Stellar Mass‐to‐Light Ratios and the Tully‐Fisher Relation

We have used a suite of simplified spectrophotometric spiral galaxy evolutionmodels to argue that there are substantial variations in stellar mass-to-lightratios (M/Ls) within and among galaxies, amounting to factors of between 3 and7 in the optical, and factors of 2 in the near-infrared. Under the assumptionof a universal spiral galaxy IMF, our models show a strong and robustcorrelation between stellar M/L and the optical colors of the integratedstellar populations. Using observed maximum disk M/Ls we conclude that aSalpeter Initial Mass Function (IMF) has too many low-mass stars per unitluminosity, but that an IMF similar to the Salpeter IMF at the high-mass endwith less low-mass stars (giving stellar M/Ls 30% or more lower than theSalpeter value) is consistent with the maximum disk constraints. We apply themodel trends in stellar M/L with color to the Tully-Fisher (TF) relation. Wefind that the stellar mass TF relation is relatively steep and has modestscatter, and is independent of the passband and color used to derive thestellar masses. The difference in slope between the optical (especially blue)and near-infrared TF relations is due to the combined effects of dustattenuation and stellar M/L variations with galaxy mass. We find that thebaryonic TF relation has a slope of 3.5 (with random and systematic errors of+/- 0.2 each). Since we have normalized the stellar M/L to be as high as canpossibly be allowed by maximum disk constraints, the slope of the baryonic TFrelation will be somewhat shallower than 3.5 if all disks are substantiallysub-maximal. [Abridged]Comment: 17 pages; 10 embedded postscript figures; to appear in the Astrophysical Journal on March 20 200