ZBLLAC: A Spectroscopic Database of BL Lacertae Objects

This paper describes the database of optical spectra of BL Lacertae (BLL) objects ZBLLAC available at the URL this https URL. At the present date, 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 (2800 Ang). We report here on the architecture of the database and on its web front-end that permits to filter, query and interactively explore the data. We discuss some properties of the objects in the present dataset 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] (3737 Ang) and [O III] (5007 Ang). Finally, we discuss on intervening absorption systems detected in 35 BLLs that allow us to set lower limits to their distance.