Galaxies

Approx. 10⁹ (one billion) observed (!!!).
Approx. 10⁵ well studied

Messier list contains about 100 galaxies.
New Galactic Catalog ∼ 10,000

When in doubt, classify!
Galaxies classified under Hubble Classification

Different shapes

• Elliptical EO -> E7
• Spiral Sa -> Sd
• Barred Spiral SBa -> SBc
• Irregulars

Different Sizes 10⁵ -> 10¹² L₀
Different Luminosities 10⁵ -> 10¹⁰ L₀

Spirals

Spirals & Barred Spirals show much smaller range of size, 10⁹ -> 10¹¹ stars.
Pop. I in arms and dust.
Pop II in core and plane

The classic is M31 (Andromeda): tightly wound spiral with 2 satellites, a bit bigger than the Milky Way
Core maybe has two black holes and billions of stars: superimposed are knots of dust	Credit & Copyright: Robert Gendler
Very bright in X-ryas, with an intense course at the centre

This shows M51 along with Hurricane Julia:

NGC 1512 is an unusual spiral: tightly wound at the centre, with "star-burst" proceeding in a narrow ring

NGC 6946: The Fireworks Galaxy. SHows the spiral structure for what it is: in this case, lots of new stars, (blue) and huge hydrogen clouds (red), and very bright small nucleus

Credit & Copyright: T. Rector (U. Alaska Anchorage), Gemini Obs., AURA

M81 and M82 in Ursa Major make an intriguing pair: M81 seems to be destroying M82 via collisions

GALEX Full Field Credit: GALEX Team, Caltech, NASA

Closeup of centre of M82 shows massive wind blowing out of centre of galaxy

Credit: M. Westmoquette (UCL), J. Gallagher (U. Wisconsin-Madison), L. Smith (UCL), WIYN/NSF, HST, NASA/ESA

>Some are seen side-on, which shows the X-sect NGC 4565 Note dust clouds surrounding disc

Barred spirals: instead of a spherical nucleus, many have a bar across the centre: NGC 1300

Barred Spiral Galaxy NGC 1300 Credit: Hubble Heritage Team, ESA, NASA

And again can be spread out SBb NGC 7424

The Arms of NGC 7424 Credit: VIMOS / VLT, European Southern Observatory

Elliptical Galaxies

Ellipticals have v. great range in size, from 10⁵ (dwarf) ->

• 10¹³ (giant)
• Little dust
• No gas
• Mainly pop. II stars
• Some giants are radio sources and have large haloes of stars.

Ellipticals look dull in pictures!

M87 is one of the largest: Note the spiral behind. (Stars have points on them).This is M87 (a giant elliptical) in Virgo. Almost perfectly spherical Has jet from centre (later) Note many globular clusters (>1000)

.

Looks simple but...short exposure shows jet + very small core

And the jet looks more complicated the smaller the scale

and it seems to be related to radio "lobes"

Irregulars

This is SMC ∼ 106 stars
Have a lot of H e.g. Magellanic clouds have more than twice visible mass as neutral H. Hydrogen bridge between them and even extends to our galaxy

These are 3 galaxies (in a group called Stefan's quintet) which are colliding
"The Cartwheel Galaxy". Ring round (100000 lightyears across) consists of new stars produced in the collision with a smaller galaxy, which has now vanished.	Credit: Kirk Borne (STScI), NASA
THe antennae galaxies are two very large galaxies in a violent collision: lats of stars being formed	B. Whitmore (STScI), F. Schweizer (DTM), NASA
Not surprisingly the very hot gas produces lots of X-rays: this is the internal part of the antenna galaxies	Credit: G. Fabbiano (CfA) et al., CXC, SAO, NASA

Centaurus A may be the result of the collision of two galaxies as well: lots of dust

Credit: Marina Rejkuba (ESO-Garching) et al., ISAAC, VLT ANTU telescope, ESO Paranal Obs.

Closeup shows the star formation round the dust

The Center of Centaurus A Credit: E.J. Schreier (STScI) et al., HST, NASA

Luminosities of galaxies

Visual (apparent) mags. range from ∼ 6 (Andromeda) to lowest detectable (∼ 25)

Absolute brightest (giant ellipticals) ∼ -23
M31 ∼ -21
Milky Way galaxy ∼ -20
dwarf ellipticals ∼ - 9 (tho. dimmer ones would probably not be detected)

What are galaxies made up of?

Ellipticals have mainly pop II stars everywhere: large number of red giants.

Spirals show complicated structure which confirms what is known about Milky Way

e.g. M31 shows old stars evenly distributed X-rays from centre (maybe black hole)

Probably variation in kinds of galaxies arises in following fashion

Hydrogen gas with small ang. mom. will condense to form large clusters of galaxies with small ang. mom. These galaxies will be roughly spherically symm. (i.e. elliptical) and stars will mop up H gas efficiently. Individual stars will be independent orbit around centre, wtih small total ang. mom.

Consistent with

1. range in size of ellipticals
2. giant clusters
3. no neutral H gas in ellipticals

H gas with large ang. mom (i.e. vortex) will contract to a disc, but ang. mom. will stop (by centrifugal force) contraction in plane. Less gas will condense into stars

Consistent with

1. no very large or very small spirals
2. observation of H gas
3. stars are all orbiting in same direction in fairly circular orbits

What we don't know is what sets the basic scale, or where the angular momentum comes from.
e.g. we can predict that stars have masses .01 M₀ < M < 100M₀
(too light and they aren't large enough to burn H, too heavy and the radiation pressure blows off outside layers)

There is no theory that tells us that all spirals have ≈ 10¹⁰ stars and radius 20 kpc Some speculations later.

Also (a related problem) don't know whether it is a top-down or bottm up problem:
i.e. is the process
H-gas ⇒ stars ⇒ galaxies
or
H-gas ⇒ galaxies ⇒ stars

Irregular galaxies: the small ones (Irr1) are probably just failed spirals.
Most of Irr II are probably formed by galactic collisions

Note: distance between stars ≈ 106 x radius, so collisions are very unlikely. For galaxies, d ≈ 10R

Note this is a galactic collision, there is (almost) no chance of stars colliding, but gas can become extremely hot⇒ X-rays

e.g. Cartwheel galaxy: Compact, heavy galaxy kicks out core