The Milky Way Galaxy

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WIMPs - Weakly Interacting Massive Particles – exotic subatomic particles predicted via supersymmetric extensions of the Standard Model.

Predicted rest mass of 200-500 times that of proton.

Being searched for in particle accelerators and large detector experiments.

MACHOs - Massive Compact Halo Objects

White Dwarf Stars, Red Dwarfs (0.2 Msun), Brown Dwarfs (<0.08 Msun), Neutron Stars, Black Holes.

Recall that the more massive remnants result from relatively rare high, mass stellar progenitors.

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The Search for Dark Matter (brown or white dwarfs): MACHOs

The faint foreground object (brown or white dwarf) bends the light of the background star because of its gravitational field

The light from the background star is focused or “lensed” by this effect and the star appears brighter.

MACHO results acount for only ~20% of dark matter

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Slight Aside on Determining Distances

We get distances to nearby planets (e.g. radar measurements)

That sets the scale for the solar system (1 AU).

Given 1 AU plus stellar parallax, we find distances to nearby stars.

Use these nearby stars, with known distances, fluxes and luminosities, to calibrate Luminosity classes in HR diagram.

Then spectral class + Flux yields Luminosity + Distance for farther stars (Spectroscopic Parallax).

Cepheids (variable stars) use P-M relation to determined distances to nearby galaxies

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The HR Diagram: Spectroscopic “Parallax”

Main Sequence

1) Determine Temperature from color or spectral type.

2) Determine Luminosity based on Main Sequence position.

3) Compare Luminosity with Flux (apparent brightness).

4) Use inverse square law to determine distance.

Example:

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Spiral Structure

Many external galaxies show spiral structure

Hard to see the morphology of the MW (since we are in it!)

Use other galaxies’ properties to determine the nature of the WM and determine we are in a spiral galaxy

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In other galaxies, HII regions and OB associations trace out spiral arms.

• Using spectroscopic parallax, we can place the nearby O and B stars at their proper distances.

• They appear to delineate spiral arms.

• Since O and B stars are young objects, spiral arms are associated with star formation.

Problem: Can’t see very far in the optical…

Spiral Structure

Slide 34

Surveys of Giant Molecular Clouds (GMCs) are likely to trace the spiral arms since these are sites of star formation