Astro 1050 Fri. Mar 26, 2004

Today: Astronomy Articles

End Ch. 12: The Milky Way

The Galactic Rotation Curve

Keplerian fall-off near center indicates compact mass at center

Flat curve throughout disk indicates much distributed mass

Lack of fall-off beyond visible “edge” indicates “dark matter”

Stellar Population and Galaxy Evolution

“Metal” abundance during time

“Metals” are elements heavier than He

A given star’s atmospheric abundance is approx. fixed at birth

Interstellar metal abundance grows with each new generation of stars

Red giants and supernova eject new heavy elements into interstellar gas

Orbits during time

A given star’s orbit is approx. fixed at birth – just plows through gas

Orbits of gas clouds evolve with time since they can collide

Orbits get more circular and disk-like with time

Traditional Model of Galaxy Evolution

Stars are stuck with their original orbits

They plow through gas like bullets

Orbits of gas can evolve

Gas clouds collide and only average motion (rotation) survives

Metal abundance grows with time

System starts out with little organized motion,few metals

Halo stars form at this time

It contracts, velocities average out leaving only rotation

Gas collapses to form the disk

Disk stars form after this has happened

Some problems with traditional model

Globular clusters not all same age

Gap in ages between halo and disk objects

Presence of some metals in even in oldest stars

System may have formed by merger of smaller galaxies

Galactic Cannibalism

M51: The Whirlpool Galaxy

Possible Origin of Spiral Arms

Differential rotation smears features out into spiral patterns

But can’t be whole story:

Number of times Sun has orbited the galaxy:

10 billion yr/200 million yr
= 50 times

Spiral arms would have been wound up very tightly

Something must continuously rebuild them

Degree of Organization
of the Spiral Arms

Different degrees of organization

Grand Design
Spirals: M51

Flocculent (“wooly”) Spirals

Tracing the Spiral Arms

Arms NOT obvious if you look at:

Old objects like the sun

Arms ARE obvious if you look at:

Maps of gas clouds

21 cm Hydrogen

Radio maps of CO

Far infrared observations of dust

Young stars

O, B stars

“HII” ionized hydrogen regions surrounding O,B stars

Clouds somehow form in arms , then dissipate between them

Short lived objects only get a short distance from their places of birth

O stars, Lifetime = few million years, at 250 km/s Þ500 pc

M51: The Whirlpool Galaxy

Density Wave Theory

SPIRAL WAVE rotates with galaxy, but slower than individual stars

Like moving traffic jam after an accident has been cleared

Gas (and stars) catch up with wave, move through it, eventually reach front

Just like cars catching up with moving traffic jam, eventually get through it

Gas is more crowded in wave – clouds collapse to form new stars

More collisions in the traffic jam

There are slightly more old stars in the arm too, because they speed up slightly coming into it and slow down slightly moving out of it.

But the best tracers are the things that mark recent cloud collapses: O,B stars, etc.

Self Sustaining Star Formation

Cloud collapse Þ New stars

New stars Þ Supernova after few million years

Supernova Þ Shock Waves

Shock Waves Þ Nearby clouds collapse

Differential Rotation twists pattern into spiral

Two limiting cases of spirals

Grand Design: Density Wave

Flocculent: Self Sust. Star Form. + Diff. Rot.

In most Galaxies you have some combination of the two

The Nucleus of the Galaxy

Likely Black hole

High velocities

Large energy generation

At a=275 AU P=2.8 yr Þ 2.7 million solar masses

Radio image of Sgr A
about 3 pc across, with model of surrounding disk

A movie of stars at the core

www.mpe.mpg.de/www_ir/GC

Very cool, and worth a look!

This is the best evidence to date for a massive black hole at the Galactic core. Now essentially “proven.”


	Keplerian fall-off near center indicates compact mass at center
	Flat curve throughout disk indicates much distributed mass
	Lack of fall-off beyond visible “edge” indicates “dark matter”


“Metal” abundance during time
	“Metals” are elements heavier than He
	A given star’s atmospheric abundance is approx. fixed at birth
	Interstellar metal abundance grows with each new generation of stars
		Red giants and supernova eject new heavy elements into interstellar gas
Orbits during time
	A given star’s orbit is approx. fixed at birth – just plows through gas
	Orbits of gas clouds evolve with time since they can collide
	Orbits get more circular and disk-like with time


	Stars are stuck with their original orbits
		They plow through gas like bullets
	Orbits of gas can evolve
		Gas clouds collide and only average motion (rotation) survives
	Metal abundance grows with time

	System starts out with little organized motion,few metals
		Halo stars form at this time
	It contracts, velocities average out leaving only rotation
		Gas collapses to form the disk
		Disk stars form after this has happened

	Some problems with traditional model
		Globular clusters not all same age
		Gap in ages between halo and disk objects
		Presence of some metals in even in oldest stars
	System may have formed by merger of smaller galaxies
		Galactic Cannibalism


	Differential rotation smears features out into spiral patterns

	But can’t be whole story:

	Number of times Sun has orbited the galaxy:
		10 billion yr/200 million yr = 50 times
		Spiral arms would have been wound up very tightly

	Something must continuously rebuild them



	Different degrees of organization
		Grand Design Spirals: M51

		Flocculent (“wooly”) Spirals


Arms NOT obvious if you look at:
	Old objects like the sun
Arms ARE obvious if you look at:
	Maps of gas clouds
		21 cm Hydrogen
		Radio maps of CO
	Far infrared observations of dust
	Young stars
		O, B stars
		“HII” ionized hydrogen regions surrounding O,B stars

	Clouds somehow form in arms , then dissipate between them
	Short lived objects only get a short distance from their places of birth
		O stars, Lifetime = few million years, at 250 km/s Þ500 pc


		SPIRAL WAVE rotates with galaxy, but slower than individual stars
			Like moving traffic jam after an accident has been cleared

		Gas (and stars) catch up with wave, move through it, eventually reach front
			Just like cars catching up with moving traffic jam, eventually get through it

		Gas is more crowded in wave – clouds collapse to form new stars
			More collisions in the traffic jam

		There are slightly more old stars in the arm too, because they speed up slightly coming into it and slow down slightly moving out of it.
		But the best tracers are the things that mark recent cloud collapses: O,B stars, etc.


	Cloud collapse Þ New stars
	New stars Þ Supernova after few million years
	Supernova Þ Shock Waves
	Shock Waves Þ Nearby clouds collapse

	Differential Rotation twists pattern into spiral


	Grand Design: Density Wave

	Flocculent: Self Sust. Star Form. + Diff. Rot.

	In most Galaxies you have some combination of the two


	Likely Black hole
		High velocities
		Large energy generation

	At a=275 AU P=2.8 yr Þ 2.7 million solar masses

	Radio image of Sgr A about 3 pc across, with model of surrounding disk


	www.mpe.mpg.de/www_ir/GC

	Very cool, and worth a look!

	This is the best evidence to date for a massive black hole at the Galactic core. Now essentially “proven.”