On the nature of sodium excess objects. I. Data and observed trends
  Several studies have reported the presence of sodium excess objects that have
neutral atomic absorption lines at 5895A (NaD) and 8190A that are deeper than
expected based on stellar population models. van Dokkum & Conroy proposed that
low-mass stars are more prevalent in massive early-type galaxies, which may
lead to a strong NaI8190 line strength. It is, however, necessary to test this
prediction against other prominent line indices in optical wavelengths. We
newly identified roughly a thousand NaD excess objects (NEOs) based on the NaD
line strength in the redshift range 0.00<z<0.08 from the SDSS DR7. The novelty
of this work is that galaxies were carefully identified through direct visual
inspection of SDSS images, and we systematically compared the properties of
NEOs and those of a control sample of normal galaxies. Note that the majority
of galaxies with high velocity dispersion (>250km/s) show NaD excess. Most
late-type NEOs have strong Hb line strengths and significant emission lines.
This implies that the presence of ISM and/or dust contributes to the increase
in NaD line strengths observed for these galaxies. In contrast, the majority of
early-type NEOs are predominantly luminous and massive systems. However, we
find that models used to reproduce the NaI8190 line strengths that adopt a
bottom-heavy IMF are not able to reproduce the observed NaD line strengths. By
comparing the observed NaD, Mgb and Fe5270 line strengths with those of the
models, we identify a plausible range of parameters. In these models, the
majority of early-type NEOs are alpha-enhanced ([a/Fe]~0.3), metal-rich
([Z/H]~0.3) and especially Na-enhanced ([Na/Fe]~0.3). Enhanced Na abundance is
a particularly compelling hypothesis for the increase in the strength of the
NaD line index in our early-type NEOs that appear devoid of dust, both in their
SDSS images and spectra.
