Chapter 5: Galaxies and Active Galactic Nuclei

Question 1

How many galaxies where known in early 1900s

Answer 1

Thought the Milky Way was the only galaxy

-we now know of trillions

Question 2

William Herschel

Answer 2

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

Question 3

Stars in disk

Answer 3

Circular orbits

• young
• lots of gas/dust/ ISM
• blue
• metallicity relatively high

Question 4

Stars in the bulge

Answer 4

• random orbits
• old
• no gas/ dust
• red
• relatively high metallicity

Question 5

Stars in the halo

Answer 5

-random orbits
-old
-no gas and dust
-red
Metal poor

Question 6

Milky Way

Answer 6

• a spiral galaxy
• due to dust can only see 1kpc of Galaxy
• this is due to interstellar extinction

Question 7

Galaxtic extinction laws

Answer 7

• photon is lost due to dust
• infrared doesn’t interact with dust very much
• Bump has is extra extinction due to carbon

Question 8

Mapping spiral arms

Answer 8

• 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

Question 9

Henrietta Swan-Leavitt

Answer 9

• 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

Question 10

Harlow Shapley

Answer 10

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

Question 11

Synchrotron radiation

Answer 11

A nonthermal process, associated with the presence of strong magnetic fields
-comes mainly from supernova remnants

Question 12

Galactic Centre

Answer 12

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

Question 13

Charles Messier

Answer 13

The first to catalog extra-galactic objects in mid 1700 , to fuzzy to be comets

Question 14

Leviathon of Parsonstown

Answer 14

Observed spiral galaxies in end of 19th century

Question 15

Shapely-Curtis debate

Answer 15

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

Question 16

Edwin Hubble

Answer 16

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

Question 17

1. Hubble’s classification scheme

Answer 17

Elliptical galaxies: no spiral arms, no dust

• spiral galaxies: spiral arms, lots dust, blue disk, red bulge
  
  1. Bared spirals
  2. Unbared spirals
• Irregular galaxies: don’t fit in with other groups properties
• Lenticular galaxies: no obvious spiral arms, disk and bulge, ISM, blue/red

Question 18

2. Hubble law

Answer 18

• 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

Question 19

Corollary

Answer 19

Recessional velocity can be used as a distance measure

Question 20

Hubble flow

Answer 20

Universe is expanding and carrying galaxies with it

Question 21

Central supermassive black holes

Answer 21

• 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

Question 22

Active galaxies

Answer 22

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

Question 23

Parts SMBH

Answer 23

Black hole, accretion disk, broad-band radiation, Torus (dusty donut), infrared radiation, radio jets

Question 24

AGN detection methods

Answer 24

Radio jets, most common in massive ellipticals but can still be found in spirals

1. x-ray emission detected around accretion disk
2. Spectroscopy of broad emission lines
   
   • broad line region: line very broad
   • narrow line region: line very narrow
   • gas shows up both narrow and broad

Question 25

Quasars

Answer 25

A clue to what switches black holes on

• so luminous that they can outshine AGN
• material funneled into black holes and merged together

Question 26

Local group

Answer 26

Group of galaxies that had two big spirals and the Magellanic Clouds and 50 other galaxies spanning 10 million lyrs

Question 27

Hierarchical growth

Answer 27

Build galaxies in a hierarchy starting with the small to the big

Question 28

Galaxy clusters

Answer 28

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

Question 29

Zwicky

Answer 29

1933 found evidence of dark matter which was ignored for 49 years

Question 30

Dark matter

Answer 30

• actual rotating curves are flat or even rising
• dark matter causes this
• most likely and exotic particle 80-90% matter

Question 31

High velocity stars

Answer 31

In halo, increase in galaxies dark velocity coming close to disk