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Slide 1
GALACTIC STRUCTURE AND FORMATION SZYDAGIS 03.13.2015 1 /
14
Slide 2
CLICKER-STYLE QUESTIONS 1. What is one of the key ingredients
needed for galaxy formation? a. dark energy b. clumps of gas c.
supernovae d. neutron stars 2. Which explanation is currently best
based on our observations? a. Top-down: big galaxies broke up b.
Bottom-up: small galaxies combined c. Combination, but bottom-up
favored d. Black holes eating dark matter 2 / 14
Slide 3
3 / 14 (not the same galaxies of course)
Slide 4
THE ORIGIN OF GALAXIES Most galaxies are very old. The very
first began forming around 12-13 billion years ago, not long after
the first stars formed Study of their formation from inner to outer
portions and continuing evolution is still an area of active
research to this day Models fall under one of two umbrellas: 1.
top-down, 2. bottom-up 1. Galaxies began as clouds of gas bigger
than galaxy 2. They started out as smaller fragments merging
together In either case, black holes (super-massive ones) are
involved, with galaxies building up around them as cores (in ->
out) These came from the collapse of the first, largest stars Local
over-density caused slower expansion (0.5% at 500,000 years after
BB, 5% at 15e6, 200% at 1.2e9): runaway effect We will learn about
this and more with the new James Webb Space Telescope, Hubbles
successor, launching in three years 4 / 14
Slide 5
THE TOP-DOWN APPROACH Gravitational collapse will overcome gas
pressure If cloud rotating slowly, stars form before collapse into
disk can occur, resulting in elliptical galaxy If rotating faster,
disk formed, so spiral galaxy (think of bulging earth) Star
formation rate determines type of galaxy that will form, or perhaps
it is other way around Variant: gas cloud fragmentation, each clump
becoming small galaxy. Clusters form first Problem: takes too long,
though explains clusters? I Zwicky 18 is a nearby dwarf galaxy that
started forming stars only 500 million to one billion years ago. It
may be an example of what the galaxies were like over 12 billion
years ago. How this galaxy remained in an embryonic state for
almost the entire history of the universe is unknown. Is it an
example of a "dark galaxy"---a dark matter clump with cold primeval
hydrogen and helium gas---in which the gas only now got compressed
enough to form stars? -http://www.astronomynotes.com/galaxy/s10.htm
A Baby Galaxy in a Grown-Up Universe! (High-density sheets of
galaxies with large voids in between are easy to predict.) 5 /
14
THE BOTTOM-UP APPROACH Little pieces merge Fragments could have
already started collapsing very early on during the history of the
universe Cluster hierarchically through mutual gravitational forces
Predicts more smaller galaxies: check! Clusters should still be
forming: check!! Variant: dark matter galaxies (hard to see). Cold
DM favors bottom-up scenario Combining theories The radio galaxy
MRC 1138-262, also called the "Spiderweb Galaxy" is a large galaxy
in the making. At 10.6 billion light years away, we see it in the
process of forming only 3 billion years after the Big Bang. Note
the small, thin "tadpole" and "chain" galaxies that are merging
together to create a giant galaxy. -astronomynotes.com 7 / 14
THE STRUCTURE OF A GALAXY Example, our own Milky Way Halo
mostly dark matter. Not perfectly spherical. Modeled as spheroid /
ellipsoidal profile Why sphere, while galaxy (if spiral) is a very
flat disk Dust-rich: spiral (flat) Dust-poor: elliptical Sensitive
to rotational degrees of freedom, initial conditions, particle
interaction probability. Also, thinner->faster rotation Center
of course black hole(s) thousands, millions, billions times more
massive than sun So, BHs = birth & death North arbitrary:
right-hand R.J. Hall http://www.cosmotography.com/images/s
upermassive_blackholes_drive_galaxy_e
volution_2.htmlhttp://www.cosmotography.com/images/s
upermassive_blackholes_drive_galaxy_e volution_2.html (Journey to
the Center) 9 / 14
Slide 10
A FEW VIDEOS Credit: Matthias Steinmetz The formation of a
"galactic fountain" in a forming disk galaxy. The simulation
includes a model for star formation and galactic winds. In this
particular galaxy, the winds generated by ongoing star formation
are confined by the gravitational potential of the dark matter
halo, giving rise to a "galactic fountain. Credit: Volker Springel
This movie shows two colliding and merging spiral galaxies. The
simulation follows dark matter, gas, and a stellar component, but
only the baryonic component is visualized. Credit: Volker Springel
http://www.mpa-garching.mpg.de/galform/data_vis/galaxy.mpg
http://www.mpa-garching.mpg.de/galform/data_vis/f10.mpg Links to a
couple of extra videos that I had trouble embedding into the
slides. The Max Planck Institute for Astrophysics (z is redshift.
Now z = 0. Beginning z~20) 10 / 14
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PARDON MY (HOT) GAS Chandra X-ray Observatory | NASA X-rays are
in left pane and optical at right. What is binding all the gases in
place to make those x-rays, considerably outside of the visible
elliptical galaxy? Best answer we have so far: dark matter 11 /
14
Slide 12
USING HYDROGEN: SEE FARTHER http://upload.wikimedia.org/wi
kipedia/commons/e/ec/M33_r otation_curve_HI.gif That is the
wavelength. Frequency=1,420 MHz 12 / 14 Lets do it again: Back to
slide 2, same question
Slide 13
HOMEWORK OVER BREAK Types and Classification of Galaxies
http://www.astro.cornell.edu /academics/courses/astro2
01/galaxies/types.htm (plus sub-link:) http://www.astro.cornell.edu
/academics/courses/astro2 01/galaxies/types.htm Also tuning fork A
Rippled Milky Way May Be Much Larger Than Previously Estimated
(from Anthony. Thanks!) http://scitechdaily.com/rippl
ed-milky-way-may-much- larger-previously-estimated/
http://scitechdaily.com/rippl ed-milky-way-may-much-
larger-previously-estimated/ Black Holes, Quasars and Active
Galaxies | Hubble & European Space Agency
http://www.spacetelescope. org/science/black_holes/
http://www.spacetelescope. org/science/black_holes/ 13 / 14
Terrifying information found on the first few pages of the
childrens book First Space Encyclopedia (owned by a friend)
Slide 14
14 / 14 Image Credit: Sergey V. Pilipenko (LPI, MIPT)LPI, MIPT)
A Redshift Lookup Table for our Universe