On the nature of sodium excess objects. I. data and observed trends

Several studies have reported the presence of sodium excess objects having neutral atomic absorption lines at 5895 Å (Na D) and 8190 Å that are deeper than expected based on stellar population models that match the stellar continuum. The origin of these lines is therefore hotly debated. van Dokkum & Conroy proposed that low-mass stars (≲0.3 M) are more prevalent in massive early-type galaxies, which may lead to a strong Na I 8190 line strength. It is necessary to test this prediction, however, against other prominent optical line indices such as Na D, Mg b, and Fe 5270, which can be measured with a significantly higher signal-to-noise ratio than Na I 8190. We identified a new sample of roughly 1000 Na D excess objects (NEOs; ∼8% of galaxies in the sample) based on Na D line strength in the redshift range 0.00 ≤ z ≤ 0.08 from the Sloan Digital Sky Survey (SDSS) DR7 through detailed analysis of galaxy spectra. We explore the properties of these new objects here. The novelty of this work is that the 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 galaxies with normal Na D line strengths. We note that the majority of galaxies with high velocity dispersions (σe > 250 km s^-1) show Na D excesses. Most late-type NEOs have strong Hβ line strengths and significant emission lines, which are indicative of the presence of young stellar populations. This result implies that the presence of the interstellar medium and/or dust contributes to the increase in Na D line strengths observed for these galaxies, which is in good agreement with the earlier study of Chen et al. who used the Na D line index to study outflow activity in star-forming disk galaxies. In contrast, the majority of early-type NEOs are predominantly luminous and massive systems, which is in agreement with the findings of van Dokkum & Conroy. However, we find that models used to reproduce the Na I 8190 line strengths that adopt a bottom-heavy initial mass function are not able to reproduce the observed Na D line strengths. By comparing the observed Na D, Mg b, and Fe 5270 line strengths with those of the models, we identify a plausible range of parameters that reproduce the observed values. In these models, the majority of early-type NEOs are "α-enhanced" ([α/Fe] ∼ 0.3), "metal-rich" ([Z/H] ∼ 0.3), and, especially, "Na-enhanced" ([Na/Fe] ∼ 0.3). An enhanced Na abundance is a particularly compelling hypothesis for the increase in the strength of the Na D line index in our early-type NEOs that appear devoid of dust, both in their SDSS images and spectra.