Several studies have reported the presence of sodium excess objects that have 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 .
Their origin is therefore hotly debated .
van Dokkum & Conroy proposed that low-mass stars ( \lesssim 0.3 M _ { \odot } ) are more prevalent in massive early-type galaxies , which may lead to a strong Na I 8190 line strength .
It is , however , necessary to test this prediction against other prominent line indices in optical wavelengths such as Na D , Mg b and Fe 5270 , which are measurable with a significantly higher signal-to-noise ratio than Na I 8190 .
We newly identified roughly a thousand Na D excess objects ( NEOs , \sim 8 % of galaxies in the sample ) based on the Na D line strength in the redshift range 0.00 \leqslant z \leqslant 0.08 from the Sloan Digital Sky Survey ( SDSS ) DR7 through detailed analysis of galaxy spectra , and then explored their properties .
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 in terms of Na D line strength .
Note that the majority of galaxies with high velocity dispersion ( \sigma _ { e } > 250 km s ^ { -1 } ) show Na D excess .
Most late-type NEOs have strong H \beta line strengths and significant emission lines , which are indicative of the presence of young stellar populations .
This implies that the presence of interstellar medium ( ISM ) 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 ( IMF ) 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 “ \alpha -enhanced ” ( [ \alpha /Fe ] \sim 0.3 ) , “ metal-rich ” ( [ Z/H ] \sim 0.3 ) and especially “ Na-enhanced ” ( [ Na/Fe ] \sim 0.3 ) .
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 .