Authors: Alister W. Graham
Abstract:
The best-fitting 2-dimensional plane within the 3-dimensional
space of spiral galaxy disk observables (rotational velocity vrot,
central disk surface brightness μ0 = -2.5log I0,
and disk scale-length h) has been constructed.
Applying a three-dimensional bisector method of regression analysis
to a sample of ~100 spiral galaxy disks that span more than four
mag arcsec-2 in central disk surface brightness yields
vrot ∝ I00.50±0.05h0.77±0.07 (B-band)
and
vrot ∝ I00.43±0.04h0.69±0.07 (R-band).
Contrary to popular belief, these results suggest that in the B-band,
the dynamical mass-to-light ratio (within 4 disk scale-lengths) is largely
independent of surface brightness, varying as
I00.00±0.10h0.54±0.14.
Consistent results were obtained when the expanse of the
analysis was truncated by excluding the low surface brightness galaxies.
Previous claims that M/LB varies with I0,B-1/2
are shown to be misleading and/or due to galaxy selection effects — not
all low-surface-brightness disk galaxies are dark matter
dominated.
The situation is however different in the near-infrared where
LK' ∝ v4 and M/LK' is shown to vary as
I0,K'-1/2.
Theoretical studies of spiral galaxy disks should therefore not assume
a constant M/L ratio within any given passband.
The B-band dynamical mass-to-light ratio (within 4 disk scale-lengths)
has no obvious correlation with (B-R) disk colour, while in the
K'-band it varies as -1.25±0.28 (B-R).
Combining the present observational data with recent galaxy model
predictions implies that the logarithm of the stellar-to-dynamical
mass ratio is not a constant value, but increases as disks become
redder, varying as 1.70±0.28(B-R).