The Mass‐to‐Light Ratio of Binary Galaxies

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Abstract
We report on the mass-to-light ratio determination based on a newly selected binary galaxy sample, which includes a large number of pairs whose separations exceed a few hundred kpc. The probability distributions of the projected separation and the velocity difference have been calculated considering the contamination of optical pairs, and the mass-to-light (M/L) ratio has been determined based on the maximum likelihood method. The best estimate of the M/L in the B band for 57 pairs is found to be 28-36 depending on the orbital parameters and the distribution of optical pairs (solar unit: H0=50 km s-1 Mpc-1). The best estimate of the M/L for 30 pure spiral pairs is found to be 12-16. These results are relatively smaller than those obtained in previous studies but are consistent with each other within the errors. Although the number of pairs with large separation is significantly increased compared with previous samples, the M/L does not show any tendency of increase but is found to be almost independent of the separation of pairs beyond 100 kpc. The constancy of the M/L beyond 100 kpc may indicate that the typical halo size of spiral galaxies is less than ~100 kpc.
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