The jet collimation profile at high resolution in BL Lacertae

Controversial studies on the jet collimation profile of BL Lacertae (BL Lac), eponymous blazar Lac objects class, complicate scenario in this already puzzling class objects. Understanding geometry, connection with kinematics and physical conditions surrounding medium, is fundamental to better constrain formation, acceleration mechanisms extragalactic jets. With aim investigating geometry innermost regions Lac, solving controversy, we explore radio source, using high resolution millimeter-wave VLBI data. We collect 86GHz GMVA 43GHz VLBA data obtain stacked images that use infer by means two comparable methods. analyze at 86GHz, discuss it context expansion. Finally consider a possible implication Bondi sphere shaping different expanding region observed along jet. found expands an overall conical geometry. A higher rate between ~5 10 pc (de-projected) from black hole. Such associated decrease brightness usually high-frequency Lac. The retrieves original expansion around 17 pc, where presence recollimation shock supported both 15GHz (MOJAVE survey). change occurring could be external pressure correspondence radius (~3.3X10$^5$$R_s$).