Steinn Sigurðsson Astro 475(W) Fall 2011

19

For this class read chapters 2 and 4 in "Galaxies..."

Our Galaxy

We discuss the solar neighbourhood and do an overview of our galaxy.

The Sun is located in the Milky Way Galaxy.

The Galaxy is a collection of about 100+ billion stars, of which the Sun is an average member - the Sun is average age, moderately average brightness and average composition compared to the other stars in the Milky Way

The Milky Way is shaped like a fried egg.
It is a thin disk with a central bulge.

It is possible the Milky Way has a central bar, in which case it may look more like M83 - M83 Image

Overview

The Sun is located about 8 kpc from the centre of the Milky Way Galaxy.
The Sun is located near the mid-plane of the thin disk of the galaxy, which is about 100 pc thick and contains about 10¹¹ stars, with a characteristic mass of about 0.7 solar masses.

Most of the stars are confined to the disk and revolve on orbits about the centre of the Milky Way in the plane of the disk.

There is also a thick disk with a scale height of about 1.4 kpc, containing maybe 1% of the mass of the thin disk.

The Galaxy also has a central bulge, with a diameter of maybe 2 kpc, a r^¼ power law profile, and a mass of about 10¹⁰ solar masses.

The dominant component of the galaxy is the halo. This has a diameter of 100-200 kpc, and a nearly isothermal density profile in the measured region. The stellar mass of the halo is about 10⁹ solar masses, and the total mass is between 5×10¹⁰ and 10¹² solar masses.
The halo appears to be dark matter dominated, with a nearly flat rotation curve.
The globular cluster population traces the galactic halo well.

The view of the sky

The Galaxy at different wavelengths

The thin disk contains a lot of gas, young, intermediate-age and old stars.
The thick disk and stellar halo contain only old stars and warm gas, the bulge contains old stars and may also contain a substantial young-intermediate age stellar population.

The Sun is located in the local fluff within the local bubble - a region of low density hot gas that is probably the remains of a supernova that went off a few million years ago. The fluff is a little gas cloud the Sun is in, embedded inside the much lower density bubble.

Local Bubble

The fluff is about 60 lightyears across, and the Sun is only 4 lightyears from the edge. The bubble surrounding the fluff is about 300 lightyears across and has gas that is at a temperature of 1,000,000 K, but very tenuous.

The Sun is drifting out of this region and will enter a region with more normal density of warm gas - in a few million years!

The disk of the Milky Way has prominent spiral arms - these are regions of higher density of stars and gas, which show up as grand spiral structure wrapping around the Galaxy. The Sun is near the Sagittarius spiral arm.

The centre of the Milky Way contains a bright nucleus, with ongoing star formation and almost certainly a low mass supermassive black hole.

The inner disk is almost certainly barred. The outer disk has a substantial warp.

The halo is dark matter dominated and probably close to axisymmetric or mildly triaxial.

About 10% of the mass of the disk is in gas, both cold, warm and hot gas. The gas distribution is strongly heterogenous with different phases intermingled.
High velocity gas may be ejected from the disk by supernovae, and there is some infall of gas, some of which may be primordial and some if fallback material.

.From the Canadian Galactic Plane Survey

Last updated 10/11

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