Spectroscopic aperture biases in inside-out evolving early-type galaxies
  from CALIFA
  Integral field spectroscopy studies based on CALIFA data have recently
revealed the presence of ongoing low-level star formation (SF) in the periphery
of ~10% of local early-type galaxies (ETGs), witnessing a still ongoing
inside-out galaxy growth process. A distinctive property of the nebular
component in these ETGs, classified i+, is a two-radial-zone structure, with
the inner zone displaying LINER emission with a H\alpha equivalent width
EW~1{\AA}, and the outer one (3{\AA}<EW<~20{\AA}) showing HII-region
characteristics. Using CALIFA IFS data, we empirically demonstrate that the
confinement of nebular emission to the galaxy periphery leads to a strong
aperture (or, redshift) bias in spectroscopic single-fiber studies of type i+
ETGs: At low redshift (<~0.45), SDSS spectroscopy is restricted to the inner
(SF-devoid LINER) zone, thereby leading to their erroneous classification as
"retired" galaxies (systems lacking SF and whose faint emission is powered by
pAGB stars). Only at higher z's the SDSS aperture can encompass the outer SF
zone, permitting their unbiased classification as "composite SF/LINER". We also
demonstrate that the principal effect of a decreasing aperture on the
classification of i+ ETGs via standard BPT emission-line ratios consists in a
monotonic up-right shift precisely along the upper-right wing of the "seagull"
distribution. Motivated by these insights, we also investigate theoretically
these biases in aperture-limited studies of inside-out growing galaxies as a
function of z. To this end, we devise a simple model, which involves an
outwardly propagating SF process, that reproduces the radial extent and
two-zone EW distribution of i+ ETGs. By simulating on this model the
spectroscopic SDSS aperture, we find that SDSS studies at z<~1 are
progressively restricted to the inner LINER-zone, and miss an increasingly
large portion of the H\alpha-emitting periphery.
