# Presentation: 471557

## The Hidden Lives of Galaxies

***Jim Lochner, USRA & NASA/GSFC***

## What is a Galaxy ?

- Solar System

***What is a Galaxy ?***

- Distance from Earth to Sun
-            =  93,000,000 miles
-            =  8 light-minutes

- Size of Solar System
-    =  5.5 light-hours

**Notes:**
To define what a galaxy is, we start with something close to home -  our own solar system - and its place in our galaxy.
The distance from the Earth to the Sun is 93 million miles (150 million km), or 8 light-minutes.  That is, the time it takes light from the sun to reach Earth is 8 minutes.
The entire solar system is 5.5 light-hours across.  That is, light takes 5.5 hours to cross the solar system.

## What is a Galaxy?

***What is a Galaxy?***

- Stellar Region

- 30
- light-years

- Sun
- (solar system
- too small to be
- seen on this scale)

**Notes:**
Neighboring stars can be grouped into stellar regions.  The solar neighborhood is about 30 light-years across.  A light year is the distance light travels in one year, and is about 6 trillion miles (9.5 trillion km)
[Note that in some regions stars can be clustered quite closely together.  Thus, some neighborhoods may be more crowded than others.]

## What is a Galaxy?

- Galaxy

- 200,000 light-years

- a massive
- collection of
- stars, gas, and
- dust kept together
- by gravity

- Sun’s Stellar Region

**Notes:**
A stellar region is just a small portion of a galaxy. A galaxy is a massive collection of stars, gas, and dust held together by its own gravity.   Galaxies can range in size from 6,000 to 350,000 light-years across.  Our own galaxy, the Milky Way, is about 105,000 light-years in diameter.
The galaxy pictured here is the Andromeda Galaxy, which is the nearest large galaxy to our own.  We think the Milky Way looks very much like Andromeda.

## What is a Galaxy?

**If our solar system was the size **
**of a cell in**
**the human body,**
**our galaxy**
**would still measure over**
**one mile across.**

**Notes:**
Galaxies are enormous in size.  This slide shows the scale of our galaxy by analogy to the size of a cell in the human body.  [A cell in the human body is about 50 microns (or 0.00005 meters) across.]
Using this same analogy, the Andromeda Galaxy would be 22 miles away.

## Types of Galaxies

- Spiral

- Barred Spiral

- Elliptical

- Irregular

- Peculiar

**Notes:**
There are different types of galaxies, each with different characteristics.
This slide presents the different galaxy types and their major features, one at time.  The slide ends by showing a few additional examples of each type of galaxy.

[This version works properly in Powerpoint for Mac OS X.  The large image of the galaxy is now grouped with its descriptive text.]

## The Hidden Lives of Galaxies

## Galaxy Formation 

- Galaxies form from the primordial density fluctuations that arise after the big bang and grow under inflation.
- These density fluctuations form filaments, and galaxies form in knots along the filaments.

## Spirals vs. Ellipticals

- Final type of galaxy depends on initial rate of star formation:
- - If stars form quickly, then galaxy becomes elliptical. Stars form within initial distribution of gas,and follow their initial orbits.
- - If stars form later, the gas has time to collapse into a disk.  Most stars from within the disk. The galaxy becomes a spiral.

## Formation via Galaxy Mergers

- In clusters, galaxies can pass close to one another.
-  Galaxies can become distorted, and often merge.
- Mergers often lead to giant elliptical galaxies at the heart of large clusters.

## Spirals in Grazing Encounter

## Antennae Galaxies

## “Invisible” Light from Galaxies

- Electromagnetic Spectrum

**Notes:**
Light is not only what we see with our eyes, but also comes in the form of radio waves, microwaves, infrared, ultraviolet, x-rays and gamma-rays.  This figure shows common images representing the major portions of the spectrum.  The gradations on the gray scale represent the changing wavelength of light as it goes from gamma-ray to radio.

Astronomers use different regions of the e-m spectrum to determine different properties of objects.  We often don’t get the whole picture until we look at an object in different wavelengths.

Note that the X-ray image is not like the other images.  The other images show the objects that emit that particular radiation.  The medical x-ray show what is absorbed by the x-rays.

## “Invisible” Light from Galaxies

**Notes:**
An X-ray picture of a galaxy looks very different from an optical picture.
On the right is an optical picture of the Andromeda Galaxy. Note the familiar spiral structure, and the dark lanes of dust.
On the left is an x-ray image of the Andromeda Galaxy taken by the ROSAT satellite.  We no longer see the spiral structure, but instead individual objects.
The inset shows a close-up of the central region of the Andromeda Galaxy taken by the Chandra X-ray Observatory.  Again we see individual sources. But in both the Chandra and ROSAT images, we also see diffuse x-ray emission caused by hot gas.

## “Invisible” Light from Galaxies

- X-ray Objects in Galaxies:

-  stars

-  supernova remnants

-  X-ray binaries

-  hot gas

**Notes:**
These are some of the objects that emit x-rays in a galaxy:
- Stars, like our Sun, emit X-rays from their hot coronas and active regions.
- A Supernova Remnant is the remains of a massive star that has exploded.  The explosion emits a shock wave that heats up surrounding gas.  The gas becomes hot enough to glow in X-rays.
- X-ray Binaries are star systems that consist of a normal star and a compact object such as a white dwarf, neutron star, or black hole.  The compact object is what remains from a star after a supernova explosion.  Material from the normal star flows toward the compact star, forming a disk of material around the compact star.  As it nears the compact star, the gas in this disk becomes hot enough to emit x-rays.
- Hot Gas permeates the galaxy and can surround a galaxy.

## Hidden Mass in Galaxies

- This X-ray image of an elliptical galaxy reveals hot, fast-moving gas even in the outer reaches of the galaxy.  The visible mass of the galaxy is insufficient to hold onto it.

- (The dark circle shows the size of the galaxy when photographed in visible light. The X-ray image shows mass far outside the visible image.)

**Notes:**
A number of elliptical galaxies are observed to have hot gas beyond the visible limits of the galaxy.
The temperature of the gas is a measure of its velocity and energy.  This gas is so hot (about 10 million degrees Kelvin, or 18 million degrees F), and consequently has such a high velocity, that it first appears to be escaping from the galaxy.   Indeed, the amount of visible mass in the galaxy is insufficient to keep it from escaping. We know the gas is bound to the galaxy, because if it wasn’t it would have dissipated long ago.  Hence, there must be some other matter that lies undetected in the galaxy.  This additional matter gives the galaxy enough mass to hold onto the gas.
This “hidden mass” is also known to astronomers as “dark matter”.

## Hidden Mass in Galaxies

- So, we have a Problem:

-  This gas gives off X-rays, which means it’s *hot!*
-  Hot gas moves at high velocities - we can measure and confirm this

-  The velocity of the gas is greater than the escape velocity of the galaxy, if we calculate the galaxy’s mass by adding up all the mass we can see at all wavelengths of light
-  So why hasn’t the gas escaped?  There *must* be more mass we *can’t *see!

## Hidden Mass in Galaxies

- Another way to look at the problem:
- We can determine the mass of an object by measuring the motion of bodies in orbit around it.
- 	Newton’s Second Law:
- 	F = Force of Gravity
- 	a = acceleration due
- 	      to circular motion

## Hidden Mass in Galaxies

- From previous slide, we have Newton’s Second Law of Motion assuming a gravitational force and acceleration due to circular motion:
- Simplifying gives:
- So, if *GM* is constant, then velocity is proportional to the inverse square root of distance.  For example ...

## Hidden Mass in Galaxies

- Rotation Curve - A Plot of Object Velocity vs. Distance

- Our Solar System

**Notes:**
To further understand the evidence for hidden mass in galaxies, we go back to examining our own solar system.
We first plot the velocity of the planets in their orbits around the sun (right hand vertical scale, in yellow) as a function of their distance from the sun (bottom scale, in yellow).  This type of plot is known as a Rotation Curve.  Ask audience members to describe how the velocity of the planets varies as the distance from the sun varies [as the distance increases, the velocity decreases].

You can use this slide with the following slide (on Activity 6a).  May work best with this as a separate transparency on an overhead. 

## Activity #6a: Evidence for Hidden Mass

- There are ____ solar system planets presented on the graph.  The planets, from the closest to the sun to the furthest, are ___________________________________ ______________________________.  Using the graph, the velocities of the solar system planets, from the lowest value to the highest value, are _____________________                                           _________________________.  Using the graph, the distances of the planets from the Sun are, from least to greatest, ____________________________________ ______________.  In general, the further a planet is from the sun the ______ its velocity.  The closer a planet is to the sun ______ its velocity.

- nine

- Mercury, Venus, Earth, Mars, Jupiter,

- Saturn, Uranus, Neptune, and Pluto

- approximately 48, 35, 30,

- 24, 13, 10, 7, 5, and 4 km/sec

- 0, 110, 150, 250, 800, 1500, 2800, 4500, and

- 6000 million km

- slower

- faster

## Hidden Mass in Galaxies

- Rotation Curve - A Plot of Object Velocity vs. Distance

- Our Solar System

**Notes:**
To further understand the evidence for hidden mass in galaxies, we go back to examining our own solar system.
We first plot the velocity of the planets in their orbits around the sun (right hand vertical scale, in yellow) as a function of their distance from the sun (bottom scale, in yellow).  This type of plot is known as a Rotation Curve.  Ask audience members to describe how the velocity of the planets varies as the distance from the sun varies [as the distance increases, the velocity decreases]. The curve described is exactly that expected from Newton’s Law of Gravity.

## Hidden Mass in Galaxies

- The previous plot showed data for planets in our solar system, illustrating the equation:
- If we solve for M:
- We can use real data (the distances and velocities of the planets) and the fact that  G = 6.67 x 10**-11** m**3**/kg-s**2**  to verify that the central mass, *M*, remains constant..

## Activity 6b, part 1

- 2.0 x 1030

- 2.0 x 1030

- 2.0 x 1030

## Hidden Mass in Galaxies

- What does this have to do with Galaxies???
- If we measure the distance and velocity of objects (eg., stars) orbiting in a galaxy, we’d expect them to obey the same laws.  As distance from the center of the galaxy increases, we should get to a point where almost all of the galaxy’s mass is inside the orbit of the furthest objects.
- At this point, the central mass would be practically constant, and we would expect a rotation curve similar to that for our solar system.

## Hidden Mass in Galaxies

**Notes:**
We now compare the rotation curve for the solar system to that of a galaxy.  Here we plot the velocities of stars going around the center of the galaxy (left hand vertical scale, in white) vs. their distance from the center of the galaxy (top scale, in white).  “Kpc” stands for “kiloparsecs”, a distance used to measure large distances in galaxies (1 kpc = 3,260 light-years).  
We expect the rotation curve of a galaxy to be linear through the central bulge.  This is because the central bulge rotates as a solid body.  Outside the bulge, we expect the rotation curve to fall off with distance according to Newton’s Law of Gravity (similar to the solar system).  
Now show the actual rotation curve for a galaxy. Again, ask audience members to describe how the velocity varies as a function of distance [it rises steadily, and then at 10 kpc it becomes nearly constant]. We find that the velocities continue to rise outside the bulge, and eventually flatten out.
The fact that velocities don’t follow Newton’s Gravity means that there is more mass in the galaxy than we can account for.

## Hidden Mass in Galaxies

- and use the actual distances and velocities observed for this galaxy, we can calculate the enclosed mass at various distances from the galaxy’s center.
- (Remember that  G = 6.67 x 10**-11** m**3**/kg-s**2** )

- If we take the same equation we used for the solar system:

## Activity 6b, part 2

- 2.1 x 1040

- 5.6 x 1040

- 8.4 x 1040

## Hidden Mass in Galaxies

- What Happened?!?!?
- The central mass never becomes constant, as it did for the Solar System.  The fact that the mass is still increasing means we haven’t yet reached a distance where all the mass is contained inside that orbit.  But we’ve plotted *all the matter we see!*
- There *must* be Missing Mass!!

## Hidden Mass in Galaxies

- Hot Gas and Rotation Curves show:
- Gas and objects move at velocities greater than can be accounted for from the gravitational effect of the visible mass of the galaxy.
- From these observations we deduce that the visible mass accounts for only 10 % of the total mass of the galaxy.
- Recent results from WMAP show ordinary matter makes up only 4% of the universe!

**Notes:**
From observations of hot gas in galaxies and data from the Rotation Curves, we conclude that gas and stars in galaxies are moving faster than what we would expect from the visible mass of the galaxy.  Note that by “visible”, we mean here mass detected at all wavelengths of the e-m spectrum, and not just optical.
We find that only 10 % of the mass of the galaxy is detectable.
WMAP results from Feb 2003 show that 4% of the universe is ordinary matter, 235 is cold dark matter, and 73% is dark energy.

## Candidates for the Hidden Mass

- Hydrogen Gas
- Very abundant, but not enough detected
- MACHOs (Massive Compact Halo Objects)
- E.g. Black Holes, Neutron Stars, Brown Dwarfs
- Not enough of them
- WIMPs (Weakly Interacting Massive Particles)
- E.g. Exotic subatomic particles
- The best candidate theoretically, but not yet observed.

**Notes:**
Scientists have various candidates for what the hidden mass in galaxies could be.  
Hydrogen gas is abundant in galaxies, but there is still not enough of it.
Massive Compact Halo Objects are objects such as black holes, neutron stars and brown dwarfs.  Again, there just aren’t enough of them.
The best candidate is Weakly Interacting Massive Particles.  These are as yet undiscovered sub-atomic particles that interact weakly with normal matter but yet have mass.  This is currently a very active field of particle physics and cosmology.

## The Hidden Lives of Galaxies

- Presentation available at
- http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/galaxies.html