The Physical Basis of the Tully-Fisher Relation (con’d)

• Ld µ (Md/Ld) -2 G Vmax 4 / I(0)
• make a number of “standard” assumptions:
• G ~ constant
• Md/Ld ~ constant, or increases slowly to early type
• I(0) ~ constant (Freeman’s law)

• ® Ld µ Vmax 4 (TF relation)

• In practice, none of the assumptions apply…

• all parameters exhibit mild dependencies on vMAX

• Ld µ Vmax n

Notes: