Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 5. The cosmic distance ladder

4. The cosmic distance ladder This lecture: Measuring distances to –things within the solar system –things within the solar neighbourhood –things within the galaxy –nearby galaxies –distant galaxies Slide 2

The problem How big is the galaxy? –and the rest of the Universe? How far away are galaxies –how luminous are they? Fundamental problem Slide 3

Measuring distances on Earth is easy. On galactic scales it’s a bit harder... Hubble’s law relates distance to redshift, but how do you measure Hubbles constant? There is no simple way to measure the distance to anywhere. Different indicators must be used for different distances The ‘distance ladder' Distance indicators Slide 4

The Moon –Laser ranging. –Used to be from Haleakala on Maui. –Apollo landers left reflectors on surface! –+-10cm several years ago. The planets –Radar ranging (usually to Venus) –Use orbital periods and Kepler’s laws to infer distances to other planets. The size of the Solar system is fundamental because... Short range measurements Slide 5

Parallax Original method for measuring distances to moon and planets –two observers on opposite side of Earth. For nearby stars, use opposite points of Earth’s orbit around the sun. –Baseline of 2 AU –Good to 30pc –limited by atmospheric distortions –Bessel, 1830s, 61 Cyg 0.3” Slide 6

Hipparcos HIgh Precision PARallax COllecting Satellite Good out to 500pc –When the catalogue was released, everything moved New Satellite GAIA will do far more, out to the other side of the Galaxy Gaia does other things besides Slide 7

Masers Interstellar gas clouds emit intense microwaves at specific frequencies. Doppler shift gives speed True speed plus proper motion gives distance Maybe 10s of Mpc but new technique Slide 8

Standard Candle Methods Many methods in this class Know absolute luminosity somehow –Apparent magnitude gives distance by inverse square law RR Lyrae variables used like this to find our place in the Galaxy. Distances that can be reached depend on the intrinsic brightness of the standard candle –and the limiting magnitude of your telescope Slide 9

Main Sequence Stars For main sequence stars luminosity and colour are related If you know the spectral type of a main sequence star, the HR diagram gives the luminosity Use inverse square law to get distance Good to 10s of kpc Also known (confusingly) as spectroscopic parallax Slide 10

Cepheids The most famous, canonical standard candle –Period luminosity relationship –know period know luminosity –Good to 30 Mpc RR Lyrae similar but lower luminosity –(around L= 100 L o ) –good to 100 kpc Slide 11

Novae Novae increase in brightness by several magnitudes rapidly (~1 day) and decay slowly. Time to dim by 2 magnitudes is related to absolute magnitude. –Standard candle Good to 300 Mpc Slide 12

Supernovae Type 1a supernova has maximum absolute magnitude M max = Supernovae formed by white dwarfs accreting too much gas Can go several Gpc Slide 13

Tulley-Fisher relation In a spiral galaxy, rotational velocity tends to constant v max at large radius spread in velocities causes doppler broadening of spectral lines  = 2 v max /c Tulley, Fisher: v max  L 1/4 For ellipticals motions are random, but spread in velocities  L 1/4 Called the Faber-Jackson relation Slide 14

Brightest Galaxies: Brightest ellipticals in rich clusters seem to have L = L o Bright so visible to 10 Gpc (pretty distant!) But problems at large distances –Observational bias –Long time ago –age dependence? Slide 15

Overview of the Distance Ladder: Slide 16

Key points Lots of ways to measure distance, but only really 3 classes: –direct (e.g. radar or laser ranging) –geometric - parallax –standard candles Short distance measurements used to calibrate the long distance ones –Interdependency –Errors in calibration mount up. Best indicators are those which are good over a wide range of distances. Slide 17