MOJAVE: Monitoring of jets in active galactic nuclei with VLBA experiments. I. First-epoch 15 GHz linear polarization images

We present first-epoch, milliarcsecond-scale linear polarization images at 15 GHz of 133 jets associated with active galactic nuclei (AGNs) from the MOJAVE (Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments) survey. MOJAVE is a long-term observational program to study the structure and evolution of relativistic outflows in AGNs. The sample consists of all known AGNs with Galactic latitude vertical bar b vertical bar > 2.degrees 5, J2000.0 declination greater than -20 degrees, and correlated 15 GHz Very Long Baseline Array (VLBA) flux density exceeding 1.5 Jy (2 Jy for sources below the celestial equator) at any epoch during the period 1994-2003. Of the 133 AGNs that satisfy these criteria, 96 are also part of the VLBA 2 cm Survey. Because of strong selection effects, the sample is dominated by blazars with parsec-scale morphologies consisting of a bright core component at the extreme end of a one-sided jet. At least one-third of the cores are completely unresolved on the longest VLBA baselines, indicating brightness temperatures above 1011 K. These cores tend to have electric vectors that are better aligned with the inner jet direction, possibly indicating the presence of a stationary shock near the base of the jet. The linear polarization levels of the cores are generally low (<5%), but many of the extended jet regions display exceedingly high fractional polarizations (>50%) and electric vectors aligned with the jet ridge line, consistent with optically thin emission from transverse shocks. The cores and jets of the radio galaxies show very little or no linear polarization. Both the weak- and strong-lined blazar classes (BL Lac objects and high-polarization radio quasars) show a general increase in fractional polarization with distance down the jet, but the BL Lac jets are generally more polarized and have electric vectors preferentially aligned with the local jet direction. We show that these differences are intrinsic to the jets and not the result of observational biases. We find that distinct features in the jets of gamma-ray-loud (EGRET) blazars are typically twice as luminous as those in non-EGRET blazars and are more highly linearly polarized. These differences can be adequately explained if gamma-ray blazars have higher Doppler-boosting factors, as the result of better alignment with the line of sight and/or higher bulk Lorentz factors.