Galaxies Island Universes

Copyright – FORS1 VLTI, European Southern Observatory

Ellipticals

Ellipticals Huge No gas. No dust. No young stars. Nothing but old stars. Random orbits.

Spirals Like Milky Way. Disks and bulge. Young stars and old. Gas and dust. Stars forming. Stars dying. M81 and M82 – Copyright R. Gendler

M63 Copyright – S. Miyazaki, Suburu

NGC1365 Copyright – VLT

M33 – Copyright NOAO

Irregulars

Distance to Near Galaxies Need a standard candle. If there is something in a galaxy which has a known luminosity we can determine a distance. Several candidates: Variable stars Supernovae “Brightest” stars

Variables in Clusters M3 Copyright – K. Stanek (Harvard)

Variable Stars For RR Lyrae stars: For Cepheid variables: Average luminosity is a standard candle Always ~ 100 x Sun For Cepheid variables: Pulsation period is proportional to average luminosity Observe the period  find the luminosity Good to 15 Mpc!

Nearby Galaxies Cepheids Period Luminosity  Mv Know mv Get Distance

The Local Group

Groups

The Virgo Cluster

Clusters

Concept Test A standard candle can be any object (or class of object) that: Always has the same luminosity. Has some means of knowing its luminosity without first needing to know its distance. Can vary in brightness (as long as it always has the same average luminosity). Has a known absolute magnitude. Always gives off the same amount of energy, regardless of distance from us.

Distant Galaxies Can’t see individual stars. Supernovae rare. Can use nearby galaxies to get distances to further galaxies. Distance ladder: Parallax  nearby stars Nearby stars  H-R diagram H-R diagram  distant stars (variables) Variable stars  nearby galaxies Nearby galaxies  Hubble’s Law

21cm Radiation Neutral hydrogen (HI) gives off light, l = 21cm. Milky Way HI emission – Copyright J. Dickey

Extragalactic HI Observe HI in other galaxies. Measure wavelength of 21 cm radiation. Doppler Shift: Get velocity away from us.

Hubble’s Law Measure the velocity of every galaxy. Ho = 71 km/s/Mpc Measure the velocity of every galaxy. Nearly all are redshifted. Use Cepheids to measure distances to nearby galaxies. Result: The faster it’s moving, the farther away it is.

Map the Universe v = HoD If you know Ho: 71 km/s/Mpc Measure v Get D Find: Voids Walls Clusters

140 Mpc 70 Mpc

Concept Test Imagine that Cepheid variables were not as luminous as previously thought. As a result, Hubble’s constant would be: Smaller than previously thought. Larger than previously thought. Unchanged since we aren’t changing either the velocity or position of the galaxy. None of the above.

Limits to Hubble’s Law Negative velocity? Galaxy pairs? Clusters? Orbits?

Homework #13 Read: Bennett Ch 22.1 – 22.4 Do Ch22: Problems: 9, 12