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.