THE IMPACT OF THE [NI] ON ELEMENT ABUNDANCE ESTIMATES
Inappropriate treatments of emission-lines can lead to an unphysical result
As a final remark, it should be noted that the necessary correction for the impact of kinematic broadening can be quite severe in the most massive galaxies
with correspondingly high values for the central stellar velocity dispersion. This occurs, in particular, for the case of indices, such as Fe5015 and Mgb,
that were typically used to estimate the metallicity of the degree of α-enhancement of stellar populations, for which not accounting for
kinematic broadening would lead to an underestimation of their values. When we considered that the MPA-JHU measurements for the Mgb index
could have been over-estimated in the presence of [NI] emission, we note that using the MPA-JHU values could lead to over-estimated values of
the Mgb/Fe5015 ratio, which is a useful gauge of α-enhancement.
To give an example of the potential extent of this problem, we considered two different subsamples:
one comprising spiral galaxies displaying nebular emission, and a second including only quiescent early-type galaxies.
These objects were morphologically selected as described in the following section and, after applying our standard A/N>4 and A/N<1 requirement
for strong lines in order to pick active and quiescent objects (in addition to requiring the detection or not of [NI] emission),
we assembled 311 and 579 objects for spirals and early-type galaxies, respectively.
Adopting an indicative luminosity-weighted age of 2 and 10 Gyr for the stellar populations of these two classes of galaxies,
we compared the position of ours and the MPA-JHU measurements for the Iron and Magnesium sensitive <Fe>(this index is defined in Gorgas et al.(1990) as
<Fe> = (Fe5270 + Fe5335)/2) and Mgb indices to the grids of Thomas et al.(2003) to determine stellar population models of varying metallicities and overabundance of α elements,
such as Magnesium. For the case of passive objects (right panel), the 1 σ contours for the distribution of ours and the MPA-JHU measurements overlapped well and indicated an α enhancement
with respect to the solar values of the [α/Fe] ratio.
On the other hand, for the case of spirals (left panel), not accounting for the presence of the [NI] lines led to an overestimation of the Mgb/<Fe> ratio,
and therefore an abundance of α elements in the central regions of spiral galaxies which, from our database, would seem to have solar abundances.
