The Optical Identification of Denis Sources

Abstract

During the last decade or so all-sky surveys have been carried out in various wavebands over the electromagnetic spectrum from radio frequencies to gamma rays. In every case an important plank in making full use of the survey has been the provision of optical identifications. For many satellite based all-sky surveys such as IRAS or ROSAT the positional errors have been too large to permit an unambiguous identification on the grounds of position alone. A number of strategies have been adopted to circumvent this problem. For example the colour of plausible candidates is sometimes assumed to narrow down the number of objects in the error box. Alternatively, maximum likelihood methods can be used to select the best candidate for the identification. Clearly, these approaches leave much to be desired, and it is only for surveys with positional accuracy of around an arcsecond that unambiguous identifications can be made. In the case of the DENIS survey this is the position which pertains, and there is thus the opportunity to provide definite classifications for all sufficiently bright objects, and to do so without preconceptions as to the nature of the source. This then opens the way for genuinely new discoveries and insights not based on current received wisdom.