Lick-index entanglement and biased diagnostic of stellar populations in
  galaxies
  The Lick-index spectrophotometric system is investigated in its inherent
statistical and operational properties to ease a more appropriate use for
astrophysical studies. Non-Gaussian effects in the index standardization
procedure suggest that a minimum S/N ratio has to be reached by spectral data,
such as S/N >= 5 px^{-1} for a spectral resolution R~2000. In addition, index
(re-)definition in terms of narrow-band "color" should be preferred over the
classical pseudo-equivalent width scheme. The overlapping wavelength range
among different indices is also an issue, as it may lead the latter ones to
correlate, beyond any strictly physical relationship. The nested configuration
of the Fe5335, Fe5270 indices, and the so-called "Mg complex" (including Mg1,
Mg2 and Mgb) is analysed, in this context, by assessing the implied bias when
joining entangled features into "global" diagnostic meta-indices, like the
perused [MgFe] metallicity tracer. The perturbing effect of [OIII](5007) and
[NI](5199) forbidden gas emission on Fe5015 and Mgb absorption features is
considered, and an updated correction scheme is proposed when using
[OIII](5007) as a proxy to appraise Hbeta residual emission. When applied to
present-day elliptical galaxy population, the revised Hbeta scale leads, on
average, to 20-30% younger age estimates. Finally, the misleading role of the
christening element in Lick-based chemical analyses is illustrated for the
striking case of Fe4531. In fact, while Iron is nominally the main contributor
to the observed feature in high-resolution spectra, we have shown that the
Fe4531 index actually maximizes its responsiveness to Titanium abundance.
