galaxies - homepages.uc.eduvazct/stars/ch 24.pdf · • elliptical galaxies have no spiral arms and...

© 2017 Pearson Education, Inc.

GALAXIES

Chapter 24

• Morphology

– Hubble’s Galaxy Classification

• The Distribution of Galaxies in Space

– Extending the Distance Ladder

• Hubble’s Law

• Active Galactic Nuclei

• The Central Engine of an Active Galaxy

• Summary


Hubble’s Galaxy Classification: Spirals

• We’re acquainted with Spiral Galaxies, the most

familiar one being the galaxy closest to us.



• Spiral galaxies do not all look the same. Hubble

classified them by the size of the central bulge.


Bode’s Galaxy

(M81)

Whirlpool Galaxy

(M51a)

Pinwheel Galaxy

(M101)


• All spiral galaxies are denoted by the letter “S”

• They are further classified by the letters “a”, “b”, “c”,

etc., according to the size of the bulge, so that:

– Type Sa has the largest central bulge,

– Type Sb has a smaller bulge, and

– Type Sc has the smallest bulge.

• The tightness of the spirals is strongly correlated

with the size of the bulge,

– Type Sa tends to have the most tightly bound spiral

arms, with types Sb and Sc progressively less tight.

• The correlation is not perfect. Andromeda is

classified as an Sb galaxy.



• The components of spiral galaxies are the same as

in our own Galaxy: halo, bulge, disk, spiral arms.

– The bulge contains older stars on the outside and

younger stars inside,

– The halos contain older stars and globular clusters,

– The galactic disks are rich in gas and dust and

contains most of the star forming regions in the spiral

arms. Most of the Luminosity of these galaxies comes

from A – G type stars in the disk.

• It turns out that the larger the bulge the less gas and

dust contained by the disk: thus Sa galaxies have

the smallest amount of gas and dust in the disk and

Sc galaxies have the most© 2017 Pearson Education, Inc.


• Most spirals will not be seen face to face. However, we do

not need to see spiral arms to classify a galaxy as a spiral

galaxy. A disk, dust and star formation is good enough.


The Spiral galaxy pair

NGC 4302 and NGC 4298.

The Sombrero Galaxy as

seen by Hubble.

Galaxy Classification: Barred Spirals

• Similar to the spiral galaxies are the barred spirals.

• They contain a pronounced “Bar” through the center.

• The spirals project from the ends of the bar.

• Barred spirals are designated the letters “SB” followed by the

letters “a”, “b”, “c”, etc. depending on the size of the bulge.

• In the case of SBc the spirals merge with the bar.


Galaxy Classification: Ellipticals

• Elliptical galaxies have no spiral arms and no disk.

They come in many sizes,

– Giant Ellipticals contain trillions of stars

– Dwarf Ellipticals contain fewer than a million stars.

• Ellipticals contain very little, if any, cool gas and

dust, and show no evidence of ongoing star

formation. They contain mostly old stars.

• Many Ellipticals, however, have large clouds of very

hot gas (millions of K), extending far beyond the

visible boundaries of the galaxy.

• The presence of this hot gas is known by X-ray

observations.



• The Giant Elliptical

NGC1316 in the Formax

Cluster.

• The Dwarf Elliptical

M110 is a satellite of

Andromeda



• Some Giant Ellipticals are known to

contain disks of gas and dust in which

stars are forming.

• Dwarf Ellipticals are the most common

type of Elliptical galaxy.

• Stars in ellipticals have more or less

random motions.

• Ellipticals are assigned the letter “E”.

• They are classified according to their

shape, from

– E0 (almost spherical) to

– E7 (the most elongated).


Galaxy Classification: Lenticulars

• Lenticular galaxies have a

thin disk and a flattened

bulge, but no gas and dust.

• They are known as

Lenticular because their

shape resembles a lens

• They are known as “S0”

galaxies or “SB0” galaxies,

if they have a bar.

• S0 and SB0 galaxies have a

disk and bulge, but no spiral

arms and no interstellar gas.


Galaxy Classification: Irregulars

• The irregular galaxies have a wide variety of

shapes. They are classified as “Irr I” and “Irr II”.

• They typically contain between 100 million and 10

billion stars.



• The stars in irregular galaxies have random motions,

like their Elliptical counterparts.

• These galaxies have vigorous star formation.

• Irr I galaxies are by far the most common. They

have hardly any nucleus and are rich in gas and

dust, so they have active star formation.

• Irr I are bluish in appearance.

• Irr II galaxies redder than Irr I galaxies and rare.

They appear more filamentary.

• This type of galaxy is thought to result from normal

galaxies that have undergone strong interactions

with other galaxies.




• Here are the Large and Small Magellanic Clouds,

which are two irregulars belonging to our local group.

Hubble’s Galaxy Classification

• A summary of galaxy properties by type


Hubble’s “Tuning Fork”

• Hubble’s “tuning fork” is a convenient way to

remember the galaxy classifications, although it has

no deeper meaning.

• Eg. Spirals are not Ellipticals that grew arms!


The Distance Ladder

• Cepheid variables allow measurement of galaxies to about 25 Mpc

away.

• However, most galaxies are farther away than 25 Mpc. New

distance measures are needed.

• There are two ways to do this:

– Tully-Fisher relation correlates a galaxy’s rotation speed

(which can be measured using the Doppler effect) to its mass,

and so to its Luminosity.

– Standard Candles: these are intrinsically bright, easily

recognizable objects whose Luminosity is known. RR Lyrae

variables and Cepheid variables are examples, but they are

generally not bright enough at very large distances. On the

other hand, Type I supernovae all have about the same

luminosity, as the process by which they happen doesn’t allow

for much variation. The same can be said for Planetary

Nebulae.© 2017 Pearson Education, Inc.

Measuring the Rotation Rate

• The rotation of a galaxy results in Doppler broadening of

its spectral lines.

• The faster the rotation the more the broadening.

• By measuring the broadening we can deduce the

rotation rate.


The Tully Fisher Relation


• The Tully-Fisher relation (TFR) is an empirical

relation between the rotational velocity and the

intrinsic luminosity of a galaxy. The relation is tighter

when the rotation rate is compared with the baryonic

mass. This latter form

is known as the

Baryonic Tully-Fisher

(BTR) relation.

By comparing the

intrinsic luminosity to

the apparent luminosity

we determine the distance.

The Distance Ladder

• With these additions, the cosmic distance ladder has

been extended to about 1 Gpc.


Distribution of Galaxies: Local Group

• Here is the distribution of galaxies within about

1 Mpc of the Milky Way. We call it our local group.


Distribution of Galaxies: Local Group

• There are three spirals in the Local Group—the Milky Way,

Andromeda, and M33 (the Triangulum Galaxy). There are

more than 54 galaxies in all most of them dwarfs.


Distribution of Galaxies: Clusters

• A collection of galaxies between 2 and 10 Mpc across is called

a Cluster. Clusters are large, eg. the Virgo Cluster is a cluster

of about 2500 galaxies, whose center is roughly 16 Mpc away,

in the constellation of Virgo, with a diameter of about 3 Mpc.


Distribution of Galaxies: Superclusters

• Clusters form Superclusters. The Laniakeia super-

cluster contains the local group and about 100,000 other

galaxies; it has a diameter of about 77 Mpc. [Video]


https://www.youtube.com/watch?v=rENyyRwxpHo

Hubble’s Law

• Universal recession:

All galaxies (with a

couple of nearby

exceptions) seem to be

moving away from us,

with the redshift of their

motion correlated with

their distance.


Hubble’s Law

• These plots show the

relation between

distance and

recessional velocity for

the five galaxies in the

previous figure, and

then for a larger sample.


Hubble’s Law

• The relationship (slope of the line) is characterized by

Hubble’s constant H0:

recessional velocity = H0×distance

• The value of Hubble’s constant today is approximately

73.4 km/s/Mpc.

• Over the past year, there’s been some confusion about

its value because recent observations from ESA’s

Planck satellite indicates that its it more like 67.4

km/s/Mpc.

• Measuring distances using Hubble’s law works better

the farther away the object is; random motions are

overwhelmed by the recessional velocity.


The Distance Ladder: Hubble’s Law

• This puts the final step on our distance ladder.


Large Scale Structure of the Universe


Active Galactic Nuclei

• About 40 percent of galaxies don’t fit well into the “normal”

category — they are far too luminous and the radiation

does not look thermal. Such galaxies are called active

galaxies. They differ from normal galaxies in both the

luminosity and type of radiation they emit.


Active Galaxies

• Most of a normal galaxy’s radiation is emitted in and

around the visible portion of the spectrum. This is

because most of the radiation is from stars.

• Radiation from stars obeys the black body curve.

• By contrast, the radiation from an Active Galaxy

does not obey the black body curve.

• On the contrary, it covers both longer and shorter

wavelengths.

• This is why the radiation from these galaxies is

called nonstellar radiation.


Starburst Galaxies

• Many luminous galaxies are experiencing an outburst of

star formation, probably due to interactions with a

neighbor. These galaxies are called starburst galaxies.

• The radiation from a starburst galaxy will be spread

throughout the galaxy and distinctly stellar in nature.

• The active galaxies we will discuss now are those whose

activity is due to events occurring in and around the

galactic center. The centers are called Active Galactic

Nuclei or AGN for short:

– Seyfert Galaxies

– Radio Galaxies

– Quasars


Active Galactic Nuclei

• We can tell when the radiation is primarily coming from

a small region by studying the variability of the light

emitted in different wavelengths.

• If the period of the variability, 𝑇, is small, then we

expect that the emitting region is also small. Strictly,

the size of a coherently emitting region cannot be

larger than the light travel time:

𝑆𝑖𝑧𝑒 = 𝑐 𝑇

• For example, if the variability is 1 day, then the size

will be 2.6 × 1010 km. This is only about 2000 times

the horizon size of the black hole at the center of our

galaxy.


AGN: Seyfert Galaxies

• Seyfert galaxies

resemble normal

spiral galaxies, but

their cores are

thousands of times

more luminous.

• Seyferts have large

radio and infrared

luminosities but look

ordinary in visible.

• A Seyfert’s luminosity

can double or halve in

a fraction of a year.


NGC 1566 lies about 13 Mpc away

AGN: Seyfert Galaxies

• The rapid variations in the luminosity of Seyfert

galaxies indicate that the core must be extremely

compact. The cores of Seyfert Galaxies have

luminosities roughly equal to the entire Milky Way,

or 5 × 1036 W


AGN: Radio Galaxies

• Radio galaxies emit very strongly in the radio

portion of the spectrum.

• They may have enormous lobes, invisible to optical

telescopes, perpendicular to the plane of the galaxy.


AGN: Radio Galaxies

• The lobes may extend

far out of the galaxy.

• They are made up of

multiple jets.

• A typical radio galaxy

has a luminosity that is

about 100 times the

Milky Way: 5 × 1038 W

• The galaxy Centaurus

A appears to be the

result of a collision.


AGN: Radio Galaxies

• Radio galaxies may also be core dominated. That

is, most of the luminosity appears to be emitted from

near the center while no lobes appear.


AGN: Radio Galaxies

• Core-dominated and radio-lobe galaxies are

probably the same phenomenon viewed from

different angles.


AGN: Radio Galaxies

• Many active galaxies have jets,

and most show signs of

interactions with other galaxies.


AGN: Blazars

• According to the theory of relativity, if a jet is

approaching the observer at speeds close to the

speed of light, the radiation emitted will be

concentrated in a cone (whose opening angle

depends on the speed) in the direction of the

motion.

• To such an observer, this jet will appear both very

intense (because the radiation is concentrated in a

cone) as well as greatly blue-shifted (by the Doppler

effect)

• If we observe them radiating gamma rays, they are

known as Blazars.


AGN: Quasars

• Quasars:

Quasi-Stellar Radio

Sources are star-like in

appearance, but have

very unusual

spectral lines.

• For a long time the

spectral lines could not

be mapped to any

element.

• The first quasars to be

detected were labeled

3C 48 and 3C 273


AGN: Quasars

• Eventually, it was realized that quasar spectra were

normal, but enormously red-shifted: 3C 48 was

redshifted about 37% and 3C 273 about 16%


AGN: Quasars

• This means that 3C 48 is receding from us at about

1/3 the speed of light and 3C 273 at 1/7 the speed of

light!

• According to Hubble’s law this puts 3C 48 and 3C

273 very far away!

• Using Hubble’s constant of 70 km/s/Mpc we find

𝑑 =𝑣

𝐻Mpc

or

– 3C 48 is 1.3 Gpc away, and

– 3C 273 is 620 Mpc away!


AGN: Quasars

• The farthest quasar observed

is 13.1 b.ly away.

• These distant quasars are very

old, dating back to an earlier

period of galactic evolution.

• They must be among the most

luminous objects in the galaxy

to be visible over such

enormous distances: 𝐿 = 5 ×1040 W.

• They are probably Radio

galaxies, in which a lobe is

viewed head on.


AGN: The Central Engine

• Active galactic nuclei have some or all of the

following properties:

– High luminosity

– Non-stellar energy emission

– Variable energy output, indicating small nucleus

– Jets and other signs of explosive activity

– Broad emission lines, indicating rapid rotation



• This is the leading

theory for the energy

source in an active

galactic nucleus: a

black hole, surrounded

by an accretion disk.

The strong magnetic

field lines around the

black hole channel

particles into jets

perpendicular to the

plane of the accretion

disk.© 2017 Pearson Education, Inc.


• In an active galaxy, the central black hole may be

billions of solar masses. Black holes have a small

horizon radius. A billion solar mass black hole has a

radius of only 3 × 109 km, which is only 20 AU.

• The black hole is probably spinning very fast.

• The accretion disk is whole clouds of interstellar gas

and dust;

• The rotational velocity of the accretion disk is very

high. We can say this because of the spectral line

broadening.

• Fast spinning, hot matter will generate powerful

magnetic fields.



• As the matter swirls around the central black hole, it

heats up by friction. The temperatures can rise to tens

of millions of degrees Kelvin.

• Accretion disks are efficient at releasing energy in the

form of radiation; they may radiate away as much as

10–20 percent of their mass energy before

disappearing into the black hole.

• Quick calculation: 20% of the mass energy of a 1 solar

mass star is 3.6 × 1046 Joules. To radiate at a rate of

about 100 times our galaxy an AGN would have to

consume about 1 solar mass every 2.3 years! At

10,000 times our galaxy, a quasar would have to

consume 1 solar mass every 8.3 days!



• The jets emerging from

an active galaxy can be

quite spectacular.

• They consist of material

(mostly charged)

blasted out from the

magnetic poles.

• They are a result of

matter being

accelerated by the

magnetic field produced

by the accretion disk.



• Measurements of the core of the galaxy M87

indicate that it is rotating very rapidly.



• One might expect the

radiation to be mostly

X-rays and gamma-rays,

but apparently it is often

“reprocessed” in the

dense clouds around the

black hole and reemitted

at longer wavelengths.



• Particles will emit synchrotron radiation as they

spiral along the magnetic field lines; this radiation is

decidedly non-thermal.


Summary

• Hubble classification organizes galaxies according

to shape.

• Galaxy types include spiral, barred spiral, elliptical,

irregular.

• Objects of relatively uniform luminosities are called

“standard candles”; examples include RR Lyrae

stars, Cepheids and type I supernovae.

• The Milky Way lies within a small cluster of galaxies

called the Local Group.

• Other galaxy clusters may contain thousands of

galaxies.


Summary

• Hubble’s law: Galaxies recede from us faster

the farther away they are.

• Active galaxies are far more luminous than normal

galaxies, and their radiation is nonstellar.

• Seyfert galaxies, radio galaxies, and quasars all

have very small cores; many emit high-speed jets.

• Active galaxies are thought to contain supermassive

black holes in their centers; infalling matter converts

to energy, powering the galaxy.


galaxies - homepages.uc.eduvazct/stars/ch 24.pdf · • elliptical galaxies have no spiral arms and...

Documents