Bar

Distribution of stars

and neutral hydrogen

Irregular Galaxies Often: result of galaxy

collisions or 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

A summary of galaxy properties by type

Distance Measurements

to Other Galaxies

a) Cepheid method: Using period – luminosity

relation for classical Cepheids:

1. Measure Cepheid’s period

2. Find its luminosity from graph

3. Measure apparent magnitude

4. Calculate its distance

Cepheid variables allow measurement of galaxies

to about 25 Mpc away.

However, some galaxies have no Cepheids and

most galaxies are farther away than 25 Mpc.

Distance Measurements to Other

Galaxies

b) Type Ia supernovae (collapse of an accreting white

dwarf in a binary system):

1. Type 1a supernovae have well known standard luminosities

2. Measure apparent magnitude

3. Calculate its distance

c) Tully–Fisher relation: correlates a galaxy’s rotation

speed (which can be measured using the Doppler

effect) to its luminosity.

All are ―Standard-candle‖ methods:

1. Determine the absolute magnitude (luminosity)

2. Measure apparent magnitude

3. Calculate its distance

Measuring Distances in Space

With these

additions, the

cosmic distance

ladder has been

extended to about

1 Gpc.

Distance Measurements

to Other Galaxies:

Hubble’s Law Edwin Hubble (1913)

found that 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.

1. Measure vr using the

Doppler effect

2. Calculate the distance.

Measuring Distances in Space

This puts the

final step on

our cosmic

distance

ladder

The last step

cannot be used

to justify

Hubble’s Law

and its

cosmological

implications.

We will look at

that later. Radar

Trigonometric

Parallax

Spectroscopic

Parallax

Cepheid

Variables

Tully-Fisher

Method

Type Ia

Supernovae

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 (d) of Mpc or even Gpc

The light we see left the galaxy millions

or billions of years ago!!

―Look-back times‖ (t = d/c)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, use Kepler’s 3rd law to infer

masses of galaxies

Adding ―visible‖ mass in stars, interstellar gas, dust, etc.,

we find that most of the mass is ―invisible,‖ just as we did

for the Milky Way.

Dark Matter

Another way to measure the average mass of galaxies in

a cluster is to calculate how much mass is required to

keep the cluster gravitationally bound.

Determining the Masses of Galaxies

Galaxy mass measurements show that galaxies need

between 3 and 10 times more mass than can be observed

to explain their rotation curves.

The discrepancy is even larger in galaxy clusters, which

need 10 to 100 times more mass. The total needed is more

than the sum of the dark matter associated with each

galaxy.

Dark Matter in the Universe

There is evidence for intracluster

superhot gas (about 106 K)

throughout clusters, densest in

the center

This head–tail radio galaxy’s

lobes are being swept back,

probably because of collisions

with intracluster gas

It is believed this gas is

primordial—dating from the very

early days of the Universe.

There is not nearly enough of it

to account for most of the matter

in galaxy clusters.

Dark Matter in the Universe

Dark Matter in the Universe This map of dark matter in and near a small galaxy

cluster was created by measuring distortions in the

images of background objects

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!

Super massive black holes

like we found for the Milky Way

The Origin of

Supermassive

Black Holes

Most galaxies seem to harbor

supermassive black holes in

their centers.

They are fed and fueled by

stars and gas from the near-

central environment

Galaxy interactions may

enhance the flow of matter

onto central black holes

Clusters of Galaxies Galaxies 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

Gravitational Lensing

The huge mass of gas in a cluster of galaxies can bend the light

from a more distant galaxy. This is an effect of the General

Theory of Relativity.

Image of the galaxy is strongly distorted into arcs.

Gravitational Lensing

This is what appeared

at first to be a double

quasar, but on closer

inspection the two

quasars turned out to

be not just similar, but

identical—down to their

luminosity variations.

This is not two quasars

at all—it is two images

of the same quasar.

Gravitational Lensing

This could happen by gravitational lensing. From this

we can learn about the quasar itself, as there is

usually a time difference between the two paths. We

can also learn about the lensing galaxy by analyzing

the bending of the light.

Gravitational Lensing

Here, an intervening galaxy has made four images of a

distant quasar.

Gravitational Lensing

Here are two spectacular images of gravitational

lensing:

Distant galaxies being

imaged by a whole

cluster

A cluster with images of

what is probably a

single galaxy.

Our Galaxy Cluster:

The Local Group

Some galaxies of our local group are difficult to

observe because they are located behind the center

of the Milky Way, which obscures our view.

Dwingaloo 1

The Local Group

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

Starburst Galaxies

Ultraluminous

infrared galaxies

Starburst galaxies are often very rich in gas and dust;

bright in infrared

Simulations of

Galaxy

Interactions

Numerical

simulations of

galaxy interactions

have been very

successful in

reproducing tidal

interactions like

bridges, tidal tails,

and rings.

Tidal Tails

Example for galaxy

interaction with tidal

tails:

The Mice

Computer simulations

produce similar

structures.

Mergers of Galaxies

NGC 7252 is

probably the

result of the

merger of two

galaxies, ~109

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

Interactions of Galaxies with

Intergalactic Matter

Galaxies may not only

interact with each

other directly, but also

with the gas between

them.

Gas within a galaxy is

stripped off the galaxy

by such an interaction.

The Furthest Galaxies

The most distant galaxies visible by HST are seen at

a time when the universe was only ~109 years old.

The Universe on Large Scales

Galaxy clusters join in

larger groupings,

called superclusters.

This is a 3-D map of the

Local Supercluster, of

which our Local Group

is a part. It contains

tens of thousands of

galaxies.

The Universe on Large Scales This slice of a larger galactic survey shows that,

on the scale of 100–200 Mpc, there is structure in

the universe – walls and voids.

The Universe on Large Scales

This survey, extending

out even farther, shows

structure on the scale

of 100–200 Mpc, but no

sign of structure on a

larger scale than that.

The decreasing density

of galaxies at the

farthest distance

comes from the

difficulty of observing

them.

Large-Scale Structure A large survey of distant galaxies

shows the largest structures in the

universe

Filaments and walls of galaxy

superclusters and voids

(basically empty space).