Lecture 20 - Galaxy Evolution and Dark Energy Flashcards
what is dark matter?
unseen form of mass that does not interact with light but has gravitational influence on motion of stars and gas clouds
what is dark energy?
unknown form of energy that is causing the expansion of the universe to accelerate
how does dark matter relate to light?
- does not interact with light –> light goes thru it
- does not produce light –> dark matter has low speed so it is cold and dark
what property of dark matter have we seen?
gravitational influence
what force does dark energy oppose?
gravity
what is the theory for calculating the center of a black hole?
what were initial predictions?
there is none!!
thought maybe matter had a negative mass but then according to Einstein, mass does not have a sign!
then they thought there is maybe a weak massive particle that we havent seen yet or missing black holes –> but haven’t detected missing black holes
how do you calculate the total mass of a system when you relate the orbital period to the avg orbital distance?
M1 + M2 = (4pi^2*a^3)/(Gp^2)
what is a rotation curve?
What does it look like for the solar system?
What does it look like for the milky way?
X: distance from orbital center
Y: orbital speed
Solar system: further away from sun = slower
Milky way: generally flat, similar speed at any distance
why is the milky way rotation curve flat?
mass is spread out over a larger region than its stars –> i.e. most of its mass is dark matter
if a galaxy’s rotational velocity rises steadily with distance beyond the visible part of its disk, what can we conclude?
it is rich in dark matter
how are old galaxies different than newer galaxies?
galaxies used to be smaller so there were more of them but they were closer together –> more collisions
what are the 2 assumptions we used when modeling galaxy formation?
- H and He with dark matter filled all of space when the universe was young
- distribution was not perfectly uniform so some areas were more dense
in the early universe, what happened at the denser areas of the universe?
5 steps to galaxy
- acted as ‘seeds’ for forming galaxies with gravity pulling gas in
- protogalactic clouds form by contraction to become galaxies
- H and He gas formed the first stars
- supernova explosions kept most of the gas
- leftover gas settled into a spinning disk due to momentum
what is the general development of an elliptical galaxy?
protogalactic cloud WITHOUT angular momentum or cloud that is dense enough to form stars before it is a disk
what is the general development of a spiral galaxy?
protogalactic cloud WITH angular momentum or cloud that is less dense so it forms stars slowly and becomes a disk
why are collisions btwn galaxies inevitable?
when did they occur more frequently? why?
what do they lead to?
collisions are inevitable because the distances btwn galaxies are similar to sizes of galaxies
more common earlier bc universe was smaller and galaxies were closer
collisions trigger bursts of star formation
what do distant red elliptical galaxies indicate?
distant red elliptical galaxies indicate that most of their stars formed EARLY in the history of the universe
how do elliptical galaxies form from spiral galaxies? (6 steps)
- 2 spiral galaxies approach each other
- the first encounter disrupts the galaxies and sends them into orbit around each other
- gas in the disk collapses into the center
- gravitational forces btwn the galaxies tear out streamers of stars called tidal trails
- centers of galaxies approach each other and merge
- creates 1 elliptical galaxy surrounded by debris
where are elliptical galaxies most common?
at the cores of dense clusters of galaxies where there are more collisions –> any spiral becomes elliptical
how do we know that elliptical galaxies likely originate from collisions?
they have stars and gas clouds with orbits as leftover pieces of galaxies
why is there little star formation in elliptical galaxies? when can star formation occur?
little amount of gas
but if it tears away gas from interacting with a spiral galaxy it can have enough gas for rapid star formation
what are shells of stars? what do they indicate?
they are stars on orbits that swing back and forth through the galaxy’s center observed around some elliptical galaxies and are from past collisions
when are elliptical galaxies prominent?
at the beginning and recently in universe
explain central dominant galaxies
giant elliptical galaxies at the centers of clusters which consumed galaxies and become v big
besides collisions, what is another way elliptical galaxies form? explain
GAS REMOVAL
the center of dense galaxy clusters have very hot gas
- when a spiral galaxy goes thru, the hot gas exerts drag forces that slow the galaxy’s gas but not its stars
- the stars move freely, leaving the gas behind
what is the result of gas removal if the disk from the dense galaxy has not yet formed many stars?
galaxy will be more like an elliptical galaxy
- disk fades, bulge and halo remain
what is the result of gas removal if the disk from the dense galaxy has formed many stars?
galaxy will look like a spiral galaxy without a disk so it will be a lenticular galaxy
describe the colour of galaxies as they evolve
start as star-forming systems –> BLUE
then galaxies merge to be larger, more luminous, and have less star formation –> RED
what is a starburst galaxy?
produce stars at very high rates!
what are the bright dots we see in a galactic disk?
emission from accretion disks around black holes or neutron stars, from recent supernovae
describe the type of radiation that is emitted when galaxies collide?
INFRARED
explain galactic wind
due to intense supernova explosions that drives away most of the gas
what is it called when the center of a galaxy is unusually bright? example
active galactic nucleus
ex. QUASAR
describe quasars
- has jet pointing straight towards us –> like pulsar but very slow
- highly redshifted so very far away
what is the luminosity of quasars relative to L_sun
> 10^12 times L_sun
what does it mean that quasars are very far away?
they were much more common early in history of universe than now, then the energy supplying the large luminosities became dormant
what 4 things can we conclude from quasars being redshifted?
- very distant
- more common in early time
- turn on via galaxy collisions
- nearby galaxies might hold dead quasars
what is a supermassive black hole?
what is the evidence for their existence?
largest type of black hole –> mass can be billions times the mass of the sun
orbital speed and distance of gas orbiting the center of galaxy M87 indicates a black hole with a mass >3 billion M_Sun but orbits of stars and direct imaging indicates a mass closer to 6 billion M_sun
describe spectra of supermassive black holes
blueshift on one side of its active nucleus indicates rotation towards us, redshift on other side indicates rotation away from us
describe the energy of a black hole (4 steps)
- gravitational PE of matter falling into black hole turns into KE
- friction in accretion disk turns KE into thermal energy (heat)
- thermal energy produces thermal radiation (photons)
- converts 10-40% of E = mc^2 into radiation
explain black holes in galaxies
most nearby galaxies have supermassive black holes at their CENTERS but they are dormant
how many galaxies had a quasar-like stage earlier in time?
ALL
what is the mass of a galaxy’s black hole related to?
what does this indicate about a galaxy with a large black hole?
the mass of a galaxy’s black hole is related to the mass of its bulge
large black hole have large bulge
can supermassive black holes disappear? what does this mean?
NO
so even though we see quasars at large distances from beginning of universe, the black holes responsible for the energy output of quasars must still be present at the center of today’s galaxies
difference btwn current and old supermassive black holes
current supermassive black holes consume less material and radiate less energy into space
RADIO GALAXIES
- what do their active nuclei do?
- size
- what does it mean when they have bent jets and swept-back lobes?
- contain active nuclei shooting jets of plasma that emit radio waves from electrons that move near light speed and influence the surrounding gas
- lobes can extend over 100,000s of light Lyr
- bent jets and swept-back lobes mean the galaxies are traveling thru surrounding hot gas
explain how we use quasar spectra
to study PROTOGALACTIC CLOUDS and how light is absorbed by galaxies
- as light from quasar passes thru clouds, H atoms absorb light of a specific wavelength
- each cloud closer to earth has diff redshifts, producing diff lines on spectra
- each absorption line in quasar spectrum indicates the composition of unique clouds
why does the pull of gravity get weaker as the universe expands?
there is more space between gravitationally attracting matter so the pull of gravity gets weaker AND the repulsion due to dark energy increases –> accelerates its expansion
what does the fate of the universe depend on?
depends on the abundances of matter and dark energy
what are the 4 possibilities for the fate of the universe?
- RECOLLAPSING –> spacetime expands then collapses until a reset –> oscillates btwn big bang, us, big bang
- CRITICAL –> enough matter to slow down expansion
- COASTING –> expanding at same rate forever
- ACCELERATING –> gravity tears galaxies away from us
UNIVERSAL FATE
- eventual fate of the universe depends upon rate of acceleration
- if it does not end in a Big Rip, it should expand forever
- all matter eventually ends up as black holes which will evaporate (via Stephen Hawking)
