Galaxies II

AST 112

Galaxies

• Billions of them!

• Islands of millions or billions of stars

• All different shapes and sizes

Hubble Deep Field

Estimate: Galaxy Count

• Hubble Deep Field is some % of the total sky– Smaller than a 1mm x 1mm piece of paper held at

arm’s length

• Count the galaxies and multiply by the ratio!

• ~130 billion galaxies in observable universe

Star Formation

What exactly is required for star formation?

Star Formation

• Cold gas is required for star formation

– Sometimes the gas “gets some help” and gets compressed by means other than gravity

Elliptical Galaxies

• Older (yellow and red) stars

• Not much structure

• Not much cold gas or dust

Spiral Galaxies

• Flat disks with arms, yellow bulges at center

• Disk and arms tend to be more blue

Lenticular Galaxies

• Like spirals: contain a disk and a bulge

• Do not contain spiral arms

• Not much star formation

• “Intermediate” between spirals and ellipticals

Irregular Galaxies

• No disk, not round

• Chaotic , “messy” structure

Spiral Galaxies

Face-On Tilted Edge-On

Spiral Galaxies

• Sizes of disk and bulge vary from spiral to spiral

M81: Larger bulge M 100: Smaller bulge

Spiral Galaxies

• Some spirals’ arms are wound more tightly than others

Spiral Galaxies

• Many spiral galaxies have a bar

• Milky Way is a barred spiral

Spiral Galaxies

• One can observe a galaxy in different types of light

• It “picks out” elements of structure, some of which cannot be seen in visible light

Andromeda In Far-Infrared

• Dust glows in far-infrared

• Where is the dust confined to?

Andromeda In Visible Light

• We see starlight when we look in visible light

• What color does the disk show more strongly than the bulge?

Andromeda in Ultraviolet Light

• Bright newborn stars glow strongly in UV

• UV is a good map of star formation

• Where is star formation happening?

Triangulum Galaxy In Far-Infrared

• Dust glows in far-infrared

• Where is the dust confined to?

Triangulum Galaxy In Visible Light

• We see starlight when we look in visible light

• What color does the disk show more strongly than the bulge?

Triangulum Galaxy in Ultraviolet Light

• Bright newborn stars glow strongly in UV

• UV is a good map of star formation

• Where is star formation happening?

Spirals: Star Formation (Observations)

• Where’s the dust?– The disk or the bulge?

• Where are the younger (bluer) stars?– The disk or the bulge?

• Where’s the star formation?– The disk or the bulge?

• Where’s the cold gas?– The disk or the bulge?

Spirals: Star Formation (Observations)

• Red HII regions and blue open clusters reveal star formation

• Where exactly do we find these elements in this galaxy?

Spirals: Star Formation

• Thus far, we can conclude that:

– The disk is full of gas and dust

– The arms are full of star formation

• Why is there enhanced star formation?

Lin-Shu Density Waves

• You might think that a spiral galaxy’s shape is a fixed structure

– If true, outer stars must orbit in same amount of time as inner stars

– It’s not. Doppler measurements don’t show this at all.

Lin-Shu Density Waves

• Kepler’s Laws:

– A star should orbit more slowly the farther out it is

– A galaxy would “wind itself up” and destroy its spiral structure well within their current ages

– No good!

Lin-Shu Density Waves

• The spiral structure is stable.

• The spiral arms are simply locations in the disk of high density– Stars move into and out of the arms!– It’s just like a traffic jam. One star enters it just as another is

exiting.

• When gas and dust slam into the “traffic jam” and slow down, they compress– That’s why spiral arms show heavy star formation

Lin-Shu Density Waves

• Animations:

http://en.wikipedia.org/wiki/Density_wave_theory

Lin-Shu Density Waves

• Why is star formation enhanced in the spiral arms?

– Gas, dust and stars accelerated toward the high-density arms, “crash into it”

– High density and compression trigger star formation

Elliptical Galaxies

• Elliptical galaxies have little or no disk

• Have little or no cold gas / dust

• Resemble the bulge of a spiral

• Most common type of galaxy in the universe

Elliptical Galaxies

• Most ellipticals are small– Dwarf ellipticals– Usually hang around

larger spirals

• But the largest galaxies in the universe are giant ellipticals– M87: 1 trillion stars

M87

Leo I

Elliptical Galaxies

• Due to lack of cold gas / dust, star formation rates are very low for ellipticals

• Blue stars have died

• So ellipticals are yellow / red

Irregular Galaxies

• Don’t have much structure

• Young stars

• More common toward beginning of the Universe

LMC

SMC

Irregular Galaxies

• Irregular galaxies often appear so because of one or more collisions

• Significant starburst activity is often seen in irregulars– Why?

Classification of Galaxies

Here are some galaxies.Try to come up with a classification scheme.

Classification of Galaxies

• We can classify elliptical galaxies by how elliptical they are

• We can classify spirals by:– Size of the bulge– Tightness of the arms– Barred or not

Hubble Sequence

Quasars

• Quasar stands for quasi-stellar object

• Galaxies look like smudges in a telescope

• There are objects that look just like stars– A point of light– Their spectra have emission lines that didn’t

correspond to anything we know of

Quasars

• They have the same redshift as distant galaxies

• Large telescopes can reveal more than just a quasi-star

• “Unknown lines” are known but highly redshifted lines

Quasars

• Distant galaxies often contain an active galactic nucleus (AGN)

• This is an SMBH that is actively devouring material

• Forms HUGE accretion disk