Tracing the vertical composition of disc galaxies through colour gradients

Optical observations of a statistically complete sample of edge-on disc-dom inated galaxies are used to study the intrinsic vertical colour gradients i n the galactic discs, in order to constrain the effects of population gradi ents, residual dust extinction and gradients in the metal abundance of the galaxies. For the majority of our sample galaxies, the colours and colour g radients in the range 1.0h(z) less than or equal to \z\ less than or equal to 3.0h(z) most likely reflect the intrinsic galactic properties (where h(z ) is the vertical scaleheight). It appears that the intrinsic vertical colour gradients are either non-exis tent, or small and relatively constant as a function of position along the major axes of the galaxies. On average, the earlier-type galaxies exhibit s maller vertical (B - I) gradients than the later types; our results are con sistent with the absence of any vertical colour gradient in the discs of th e early-type sample galaxies. In most galaxies small-scale variations in th e magnitude and even the direction of the vertical gradient are observed: a t larger galactocentric distances they generally display redder colours wit h increasing z height, whereas the opposite is often observed in and near t he galactic centres. For a significant fraction of our sample galaxies another mechanism in addi tion to the effects of stellar population gradients is required to explain the magnitude of the observed gradients. The non-zero colour gradients in a significant fraction of our sample galaxies are likely to be (at least) pa rtially due to residual dust extinction at these z heights, as is also evid enced from the sometimes significant differences between the vertical colou r gradients measured on either side of the galactic planes. We suggest that initial vertical metallicity gradients, if any, have probab ly not been accentuated by accretion or merging events over the lifetimes o f our sample galaxies. On the other hand, they may have weakened any existi ng vertical metallicity gradients, although they also may have left the exi sting correlations unchanged.