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 
M 
45 M
pc-2 (stars and stellar remnants) and LV
15 L
pc-2
(Binney & Tremaine 1987;
Bahcall & Soneira 1980).
GBF find that the
mass column density must be reduced
to M 27.3 M
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
1.82 or M/LB
1.50 for the disk with B
- V 0.44
(de Vaucouleurs & Pence
1978).
This is not very different from the early estimate by
Mihalas & Binney (1981),
M/LB 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 
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
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
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
