# Presentation: 116023

## The Hidden Lives of Galaxies

***Jim Lochner and Gail Rohrbach, NASA/GSFC***

- NSTA   March 24, 2001

## 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

***Types of Galaxies***

- Spiral

-    disk-like appearance with arms of stars and dust forming a spiral pattern

- Barred Spiral

-    similar to spirals but with a bright bar of stars and gas through the center

- Elliptical

-    elliptically-shaped, with less gas and dust than spirals; no disk or “arms”

- Irregular

-    neither elliptical nor spiral in shape; gas and dust as in spirals but no defined “arms”

- Peculiar

-    distorted form of one of the above types, often due to collision with another galaxy or similar catastrophic event

**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.

## The Hidden Lives of Galaxies

- NSTA   March 24, 2001

## “Invisible” Light from Galaxies

**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.  The wavelength of light decreases as we move up the electromagnetic spectrum from radio to gamma rays.  This figure compares the wavelength with common objects.  
In addition, the diagram shows one of the ways that ultraviolet light, x-rays, and gamma-rays can be emitted: UV light is emitted from molecules; x-rays are emitted by processes involving electrons in atoms; and gamma-ray are emitted from processes occurring in the nuclei of atoms.
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.

## “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.)

**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

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

**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.
Next examine the plot for the galaxy F563-1.  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).  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].
In a galaxy, we expect the velocities to rise steadily from the center, as if near the center the galaxy were a solid body.  But once we are outside the majority of the mass of the galaxy, we expect the velocities to fall off according to Newton’s Law of Gravity.  The fact that it doesn’t means that there is more mass in the galaxy than we can account for.

## 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

- 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.

**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.

## 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.