Elements Abundance Estimates

:: The Impact of [NI]λ5198, 5200 Doublets on Elements Abundance Estimates

Here’s a dramatic example of elements abundance estimates. we considered two different sub-samples: 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.



Figure : Effect of [NI] emission on the Mgb absorption-line index and connected estimates of the α-element abundance. The left panel shows a comparison of the position of a sample of spiral galaxies with strong nebular emission (shown by the 1 σ contour level of the data) in the Mgb vs. <Fe> diagram with the model grids of Thomas et al.(2003). The solid and dashed contours indicate the distribution of the data from our catalogue and from that of the MPA-JHU database. Similarly, the right panel shows the same quantities, but for a sample of quiescent early-type galaxies. The model grids assume a luminosity-weighted mean age of 2 Gyr and 10 Gyr for the spirals and early-type galaxies, respectively.