Astrophysics from Space

Lecture 4: The extragalactic distance scale

Prof. Dr. M. Baes (UGent)

Prof. Dr. C. Waelkens (KUL)

Academic year 2014-2015

The expanding Universe

One of the most important cosmological discoveries: the Universe is expanding.

Vesto Slipher Georges Lemaître Edwin Hubble

The expanding Universe

One of the most important cosmological discoveries: the Universe is expanding.

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)

Hubble’s constant

1960s – 1970s: two groups advocating

two distinct values (50 and 100 km/s/Mpc)

Cosmic distance ladder

Each rung of the ladder provides information that can be used to

determine distances at the next higher rung.

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

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

Radar echo measurements

From 1960s: radar echo measurements on Venus and other nearby planets and asteroids

AU = 149 597 870 691 ± 30 m

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

Standard candles: cepheids

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

Standard candles: type Ia SN

White dwarfs in a binary system, where infalling matter pushes it over the Chandrasekhar limit.

Consequence: thermonuclear explosion.

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

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

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

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

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.

HST Key Project

Final result based on cepheids and cepheid-calibrated secondary methods: H0 ≈ 72 ± 8 km/s/Mpc