From the Blazar Sequence to the Blazar Envelope: Revisiting the
  Relativistic Jet Dichotomy in Radio-loud AGN
  We revisit the concept of a blazar sequence that relates the synchrotron peak
frequency ({\nu}peak) in blazars with synchrotron peak luminosity (Lpeak, in
{\nu}L{\nu}) using a large sample of radio-loud AGN. We present observational
evidence that the blazar sequence is formed from two populations in the
synchrotron {\nu}peak - Lpeak plane, each forming an upper edge to an envelope
of progressively misaligned blazars, and connecting to an adjacent group of
radio galaxies having jets viewed at much larger angles to the line of sight.
When binned by jet kinetic power (Lkin; as measured through a scaling
relationship with extended radio power), we find that radio core dominance
decreases with decreasing synchrotron Lpeak, revealing that sources in the
envelope are generally more misaligned. We find population-based evidence of
velocity gradients in jets at low kinetic powers (~ 10^42-10^44.5 erg/s),
corresponding to FR I radio galaxies and most BL Lacs. These low jet power
'weak jet' sources, thought to exhibit radiatively inefficient accretion, are
distinguished from the population of non-decelerating, low synchrotron-peaking
(LSP) blazars and FR II radio galaxies ('strong' jets) which are thought to
exhibit radiatively efficient accretion. The two-population interpretation
explains the apparent contradiction of the existence of highly core-dominated,
low-power blazars at both low and high synchrotron peak frequencies, and
further implies that most intermediate synchrotron peak (ISP) sources are not
intermediate in intrinsic jet power between LSP and high synchrotron-peaking
(HSP) sources, but are more misaligned versions of HSP sources with similar jet
powers.
