Description of the library

BL Lac objects (BLL) are active nuclei, hosted in massive elliptical galaxies, the emission of which is dominated by a relativistic jet closely aligned with the line of sight. BLL are characterized by luminous, rapidly variable UV-to-NIR non-thermal polarized continuum emission. In the radio band, they appear as compact and strong sources with a flat emission spectrum (for a recent review see Falomo, Pian and Treves 2014 ). A well known characteristic is the manifestation of the superluminal motion from the knots of their relativistic jets. From the spectroscopic point of view, BL Lacs are characterized by quasi featureless optical spectra. In fact their emission is strongly dominated by the non-thermal continuum which arises from the relativistic jet aligned towards the line of sight of the observer. The quasi featureless nature of their optical spectra makes the determination of their redshifts (and, as a consequence of their distance) a very challenging task. This site is aimed to provide an inventory of BL Lacsoptical spectra and their redshift.

Deep imaging studies of BL Lac objects carried out using both ground based and space observatories show that they are hosted in luminous giant ellipticals (see e.g. Urry et al 2000Falomo et al 2000 and references therein). For this reason, in addition to the non thermal emission, there is always a thermal contribution due to the stellar component of the host galaxy (see example in Figure 1) . Sometimes, like in other AGN, some emission lines generated by fluorescence in gas clouds surrounding the central black hole, can be present.

Finally as in the case of QSOs, absorption lines due to intervening gas in the halo of foreground galaxies can be observed in the spectra of BL Lacs (e.g. Stocke & Rector 1997). In this latter case, if the BL Lac object does not exhibits other intrinsic features the intervening absorptions put a lower limit to the redshift of the source.



Simulation of a typical BLL Spectrum

Figure 1: The simulated optical spectrum of a BL Lac object at z =0.1 assumin a nucleus to host ratio (N/H) =2. The observed spectrum (black line) is composed of the non-thermal emission (yellow line) plus the host galaxy spectrum (red line) using an ellitical galaxy template.

In the past decade a number of projects were carried out to derive the redshift of BL Lac objects. Most of these works were based on optical spectra collected with 4 m class telescopes, and are therefore limited by relatively low signal-to-noise ratio, low spectral resolution and limited wavelength range (e.g. Falomo et al. 1993; Stickel & Kuhr 1993; Veron-Cetty & Veron 1993; Bade et al. 1994; Falomo et al. 1994; Falomo 1996; Marcha et al. 1996; Drinkwater et al. 1997; Laurent-Muehleisen et al. 1998; Landt et al. 2001; Rector & Stocke 2001; Londish et al. 2002; Carangelo et al. 2003; Hook et al. 2003).

The detectability of spectral features depends on the signal-to-noise ratio (SNR) and on the brightness of the nuclear source. In fact, during low brightness states, intrinsic absorption features can be more easily revealed, while during high states one can better discover intervening absorption systems. Nevertheless, the strong contribution from the non-thermal continuum, illustrated in the previous paragraph, drastically lowers the equivalent width of the intrinsic spectral features making their detection a challenging task.

Optical spectra of BL Lacs

Spectrum of 1553+113

Figure 2: Flux calibrated (upper panel) and normalized to the continuum (lower panel) spectrum of 1553+113. The spectrum is lacking any intrinsic spectral feature. Absorptions from our galaxy ISM are labeled in green. The telluric absorption are marked.

Spectrum of 2214

Figure 3: Optical spectrum of the BL Lac object (2214-313) in which the features associated to its host galaxy (e.g. CaII absorptions) are clearly detected (z = 0.460).



For all of these reasons, a more systematic work aimed at obtaining high SNR spectra of BL Lacs  has been started up (Sbarufatti et al. 2005; 2006). In particular, exploiting the capabilities of  FORS1 and FORS2  at the 8-m Very Large Telescopes (VLT) of the European Southern Observatory (ESO) spectra for a sample of 69 objects, extracted from the  lists of BL Lacs by Padovani & Giommi the Sedentary Survey, have been secured. The ensemble of these high quality spectroscopic data forms the first collection of homogeneous and high quality spectra of BL Lacs from which a number of properties (in addition to the redshift or a lower limit) can be extracted (see e.g. Sbarufatti et al. 2005; 2006; 2009; Landoni et al 2013). In particular, for the 69 observed BL Lacs we firmly measured the redshift for 23 of them. In more details, for 12 sources we clearly detected the absorption features imprinted in the spectrum by the host galaxy while in 15 sources emission lines have also been measured (broad line such as Mg II of C III] and narrow emission lines such as [O II] 3727 A and/or [O III] 5007,4958 A).

Figure 2 and Figure 3 show examples of optical spectra for two BL Lacs objects . In the first case we report the spectrum of the well known TeV source PG1553+113 obtained with VLT + FORS2. As shown, no intrinsic features are apparent yielding this powerful source with an unknown redshift. In the second case, the absorption features of the host galaxy (CaII H K and the stellar G Band) of 2214-313 are detected with high confidence allowing us to derive a firm value for the redshift of the source.Description of the database

Since the quality of the retrieved data with large telescope is quite high, we promptly developed a public website in order to allow the astrophysical community to access our data for further works and paper. In particular, the database is organized as follow: for each object we give the coordinates at J2000 equinox (RA,DEC), the V band magnitude, the redshift (or a lower limit to it), the spectrum (in pdf and ascii table format) and details on the setup used. In general the best available optical spectrum is linked in the main page of the database while additional spectra are appended and linked in separate pages.

Latest revision on May 8, 2020