What Produces Spiral Structure?

Why Spiral Arms?

Why not just an undisturbed uniform disk? Well, remember that the innermost parts

rotate faster than those farther out. This suggests one obvious possibility: some

linear structure forms, then ‘winds up’ because of the differential rotation.

Like So… [if, by chance, stars A,B,C,D start out all in a line]

Analogy

Stir a cup of black coffee to set it swirling. Dribble in a dollop of cream. Note the ‘spiral structure’ that forms

briefly https://www.youtube.com/watch?v=rxZYSih3r6M

The Problem

Galaxies have been around for 10 billion years, but

rotate every 250 million years. That is time enough for 40 complete turns! Why isn’t the spiral structure ‘all wound up’?

There Are Actually Two Kinds of Spirals ‘Grand design’ spirals ‘Flocculent’ spirals

1. Grand Design Spirals: Density Waves

There are waves (like sound waves) passing through the

gas, compressing it. These waves are in a sustained, long-lived spiral pattern for reasons we understand.

Where the gas is compressed, new stars form and we see a

prominent spiral arm. (Note: there are lots of stars outside the arms as well! But the bright ones stand out)

The wave moves on and the hot young stars soon evolve

and fade away. More bright stars form in the new location of enhanced gas density.

Note the Implication

At any given time, the spiral arm is delineated by short-lived stars that do not move with it! The wave passes on, forming a set of new young stars. Those that were formed somewhat earlier, now left behind, fade in brilliance and soon become inconspicuous (because massive stars evolve so quickly).

`The Wave’

http://www.youtube.com/watch?v=H0K2dvB-7WY

Moving ‘Clumps’ of Cars on the Motorway

This Model is Supported by Good Evidence

2. Flocculent Spirals:

Stochastic (Random) Star Formation A star (or a cluster of stars) forms, and the most

massive ones evolve quickly, exploding as supernovae. The blast wave compresses nearby gas, leading to more local star formation.

In this way, a fairly broad region of continuing star

formation can happen in quick succession. The rotation of the galaxy stretches this complex of young stars out into a stubby disconnected spiral arm – nothing like the ‘Grand Design’ spiral.

That Analogy Again

It’s like dropping a fresh dollop of cream into a swirling cup of coffee.

There is a momentary appearance of a short

spiral segment… It is not widespread, striking, or long-

lasting.

3. Yet One More Possibility

We see spiral structure in some galaxies that have close companions.

Can interactions and tides create spiral structure?

Like So? These figures are based on some very early computer simulations, about 40 years ago.

Answer n  Yes, interactions can be important! (We will

soon explore how galaxies interact, and address this question again.)

n  But the biggest, most massive galaxies, even if isolated in space (no interactions), seem to be ‘unstable’ and will spontaneously form spiral structure sustained by density waves in the gas

n  Our Milky Way may be one of these beautiful ‘Grand Design’ spirals!

In Fact…

Recent studies in the highly obscured galactic centre region (top left), working at long wavelengths to peer through the interstellar dust, seems to show that our galaxy has a central bar. An artist’s impression of its likely structure is shown at the top right.