The redshift of BL Lacertae objects
|Figure 1: EXO 00556.4-3838: In this case the spectrum clearly shows
the host galaxy spectral features (Caii 3 934, 3 968, G-band 4 305, and Mgi 5175 absorption lines)
at z = 0.302. Absorption features in our galaxy are labelled by DIB (diffuse interstellar band).
BL Lacs are radio loud active galactic nuclei (AGN) in which the relativistic jet points toward the direction of the observer,
so the continuum emission is significantly enhanced while the line equivalent
width is depressed. These effects in several cases hinder the determination of their distance and
the assessment of the properties of the class.
BL Lac objects thus offer one of the best opportunities to study relativistic jets
from the radio- band to 100 MeV gamma-rays. The atmospheric Cherenkov technique has demonstrated that
BL Lacs emit also in the TeV band. High signal-to-noise optical spectra of these sources obtained
with the VLT help us to overcome these difficulties.
This allows us to determine new redshifts and to set tight
limits on the distance for pure lineless objects
(Sbarufatti et al. 2005,
Sbarufatti et al. 2006).
Several high S/N spectra of BL Lacs (see example in Figure) were taken and analysed through a new method, based on the properties
of the host galaxies. This method allows to determine significant limits for
objects with featureless optical spectra . Under the assumption that the host galaxy
luminosity is confined into a narrow range (e.g. Sbarufatti et al. 2005) from the EW limits of spectral
features, it is possible to constrain the position of the source on the nucleus-to-host flux ratio (ρ)
vs redshift plane.
The optical spectra of BL Lacs obtained at the VLT have been made available to the astronomical community
through a spectroscopic library ZBLLAC. These web pages
are open to external contributions.
Collaboration: B. Sbarufatti, A. Treves (Insubria),
J. Kotilainen (Tuorla,FIN),
R. Scarpa (ESO)
Falomo et al. (2007), Msngr 129,42;
Sbarufatti et al. (2009), AJ 137,337