Chapter 5: Galaxies and Active Galactic Nuclei Flashcards
How many galaxies where known in early 1900s
Thought the Milky Way was the only galaxy
-we now know of trillions
William Herschel
In 1800 counted stars to sketch the Milky Way
- he put sun at centre Milky Way
- not round
- was not completely accurate though no (ISM) cause dust blocks our view
Stars in disk
Circular orbits
- young
- lots of gas/dust/ ISM
- blue
- metallicity relatively high
Stars in the bulge
- random orbits
- old
- no gas/ dust
- red
- relatively high metallicity
Stars in the halo
-random orbits
-old
-no gas and dust
-red
Metal poor
Milky Way
- a spiral galaxy
- due to dust can only see 1kpc of Galaxy
- this is due to interstellar extinction
Galaxtic extinction laws
- photon is lost due to dust
- infrared doesn’t interact with dust very much
- Bump has is extra extinction due to carbon
Mapping spiral arms
- has near hot stars (HII region)
- strongest emission lines are in optical, so still suffer from steal extinction
- study wavelengths in radio
- HI 21 cm is immune to dust l
- electrons have two different energy spins corresponds to 21 cm
- can map arms in infrared but has to be from space
Henrietta Swan-Leavitt
- Cepheid variable stars pulsate
- period-luminosity relation discovered 1910. Distance measure beyond parallax
- she plotted period of peak vs luminosity which made a linear relation l
- by knowing period we can find true luminosity, then apparent brightness from which we can find distance
Harlow Shapley
Early 20th century found the extend of MW by using distances to globular stars, spherically distributed over many Kpc
- sun was not at centre
- they were RRLyra stars not Cepheid
Synchrotron radiation
A nonthermal process, associated with the presence of strong magnetic fields
-comes mainly from supernova remnants
Galactic Centre
X-ray images find Sagitarius A which is a central black hole
- lots of radiation coming from such compact centre
- have a massive black hole in the centre
Charles Messier
The first to catalog extra-galactic objects in mid 1700 , to fuzzy to be comets
Leviathon of Parsonstown
Observed spiral galaxies in end of 19th century
Shapely-Curtis debate
1920
- Shapley argues spiral nebulae that we’re in our galaxy and putting them outside would regress Einstein’s theory
- Curtis claimed that it was a mistake and that they were outside and that there were many other galaxies
Edwin Hubble
Settles the great debate in 1923
- first to make distances to these galaxies
- showed that it was far away and Curtis was correct
1. Hubble’s galaxy classification scheme
2. Hubble’s law
- Hubble’s classification scheme
Elliptical galaxies: no spiral arms, no dust
- spiral galaxies: spiral arms, lots dust, blue disk, red bulge
- Bared spirals
- Unbared spirals
- Irregular galaxies: don’t fit in with other groups properties
- Lenticular galaxies: no obvious spiral arms, disk and bulge, ISM, blue/red
- Hubble law
- distance vs velocity (linear)
- where H is constant that describes the gradient of the slope called Hubble’s constant
- most galaxies moving away from us
- the farther away the faster it travels
- universe is expanding
Corollary
Recessional velocity can be used as a distance measure
Hubble flow
Universe is expanding and carrying galaxies with it
Central supermassive black holes
- Sag A in centre MW
- all massive galaxies have SMBH whose mass is proportional to the mass of stars in the bulge (scales with rest of galaxy)
- low mass shouldn’t have one but some do
Active galaxies
Galaxies in which the black hole and switched on and producing large amounts of energy, cores called active galactic nuclei
-AGN fuelling: accretion disks, processes in accretion disk/ jet make black holes viable
Parts SMBH
Black hole, accretion disk, broad-band radiation, Torus (dusty donut), infrared radiation, radio jets
AGN detection methods
Radio jets, most common in massive ellipticals but can still be found in spirals
- x-ray emission detected around accretion disk
- Spectroscopy of broad emission lines
- broad line region: line very broad
- narrow line region: line very narrow
- gas shows up both narrow and broad
Quasars
A clue to what switches black holes on
- so luminous that they can outshine AGN
- material funneled into black holes and merged together
Local group
Group of galaxies that had two big spirals and the Magellanic Clouds and 50 other galaxies spanning 10 million lyrs
Hierarchical growth
Build galaxies in a hierarchy starting with the small to the big
Galaxy clusters
Can have many thousand galaxies, more then groups
- lots of x-ray emission between galaxies due to elliptical galaxies and the intra-cluster medium
- more clusters of ellip then spies
Zwicky
1933 found evidence of dark matter which was ignored for 49 years
Dark matter
- actual rotating curves are flat or even rising
- dark matter causes this
- most likely and exotic particle 80-90% matter
High velocity stars
In halo, increase in galaxies dark velocity coming close to disk
