Goal: To understand the structure and makeup of our own Milky Way Galaxy

Objectives: 1) Viewing our galaxy in the optical 2) Viewing our galaxy at other wavelengths (to understand what can they tell us that the optical cannot) 3) To learn about Formation of our galaxy 4) To understand Structure of our galaxy 5) To learn about Movements in our galaxy

In the optical

• Why does it look like this?

In the optical

• We are looking through our galaxy – much like looking through a fog or through a forest.

• Much like a forest, nearby brush masks our view of the surrounding forest also (thus the stars all over).

• We live in a barred spiral galaxy.

• We live halfway out.

• The bright clumps are star forming regions – notice how they lie on the spiral arms.

From far away

But how do we know if we can’t see through the dust?

• Dust is a big problem when observing in the optical.

• http://www.astro.livjm.ac.uk/courses/one/TEXTBO/INTE RS02.HTM

• So, how do you get around the dust?

Infrared and Radio!

• Infrared and Radio work great because radio waves are bigger than the size of the dust.

• The dust can’t absorb the radio (would be similar to an ant trying to catch a basketball).

Still can’t see everything

• What are the arcs above and below?

How do we see all our galaxy and map it? • Use radio!

• Some wavelengths of radio give us specific molecules (emission) – below is Hydrogen

Carbon Monoxide

But we still don’t get the full picture.

• How far away is everything? • We want to know things in 3D not 2D!

• How do we do that?

• To help lets consider this.

• Imagine a star system with no planets.

• An alien species colonizes this star system by building orbiting homes.

• The homes each have an optical light to light up their own home.

• However, the walls of their home are transparent to optical light.

• Their homes also emit infrared light, but the infrared light gets absorbed by all the other houses.

• How does one alien – without leaving his home map the system of homes?

Map infrared and optical!

• Well, the alien can figure out where the homes are (in the sky) – sort of.

• The alien will see a large bar of optical light which represents the plane in the system all the homes orbit in.

• There will be some points above and below this bar because of the nearest houses.

• The infrared map will have thick dark areas where there are homes absorbing light – and we won’t see very far there.

• However, the alien still can’t map them. • Could we use a trick? Do the homes move with respect to the alien’s home?

Orbits!

• The homes are in orbit around the star.

• So, the homes will move with respect to the alien’s home.

• At any given position in the sky the radial (outward/inward) and tangential (sideways, or motion in the sky) motions of the other homes will depend on their distance from the star!

• Can you detect the radial and/or tangential motions somehow?

• A) no for both • B) no radial and yes tangential • C) yes radial and no tangential • D) yes for both

Movement

• For the tangential direction – in a star system if you could see individual objects you MIGHT be able to watch them move.

• This is how we find asteroids and Trans Neptunian Objects (TNOs) such as Pluto.

• However, if you have soooo many homes they all merge together, then you won’t.

• Also, if the size of the system is so big that the motions are too small to detect then you won’t be able to see the tangential movement either.

Radial velocity

• However we can observe radial velocity!

• How? By using the Doppler effect!

• When an object moves towards us, the wavelengths of light it emits (or sound on earth) decrease (because the object is closer to us when the wave finishes than when it starts – so the shrink in the wave is the distance the object travels in the time it takes to make the wave).

• When it moves away from us, the wavelength increases.

• The fraction of the increase/decrease of the wavelength just depends on the velocity of the object!

Radio and Doppler shift

• In the radio, you are looking at specific wavelengths.

• Hydrogen for example has a very strong line at 21 cm.

• So, if you look near 21 cm you can get a spectrum from say 20 cm to 22 cm.

• If you get a peak at 21.1 cm then you know the Hydrogen you are looking at is moving away from us (away because the wavelength is increased) at 1400 km/s.

• By using this we can map our the homes, and our galaxy.

• Everything in our galaxy orbits around the center of our galaxy – so we have one really big system.

Mapping the galaxy

• So, in each direction we look for the brightness at each wavelength near specific bands.

• You compare that to where you expect each part to be in orbits.

• This gives you a map.

Other wavelengths useful

Formation of our galaxy

• Formations of spiral galaxies are very much like the formation of an individual star.

• You start with really huge area of gas with some spin.

• It collapses to a plane (except the center which have orbits in the 3 rd dimension so are move oval).

• Somehow you also form the globular clusters.

http://www.ldps.ws/Mirror/Universe/galaxy.html

Components - Bulge

• The central 13000 light years of our galaxy contains the Bulge.

• The Bulge is a bar aimed 45 degrees away from where we are in the galaxy.

• On the outside of the bulge is a ring (called the “5-kpc ring”) and contains a large fraction of the molecular Hydrogen in our galaxy. • This is also the location of the greatest amount of star formation in our galaxy and would be the Milky Way’s brightest feature to anyone in any other galaxy.

http://www.ldps.ws/Mirror/Universe/galaxy.html

Disk

• The disk is about 2000 light years in total thickness. • The disk contains most of the stars in our galaxy.

• Our galaxy has somewhere between 200 400 billion stars!

• The diameter of the disk is 100,000 light years.

Spiral Arms

• A common feature in discs of Spiral Galaxies are Spiral Arms.

• Spiral Arms are density waves which pass through the galaxy.

• They also rotate around the galaxy, but only with a period of 160 million years or so.

• So, the materials in the galaxy actually plow into the Spiral arms.

Structure of a Spiral Arm

• (diagram on board) • It starts with dust running into the arm.

• This is called a dust lane.

• The dust gets compressed by a factor of 4 at the start of the wave.

• Just behind that you start star formation.

Structure of a Spiral Arm 2

• The massive stars die in a few million years.

• So, just behind the start of star formation you have supernovae.

• Just behind that you have bubbles from where all the supernovae have merged.

• After that you are left with normal stars and normal space which slowly cool until they hit the next spiral arm in a few hundred million years.

• With this process, the Milky Way produces about 7 new stars per year.

3

rd

component: Halo

• Surrounding our galaxy is our Halo.

• This is where the Globular Clusters all lie.

• There are also a couple of dwarf galaxies that our galaxy is currently absorbing: • Sagittarius Dwarf Galaxy • Canis Major Dwarf Galaxy • But there is more…

Rotation curve

• In our solar system, as you move further from the sun, your orbital velocity decreases.

• V 2 orbital= G Msun / R • In a galaxy though, as you go further out the amount of mass on the inside of your orbit goes up.

• So, if you plotted orbital velocity with distance, what would you expect it to look like?

• A) should fall, but not as fast as for the solar system • B) should fall, but very slowly • C) should stay constant • D) should go up

Expected

• From the mass we can see, you expect a gradual decrease in the orbital velocity.

However

• The velocities stay flat or even INCREASE!

http://spiff.rit.edu/classes/phys301/lectures/mw/mw.html

Dark matter problem

• It turns out that most of our galaxy is DARK MATTER.

• If you look at stars and gas and dust the mass of our galaxy is about 100 billion solar masses.

• However, the gravitational mass (M = V 2 * R / G) is 1 trillion solar masses!

• 80-90% of our galaxy is mass we cannot see!

What is dark matter?

• We have no idea.

• The ideas are: • MACHOs – large objects which are too dim to see.

• WIMPs – large atomic particles which would not emit light • Maybe others? We just don’t know.

• And the dark matter for our galaxy seems to go out to 300,000 light years.

Back to stuff we DO know!

• Stars!

• There are 2 populations of stars in our galaxy.

• Guess what we call them (hint REALLY lame astronomy name coming up)?

Star populations

• Population I stars – told you it was lame – are stars on the disk of our galaxy. • They are newer stars (newer than Population II).

• They have higher metals than Population II – and are usually metal rich.

• Our sun is a Population I star.

Thick disc stars

• There are some stars that are sort of half way between population I and II. These are called the Thick disc stars.

• These are stars who have orbits which take them into and out of the plane of the galaxy and often are very elliptical.

• In essence they fill a thicker disc that the disc stars.

• These stars are usually very old and have low amounts of metals.

• Arcturus is a thick disc star.

Population II stars

• Population II stars are all very old (12 billion years).

• They are all located in the halo of our galaxy (and most are in Globular Clusters).

• All have very low metallicities.

• All Globular Cluster stars are Population II stars.

Conclusion

• To understand our galaxy you need to look at a multitude of wavelengths.

• Radio is the best type of light to map our galaxy.

• Our galaxy has 3 components: bulge, disk, and halo.

• There are 2 populations of stars.

• However most of our galaxy is made of dark matter.