The Milky Way, Local Galaxies, and the Infrared Tully‐Fisher Relation

Using the near infrared fluxes of local galaxies derived from COBE/DIRBEJ(1.25 $\mu$m) K (2.2 $\mu$m) \& L (3.5 $\mu$m) band maps and published Cepheiddistances, we construct Tully-Fisher (TF) diagrams for the nearby galaxies. Themeasured dispersions in these luminosity-linewidth diagrams are remarkablysmall: $\sigma_J = 0.09$ magnitudes, $\sigma_K = 0.13$ magnitudes, and$\sigma_L = 0.20$ magnitudes. For the J and K bands, Monte Carlo simulationsgive a 95\% confidence interval upper limit on the true scatter in the T-Fdiagram of $\sigma_J \le 0.35$ and $\sigma_K \le 0.45$. We determine MilkyWay's luminosity and place it in the T-F diagram by fitting a bar plus``standard'' values of its size and circular speed (Sun-Galactic centerdistance $R_0 = 8.5 \kpc$ and $\Theta_0 =220 \kms$), the Milky Way lies within $1.5 \sigma$ of the TF relations. We can use the TF relation and the Cepheiddistances to nearby bright galaxies to constrain $R_0$ and $\Theta_0$.Alternatively, we can fix the parameters of the Galaxy to their standardvalues, ignore the Cepheid zero-point, and use the Tully-Fisher relation todetermine the Hubble Constant directly: $H_0 = 66 \pm 12$ km/s/Mpc.