Our Place in the Cosmos

Lecture 14Galaxies

Nebulae

• The planets and stars we have studied so far have all been part of our own galaxy, the Milky Way, which we will come back to in a later lecture

• Deep images of the sky, such as the Hubble Space Telescope Ultra-Deep Field show many faint and diffuse smudges of light

• Diffuse patches of light in the night sky were originally referred to as nebulae due to their nebulous appearance

Hubble Space Telescope Ultra-Deep Field

Messier Catalogue

• In 1784 the French astronomer Charles Messier published a catalogue of 110 nebulae

• [Primary motivation was as an aid to comet hunters - nebulae and comets are similar in appearance, but nebulae do not move]

• Within another 20 years, largely thanks to Caroline and William Herschel, the number of known nebulae had reached 2500

• Three types of nebulae became apparent - diffuse and amorphous, elliptical and spiral

Nature of the Nebulae

• Many astronomers pre-1920 thought that the Universe consisted entirely of the Milky Way Galaxy

• It was suggested that the spiral nebulae might be planetary systems in formation

• Immanuel Kant (1724-1804) proposed that the nebulae were island universes separate from our own Galaxy, a belief shared by William Herschel, but which could not be proved with existing telescope technology

Nature of the Nebulae

• We now know that Kant was correct• Our Milky Way Galaxy is just one of Kant’s island

universes, which are now referred to as galaxies• The word Universe now refers to the full expanse

of space and its contents• While most diffuse nebulae are nearby clouds of

gas and dust within the Milky Way, the elliptical and spiral nebulae are galaxies located well beyond the limits of the Milky Way

• The Universe contains uncounted billions of galaxies, each of which in turn contains millions to hundreds of billions of stars

The Great Debate

• The key to understanding the nature of nebulae lay in determining their distances and hence sizes

• Early astronomers did not know of the existence of dust which limits our view, and under-estimated the size of our Galaxy as about 6,000 light years across

• In the mid 1910s Harlow Shapley determined that the Galaxy is over 300,000 light years in size

• Due to this large size, Shapley believed our Galaxy to contain the observed nebulae

• In 1920 the Great Debate took place between Shapley and Heber Curtis, who argued for the island universe hypothesis

Edwin Hubble

• The debate itself was inconclusive, but it did focus attention on the nebulae

• The debate was resolved in 1924 by Edwin Hubble who used the Mt Wilson 100-inch telescope to observe Cepheid variable stars in the Andromeda Nebula

• Using the period-luminosity relation for these stars, and comparing with their observed (apparent) brightness, Hubble was able to estimate their distances to be far greater even than Shapley’s measurement of the size of the Milky Way

Edwin Hubble

• Hubble’s estimate of the distance to Andromeda (M31) showed it to be of comparable size to the Milky Way

• Almost overnight our world view of the Universe changed - the Milky Way became but a speck adrift in a Universe full of galaxies

Galaxy Classification

Galaxies are like a handful of coins thrown into the air - some are seen face-on, some edge-on and most somewhere in between

The intrinsic 3d shape is seen in projection on the sky

HST image of a group of galaxies

Galaxies come in a wide range of shapes and sizes

Hubble Tuning Fork

Ellipticals: E0 E5 more elongatedSpirals: Sa Sd more open, knottier arms, smaller bulgeLenticulars: S0 - lenticular shape but no spiral armsIrregulars: Irr - none of the above

Elliptical Galaxies

• Galaxies are not solid objects but collections of stars, gas and dust orbiting under the galaxy’s gravitational field

• In elliptical galaxies stars are moving in all directions on different shaped orbits

• Random motions spherical galaxy• Faster motions in any direction elongate the

galaxy in that direction• Note that an E0 (apparently round) elliptical

galaxy is not necessarily spherical - it could be a rugby-ball shape seen end-on

Spiral Galaxies

• Spiral galaxies possess a flattened rotating disk• Most stars in the disk follow nearly circular orbits

in the same direction about the centre of the galaxy (much like the planets in our Solar System)

• The disk also contains the eponymous spiral arms• Spiral galaxies have a central bulge that is quite

similar to a small elliptical galaxy• Some bulges are bar shaped• Barred spirals are classified in parallel with

unbarred spirals• Sa or SBa tightly-wound arms, prominent bulge• Sd or SBd loosely-wound arms, small bulge

Gas and Dust

• Spiral galaxies contain large amounts of molecular gas and dust within their disks

• This can sometimes be seen as a dust lane in an edge-on spiral galaxy

• Gas in ellipticals is heated by supernovae and is unable to cool

Lenticular Galaxies

• Lenticular galaxies lie between spirals and ellipticals

• They have a lenticular (lense-shaped) stellar disk but no spiral arms

• The zero in their designation S0 denotes the lack of spiral arms

• Modern observations show that most elliptical galaxies contain small rotating disks

Star Formation

• Stars form from dense clouds of molecular gas, found in spiral and irregular galaxies but not ellipticals

• Spiral and irregular galaxies are thus still forming stars today, whereas ellipticals contain mostly old stars

• This difference is seen in colour:• Spiral disks are relatively blue due to light from

hot, massive, young stars• Elliptical galaxies are relatively red due to the

dominant population of older, lower-mass stars

Luminosity and Size

• Galaxy luminosities range from around 1 million to 1000 billion Solar luminosities (106-1012 L)

• Sizes range from about 3,000 to hundreds of thousands of light years

• No strong dependence of luminosity and size on Hubble type

• Mass is not so important in determining the appearance of a galaxy as it is for a star

• A large, distant galaxy looks similar to a nearby, smaller galaxy

Which galaxy is 4 times larger and 10 times more luminous than the other?

Large distant galaxy Small closer galaxy

Spiral Arms

• What are the spiral arms?• The overall distribution of stars is not

strongly concentrated in spiral arms, but the young, massive and luminous stars are concentrated here

• This is apparent when comparing the image of a spiral galaxy seen in ultraviolet (UV) light compared with red light

• The arms are much more prominent in UV light, and also in hydrogen gas seen with a radio telescope

Three Views of a Spiral Galaxy

UV light is dominated by massive, hot young stars which are concentrated in the armsHydrogen gas is also concentrated in the armsOlder stars seen in red light are less concentrated in arms

Spiral Structure

• Spiral structure is almost inevitable in a disk which is rotating differentially

• Material close to the centre takes less time to complete a revolution about the galaxy than material further out [cf uniform rotation of a CD]

• If we started with a single linear arm, it would soon be wound up into a spiral pattern as those parts of the arm closer to the centre rotate faster than outer parts

Spiral Structure

• Any “kick”, such as a gravitational interaction with another galaxy, or an internal burst of star formation, will lead to spiral structure

• Note that the spiral from a single “kick” will eventually be wound up and disappear

• A repetitive kick, such as that from a rotating central bar, is able to sustain two-armed spiral structure

• These disturbances are known as spiral density waves - they propagate as regions of increased density

Spiral Density Waves

• The density wave does not necessarily rotate at the same rate or even in the same direction as the material in the disk

• The density wave thus passes through the material in the disk, much like ocean waves pass through water

• As a density wave passes, gas is compressed, leading to enhanced rates of star formation

• The most massive stars die out by the time the wave has passed and so are only seen in the arms

• Lower mass stars live on and are seen throughout the disk

Dark Matter in Galaxies

• The total mass of a galaxy may be estimated by studying the effects of its gravity on motion

• We can apply Kepler’s laws to the rotation of a spiral disk to estimate the mass contained within a given radius

• One can also predict how the mass should be distributed by measuring the light distribution

• The predictions say that rotation speed should decrease with radius, whereas the observed rotation curves are roughly constant to large distances

Dark Matter Halos

• The “flatness” of the observed rotation curves implies that most of the mass (up to 95%) of a galaxy is contained in a “halo” surrounding the luminous material

• Since this matter is not luminous it is known as dark matter

• The luminous part of a galaxy is akin to the tip of an iceberg seen above the water - most of the mass is hidden within the dark matter halo

Summary

• Galaxies were shown in 1924 by Edwin Hubble to be separate from our own Milky Way

• They are classified into elliptical, lenticular, spiral and irregular

• Spiral galaxies are dominated by a rotating disk with often-spectacular spiral arms

• Stars in elliptical galaxies are on more random orbits

• The masses of all galaxies are dominated by dark matter