Dark Matter

Rotation Curve

Now consider the Milky Way galaxy. Using radio observations, it is possible to measure the orbital speed as a function of the distance from the center.

Rotation Curve Compare the two graphs:The rotation curve for

the Solar system is falling with radius.

The rotation curve for the Milky Way is flat or even rising with radius.

The mass distribution of the Milky Way is not a simple “point mass” distribution.

Stars in the outer parts are orbiting at a much higher velocity than expected, based on the amount of visible matter (e.g. stars). There must be dark matter in order to account for this.

But, in our galaxy the orbital speeds continue to climb well above the visible edge of the galactic disk, so there must be more gravitational force acting on the stars & clouds.

This is not a single gravitational source like the Sun, so the speed near the center will be small and rise quickly. As you get to the edge of the visible galaxy, the velocity should drop down.

What is Dark Matter?

Dark matter is found in the halo and far beyond the luminous disk

There must be sufficient matter to provide gravitational force to bind galaxies together in clusters. No clusters of galaxies contain enough visible matter to keep them bound together.

If we trust our theory of gravity... there may be 10 times more dark than luminous matter in our Galaxy

Dark Matter can be anything that gives off no light.

Possibilities :

1. Ordinary matter is called baryonic and consists of MACHOS (Massive Compact Halo Objects). These objects could be Brown Dwarfs, White Dwarfs, Jupiters , protons and neutrons. Protons and neutrons are composed of baryons.

2. Non-baryonic matter called WIMPS (Weakly Interacting Massive Particles). Neutrinos are now thought to have some mass, so they are a possibility. There may be some sub-atomic particles yet to be discovered that could be the answer.

4. Maybe our gravity works differently for

The other question about the nature of dark matter is, the matter:

1. “hot” fast moving like neurtinos

2. “cold” slow moving

The type of dark matter determines when structure could actually form. It is hard to form structures in a hot Universe and easier in a cold Universe.

3.The answer could be a combination of both.

massive galaxies.

0

0

z

If Z > .1, you need to use the Relativistic form.

0

0

vZ

c

1c v

zc v

2

2

1 1

1 1

zv

c z

Or more useful

If z=1 , then v = c , if z = 3 then v=3c . Can’t be moving faster than light. Yet, there are objects with z = 7 or 8. The answer is you must use the Relativistic form below.

Redshift Recessional velocity Distance

0

0 0

0.2 0.180 2.41

0.4 0.324 4.26

0.75 0.508 6.57

1 0.6 7.73

2 0.8 10.3

3 0.882 11.5

4 0.923 12.1

5 0.946 12.5

10 0.984 13.2

Infinite 1 13.7

z v/c Bly

Active GalaxiesActive Galaxies are divided into 4 main categories based on observational characteristics.

• Radio Galaxies

• Seyfert Galaxies

• BL Lacertae Galaxies (Blazars)

• Quasars

•These objects are by far the most powerful objects in the universe (the energy output easily exceeds the total output of all of the stars in the Milky way).

•AGN are powerful sources of energy, usually associated with the center of a distant galaxy.

PROPERTIES OF ACTIVE GALAXIES• High luminosity• Small, luminous core. • Jets or explosive appearance • Non-stellar emission features

•Observationally, AGN are powerful sources of energy, usually associated with the center of a distant galaxy.

•Multi-wavelength observations, X-ray and radio observations, are essential for understand what is happening.

Seyfert Galaxies• Early in the 20th century Carl Seyfert and others

cataloged galaxies with optically bright nuclei called “Seyfert Galaxies”.

• These objects have large amounts of rapidly moving outflowing gas (velocities up to 10,000 km/sec), much more than normal galaxies.

The galaxy at the left is normal and the one on the right has a very bright nucleus.

Seyfert Galaxies are thought to be spirals with an abnormally bright cores. Variable brightness on short time scales (small source)

•All indicators point to violent explosive activity in the galactic nucleus.

•10% of most luminous galaxies are Syferts.

•Nuclei show emission lines of ionized gas.

•About 10% are radio sources.

RADIO GALAXIES

• Are double lobed or compact radio source

• They often have visible or radio jets

•Most often Radio Galaxies are giant elliptical galaxies

•Radio/Visible luminosity ratio is very high

RADIO GALAXIES:

AGN lie at the center of double radio sources. Gases ejected from the galaxies create two radio lobes.

Radio galaxies are called “core-halo “ or “lobe”, depending upon their angle from us.

Radio galaxies emit most of their energy at radio wavelengths. Stars can not do this.

This galaxy has HUGE radio lobes, which occur where the jets plow into intracluster gas.

The thin line through the galaxy is a jet ejected from the nucleus.

BL LACERTAE GALAXIES (BLAZARS)

• Elliptical galaxies with very bright nuclei

• Featureless continuous spectrum

• Short periods of variability

• Highly red shifted spectra

• Viewing directly into jet

Extremely luminous galaxy cores with no spectral features

But wait, there’s more, the discovery of Quasars•In the early 1960s, Maarten Schmidt identified the radio source 3C 273 with a faint blue star. using an optical telescope.

•Large red shifts indicate that quasars are the most distant object known in the Universe. •Some have radio or optical jets.

•1,000 brighter than the entire Milky Way Galaxy

So what did Schmidt discover about 3C273 ?

Using the Doppler Formula, he found the velocity to be 48,000 km/sec.

If we use Ho = 71 km/sec/Mpc in the formula d= v/Ho, the distance turned out to be 676 Mpc or 2.1 Bly.

Extremely distant, but brighter by a factor of 100-1000 than normal galaxies at those distances, and they look star like.

Quasars

• These objects appear relatively bright, and the great distances imply exceedingly large luminosities.

• It is also known that they can vary in brightness over timescales of days to weeks. This implies that the physical size is relatively small.

•The farther away we look out in distance, the farther back we look in time! Quasars existed only in the early Universe!

Quasars• The timescale of

the variability sets a limit on the physical size of the object.

• Most quasars are smaller than 1 light-month.

Quasars are associated with distant galaxies

AGN and QuasarsWe have two types of unusual sources.

•In one case, the nucleus of a relatively nearby galaxy is producing large amounts of energy.

•In the other case, an unresolved source is doing the same, at great distances out in space. This equates to a long time ago.

By process of elimination, only possible answer: huge BHs are in the center of galaxies.

Quasars can only be seen in the early years of the Universe, there are not any around today.

They glow only while they are accreting, for quasars this was long ago. They faded with time , as the gases were used up.

It’s all a matter of how you look at it.

Forming Jets from a blackhole

•Synchrotron radiation occurs when electrons move rapidly (near the speed of light) through a magnetic field.

•Charged particles are forced to spiral (accelerate) around the magnetic field lines.

Gamma-Ray Bursts

Brief bursts of gamma rays coming from space were first detected in the 1960s by satellites placed in orbit to monitor nuclear explosions.

The satellites turned out to be detecting gamma ray bursts from out in space.

The energies involved were the most powerful explosions in the universe.

More than 3,000 GRBs have been observed. A long lived GRB emits as much energy in 100 seconds as the sun will emit over its life time.

• Observations in the 1990s showed that many gamma-ray bursts were coming from very distant galaxies, and they must be among the most powerful explosions in the universe.

• Observations show that at least some gamma-ray bursts are produced by supernova explosions

Sources of GRBs are believed to be from powerful supernovae. Other sources are collisions of black holes, a black hole swallows a neutron star, or when a pair of neutron stars collide and form a black hole. Calculations confirm that such collisions should lead to an intense burst of gamma rays.

BINARY BLACK HOLES

• Astronomers have found proof of two super massive black holes together in the same galaxy.

• These black holes are orbiting each other and will merge several hundred million years from now.

• The event will unleash intense radiation and gravitational waves and leave behind an even larger black hole than before.

Radar

~ 1 AU

Radar

Parallax

~ 1 AU

~ < 500 pc

Distances to the near by stars in Parsecs

The unit Parsec makes it easier to measure distances to the closer stars. Measure angle

in arc seconds.1d

p

Limitations: Parallaxes out to 50 pc only from Earth. Hipparcos satellite (ESA) (1990–93):Parallaxes with greater accuracy out to ~500 kpc.

Radar

Parallax

Spectroscopic Parallax

~ 1 AU

~ < 500 pc

40 pc to 10Kpc

40,000 20,000 10,000 5,000 2,500

106

104

102

1

102

104

Temperature (K)

Lu

min

osi

ty (

Lsu

n)

m – M = 5 log(d)-5 or log(d) = (m-M+5)/5

Radar

Parallax

Spectroscopic Parallax

Cepheid Variables

~ 1 AU

~ < 500 pc

~ 1Kpc to 30 Mpc

40 pc to 10Kpc

Cepheid & RR Lyrae Variables

Distances Using Cepheid VariablesThese variable stars show intrinsic brightness variations.

Cephei

Cepheid variables are good standard candlesThe period directly linked to its average

brightness: the longer the period, the brighter the star.

From the `light curve', you can tell that it is a Cepheid or RR Lyrae variable.

RR Lyrae

Cepheid

The period is simple to measure, as is the apparent maximum brightness.

Metal Rich

Metal Poor, fainter

There are two types of Cepheids

To find the distance to a Cepheid Variable:

Measure period of pulsation of the star.Magnitude (Mv) from the Period.

Determine that the star is a Cepheid Variable

Measure the apparent magnitude m

Calculate the distance to the star using

m – M = 5 log(d)-5

Radar

Parallax

Spectroscopic Parallax

Cepheid Variables

~ 1 AU

~ < 500 pc

~ 1Kpc to 30 Mpc

40 pc to 10Kpc

Supernovae 1a, II 1 to over 1,000Mpc

Supernovae Type Ia has all the right characteristics for standard candles:

Easily detected, but rather rare (3 SN per galaxy per century).

Extremely bright, outshining the entire galaxy of stars.

White DwarfEvolving (dying) star

Roche Lobes

Evolving (dying) star White Dwarf

Accretion Disk

Roche Lobe filled

Evolving (dying) star

I

II

III

A lone white dwarf is a cooling corpse, but a white dwarf in a binary system can become a Type 1a SN.

Sirius

Type Ia: No hydrogen lines in the spectrum Type II: Hydrogen lines in the spectrum

You can determine M from the graph and m is known, now you can get distance. m – M = 5 log(d)-5

Radar

Parallax

Spectroscopic Parallax

Cepheid Variables

Supernovae 1a, II

~ 1 AU

~ < 500 pc

~ 1Kpc to 30 Mpc

40 pc to 10Kpc

Tully Fisher ~ 700Kpc to 100Mpc

1 to over 1,000Mpc

Tully-Fisher Relation

The greater a spiral galaxy’s mass, the greater it’s luminosity and the faster it rotates.

The rotational velocity of a galaxy can be determined by observing the 21cm emission line width of neutral hydrogen

Superposition of blue- and red shifted 21cm line gives broad emission line. Velocity can be calculated from the from line width (dispersion).

V V

The luminosity is related to its absolute magnitude (M) and its apparent magnitude (m) can be observed.

Use the distant formula to find the distance

m – M = 5 log(d)-5

Specifically Luminosity is proportional to V^4.

Tully-Fisher fails when you can’t determine the size or orientation of the spiral galaxy

Radar

Parallax

Spectroscopic Parallax

Cepheid Variables

Hubble’s Law

~ 1 AU

~ < 500 pc

~ 1Kpc to 30 Mpc

> 1 Mpc

40 pc to 10Kpc

Tully Fisher ~ 700Kpc to 100Mpc

1 to over 1,000Mpc

Supernovae 1a, II

A red shifted galaxy spectrum

czv

z

0

0

Hubble’s Law and Red Shift

d=V/Hd=V/H

Hubble’s Law : Recession velocity is related to Distance.

Hubble’s Law

The

End The

End