Context. The advent of Fermi is changing our understanding on the radio and
gamma-ray emission in Active Galactic Nuclei. Contrary to pre-Fermi ideas, BL
Lac objects are found to be the most abundant emitters in the gamma-ray band.
However, since they are relatively weak radio sources, most of their
parsec-scale structure and their multi-frequency properties are poorly
understood and/or have not been investigated in a systematically fashion. Aims.
Our main goal is to analyze the radio and gamma-ray emission properties of a
sample of 42 BL Lacs selected, for the first time in the literature, with no
constraint on their radio and gamma-ray flux densities/emission. Methods.
Thanks to new Very Long Baseline Array observations at 8 and 15 GHz for the
whole sample, we present here fundamental parameters such as radio flux
densities, spectral index information, and parsec-scale structure. Moreover, we
search for gamma-ray counterparts using data reported in the Second Catalog of
Fermi Gamma-ray sources. Results. Parsec-scale radio emission is observed in
the majority of the sources at both frequencies. Gamma-ray counterparts are
found for 14/42 sources. Conclusions. The comparison between our results in
radio and gamma-ray bands points out the presence of a large number of faint BL
Lacs showing "non classical" properties such as low source compactness, core
dominance, no gamma-ray emission and steep radio spectral indexes. A deeper
multiwavelength analysis will be needed.Comment: 19 pages, 6 figures, 6 tables, accepted for publication in A&
