Lecture 28 Flashcards
what is a black hole
an object so dense that nothing can escape it, including light
what can you think of a black hole as and why
extreme gravitational lensing - light is deflected so much that it enters into orbit around the singularity
what is at the centre of black holes
we do not know, laws of physics break down
what is the singularity in black holes
infinitely dense point with no size
why cant light escape a black hole
escape velocity of the black hole is greater than the speed of light
what is the event horizon
distance within which if light, or any object, passes closer, it will be captured by the black hole
what is the distance between centre of black hole and the event horizon
radius of the black hole which is known as the schwarzchild radius R_s
how big are black holes generally
very small
what would you have to compress the eaths mass to for it to make a black hole
less than 1cm
how to tell if a mass would be a black hole
calculate the corresponding R_s, if the mass’s radius is smaller than this then it will be a BH
how was M87’s SMBH event horizon captured and what did it confirm
planet wide telescope called event horizon telescoped imaged it, this confirmed GR predictions
what quantities is a black hole defined by
mass, spin (not very important for galaxies), charge (has little charge)
which BH quantity is most important?
mass - determines the radius, the mass only grows larger overall
what are the 4 types of black holes?
stellar, supermassive, intermediate, primordial
mass and observations/origin for stellar black holes
mass: 1 - 1000 M
from death of a massive star( >25M)
mass and observations/origin for intermediate mass BH
1000 - 100000 M
never been saw
mass and observations/origin for supermassive BH
mass: 10^5 - 10^10M
saw at centre of galaxies
mass and observations/origin for primordial BH
mass: unknown for sure, generally small
made at big bang - speculative
what does compressing matter enough to make a BH require?
an extreme event
what is the only known way for mass to be compressed to form a BH, and give some info
death of a super massive star, only forms a BH wit 10’sM, Since SMBH are millions or billions of M, BH’s grow a lot and quickly
what is the black hole accretion disk
matter pulled in, gas radiates away energy but retains angular momentum forming a disk, gas is accelerated to near C due to how strong the gravity is
what do black hole accretion disks emit and why
extremely hot and bright, can each 10^8K or more, emits thermally in hard X-rays, lots of energy released
what is the Eddington limit (Ledd)
luminosity at which radiation pressure = gravity
what happens if Lbh > Ledd
black hole blows gas away and cant grow, but this assumes spherical but we know it is a disk, Ledd can be exceeded somewhat
what happens when a black hole is accelerating
it will release energy and shine - active galactic nucleus
are most black holes active or dormant?
dormant - these are BH that are not AGN
what are the brightest AGN and where are they associated with
quasars, typically associated with galaxy mergers or ellipticals
what are modestly bright AGN
seyferts, mostly in spirals
what are black hole winds
accretion disks have super heated gas emitting energetic photons, gas and dust absorbs photons, get pushed into an accretion disk wind
how do black hole jets form
matter concentrates into a strong magnetic field
charged particles flow along the field lines
they get accelerated into a tightly collimated beam - a jet
what radiation do black hole jets release
synchrotron radiation - broad spectrum and not peaked at particular frequency, jets are prominent in the radio frequency
what is AGN feedback
energy released via winds and jets
what % of material in accretion disk falls in and what happens to the rest? what happens to energy?
10% falls in, the rest is ejected. Generally, lots of energy is put out
what are two possible causes for supermassive black holes growing so big
massive seed - exotic, huge first stars?
super-edd accretion - Ledd assumes spherical accretion, but accretion is from a disk
