Download Searching for the Most Distant Black Holes in the Early

Searching for the Most Distant
Black Holes in the Early Universe
Anton Koekemoer
(Space Telescope Science Institute)
June 6, 2006
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A Bit of History...
1687: Isaac Newton
– what goes up, comes down...
– an object moving away from the earth
faster than ~25,000 mph can escape
1783: John Michell
– considered speed of light (186,000 miles/second)
– asked how massive an object can be until light can’t
escape - first description of black hole
1796: Pierre-Simon Laplace
– independently predicted the properties
and possible existence of such objects
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How are Black Holes made?
• Normal stars:
– the sun has 332,000 times more mass than earth
– what prevents it from collapsing?
• nuclear fusion reactions at its core: 15 million deg K
• heat generates pressure to balance gravitational collapse
• When nuclear fuel runs out, a star collapses
(and explodes), leaving a remnant that is either:
– white dwarf
– neutron star
– black hole
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• White Dwarf: up to 1.44 Msun
– “electron degenerate” matter
– 1 teaspoon would weigh ~ 5 tons!
– all the mass of the sun, about the size
of earth...
• Neutron Star: 1.44 to 2-3 Msun
– “neutron degenerate” matter
– 1 teaspoon would weigh ~100 million tons!
– the mass of the sun, about the size of a city...
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• Black Hole: above ~2-3 Msun
– neutrons can no longer be kept apart
– current physics predicts gravitational collapse to
point-like “singularity”
– collapse might instead be to some new form of
condensed matter (eg degenerate quarks) - but still
inside event horizon, thus effectively a black hole
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• What might a black hole look like?...
not quite!
• Black holes can be
messy - may often
have disks of gas and
dust around them
Einstein: Gravity can
bend light rays...
(light has mass... E=mc2)
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So, how do we find Black Holes?...
• Black hole candidates in our own Milky Way
galaxy:
–
–
–
–
Cygnus X-1
Circinus X-1
SS433
...
• each one is a likely product of a supernova
• display signatures too unusual for white
dwarfs or neutron stars
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• Compact objects (black holes, neutron stars)
are often surrounded by “accretion disks”
–
–
–
–
Gas spirals in toward the black hole
heats up as it gets close
emits energetic UV light and X-rays
material can be
ejected in “jets”
Cygnus XR-1 jet
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• Why might these objects be black holes?
– Spectroscopy show that the main star revolves
around their companion every few days, thus
companion has mass up to 10 Msun
– However, it’s invisible! - can’t be normal star
– Too massive for neutron star
– X-ray and UV properties can show relativistic
signatures
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• SS433:
– twin “jets”
– rotate with fast
precession speed
– likely too powerful
to be neutron star
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• Cygnus X-1
– timing measurements
of UV emission from
hot gas around it
– gas is rotating, but
with shorter rotation
period each time
– eventually, emission
fades as gas spirals in
• is only consistent
with a black hole
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• The Black Hole at the Center of our Milky
Way Galaxy
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• What is the Central Object in our Milky Way
Galaxy?
– Very massive: almost 4 million Msun
– Compact: diameter ~14 million miles
• Smaller than the orbit of Mercury!
– Strong radio and X-ray emission
– No optical emission:
Radio image
• can’t be a conventional object
(massive star cluster, etc)
• Black hole is the best
candidate that explains the
available data
Chandra X-ray image
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• MCG-6-30-15: A Galaxy with a candidate
active central black hole
Optical image
Infra-red image
X-ray spectrum
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Observed spectrum of
MCG-6-30-15
A single emission line (at
6.7 keV) is “Doppler
shifted” to lower energy
with velocities up to
60% the speed of light!
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• Radio Jets!: The “Radio Galaxy” Centaurus A
– distance: ~ 10 million light years
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• Radio Jets!: The “Radio Galaxy” Centaurus A
– distance: ~ 10 million light years
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• Radio Jets!: The “Radio Galaxy” Centaurus A
– distance: ~ 10 million light years
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• Radio Jets!: The “Radio Galaxy” Centaurus A
– distance: ~ 10 million light years
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• M87, another radio galaxy with jets
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• M87, another radio galaxy with jets
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• Other active “radio galaxies”: more distant,
more powerful:
3C433
Fornax A
3C66B
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• Images in radio emission (100,000s light years)
3C353
Cygnus A
3C31
3C288
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• A zoo of different types of active galaxies:
–
–
–
–
–
Seyferts Type 1, 2
quasars
blazars
BL Lacs
Radio galaxies; Fanaroff-Riley Type I, II
• Basic ingredients:
–
–
–
–
–
black hole
accretion disk
diffuse gas clouds
thick torus
jets (optional extra)
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• Measuring Black Hole masses
– Study stars, gas around black holes in galaxy cores
– Use basic “Doppler shift” technique:
• emission towards us is blue-shifted
• emission away from us is red-shifted
– Compare difference, measure size, get the mass!
– eg black hole
in M84
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Black Hole and Galaxy Masses - A connection!
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• What determines the BH/bulge mass relation?
– do black holes “grow” along with galaxies, eg
when galaxies merge?
– or do black holes form as-is, early in the universe,
and then fix the properties of galaxies that form
around them?
• These are two very different scenarios gradual growth with cosmic time vs static
early-on determination
• Both probably apply to some extent; therefore,
aim to determine the dominant mechanism
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• Black Holes can Grow when Galaxies Merge...
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• Massive black holes appear to exist at the
highest observed distances (earliest epochs):
– more than a dozen at redshift > 6; age of the
universe was less than 1 billion years
– these black holes may be up to 1 billion Msun
How do they grow so fast in the early universe?
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• Track black hole growth with cosmic time
• Luminosity functions:
Number of objects
– simply count active galaxies of different luminosities
Luminosity
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• Track the change in luminosity function at
higher redshift (earlier cosmic times):
– low-luminosity objects more numerous later
– high-luminosity objects more numerous earlier
-3
-3
Lx = 42.0
Lx = 43.0
-5
Lx = 44.0
d(phi)/d(logL) (Mpc-3)
Number of objects
-5
-7
-9
z = 0.015 - 0.2
-11
z = 0.2 - 0.4
Lx = 45.0
-7
Lx = 46.0
Lx = 47.0
-9
Lx = 48.0
- 11
z = 0.4 - 0.8
-13
- 13
z = 0.8 - 1.6
z = 1.6 - 3.2
z = 3.2 - 4.8
-15
41.5
- 15
0.1
42.5
43.5
44.5
45.5
Log Lx (e rg/s)
Luminosity
46.5
47.5
0.6
2.4
redshift
Higher redshift
(earlier cosmic times)
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• So, low-luminosity black hole nuclei reach
their peak at recent cosmic times
• We also know that the rate of star formation in
galaxies reaches a peak at late cosmic times
• One mechanism that can explain both:
– minor mergers (large galaxy swallowing small
galaxy, or passing interactions)
• Small amounts of gas trigger star formation,
and yield low luminosity black hole nuclei
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• What about high-luminosity black holes?
– reach their peak at much earlier cosmic times
– suggests the fuelling mechanism may be different
• One possible mechanism:
– major mergers (two large galaxies colliding)
– large amounts of gas fuel high luminosity black
hole nuclei
• How to answer this?
– up till now, only had ~10 black holes at high
redshifts (z ~ 6, or cosmic age < 1 billion yrs)
– need to find a lot more black holes at these early
epochs, to enable statistical tests
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• How to find distant black holes...
– they are very rare, so we need a large area
– they are very faint, so we need to go deep...
– they emit across the spectrum, so need X-ray,
optical, infrared, radio, ...
• do everything! deep, wide, multiband surveys
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• Hubble / ACS for ~1 million seconds exposure
• deepest exposures ever taken (~30th magnitude)
• Very small area (~3 arcminutes on a side):
– 1/10 diameter of full moon
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• Two matching fields, north and south
• 30 times more area than UDF
• Each field is 10x16 arcminutes:
– 30 times more area than UDF
– about half the size of the full moon
• Deepest ever X-ray and infrared exposures
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• X-ray view of the two GOODS fields:
– each is about half the size of the full moon
– 1 million and 2 million seconds with Chandra
South
North
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• Largest ever survey obtained with Hubble
– 600 orbits (1.5 million seconds)
– 2 square degrees
– 10 times the area of the full moon
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• Largest ever survey obtained with Hubble
– 600 orbits (1.5 million seconds)
– 2 square degrees
– 10 times the area of the full moon
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• Predicted properties of distant black holes?
– should be strong X-ray sources (from hot gas)
– should be strong infrared sources (from hot dust)
– should have very faint (or none) optical emission
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• These black hole nuclei appear to be at very
large distances, and they are 1000x fainter
than the bright quasars found previously
• Compare their numbers to those found at
lower redshifts (later cosmic times):
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• Results:
– find that the faint, low-luminosity black hole
nuclei are indeed much rarer at these epochs
– confirms the prediction that black holes in the
early universe grow mostly by major mergers
between galaxies
– suggests that the relationship between black holes
and galaxy bulge mass is also driven by mergers
• Remaining question: What happens earlier???
• Next steps:
– probe to redshifts > 6 (epochs < 1 billion years)
– probe to fainter limits (lower-luminosity objects)
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• The Future...
Wide-Field Camera 3
– scheduled for Servicing Mission 4
(late 2007 / early 2008)
James Webb Space Telescope
– currently being built
– scheduled for launch in 2013
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