1. Galaxies • Classification • Clusters • Collisions
• Active galaxies and quasars

2. Classification The Hubble sequence: • ellipticals E0 – E9
• spirals Sa – Sc
• barred spirals SBa – SBc
+ irregulars

3. Global properties Spirals Ellipticals Irregulars
Classification - 2
Global properties
Spirals Ellipticals Irregulars
Visible mass (M ) 109 – – – 1010
Diameter (103 LY) 20 – – – 30
Stars’ages all ages old all ages
Stellar orbits circular elliptical
Interstellar matter yes no yes
It was first thought that the Hubble sequence corresponds to an evolutionary path
This is no more the case

4. Classification - 3
M74
Type Sc
(Gemini)

5. Classification - 4
NGC4565
Type Sb
(R. Gendler)

6. Classification - 5
M81
Type Sab
(G. Benintende)

7. Classification - 6
M104
`Sombrero´
Type Sa
(HST)

8. Classification - 7
NGC1300
Type SBbc
(HST)

9. (SSRO/PROMPT and NOAO/AURA/NSF )
Classification - 8
NGC1365
Type SBb
(SSRO/PROMPT and NOAO/AURA/NSF )

10. M31 `Andromeda nebula´ Type Sb and companions M32 NGC205
Classification - 9
M31
`Andromeda nebula´
Type Sb
and companions
M32
NGC205
(dwarf ellipticals)
(R. Gendler)

11. Main galaxy in the Virgo cluster
Classification - 10
M87
Giant elliptical
(CFHT)
Main galaxy in the Virgo cluster

12. Intense stellar formation
Classification - 11
M82
Irregular
(HST)
Intense stellar formation
`Recent´ interaction with M81

13. Satellite of our Galaxy
Classification - 12
Sagittarius dwarf
Dwarf irregular
(HST)
Satellite of our Galaxy

14. Large Magellanic Cloud Dwarf irregular + bar
Classification - 13
Large Magellanic Cloud
Dwarf irregular
+ bar
(Wei Hao Wang, IfA, Univ. Hawaii)
Main satellite de of of our Galaxy

15. Galaxy clusters
Many galaxies belong to associations of variable size: groups or clusters
• the local group contains ~ 30 galaxies
with 2 major galaxies
it is a satellite of the nearest cluster:
• the Virgo cluster contains ~ 2000 galaxies
with ~ 100 major galaxies
• elliptical galaxies are more numerous inside clusters
• spiral galaxies are more numerous outside clusters

16. The Virgo cluster at ~ 18 Mpc
Galaxy clusters - 2
The Virgo cluster at ~ 18 Mpc

17. The Coma cluster at ~ 100 Mpc (Jim Misti)
Galaxy clusters - 3
The Coma cluster at ~ 100 Mpc
(Jim Misti)

18. The Abell 1185 cluster at ~ 400 Mpc (CFHT)
Galaxy clusters - 4
The Abell 1185 cluster at ~ 400 Mpc
(CFHT)

19. Galaxy clusters - 5
Large structures
The galaxy clusters are associated into superclusters which concentrate
on irregular `surfaces´ (walls) surrounding nearly empty `bubbles´ (size ~ 1 Mpc)
That structure is revealed by ambitious 3D cartography projects:
• 2 coordinates are deduced from the position of the galaxy on the sky
• distance is deduced from the spectral redshift
→ spectra of thousands of galaxies
Large scale distribution of galaxies

20. The Hubble deep fields Hubble Ultradeep Field:
Galaxy clusters - 6
The Hubble deep fields
Hubble Ultradeep Field:
Field without bright stars
11.3 days of exposure with ACS (visible) and 4.5 days with NICMOS (IR)
~ galaxies in a field of 36.7 arcminutes squared
↔ ~ 100 billion galaxies in the observable Universe

21. Galactic collisions • Collisions of planets: very unlikely
d (planets) ~ 1000 RP
• Collisions of stars: even more unlikely
d (stars) ~ R*
• Collisions of galaxies: frequent in groups and clusters
d (galaxies) ~ 10 to 100 RGal
– unlikely to give rise to collisions of stars
– but perturbation of orbits → some stars are ejected from the galaxies
(~10 to 30% of stars in the Virgo cluster are outside galaxies)

22. M51, spiral galaxy interacting with NGC5195, a lower mass companion
Galactic collisions - 2
M51, spiral galaxy interacting with NGC5195, a lower mass companion

23. Galactic collisions - 3
Arp295, two galaxies that passed close to each other, giving rise to `tidal tails´
(USNO, Flagstaff)

24. (Daniel Verschatse – Antilhue Observatory)
Galactic collisions - 4
NGC4038 and 4039, `the antennae´, two colliding galaxies, with intense star formation and spectacular `tidal tails´
(Daniel Verschatse – Antilhue Observatory)

25. Galactic collisions - 5
NGC520, probably the result of two spiral galaxies that collided 300 millions years ago and have now nearly merged
(Gemini obs.)

26. Galactic collisions - 6
AM0644−741, with ring of diameter ~ LY, showing intense star formation (result of a `head-on´ collision)
(HST)

27. `Cartwheel´ galaxy with gas ring and intense star formation (HST)
Galactic collisions - 7
`Cartwheel´ galaxy with gas ring and intense star formation
(HST)

28. Galactic collisions - 8
Neutral hydrogen contours superimposed onto the optical image of the `Cartwheel´ galaxy → reveal a `bridge´ of matter

29. Consequences of galactic collisions
• few perturbations onto stars, apart from their orbits
• strong perturbations of interstellar clouds
→ star formation (and more… see AGNs below…)
• Computer simulations:
→ reproduce the observed shapes
→ collision of 2 spirals often results in an elliptical
→ giant ellipticals might result from successive collisions
(in agreement with their predominance in clusters)
→ what happened to the gas?

30. Active galaxies and quasars
What is an AGN?
AGN = Active Galactic Nucleus
Some galaxies have several peculiarities:
• very luminous center
• output of a huge energy amount in a small volume
• strong emission lines
4 main AGN categories:
Seyfert galaxies, radiogalaxies, blazars and quasars
Center of active galaxy NGC1097 (VLT)

31. Active galaxies and quasars - 2
Seyfert galaxies
1943: Carl Seyfert → catalogue of spiral galaxies showing a particulary bright nucleus
Luminosity of nucleus highly variable on time scales < 1 year
→ size < ~1 LY
2 subcategories according to spectrum:
• Type 1: intense continuum broad permitted emission lines + narrower forbidden lines (lower Doppler broadening)
• Type 2: only narrow emission lines
Spectra of Seyfert galaxies

32. Active galaxies and quasars - 3
Radiogalaxies
= giant elliptical galaxies + strong emissions at radio frequencies
(~ stronger than normal galaxies)
Nucleus does not always have an optical counterpart
Radio waves emitted by highly energetic electrons moving inside a magnetic field
→ synchrotron radiation
Radiogalaxy Centaurus A

33. Active galaxies and quasars - 4
Synchrotron radiation
Charged particle moving at relativistic speed in a magnetic field:
→ the particle spirals around the field lines
→ radiation depending on the intensity of the magnetic field and of the velocity distribution of the particles : Fν ~ ν−α (non thermal)

34. Parts of the radio galaxy Cygnus A
Active galaxies and quasars - 5
Parts of the radio galaxy Cygnus A
Radio emission
Radio flux < radio lobes
(total extent ~10 times the extent of the galaxy)
Lobes sometimes linked to nucleus by thin filaments called radio jets
(relativistic: particles moving at velocities close to c)
2 types: Narrow-Line & Broad-Line Radio Galaxies (NLRG/BLRG) according to the absence or presence of broad permitted lines in the optical spectrum
Radiogalaxy M87

35. Active galaxies and quasars - 6
Quasar = QUAsi-Stellar Astronomical Radio Source
Neologism invented in the 1950s: strong point-like radio sources, some without any optical counterpart
First hypothesis: new type of stars in our Galaxy, with emission lines that do not correspond to any known chemical element?
1963: Maarten Schmidt realizes that quasars are very distant, thus extremely luminous
→ strongly redshifted emission lines
Maarten Schmidt

36. Active galaxies and quasars - 7
Quasars and QSOs
Not all quasars emit strong radio waves
→ radio-loud quasars & radio-quiet quasars
→ alternate name for radio-quiet: Quasi-Stellar Objects (QSOs)
The brightness of the nucleus and the distance mask the host galaxy
→ appear as point sources unless observed at very high angular resolution
Unclear distinction between (radio-quiet) QSOs and Seyfert galaxies
→ adopted definition: QSO if MV < −23
A quasar and its host galaxy (HST)

37. Active galaxies and quasars - 8
Quasars and host galaxies
Quasars are found at the centers of massive galaxies of various types, but often perturbed by gravitational interactions
~ known quasars
~ 10% are intense radio emitters
Redshift 0.06 < z < 7.1
→ located in between 800 millions and 13 billions LY
→ useful tools to probe the evolution of the Universe
Quasar host galaxies (HST)

38. Quasar spectra Luminosity ~ 1012 to 1015 times the solar luminosity
Active galaxies and quasars - 9
Quasar spectra
Luminosity ~ 1012 to 1015 times the solar luminosity
Spectral distribution: non thermal continuum (≠ black body law)
→ synchrotron radiation
• Type 1 quasars: broad + narrow lines
• Type 2 quasars: only narrow lines, weaker continuum (less frequent)
↔ Seyfert galaxies
Typical spectrum of a type 1 quasar

39. Active galaxies and quasars - 10
Blazars
BL Lacertae (prototype discovered in 1929) + quasar = blazar
Characteristics: appear point-like at low resolution, strong radio emitters, highly variable, found in the center of elliptical galaxies
2 sub-categories:
• BL Lac: absence of broad lines
• OVV = Optically Violently Variables: weak broad lines
Intensity fluctuations of the blazar

40. Active galaxies and quasars - 11
AGN unification model
Basic idea:
Different types of AGNs are variants of the same phenomenon:
• at various luminosities
• seen under different angles
Taking into account:
• anisotropy of the AGN radiation
• extinction by dust
Artist view of an AGN

41. Active galaxies and quasars - 12
Basic ingredients of the model
At the center of the AGN: supermassive black hole (106 to 109 M )
• accretion of matter
• conservation of angular momentum → accreted matter forms a rotating accretion disk
• viscosity (friction) inside the disk heats the matter
→ emission of a continuum radiation, carrying a huge amount of energy
Other artist view of an AGN

42. Energy radiated by AGNs
Active galaxies and quasars - 13
Energy radiated by AGNs
Typical power (luminosity) of an AGN: ~1040 W
→ `swallow´ a mass ~10 M /year
(up to 100 for the most luminous quasars)
• AGNs turn `on´ and `off´ depending on the available matter
• When all surrounding matter is exhausted, the AGN becomes invisible and its host a `normal´ galaxy
• AGNs were more numerous in the past
• The Milky Way may have experienced a (rather mild) AGN phase in the past
Illustration of a black hole

43. Active galaxies and quasars - 14
Variability of AGNs
Variation of the rate at which the black hole is fed
→ variation of the AGN luminosity
Depends:
• on the presence of matter in the vicinity of the black hole
• on mechanisms bringing matter close enough to the black hole:
– bar in a spiral galaxy
– collision between galaxies

44. Active galaxies and quasars - 15
Emission lines regions
Gas clouds in orbit around the black hole
→ responsible for emission lines observed in the spectrum
• Broad-Line Region (BLR): dense clouds close to the black hole
→ fast motion
→ high velocity dispersion
→ broad lines
• Narrow-Line Region (NLR):
lower density clouds, further away from the black hole

45. Active galaxies and quasars - 16
Around the black hole
Dust torus
• surrounds the fast rotating dense clouds
• located in the same plane as the accretion disk
• opaque to visible and UV radiation
Possible radio jets
• particles accelerated along the rotation axis to v ≈ c
• decelerated when colliding with matter → radio lobes

46. Active galaxies and quasars - 17
Influence of orientation with respect to the line-of-sight
The BLR is visible:
→ quasar or type 1 Seyfert
The BLR is hidden:
→ type 2
One or two jets viewed from the side
→ radiogalaxy (NLRG or BLRG as above)
A jet points towards us
→ blazar