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