We investigate the [ 3.6 ] - [ 4.5 ] Spitzer-IRAC colour behaviour of the early-type galaxies of the SAURON survey , a representative sample of 48 nearby ellipticals and lenticulars .
We investigate how this colour , which is unaffected by dust extinction , can be used to constrain the stellar populations in these galaxies .

We find a tight relation between the [ 3.6 ] - [ 4.5 ] colour and effective velocity dispersion , a good mass-indicator in early-type galaxies : ( [ 3.6 ] - [ 4.5 ] ) _ { e } = ( -0.109 \pm 0.007 ) log \sigma _ { e } + ( 0.154 \pm 0.016 ) .
Contrary to other colours in the optical and near-infrared , we find that the colours become bluer for larger galaxies .
The relations are tighter when using the colour inside r _ { e } ( scatter 0.013 mag ) , rather than the much smaller r _ { e } /8 aperture ( scatter 0.023 mag ) , due to the presence of young populations in the central regions .
We also obtain strong correlations between the [ 3.6 ] - [ 4.5 ] colour and 3 strong absorption lines ( H \beta , Mg b and Fe 5015 ) .
Comparing our data with the models of Marigo et al. , which show that more metal rich galaxies are bluer , we can explain our results in a way consistent with results from the optical , by stating that larger galaxies are more metal rich .
The blueing is caused by a strong CO absorption band , whose line strength increases strongly with decreasing temperature and which covers a considerable fraction of the 4.5 \mu m filter .
In galaxies that contain a compact radio source , the [ 3.6 ] - [ 4.5 ] colour is generally slightly redder ( by 0.015 \pm 0.007 mag using the r _ { e } / 8 aperture ) than in the other galaxies , indicating small amounts of either hot dust , non-thermal emission , or young stars near the center .

We find that the large majority of the galaxies show redder colours with increasing radius .
Removing the regions with evidence for young stellar populations ( from the H \beta absorption line ) and interpreting the colour gradients as metallicity gradients , we find that our galaxies are more metal poor going outward .
The radial [ 3.6 ] - [ 4.5 ] gradients correlate very well with the metallicity gradients derived from optical line indices .
We do not find any correlation between the gradients and galaxy mass ; at every mass galaxies display a real range in metallicity gradients .

Consistent with our previous work on line indices , we find a tight relation between local [ 3.6 ] - [ 4.5 ] colour and local escape velocity .
The small scatter from galaxy to galaxy , although not negligible , shows that the amount and distribution of the dark matter relative to the visible light can not be too different from galaxy to galaxy .