Based on data obtained at the VLT through the ESO program 178.B-0838.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

For the $A_{\rm V}$ -metallicity fit we used the Milky Way extinction curve (Cardelli et al. 1989, $R_{\rm V}=3.1$ ), which is often preferred for the nebular lines.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... points

Note that, to avoid the fit to be dominated by the mass intervals with largest number of objects, we first obtained metallicity medians within mass bins and then fitted the resulting values with Eq. (2).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...Shapley et al. (2001)

Note that Shapley et al. (2001) did not have access to Spitzer-IRAC data, and therefore their masses may be subject to significant uncertainties.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.