galaxies - texas a&m...

Galaxies

• 01_Zoom26OrdersMag.htm

What is a galaxy?

A gravitationally bound system of:

• stars — numbering typically 109-1012, distributed over 10,000s of lightyears.

• gas — primarily hydrogen and helium; fuels star formation.

• dust — actually it’s more like soot. It obscures stellar light, influencing the visual appearance.

• exotic objects — stellar-mass black holes, supermassive black holes, neutron stars, etc.

• dark matter

What is a galaxy?

What kinds of galaxies are there?

• Galaxies are unique, complex systems

• But nonetheless there are regularities that tell us something important

What kinds of galaxies are there?

The “tuning fork” diagram:

What kind of galaxy is the Milky Way?


Spiral galaxies have several different components:



Most of the stars are in the disk. That is also where the dust and gas are, and so that’s where new stars can form.



The sun is located in the suburbs of the disk. The farthest stars that we can see with the unaided eye are still only in our close vicinity — a few thousand lightyears away



The bulge is rounder and has a greater fraction of older (redder) stars.



The halo is a very large and diffuse collection of old stars. It also has a lot of dense clusters of stars called globular clusters.

Globular clusters

What does the Milky Way look like?


It depends on what wavelength of light you’re looking at!


Visible light shows where the stars are — but it is easily absorbed by dust.


Infrared light also comes from stars, but it penetrates through the dust much more eaily.

Infrared

Visible


X-rays reveal the location of hot gas. Some of that gas is bubbling up outside of the disk, but the bright spots are gas that’s orbiting really fast around black holes.


Radio waves (particularly with a wavelength of 21cm) show the location of Hydrogen gas


Radio waves at other wavelengths show locations of the densest clouds of gas, where stars can form.


Infrared light with long wavelengths reveal the thermal emission from dust that is heated by young stars.


You can learn a lot by looking at galaxies at different wavelengths of light.

• No homework next week — have a nice break!

Review: looking back in time

This is Andromeda, which is 2.5 million ly away. This means that right now we are seeing what Andromeda looked like 2.5 million years ago… and we have no way of knowing what Andromeda looks like right now.

Review: what kinds of galaxies are there?

Review: what kind of galaxies are there?

Spiral galaxies:• disk of stars with spiral arms• bluer colors

Elliptical galaxies:• round(-ish) shape• redder colors

After ~107 years

After ~109 years

Review: what kind of galaxies are there?

Spiral galaxies:• disk of stars with spiral arms• bluer colors• active star formation with lots

of gas and dust to provide the fuel

Elliptical galaxies:• round(-ish) shape• redder colors• little star formation, and little

gas and dust

Review: what does the Milky Way look like?

It depends on what wavelength of light you’re looking at!

Review: what does the Milky Way look like?

• IF_15_12_MultiwaveMilkyWay.htm

Review: what kind of galaxy is the Milky Way?


Where do stars form in galaxies?

Ionization nebulae are the birth-clouds of gas that are being heated and destroyed by short-lived high-mass stars, signifying active star formation.


Where do stars tend to form in galaxies?

Disk: there is plenty of gas → star formation → O and B-type stars → ionization nebulae

Where do stars tend to form in galaxies?

Disk: there is plenty of gas → star formation → O and B-type stars → ionization nebulae

Halo: there is little gas → no star formation → only old and low-mass stars are present → no ionization nebulae

dust clouds obscurevisible light

ionization nebula show theregions where young, hot starsare ionizing their birth-clouds

young stars that have burnedaway their birth-clouds

spiral arms are waves ofstar formation; by the timestars emerge from the spiralarm, they have burned awaythe birth-clouds

Q: Where do you expect to find the stars with the greatest amount of “heavy” elements (carbon, oxygen, silicon, iron, etc.)?

A. the diskB. the halo

Q: Where do you expect to find the stars with the greatest amount of “heavy” elements (carbon, oxygen, silicon, iron, etc.)?

A. the diskB. the halo

Hint: think about how the heavy elements (heavier than hydrogen and helium) build up over time.

Chemical evolution of the universe

We are made of starstuff — Carl Sagan

How are galaxies grouped together

Spiral galaxies are often isolated or found in small groups of galaxies (up to a few dozen per group)

How are galaxies grouped together

Elliptical galaxies are often found in large clusters of galaxies (hundreds or thousands per cluster)

The expansion of the universe

We didn’t always know that the “sprial nebulae” are actually other galaxies. It was thought that perhaps they were rotating nebulae within our own galaxy.

But interestingly, many of them were observed to have large redshifts.


The key to figuring out the nature of the spiral nebulae came when Edwin Hubble found a Cepheid variable star in Andromeda.

Review: distance measurementsCepheid variable stars pulsate, getting brighter and dimmer with time. The period of this pulsation is related to the star’s luminosity

So if you can measure the apparent brightness and the period then you can figure out the distance using:

apparent brightness = luminosity4𝛑×distance2


The key to figuring out the nature of the spiral nebulae came when Edwin Hubble found a Cepheid variable star in Andromeda.

Hubble realized that Andromeda is much further away then anything within our galaxy — therefore it must be an entirely separate galaxy!


By measuring the distances to other galaxies, Hubble found that their recessional velocity is proportional to their distance

Hubble’s law:v=Ho×dwhere Ho is Hubble’s constantHo=22/km/s/Mly

• Q: Galaxy X is moving away from me twice as fast as Galaxy Y. That probably means it's twice as far away.A. Yes, the distance of a galaxy from you is

proportional to the velocity that it moves away from you.

B. No, it's the other way round: Galaxy Y is twice as far as Galaxy X.

C. No, the velocity that a galaxy moves away from you is independent of its distance from you.

D. No, you have to measure the distance to the galaxies using standard candles before making this statement.

Q: By measuring a galaxy’s redshift you figure out that it has a recessional velocity of 220 km/s. How far away is this galaxy?

A. 1 MlyB. 10 MlyC. 22 MlyD. 100 MlyE. 220 Mly


This is what we mean by the expansion of the universe: we see that everything is flying away from everything else, unless gravity is strong enough to overcome the expansion.


This is what we mean by the expansion of the universe: we see that everything is flying away from everything else, unless gravity is strong enough to overcome the expansion.

This also gives us a way to estimate the distances to very distant galaxies. If we can measure the redshift, then we can get a very good estimate of the distance.

How do we study galaxy evolution?

Problem: galaxies evolve over very long timescales. You can’t just sit and watch a galaxy evolve. So how do we figure out how galaxies change over time?

1. lookback studies2. computer simulations


Some of these galaxies are really far away, so we actually see what they looked like a long time ago

Other are relatively nearby


Lookback studies: while we can’t watch individual galaxies evolve, we can look backwards in time to watch the overall galaxy population evolve:

• Looking at galaxies that are 1 billion lightyears away shows us what galaxies looked like 1 billion years ago

• Looking at galaxies that are 2 billion lightyears away shows us what galaxies looked like 2 billion years ago

• And so on, almost all the way back to the big bang.

Inferring the growth of the Milky Way galaxy

Galaxy evolution: theory

We used to think that galaxies formed out of a huge collapsing gas cloud — almost like a very scaled-up version of how a cloud of gas collapses to form a star and planets.

Galaxy evolution: theoryBut we now know that galaxy evolution is a much more complex and dynamic process:


Indeed, when we look back in time galaxies often look irregular and disturbed


And large, well-organized spiral galaxies take a long time to develop.

Galaxy evolution: collisions

But what about really weird, irregular objects that we see in the present-day universe?

Galaxy evolution: collisionsSpiral galaxies can form from the collapse of a large gas cloud and can continue to growing by accreting other galaxies. But sometimes galaxies undergo major collisions:

Galaxy evolution: theorySpiral galaxies can form from the collapse of a large gas cloud and can continue to growing by accreting other galaxies. But sometimes galaxies undergo major collisions:

And this is how you can make an elliptical galaxy.

Midterm #2

• Mean score: 74/100

• Approximate curve (using the score written on the back of the exam):

A: 82-100

B: 72-81

C: 62-71

D: 51-60

L-QG-KC-FA-B R-Z

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