Questions of the Day What is a galaxy? How is our Galaxy (the Milky Way) structured? Where are we located in our Galaxy? What are the bulge, disk, and halo? Why do rotation curves imply dark matter? How do stars and gas move in the bulge, disk and halo? What kind of galaxy is the Milky Way?

A walk outside the Solar Neighborhood… Today we explore our Galaxy, the Milky Way! Our Galaxy has three major components: The Bulge The Disk The Halo

Our View from Within Messier was a comet hunter, It’s hard to figure out what something looks like when you’re inside of it! Messier was a comet hunter, but kept finding other fuzzy objects… Charles Messier 18th century French guy …so he catalogued them! “Messier Objects” are a mixed bag, but made up largely of star clusters, galaxies and nebulae.

Messier Objects “Spiral Nebulae”

Early theories on the Milky Way Wright: The Earth was at the center of two spheres-- all the stars were between them Kant: The Galaxy is like the Solar System, with stars moving around in orbits Hershel: Counted stars, assumed a uniform distribution and estimated distances. Noticed fuzzy objects could sometimes be resolved into individual stars. Kapteyn: “Kapteyn’s Universe”, a flattened disk of stars about 10,000 parsecs across, centered on Earth

The Great Debate Heber Curtis Harlow Shapley Curtis: Shapley: The pro versus the prodigy: Heber Curtis v. Harlow Shapley “You want a piece of this, kid?” “Bring it, Gramps!” Curtis: Used star counts like Kapteyn Thought Kapteyn’s model for the Galaxy was right but that spiral nebulae were “island universes”, outside our Galaxy Shapley: Used globular clusters to map Galaxy, said center was not Earth Thought spiral nebulae were objects in our Galaxy and our Galaxy was the whole Universe

Both right, both wrong! Shapley:  The Solar System is not the center of the Galaxy  The Galaxy is the whole Universe  The “spiral nebulae” are objects in our Galaxy Curtis:  The Solar System is the center of the Galaxy  The “spiral nebulae” are actually other galaxies outside our own!  The Universe is HUGE

The modern view of our Milky Way You are here! 1 kiloparsec = 1000 parsecs

Optical In Pictures… Disk Bulge Infrared 8 kpc

The Galactic Disk Stars Gas Dust The disk of our Galaxy is made up of three main components: Stars Gas Dust

The gas in between stars is the “Interstellar Medium” (or “ISM”) Like everywhere, most gas in the disk is hydrogen. Molecular (H2): Cold, dense, tightly clumped. Stars form within molecular clouds. “Neutral” or Atomic (HI): Cool, less dense, less tightly clumped. Most common phase; the reservoir for forming the next generation of stars. “Ionized” (HII): Hot, more diffuse. By-product of forming young stars. Note: “HI” reads as “H-one”, “HII” as “H-two”.

Atomic Gas (neutral hydrogen) Most of the gas in the disk is atomic hydrogen (HI)… Optical Image (stars & dust) …and there’s a lot of it! Atomic Gas (neutral hydrogen)

Young massive O-stars form, and ionize the gas How are the three phases of gas inter-related? Young massive stars die out, and electrons and nuclei recombine Gas is compressed, and cools Atomic HI Ionized HII Molecular H2 Young massive O-stars form, and ionize the gas

Dust in the Milky Way HII Regions Powered by O-stars

Dust in the Milky Way The dust shows where the molecular gas is, and where star formation can take place

A map of the stars in the Milky Way Stars appear red in directions where there is a lot of dust along the line of sight If there’s too much dust, not even the red light can get through From “2MASS”, an all-sky near-infrared survey

Stars in the Galactic Disk The disk is the site of star formation Disk is mostly young stars, formed from gas “polluted” by now-dead stars… Stars play an important role in processing gas and spitting it back out in stellar deaths, “enriching” the Galaxy!

Spiral Arms: Not All Wound Up “Spiral Density Waves”: Coiled compression waves moving through gas of our Galaxy Density waves compress the gas and initiate star formation! Hot young blue stars!

The Bulge: A Very Crowded Neighborhood The density of stars in the bulge is about 50,000 per cubic parsec By comparison, the nearest star to the Sun is 1.3 pc away! What would the sky look like if we were located in the bulge?

The Galactic Bulge  Mix of young and old stars, brightest stars are red giants (gives the bulge a redder color than the disk!) Shaped like a flattened sphere, sort of like an M&M

Milky Way is believed to have a bar at its center… No, not that kind of bar. This is the bar. The bulge is also home to the Galactic Nucleus, which contains a SUPERMASSIVE BLACK HOLE!

The Supermassive Black Hole at the Center of the Galaxy Stellar Black Holes: 5-10 solar masses Supermassive Black Holes: 5-10 MILLION solar masses! EVIDENCE: The stars in the movie above are moving a lot and FAST There must be some kind of very strong gravitational force on them!

Black holes can be some of the most luminous things in the Universe HUH? Gas near the black hole can spiral into the center, becoming heated to millions of degrees! We don’t see the black hole or actual stuff spiraling in, but we see the glow from it in X-rays

The Galactic Halo: Ancient Stars No gas, so no star formation… …just a bunch of old stars, either by themselves or in globular clusters. Halo stars are “metal poor”, meaning they have fewer metals than disk stars. Do you know why?

The Dark Matter Halo DARK MATTER: The major component of the halo is NOT stars… Mass that emits no light, but interacts with other matter through gravity. DARK MATTER: Most light comes from stars, but most MASS in the Universe is DARK! How do we know there’s dark matter if it’s so dark?

First, we need to understand Galactic motions. Everything moves! Stars, gas, everything. Motion implies a force at some point: GRAVITY! Galaxies form with motion, but the interaction of mass due to gravity governs how stuff moves after that.

Ordered Rotation Randomly Stuff in Galaxies moves in two basic ways… Ordered Rotation Nearly circular orbits (like planets around the Sun) Randomly Highly elliptical orbits, plunging in and out of the center of the galaxy, at random orientations.

Different Motion = Different Structures Ordered Rotation Makes flat, circular structures like our disk! Random Motion Makes 3-D, spherical structures like our bulge and halo

Bulge is spherical because its stars are moving randomly. Disk is flat because it’s rotating. Bulge is spherical because its stars are moving randomly.

How do we measure motion? “Doppler shifts”

Doppler Shifts Objects moving towards you catch up with the waves they’re emitting. Shortens wavelength  higher frequency

Doppler Shifts Objects moving away from you move away from the waves they’re emitting. Lengthens wavelength  lower frequency

Same effect happens for light! “Blueshift” “Redshift” Note: Only motion directly towards you or away from you matters!

Emission & Absorption Lines shift in wavelength depending on motion: “Redshift” = moves away from us. “Blueshift” = moves towards us. No shift = no motion along the light of sight Anything that has emission & absorption lines can show this shift! Ha emitted by star formation regions. HI produces 21cm emission line.

Same spectrum, just shifted… Spectrum of a stationary object Spectrum of the same object, moving away Spectrum of the same object, moving away faster! Spectrum of the same object, moving closer

Bigger Shift  Bigger Speed! Fractional change in wavelength Velocity as a fraction of the speed of light Dl If the velocity is 10% the speed of light, the wavelength shifts by 10%

If different parts of galaxy are moving at different velocities, we see different wavelength shifts at different positions. Compared to the spectrum you’d see looking at “B”, the spectrum you’d see at “A” would be blueshifted, the spectrum you’d see at “C” would be redshifted.

The rotation curve also tells us how the mass is distributed! By measuring these shifts, we can tell how stars in the Galaxy are moving: This graph is called a rotation curve because it shows how the Galaxy rotates. The rotation curve also tells us how the mass is distributed! (it’s the mass that’s making things move, through the influence of gravity… remember?)

This may not bother you at first. Stars and gas in galaxies are extended, so of course there is mass outside of the center. However, there is less and less normal stuff at large radii!

The Milky Way’s rotation doesn’t slow down where it’s supposed to! There must be invisible mass out here! There is very little visible matter in the Milky Way beyond 15 kpc (just trace amounts of HI).

Turns out there’s a lot of this invisible matter.

Dark matter is not normal! It can’t be gas. Gas would always glow… It isn’t very faint stars (white dwarfs, for example). They would still emit too much light. It isn’t in the form of dust, dark rocks, or planets. They would be warm enough that they’d still glow in the infrared.

SO, what would the Galaxy look like from the outside? A Disk with three kinds of Gas, Young Stars and Spiral Arms A Stellar Halo with old, metal poor Stars A Central Bulge with a mixture of Stars and a Supermassive Black Hole A Dark Matter Halo we can’t see We know it has: We can look elsewhere in the Universe and see if anything matches this description!

The Milky Way: A Barred Spiral Galaxy The Milky Way is a “spiral” galaxy, sometimes also called a “late type” galaxy.

Top View of a Real Late-type Spiral Galaxy HII regions and young stellar clusters make spiral arms clumpy This particular galaxy doesn’t have a bar, but otherwise they’re similar.

These galaxies are also probably similar to the Milky Way Side View “edge-on” Top View “face-on”

Overall Properties of the Galactic Disk, Halo, and Bulge Property Disk Halo Bulge Shape highly flattened like a frisbee roughly spherical; mildly flattened M&M shaped Star Types young and old only old stars young and old stars; more old stars at greater distances from center ISM gas and dust no gas or dust gas and dust, especially in inner regions New Stars? ongoing star formation no star formation for past 10 billion years ongoing star formation in inner regions Dynamics contents move in circular orbits in the Galactic plane stars have random orbits in three dimensions largely random orbits with some net rotation about the center Substructure spiral arms no obvious substructure nucleus; ring of gas and dust near center; bar Color overall white color with blue spiral arms stars reddish in color due to old age and cool temperatures yellow-white due to mix of stars