Cosmic Distance Determinations

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Sybil Richardson
5 years ago
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1 Cosmic Distance Determinations Radar (works for inner solar system) Parallax D(pc) = 1 p(arcsec) GAIA satellite (2013) 20 micro-arcsec resolution! Thus D < 10 kpc Beyond Parallax: Standard Candles Use measured flux and known luminosity of object. r L d = 4 F

2 Spectroscopic Parallax For each individual star, determine location on HR diagram using spectrum and luminosity class (determined from line widths) Use on a cluster of stars to reduce uncertainties D < kpc

3 Cepheid variables are pulsating variable stars (similar to the RR Lyraes) due to a dynamical instability (between radiation pressure and gravity). Oscillation period correlated to luminosity D < 25 Mpc Cepheid Variables

6 Cepheids in Andromeda Edwin Hubble used 100-inch telescope on Mount Wilson Found a cepheid in Andromeda nebula Proved that Andromeda was a whole galaxy completely separate from the Milky Way. 770 kpc away Firm evidence for the island universes hypothesis The Andromeda galaxy (M31)

7 Cepheids in the Virgo galaxy cluster with Hubble Space Telescope (16 Mpc away )

8 Planetary Nebula For a particular galaxy, determine the number of PN as function of apparent magnitude (flux) of O III (5007 A). Fit to planetary nebula luminosity function (PNLF) based on statistical distribution in terms of absolute magnitude (luminosity)

9 White Dwarf Supernova Most luminous standard candle

High-Z Supernova z = 1.71 10 Billion light years away! http://newscenter.

10 High-Z Supernova z = Billion light years away!

13 Structure of Galaxy Distribution Milky Way is one of roughly 30 galaxies in the local group. 2 spirals 1 small spiral 2 ellipticals 13 irregulars 14 dwarf ellipticals 90% of mass is due to M31 and Milky Way diameter about 10 million ly

15 Nearby Clusters Other nearby clusters are also poor: Sculptor group (d=1.8 Mpc): 6 spiral galaxies M81 group (d=3.1 Mpc): 8 spiral galaxies Centaurus (d=3.5 Mpc): 17 spiral galaxies M101 (d=7.7 Mpc): 5 spiral galaxies Roughly 20 small groups within 15 Mpc Nearest Rich Clusters: Virgo (d = 16 Mpc): 250 large galaxies, 2,000 small galaxies, size = 3 Mpc; M87 in center (which has a mass 3 x M solar )

16 Virgo Cluster

17 Coma Cluster

18 Structure of Galaxy Distribution Coma Cluster (d=100 Mpc): Roughly 10,000 galaxies; Most galaxies are ellipticals One of the largest clusters known: 7 Mpc across Mass roughly 7 x M Solar Superclusters: Clusters of clusters Combined mass of M solar Size roughly 40 Mpc 100 Mpc Milky Way in Local supercluster (Virgo supercluster) Voids Size roughly 50 Mpc

Hubble s Law z = 0 = 0 1 1929: Hubble discovered that all far-away galaxies

20 Hubble s Law z = 0 = : Hubble discovered that all far-away galaxies are redshifted : The redshift is proportional to the distance to the galaxy

21 Interpretation was that the redshifts are due to Doppler shifts z = s For small redshifts, z v/c 1+v/c 1 v/c Hubble s Law: Recession velocity is proportional to distance to galaxy v = H 0 d 1

22 z = / emit = obs / emit 1= s (1 + v/c) (1 v/c) 1 Note that the relationship between velocity and redshift is not linear for relativistic velocities.

23 Peculiar Velocities Galaxies are moving due to gravitational interactions with other galaxies. Without peculiar velocities, we could easily measure Hubble s constant using nearby galaxies. Typical peculiar velocities are Thus velocities due to Hubble s law dominate when H 0 d >> 100 km/s, or d >> 1 Mpc. v p 100 km/s

24 Hubble s Constant Lots of research went into determining H 0 H 0 = 74 ± 3 km/s/mpc

25 New way to get distance! v = H 0 d d = cz H 0 = km/s/mpc H 0 z Mpc

26 Supercluster Distribution

2dF Survey 1997-2002 Anglo-Australian Observatory

28 2dF Survey Anglo-Australian Observatory 221,414 galaxies with reliable redshifts 1500 square degrees

30 6dF Galaxy Survey 136,304 spectra, 110,256 redshifts (southern sky) 10 times the sky area of the 2dF UK Schmidt Telescope Multi-object fibre spectograph

32 Sloan Digital Sky Survey Dedicated 2.5 m telescope at Apache Point (New Mexico) 2000 present 500 million photometric observations; over 3 million spectra 35% sky coverage

Learning Objectives. distances to objects in our Galaxy and to other galaxies? apparent magnitude key to measuring distances?

Learning Objectives. distances to objects in our Galaxy and to other galaxies? apparent magnitude key to measuring distances? The Distance Ladder Learning Objectives! What is the distance ladder? How do we measure distances to objects in our Galaxy and to other galaxies?! How are the concepts of absolute magnitude and apparent