AS1001:Extra-Galactic Astronomy Hubble's Discovery Hubble's Law ...

!
Peculiar Velocities!
• We measure from spectra:
V OBS = c z = c " # " 0
" 0
= H 0 d + V PEC
VIRGO
V OBS
d
Correcting for peculiar velocity!
• MW falls toward a Virgo cluster galaxy:
V pec = -1000 km/s (approaching).
• Distance is d = 50 Mpc (from Cepheids)
• If the redshift, z = 0.01, what is H 0 ?
V OBS = c z = H 0 d + V PEC
H 0 = c z "V PEC
d
It depends on the direction of the peculiar velocity:
! (towards us VPEC < 0, away VPEC > 0.)
= 3000 +1000
H 0 = 80 km/s/Mpc
Note: Take care with the sign of VPEC.
( ) km/s
50 Mpc