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Astrophysics from Space
Lecture 4: The extragalactic distance scale
Prof. Dr. M. Baes (UGent)
Prof. Dr. C. Waelkens (KUL)
Academic year 2014-2015
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The expanding Universe
One of the most important cosmological discoveries: the Universe is expanding.
Vesto Slipher Georges Lemaître Edwin Hubble
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The expanding Universe
One of the most important cosmological discoveries: the Universe is expanding.
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Hubble’s law
Hubble’s first determination resulted in H0 = 500 km/s/MpcResulting age of the Universe: about 2 Gyr
Radioactive dating of Earth rocks (> 3Gyr)
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Hubble’s constant
1960s – 1970s: two groups advocating
two distinct values (50 and 100 km/s/Mpc)
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Cosmic distance ladder
Each rung of the ladder provides information that can be used to
determine distances at the next higher rung.
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Fundamental distance measurements
Methods in which distances are measured directly, with no physical assumptions about the nature of the object.
Fundamental distance: scale of the Solar SystemAstronomical Unit (AU)
Kepler’s third law: if we knowthe distance to one planet, all distances in the Solar System are known
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Planetary parallax measurements
Late 1800s: efforts concentrated on Venus at inferior conjunction (in particular during Venus transit)
http://www.vt-2004.org
Early 1900s: Mars and Eros
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Radar echo measurements
From 1960s: radar echo measurements on Venus and other nearby planets and asteroids
AU = 149 597 870 691 ± 30 m
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Stellar parallax measurements
High astrometric precision is necessary to measure large distances
Hipparcos: • precision of milli-arcsec• accurate distances out to
several 100 pc• also accurate distances to
next distance ladder objects (Cepheids) Contribution of Hipparcos to
the cosmic distance ladder has been crucial
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Standard candles: cepheids
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Standard candles: cepheids
1908: Henrietta Leavitt discovers period-luminosity relation in cepheids (studying the LMC and SMC)
1915: Harlow Shapley uses cepheids to determine the size of the Milky Way
1924: Edwin Hubble uses cepheids to determine distance to Andromeda
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Standard candles: type Ia SN
White dwarfs in a binary system, where infalling matter pushes it over the Chandrasekhar limit.
Consequence: thermonuclear explosion.
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Standard candles: type Ia SN
The width of the light curve correlates with the peak luminosity.
Huge advantage: type Ia supernovae are extremely bright.
Type Ia supernovae are ideal standard candles for
cosmological studies
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Standard candles: caution…
(1) Calibration issues: what is the absolute magnitude(requires thorough definition of a class and enough members in that class)
(2) Confusion with similar objects (different SN types, novae versus supernovae…)
(3) Interstellar extinction
(4) How standard are standard candles ?For example: there are two classes of cepheids….
Walter Baade
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Secondary distance indicators
Indirect distance indicators, often based on statistical relations in galaxies. To be used when no primary indicators can be used.
Prime example: Tully-Fisher relation
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HST Key Project
HST Key Projects• large observations projects with significant impact• separate time budget (no competition)• guaranteed completion in the early years
HST Key Project on the Extragalactic Distance Scale• measure direct distances to 25-30 galaxies using
cepheid variables• use these distances to calibrate secondary distance
indicators (TF) to probe structure of the Universe
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HST Key Project
Spectacular results obtained as soon as the optics were refurbished (late 1993).
8 observations of each target galaxy to detect cepheids with periods of 10 – 50 days.
E.g. more than 80 cepheids discovered in M100.
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HST Key Project
Final result based on cepheids and cepheid-calibrated secondary methods: H0 ≈ 72 ± 8 km/s/Mpc