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The known population of BL Lacs consists of objects which have been selected in radio surveys (RBLs), and those which have been selected in X-ray surveys (XBLs). Both types of objects seem to show apparently different statistical properties such as different rates of cosmological evolution and different continuum spectral properties. We have investigated whether or not selection effects involved in the recognition of BL Lacs originating from surveys at different frequencies can account for these differences.
BL Lacs are found in the nuclei of elliptical galaxies and recognition of the BL Lac is only possible if it is bright enough compared to the host galaxy. Our simulations of this process show that significant numbers of BL Lacs will get missed from existing samples for this reason. One of the consequences of missing objects is that it will produce a redshift dependent cut-off in the BL Lac luminosity function which can mimic cosmological evolution. Our analysis shows that some spurious evolution can arise from this effect but not enough to account for the differences between XBLs and RBLs. Another consequence of having missed objects from the sample is that their spectral properties will also be affected. In fact, the presence of host galaxy light and recognition effects will influence the spectral properties of objects selected at different wavelengths. We have used our recognition analysis to predict the distribution of the ratio of BL Lac to galaxy light in different samples. On average existing X-ray sample will have a much higher starlight fraction, consistent with observations. We can predict the distribution of infrared colours and find that we can reproduce the observational separation between XBLs and RBLs in the infrared luminosity/infrared colour plane.
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