The redshift of BL Lacertae objects

 

BL Lac objects (see Falomo et al 2014 for a recent review) are Active Galactic Nuclei (AGN) characterized by luminous, rapidly variable UV-to-NIR non-thermal polarized continuum emission. In the radio band, they appear as a compact and strong sources with a flat emission spectrum. 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 a featureless optical spectra. In fact their emission is strongly dominated by the non-thermal continuum that arises from the relativistic jet aligned towards the observer. The featureless nature of their spectra make the determination of their redshifts (and, as a consequence, their distance) a rather challenging task. Deep imaging studies of this class of AGN demonstrated that BL Lac objects are hosted in luminous giant ellipticals (e.g. Urry et al 2000 ,  Falomo et al 2000). 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. Sometimes, like in other AGN, some emission lines generated by fluorescence in gas clouds surrounding the central black hole, can be present. 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 (Stocke & Rector 1997). In this case, if the BL Lac object does not exhibits intrinsic feature that allow a firm determination of its redshift, the absorber systems put a lower limit to the redshift of the source.

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Several high S/N spectra of BL Lacs (see example in Figure) were taken at VLT 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. 2005Sbarufatti et al. 2006 ) 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.
 
 
 
 
 
 
 
 
Figure 1: The optical spectrum of the BL Lac object EXO 00556.4-3838. 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).

 

 




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.

  

 zbllac home

 


 

People: R. Falomo  

Collaboration: B. Sbarufatti, A. Treves (Insubria, Como), J. Kotilainen (Tuorla,FIN), R. Scarpa (ESO)  M. Landoni (Insubria, Como), A. Sandrinelli (Insubria, Como)

Recent Publications:  Falomo et al. (2007), Msngr 129,42;   Treves, A. &  Falomo, R. Uslenghi, M.  2007 A&A 473 L17Sbarufatti et al. (2009), AJ 137,337  ,  Landoni, M. Falomo, R. Treves, A. et al., 2012  A&A  543 116Landoni et al  2013 AJ....145..114L, Sandrinelli, A. Falomo, R., Treves, A. et al, 2014 , MNRAS 444 1835 (PDF

 

 

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