Download Quasars

Galaxies
• Classification
• Clusters
• Collisions
• Active galaxies and quasars
Classification
The Hubble sequence:
• ellipticals E0 – E9
• spirals Sa – Sc
• barred spirals SBa – SBc
+ irregulars
Classification - 2
Global properties
Spirals
Ellipticals
Irregulars
Visible mass (M )
109 – 1011
106 – 1013
107 – 1010
Diameter (103 LY)
20 – 150
2 – 500
5 – 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
Classification - 3
M74
Type Sc
(Gemini)
Classification - 4
NGC4565
Type Sb
(R. Gendler)
Classification - 5
M81
Type Sab
(G. Benintende)
Classification - 6
M104
`Sombrero´
Type Sa
(HST)
Classification - 7
NGC1300
Type SBbc
(HST)
Classification - 8
NGC1365
Type SBb
(SSRO/PROMPT and
NOAO/AURA/NSF )
Classification - 9
M31
`Andromeda nebula´
Type Sb
and companions
M32
NGC205
(dwarf
ellipticals)
(R. Gendler)
Classification - 10
M87
Giant elliptical
(CFHT)
Main galaxy in
the Virgo cluster
Classification - 11
M82
Irregular
(HST)
Intense stellar
formation
`Recent´
interaction with
M81
Classification - 12
Sagittarius dwarf
Dwarf irregular
(HST)
Satellite of our
Galaxy
Classification - 13
Large Magellanic Cloud
Dwarf irregular
+ bar
(Wei Hao Wang,
IfA, Univ. Hawaii)
Main satellite
of our Galaxy
de of
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
Galaxy clusters - 2
The Virgo cluster at ~ 18 Mpc
Galaxy clusters - 3
The Coma cluster at ~ 100 Mpc
(Jim Misti)
Galaxy clusters - 4
The Abell 1185 cluster at ~ 400 Mpc
(CFHT)
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
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)
~ 10 000 galaxies in a
field of 36.7 arcminutes
squared
↔ ~ 100 billion galaxies
in the observable
Universe
Galactic collisions
• Collisions of planets: very unlikely
d (planets) ~ 1000 RP
• Collisions of stars: even more unlikely
d (stars) ~ 10 000 000 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)
Galactic collisions - 2
M51, spiral galaxy interacting with NGC5195, a lower mass
companion
Galactic collisions - 3
Arp295, two galaxies that passed close to each other, giving rise to
`tidal tails´
(USNO, Flagstaff)
Galactic collisions - 4
NGC4038 and 4039, `the antennae´, two colliding galaxies, with
intense star formation and spectacular `tidal tails´
(Daniel Verschatse
– Antilhue
Observatory)
Galactic collisions - 5
NGC520, probably the result of two spiral galaxies that collided 300
millions years ago and have now nearly merged
(Gemini obs.)
Galactic collisions - 6
AM0644−741, with ring of diameter ~ 150 000 LY, showing intense
star formation (result of a `head-on´ collision)
(HST)
Galactic collisions - 7
`Cartwheel´ galaxy with gas ring and intense star formation
(HST)
Galactic collisions - 8
Neutral hydrogen contours superimposed onto the optical image of the
`Cartwheel´ galaxy → reveal a `bridge´ of matter
Galactic collisions - 9
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?
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)
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
Active galaxies and quasars - 3
Radiogalaxies
= giant elliptical galaxies + strong emissions at radio frequencies
(~10 000 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
Active galaxies and quasars - 4
Synchrotron radiation
Charged particle moving at relativistic speed in a magnetic field:



 F  qv  B
→ 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)
Active galaxies and quasars - 5
Radio emission
Radio flux < radio lobes
(total extent ~10 times the
extent of the galaxy)
Parts of the radio galaxy Cygnus A
Lobes sometimes linked to nucleus by thin
filaments called radio jets
(relativistic: particles moving at velocities
close to c)
Radiogalaxy M87
2 types: Narrow-Line & Broad-Line Radio
Galaxies (NLRG/BLRG) according to the
absence or presence of broad permitted lines
in the optical spectrum
Active galaxies and quasars - 6
Quasars
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
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)
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
~ 60 000 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)
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
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 1156+295
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
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
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
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
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
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
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