Significance of Tully-Fischer (Spirals)
• M(total) = V2rot R/G
• Recall M/L = constant (at least within class – and is
more true across classes for H-band)
• L ~ V2rot R/G
• Also, surface brightness, μ, L/R2 ~ constant
(“Freeman’s Law,” μ(r) = μ0exp(-r/r0))
• Spiral central surface density = 21.65 B-mag arcsec-2
• Together these give L ~ V4, or in terms of Absolute
magnitude, M = -10logV + constant
• Next chapter, Faber-Jackson equivalent for ellipticals