Chapter 15, Galaxies

G

alaxies come in different size and shape. In the previous chapter, we talked about how galaxies provide an environment for the stars to be born and die, and enrich the heavy element content of the galaxy. In this chapter, we will talk about • • • • Galaxy Classification Location of Galaxies in the Universe Galaxy Evolution Quasars and AGN (Active Galactic Nuclei)…

• NGC 1300 – barred spiral

Spiral Galaxies

• NGC (New General Catalog) 4594… • NGC 4594 – Sombrero Galaxy. Large bulge, small disk…

Elliptical and Irregular

• M87 – Elliptical Galaxy in Virgo • Large Magellanic Cloud, irregular galaxy

Galaxy Classification

G

alaxies come in different size and shape…

1.

2.

3.

• • •

Spiral Galaxies Barred Spiral : Lenticular galaxy:

Spiral galaxy with a bar. Spiral galaxy without spiral arms.

Formed by gas clouds with large initial angular momentum.

• • •

Elliptical Galaxies

Similar to the bulge of the spiral galaxies. Formed by gas clouds with small initial angular momentum. Formed by high density clouds—more efficient cooling, faster star formation, • exhausting the gas supply before the galaxy has time to collapse into the disk. Formed by collision and merging of galaxies. • • •

Irregular Galaxies

Usually more distant galaxies.

Starburst galaxies

– Galaxies with high star forming rate. Merging galaxies?

Quasars, Active Galactic Nuclei

– Distant objects. Extremely luminous—could be 1,000 times the luminosity – of the Milky Way.

Supermassive Black holes?

Galaxy Facts

• •

Grouping…

Spiral galaxies

are usually found in loosely associated small

group

(tens) of galaxies

Ellipticals

are commonly found in large

cluster

of galaxies containing hundreds or thousands of galaxies extending over tens of millions of light-years. • • •

Size…

Most of the

large galaxies

are

spiral galaxies

While some of the largest galaxies are

giant

…

elliptical

galaxies, the

most common type of galaxy in the universe is small elliptical galaxy

.

Very small ellipticals (or

dwarf spheroidals

, less than a billion stars) are often found near large spirals – our Milky Way galaxy has 10 or more nearby…

Where are the Galaxies?

W

here are the galaxies located? Are they located within the Milky Way, or are they much further away from us than the stars?

• Before the 1920s, there were no reliable methods of measuring the distance to the galaxies.

Many people believed that the galaxies were

located within the Milky Way…

How do we measure the distance of objects far away in the universe, much farther than the distance that can be measured by

stellar parallax

?

•

Measurement of distance farther than the reach of stellar parallax rely on our ability to find objects with known luminosity…

• In 1924, Edwin Hubble determined the distance to the Andromeda galaxy using

Cepheid variables

, thus proving that the galaxies are located far beyond the stars in the Milky Way galaxy.

Measuring Cosmological Distance

T

he principle method of measuring astronomical distance is the

distance-luminosity relation

• • However, the apparent brightness

B

is the only thing we can measure accurately in most of the cases.

L

 4 

D

2

B

  If we know the distance Need to find

D

Standard Candles

, we can determine the luminosity

—

L

. If we know the luminosity

L

, then we can determine the distance

D

. astronomical objects with known luminosity.  Main sequence stars.  Cepheid variables.   White dwarf supernovae. Galaxies (using Tully-Fisher Law).

The Cosmological Distance Ladder

M

ethods of Measuring Distance and their useful range… •

Radar ranging

– D < 10 -4 light-years • •

Parallax

– D < 10 3 light-years •

Standard Candles

 Main sequence stars – D < 10 5  Cepheid variables – D < 10 7 light-years light-years  White dwarf supernovae – D < 10 10 light-years

Hubble’s Law – 10 10 ly and beyond…

Calibrating the Cosmic Tape Measure

W

e rely heavily on the

standard candles

for the measurement of the cosmological distance. How do we make sure that these standard candles are truly standard?

1.

2.

3.

Use independent measurements to check the luminosity of the standard candle.

For example, we can use parallax measurements of the distance to main sequence stars to check measurements of distance using

main-sequence fitting

. If we do this for a few of them, then we can verify the assumption that the main sequence stars are good standard candle. However, this method works only for stars that are relatively close by.

The standard candles we had verified in close range can now be used (by extrapolation ) to measure the distance to more remote objects.

This new distance measure then allows us to calibrate the next standard candle. For example we used the distance measured by observation of Cepheid variables to check the assumption of the constancy of the white dwarf supernovae. Keep going…

Main Sequence Fitting

Main sequence stars with the same color should

have the same luminosity.

So, if we compare the (pseudo) H-R diagram of a star cluster with unknown distance (using their apparent brightness instead of luminosity) to that of a • group of main sequence stars with known distance, then we can determine the distance to • this new cluster.

For example… Hyades

(in constellation Taurus) is a open cluster about 150 light years away. It’s distance is close enough to be measured by stellar parallax. Comparing the H-R Diagrams of Hyades with that of Pleiades, we can determine that • the distance of

Pleiades

should be 2.75 times farther than Hyades…or, 150*2.75 =

410 light years

. New parallax measurement of Hyades by Hipparcos (ESO Space Interferometry mission) yielded a distance of

438 light years

The two parallel lines shows that the apparent brightness of all the different type of main sequence stars are decreased by a factor of 7.5

Cepheid Variables

C

epheid variable stars are population I (metal-rich) yellow giant stars with

periodic

• • • •

luminosity variation

….

Their periods range from a few days to over 100 days, Their luminosities range from 1000 to 30,000

L

⊙ , The high luminosity makes it possible to identify them from a large distance… Their luminosity and period are strongly correlated. Therefore,

we can determine their luminosity by simply measuring their periods!

The luminosity of Cepheid variabls are strongly correlated to their periodicity…

Cepheid Variables in M100

T

he period-luminosity relation of Cepheid variables were discovered Henrrietta Leavitt in 1912. Edwin Hubble identified Cepheid variables in Andromeda galaxy (about 2.5 million light-years away) in 1924, and used the luminosity-distance relation to demonstrate that galaxies are much farther than the stars….

• There are well over 1,000 Cepheid variables known today…for example, the Polaries!

• In 1994, Hubble Space Telescope observed a Cepheid variable in the face-on spiral galaxy M100 in Virgo Cluster located at a distance of 56 million light-years…this is the most distant distant Cepheid observed so far….

http://hubblesite.org/newscenter/archive/releases/1994/49/

Tully-Fisher Relation

A

lthough this is not discussed in our text book, the

luminosity

of the spiral galaxies are related to their

rotational speed

,. This was discovered by B. Tully (of UH/IfA) and J.R. Fisher in 1977. Therefore, the luminosity of the spiral galaxies can be determined simply by measuring their rotational speed… 

Spiral galaxies are good standard



candles also!

The slope of the luminosity-rotation rate curve is different for different type of spiral galaxies…

White Dwarf Supernova

E

very time the hydrogen shell is ignited, the mass • • • • of the white dwarf may increase (or decrease, we don’t know for sure yet). The mass of the white dwarf may gradually increase, At about 1

M

⊙ , the gravitation force overcomes the electron degenerate pressure, and the white dwarf collapses, increasing temperature and density until it reaches

carbon fusion temperature

. The carbon inside the white dwarfs are simultaneously ignited. It explodes to form a

White dwarf supernova

. (Type I).

Nothing is left behind from a white dwarf supernova explosion (In contrast to a massive-star supernova, which would leave a

neutron star

or

black hole

behind). All the materials are dispersed into space.

White Dwarf Supernova is a very important standard candle for measuring cosmological distance…

White Dwarf and Massive Star Supernovae

B

ecause the mass of white dwarfs when they explode as supernovae is always around 1.0 M

⊙

, its luminosity is very consistent, and can be used as a standard candle for the measurement of distance to distant galaxies

(Chapter 15).

The amount of energy produced by white dwarf supernovae and massive star supernovae are about the same. But the properties of the light emitted from these two types of supernovae are intrinsically different, allowing us to distinguish them from a distance.

• • • Massive star supernovae spectrum is rich with hydrogen lines (because they have a large outer layer of hydrogen). White dwarf supernovae spectra do not contain hydrogen line (because white dwarfs are mostly carbon, with only a thin shell of hydrogen).

The

light curve

is different.

Supernovae from Distant Galaxies

T

hese snapshots, taken by NASA's Hubble Space Telescope, reveal five supernovae, or exploding stars, and their host galaxies. – The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. The supernovae exploded between 3.5 and 10 billion years ago.

Distance and Redshift

I

n addition to distance, Hubble also measured the

redshift

of the galaxies…and when combined with distances derived from observation of Cepheid variables and the brightest stars in galaxies, Hubble found that,

the more distant a galaxy, the greater its redshift is, and hence the faster it is moving away from us…

→

the universe is expanding!

Hubble’s Law

F

rom the redshift and distance measurements, we can express the recession speed

V

of a galaxy located at a distant

d

away from us by

V

=

d



H

0 The value of the

Hubble’s Constant is H

0 = 20~24 [km/sec] / million light-year Once the value of

H

0 is determined, we can use measured recession speed to infer the distance of galaxies using the formula

d

=

V / H

0