Galaxies: the essentials

The Milky Way

Just as the Sun is "our" star, the Milky Way is "our" galaxy.
Unfortunately we see it from the inside, so first a look at a galaxy which is reasonably similar: M100. Large spiral galaxy, Note:
  • ∼ 100 billion stars
  • Large spiral arms outlined by hot stars
  • Large dust lanes
  • Bright centre
  • Size: ∼ 50000 pc (50 kpc) across
  • Bright patches are clumps of young stars

M100: A Grand Design Credit: NASA

Looked at from the side, the Milky way looks a bit like NGC 4565 (ignore the foreground stars): note
  • bulge
  • dust
  • Thickness of arms ∼ 1 kpc

Credit: Bruce Hugo and Leslie Gaul, Adam Block (KPNO Visitor Program), NOAO, AURA, NSF
So the Milky Way probably looks like this

Illustration Credit & Copyright: Mark Garlick (Space-art

Most of the whole Milky Way from Chile

The Milky Way Credit:© Serge Brunier
and we can pick out the same general structure in radio waves, but note very intense source at centre

Credit: C. Haslam et al., MPIfR, SkyView
and γ-rays from the EGRET satellite

Credit: EGRET Team, Compton Observatory, NASA

Galactic Centre Not visible directly (too much dust)

Credit: W. Keel (U. Alabama, Tuscaloosa), Cerro Tololo, Chile

but strong radio source

We can see through the dust (partially) with infra-red: note how dense the star field is

Credit: 2MASS Project, UMass, IPAC/Caltech, NSF, NASA
and X-rays

Credit: Fred Baganoff (MIT), Mark Morris (UCLA), et al., CXC, NASA

Close to centre a lot of rapidly moving (300 km/s) hot (i.e. ionised) gas(Gravitational field at centre of galaxy should be very small, so would expect velocities to be small.)

and hot stars. Could be very dense cluster of stars..........but note M31 (Andromeda), M100 and many others show a star-like central nucleus

and very rapidly moving stars

Credit: A. Eckart (U. Koeln) & R. Genzel (MPE-Garching), SHARP I, NTT, La Silla Obs., ESO

Size of centre < 1pc
Mass of object ∼ 3000000 M₀

Whole picture is consistent with a very large black hole at centre, but not nearly as active as we see in other galaxies: e.g. this shows gas at the centre of NGC 1365

Credit: Ground-based image: Allan Sandage (Carnegie Observatories), John Bedke (CSC, STScI) WFPC2 image:John Trauger (JPL), NASA NICMOS image: C. Marcella Carollo (JHU, Columbia U.), NASA, ESA
Suspect that there are black holes (1 million to 100 million Mo at the centre of all galaxies: these are very different from BH left over from supernovae.

Galaxies

Spirals

Spirals & Barred Spirals show much smaller range of size, 109 -> 1011 stars.
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-rays, with an intense course at the centre
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

Elliptical Galaxies

Ellipticals have v. great range in size, from 105Lo (dwarf) ->1013 Lo(giant)<

Ellipticals look dull in pictures!
M87 is one of the largest: Note the spiral behind. (Stars have points on them). Almost perfectly spherical

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"
Best interpretation is that a very large black hole at the centre of the galaxy is producing a jet which pulsates and interacts with surrounding gas.

Irregulars

come in many varieties.
Some (Irr. I) seem to be similar to spirals, e.g. Magellanic clouds.
i.e. show pop I stars, dust, H gas...

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
Since galaxies are quite large and move fast, ,it is not surprising they interact and collide: e.g.
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

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)

Radio Galaxies:

Centaurus A
M87 jet turns into intense radio source
Cygnus A appears to be double galaxy with radio emission coming from distant lobes
Brightest radio object in sky: took a long time to find optical counterpart, since it is a distant galaxy. Emission is synchrotron from electrons ejected from centre

Note jet
Nowhere near largest radio source!

Seyfert galaxies

NGC4151
  • v. bright central nucleus (arms usually hard to see)
  • broad emission lines implies there must be a lot of hot gas in violent motion
  • sometimes radio, usually strong I.R.
  • short term variability

Quasars:


Quasi-stellar (radio) source, QSS. Large number of radio sources could not be identified with known objects. were originally discovered by radio: 3C48 found near blue "star" (1960)
  • Variable output (flickering over a few minutes, 3 mags. change over a year)
  • Optical and radio variability not related.
  • spectrum very odd: strong emission lines correspond to no known element
  • huge redshifts,
  • massive power output ∼ 100 x large galaxy
  • SIze ∼ few light hours ∼ size of solar system

i.e. light output occurs from same size region as supergiant star, but energy ∼ 1010 larger

What are they?


Best Bet Model! Supermassive black hole, consuming 10-20 M₀/year Surrounding galaxy produces emission lines "Flickering" from stars falling into black hole
Hence expect following scenario:

Quasars very plentiful in early universe. We see Seyfert galaxies, which seem to be intermediate, so quasars evolve to Seyferts or radio galaxies as black hole consumes most of central core, and then to normal spiral as central core becomes used up

Gravitational lensing...

About 10000 quasars known, and each is very characteristic: red-shifts and spectrum are very distinct.

However several pairs which lie very close in sky: e.g. 0957 +561A & 0957 +561B are 6" apart in sky and have identical red-shifts. Note "fuzz" sticking out of lower one

THis one is imaged 4 times (the Einstein cross) Can just see the galaxy. Allows us to measure mass of intervening galaxy, but not easy in practice.

Credit & Copyright: J. Rhoads (STScI) et al., WIYN, AURA, NOAO, NSF

Gamma-ray bursters

And finally amongst the wierdos: Gamma-ray bursters

Found originally by Vela satellite (designed to look for γ's from nuclear explosions). Can identify source by using timinng with various satellites

Bursts last 1/10 - 100s, no particular pattern

Problems: Could not be identified with any known object

Possibly collisions between stars in very distant galaxies, or some unusual form of supernova

Image Credit: S. Kulkarni, J. Bloom, P. Price, Caltech - NRAO GRB Collaboration
Before we can start asking the profound questions, we need to talk about the sizes of things again: what is the large scale structure of the universe?