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2.1.4. Mass-to-Light Ratio of the Disk Component

One way to estimate the mass-to-light ratio of the disk stars is to use estimates of the mass and luminosity column densities from surveys of stars and remnants in the solar neighborhood. Commonly used values are SigmaM appeq 45 Msun pc-2 (stars and stellar remnants) and SigmaLV appeq 15 Lsun pc-2 (Binney & Tremaine 1987; Bahcall & Soneira 1980). GBF find that the mass column density must be reduced to SigmaM appeq 27.3 Msun pc-2 because of the turnover of the luminosity function towards the faint end, as noted above. With this new value we have M/LV appeq 1.82 or M/LB appeq 1.50 for the disk with B - V appeq 0.44 (de Vaucouleurs & Pence 1978). This is not very different from the early estimate by Mihalas & Binney (1981), M/LB appeq 1.2.

The solar neighborhood value may be compared with M/LB from population synthesis calculations. The Tinsley (1981) 10-Gyr model with a constant star formation rate gives M/LB = 1.2 after a 20% upward correction in the mass integral of the IMF to match GBF. Shimasaku & Fukugita's (1997) gas infall model (with effective disk age approx 10 Gyr) gives M/LB = 1.9 after a 40% reduction in M to correct the IMF. From a study of rotation curves for spiral galaxies, Persic and Salucci (1992) find M/Lb = 1.24h. We conclude that there is reasonably small scatter among these different approaches to the disk mass-to-light ratio, and we adopt

Equation 8 (8)

There is good evidence that stars in gas-rich irregular (Im) galaxies are young. (For reviews see Fukugita, Hogan, & Peebles 1996; Ellis 1997). If the mean age of irregular galaxy stars is approx 5 Gyr, the B luminosity is 0.5 mag brighter and the mass in star remnants is about 10% smaller than for a 10 Gyr age disk. These corrections reduce M/L in equation (8) to M/LB = 1.1 for irregular galaxies. A population synthesis model for 8 Gyr age (Shimasaku & Fukugita 1997) also gives M/LB = 1.1 after the correction to the IMF. We take

Equation 9 (9)

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