NH and Mg Index Trends in Elliptical Galaxies
  We examine the spectrum in the vicinity of the NH3360 index of Davidge &
Clark (1994), which was defined to measure the NH absorption around 3360 \AA
and which shows almost no trend with velocity dispersion (Toloba et al. 2009),
unlike other N- sensitive indices, which show a strong trend (Graves et al.
2007). Computing the effect of individual elements on the integrated spectrum
with synthetic stellar population integrated spectra, we find that, while being
well correlated with nitrogen abundance, NH3360 is almost equally well
anti-correlated with Mg abundance. This prompts the definition of two new
indices, Mg3334, which is mostly sensitive to magnesium, and NH3375, which is
mostly sensitive to nitrogen. Rather surprisingly, we find that the new NH3375
index shows a trend versus optical absorption feature indices that is as
shallow as the NH3360 index. We hypothesize that the lack of a strong index
trend in these near-UV indices is due to the presence of an old metal-poor
component of the galactic population. Comparison of observed index trends and
those predicted by models shows that a modest fraction of an old, metal-poor
stellar population could easily account for the observed flat trend in these
near-UV indices, while still allowing substantial N abundance increase in the
larger galaxies.
