A fit to the Johnson U data gives (
We assume that the contribution to the scatter from the measurement errors is the same in the different passbands. This is supported by the internal comparison of the photometry: we found an rms scatter of for Gunn r and for Johnson B (Table , p. ). This implies that the intrinsic scatter in the three passbands is approximately the same. This is also what JFK96 found for the passbands Gunn r, Gunn g, Johnson B, and Johnson U. As JFK96 noted, one consequence of this is that intrinsic dust absorption in the galaxies cannot be a major source of the intrinsic FP scatter, since dust absorption depends on wavelength.
Another consequence is related to the question whether the
intrinsic FP scatter is produced by an age scatter or a metallicity scatter.
If the galaxies have similar structure,
we can translate the FP scatter
into a scatter in the
(cf. Sect. ),
where b can be calculated from
The scatter in the quantity
(i.e. the zero point in the
0.127 dex in Gunn r and Johnson B,
0.075 dex in Johnson U, and
0.103 dex in Gunn r selected as Johnson U.
This includes scatter from the measurement errors.
The typical measurement error on
is 0.074 for
the 45 HydraI galaxies with Gunn r and Johnson B photometry,
and 0.068 for the 19 HydraI galaxies with Johnson U photometry.
This is based on the values in
Table , p. ,
and corrected for the fact that the 19 galaxies have lower
uncertainties than all the 45 galaxies.
As an approximation, we estimate the intrinsic scatter in the
relation by subtracting in quadrature
the typical measurement error on
0.103 dex (24%) in Gunn r and Johnson B,
0.020 dex (5%) in Johnson U, and
0.078 dex (18%) in Gunn r selected as Johnson U.
By means of stellar population models,
we can translate the scatter in
into a scatter in age or metallicity.
the models by Vazdekis et al. (1996)
with a bi-modal IMF with high mass slope
can for ages > 5 Gyr be well approximated by
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (firstname.lastname@example.org)