The Milky Way: How do we know what it looks like? Bryan Hill

What is this?

What does our galaxy look like?

Edge-On View of the Milky Way Galaxy

The thick disk (z_thick = 1000pc) is an older star population with lower density of stars. The thin disk (z_thin = 350) is a younger star population with a higher density of stars.

Thin/Thick Disks Thin and thick disks not just determined by scale heights and stellar number densities. Disk are also distinguished by metal concentrations in stars. Population I- metal-rich Z~0.02 Population II- metal-poor Z~0.001 Population III- no metal Z~0

Thin/Thick Disks Population I- metal-rich Stars similar to the Sun Population II- metal-poor Stars that exploded to form Population I stars Population III- no metal Stars that exploded to form Population II stars

Population I Population III Population II

Metallicity Metallicity is a measure of the iron to hydrogen ratio in the atmosphere of a star compared to the Sun.

The older the star, the more negative the value of [Fe/H]. The younger the star, the more positive the value of [Fe/H]. For stars with the same metal abundance as the Sun, [Fe/H]=0. [Fe/H]≡ log 10 [((N Fe /N H ) star )/((N Fe /N H )  )]

How do we know there are spirals?

How do stars orbit in our galaxy?

Stars in the disk all orbit in the same direction with a little up-and-down motion

Orbits of stars in the bulge and halo have random orientations

Thought Question Why do orbits of bulge stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

Why do orbits of bulge stars bob up and down? A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk

Star Halo

Halo Stars: % heavy elements (O, Fe, …), only old stars Disk Stars: 2% heavy elements, stars of all ages Halo stars formed first, then stopped Disk stars formed later, kept forming

Matter inside stars:~ 0.6% Matter outside stars: ~ 3.8% What is the galaxy made of? What’s wrong with these numbers?

Where did all the Matter Go? The “Missing Matter” caused scientist to hypostasize the existence of Dark Matter

Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

“Normal” Matter: ~ 4.4% –Normal Matter inside stars:~ 0.6% –Normal Matter outside stars:~ 3.8% Dark Matter: ~ 25% Dark Energy~ 71% Why do we think it is there?

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter.

Does dark matter really exist?

Our Options 1.Dark matter really exists, and we are observing the effects of its gravitational attraction 2.Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

Our Options 1.Dark matter really exists, and we are observing the effects of its gravitational attraction 2.Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter Because gravity is so well tested, most astronomers prefer option #1

The First Option MACHOs- Massive Compact Halo Objects act as a Gravitational Lens

The Milky Way: How do we know what it looks like? Bryan Hill