Tracing the vertical composition of disc galaxies through colour gradients

Optical observations of a statistically complete sample of edge-on disc-dominated galaxies are used to study the intrinsic vertical colour gradients in the galactic discs, in order to constrain the effects of population gradients, residual dust extinction and gradients in the galaxies’ metal abundance. For the majority of our sample galaxies, the colours and colour gradients in the range 1.0hz ≤ |z| ≤ 3.0hz most likely reflect the intrinsic galactic properties (where hz is the vertical scale height). It appears that the intrinsic vertical colour gradients are either non-existent, or small and relatively constant as a function of position along the galaxies’ major axes. On average, the earlier-type galaxies exhibit smaller vertical (B − I) gradients than the later types; our results are consistent with the absence of any vertical colour gradient in the discs of the early-type sample galaxies. In most galaxies small-scale variations in the magnitude and even the direction of the vertical gradient are observed: at larger galactocentric distances they generally display redder colours with increasing z height, whereas the opposite is often observed in and near the galactic centres. For a significant fraction of our sample galaxies another mechanism in addition 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 (at least) partially due to residual dust extinction at these z heights, as is also evidenced from the sometimes significant differences between the vertical colour gradients measured on either side of the galactic planes. We suggest that initial vertical metallicity gradients, if any, have likely not been accentuated by accretion or merging events over the lifetimes of our sample galaxies. On the other hand, they may have weakened any existing vertical metallicity gradients, although they also may have left the existing correlations unchanged.