tokens: the values. The values are taken from Dunlop et al. ( 1993 , 2003 ). 3.1. Evolution of the bolometric luminosity with time To demonstrate how luminous progenitors of UCDs could have been, we compute the bolometric luminosity, , as a function of time. The dependency is shown in Fig. 3 . The benchmark initial mass is . For the cases of the CAN, SAL, and vDC IMF, the values are proportional to . The MKDP IMF is a function of and therefore we plot also for . The main difference between different MKDP IMFs (for different ) is the slope of the luminosity as a function of time. For a more top-heavy IMF, the decrease of the luminosity with time is steeper. Fig. 4 Time evolution of for all IMFs considered (vDC, SAL, CAN, MKDP). We assume that the system is gas free and that neither stars nor remnants are leaving (the dashed lines show the change of the value if only certain fractions of remnants are kept). The upwards-pointing blue arrow demonstrates that if we would assume more massive remnants (e.g. due to implosion directly to a BH without a SN explosion), then this would lead to larger values of . The models are computed for the case of , however, results are mass independent. The only exception is the MKDP IMF, which depends on initial object mass, , and also contains a large fraction of mass in high mass stars, and is plotted for (, bottom to top) and different fractions of remnants retained (100%, 10%, 0%). Left panel : the evolution for [Fe/H] = 2. Right panel : the evolution for [Fe/H] = 0. The grey band indicates the span of the observed present-day values, approximately five to ten for the majority of UCDs ( Mieske et al. 2008 ; Dabringhausen et al. 2009 ). The scales are identical for both panels. There are several noticeable features in this figure: (i) for the MKDP IMF, UCDs with initial stellar mass are as bright as quasars ( Dunlop et al. 1993 , 2003 ; Souchay et al. 2015 ) for the first few yr. We use low-redshift quasars because for these we were able to find bolometric luminosities, but also absolute magnitudes in the and band. We note, however, that high redshift quasars show very comparable luminosities as shown for example by Mortlock et al. (2011) . This is mainly due to the presence of a large number of O and B stars () in the system. Due to stellar evolution and core-collapse supernova explosions, these UCDs will be variable on a timescale of months. As already suggested by Terlevich Boyle (1993) , such objects might be confused with quasars, especially for example in large photometric surveys. (ii) After less than a Myr, a strong degeneracy between the IMF and appears. That is, for different and different IMFs, similar luminosities comparable to those of observed UCDs occur. (iii) The metallicity is a second-order effect. Fig. 5 Values of the ratio as a function of for three different times . The case of the MKDP IMF is not constant since the MKDP IMF is a function of initial mass. The plotted curves assume all remnants are
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