Chemical element ratios of SDSS early-type galaxies
  We discuss chemical enrichments of ~4000 SDSS early-type galaxies using as
tracers the element abundance ratios [C/Fe], [N/Fe], [O/Fe], [Mg/Fe], [Ca/Fe]
and [Ti/Fe]. We utilise the stellar population models of absorption line
indices from Thomas, Maraston & Johansson (2011) which are based on the MILES
stellar library. We confirm previous results of increasing age, [Z/H] and
[O/Fe] ratios (most often represented by [{\alpha}/Fe] in the literature) with
velocity dispersion. We further derive identical correlations with velocity
dispersion for the abundance ratios [O/Fe], [Mg/Fe] and [C/Fe], implying that
C/Mg and C/O are close to solar values. This sets a lower limit on the
formation time-scales and star-burst components of early-type galaxies to ~0.4
Gyr, which is the lifetime of a 3Msun star, since the full C enrichment must be
reached. [N/Fe] correlates with velocity dispersion, but offset to lower values
and with a steeper slope compared to the other element ratios. We do not find
any environmental dependencies for the abundances of C and N, contrary to
previous reports in the literature. [Fe/H] does not correlate with velocity
dispersion over the entire parameter range covered, but for fixed age we find a
steep trend for the [Fe/H]-{\sigma} relation. This trend is weaker than the
analogous for total metallicity (which also shows steeper trends at fixed age)
owing to the lower Fe contribution from SN Ia for more massive early-type
galaxies. We find [Ca/Fe] ratios that are close to solar values over the entire
velocity dispersion range covered. Tentative, due to large scatter, the results
for [Ti/Fe] indicate that Ti follows the trends of Ca. This implies a
significant contribution from SN Ia to the enrichment of heavy
{\alpha}-elements and puts strong constraints on supernova nucleosynthesis and
models of galactic chemical evolution.
