tween 1500–2800Å, assuming a @salp55 IMF. This would lead to an expected UV colour for our objects of $(m_\mathrm{{F606W}} - m_\mathrm{{F850LP}})=0$ or spectral index $\beta=-2$, more or less independently of redshift (within the $z$ range of our sample), inconsistent with our observations for most objects (see previous subsection, Figure \[fig:SSP\_colours\_AB\] and Table \[tab:colours\]). In other words, most detected host galaxies of our sample have UV colours that are too ‘red’ for a simple continuous star formation scenario, while the stacked host galaxy is roughly consistent. If for the resolved hosts the [F850LP]{} band fluxes were used instead of the [F606W]{} band, SFR values would be higher by roughly a factor of 2.

This apparent inconsistency could be resolved in several ways. The initial assumption could be wrong, and the UV flux originates not from freshly formed stars, but from a passively evolved starburst as outlined in Sect. \[sec:starburst\]. The star formation rate might not have been constant over the past, so that varying amounts of stars of different masses and ages would have been formed; such a configuration is always possible, and our only argument against it would invoke Occam’s razor. Finally, as discussed in the previous section, there could be an underlying older stellar population contributing more to [F850LP]{} and less to [F606W]{} (but note that by sample design, also [F850LP]{} is completely below the Balmer jump at all relevant redshifts). This is the scenario that is favoured by our intermediate redshift data from [[gems]{}]{} [@sanc04a].

Clearly, in this study our current data set of just two UV bands is insufficient to settle this ambiguity. However, in all three cases there would be no continuous star formation as inferred for LBGs. The blue light would be a result not of a continuous process but of one or more events in the past of the host galaxy that triggered star formation. Whether galaxy interaction or merger incidents were responsible or the format