Abstract
Abstract This paper describes the database of optical spectra of BL Lacertae (BLL) objects (Z BLL objects) available at https://web.oapd.inaf.it/zbllac/. At present, it contains calibrated spectra for 295 BLL. For about 35% of them, we report a firm measure of redshift z, while for 35 sources we set a lower limit on z based on the detection of intervening absorption systems, mainly ascribed to Mg ii (λ2800 Å). We report here on the architecture of the database and on its website front-end that permits us to filter, query, and interactively explore the data. We discuss some properties of the objects in the present data set by giving the distribution of the redshifts and reporting on the detected emission lines, which turn out to be mainly forbidden and ascribed to [O ii] (λ3737 Å) and [O iii] (λ5007 Å). Finally, we discuss on intervening absorption systems detected in 35 BLLs that allow us to set lower limits to their distance.
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