The flux ratio of the [N II]$\lambda\lambda$ 6548, 6583 \AA\ lines in sample of Active Galactic Nuclei Type 2

TL;DR

This study measures the flux ratio of [N II] emission lines in 250 Type 2 AGN spectra from SDSS, finding a mean ratio consistent with theoretical predictions, addressing previous measurement challenges.

Contribution

First systematic and precise measurement of the [N II] flux ratio in a large sample of AGNs using SDSS data, confirming theoretical calculations.

Findings

Mean flux ratio of 3.049 ± 0.021 found.

Results agree with relativistic theoretical calculations.

Addresses previous measurement difficulties.

Abstract

In spectra of the Active Galactic Nuclei (AGNs), the [N II] 6548, 6583 A lines are commonly fitted using the fixed intensity ratio of these two lines (R[N II]=I$_{6583}$/I$_{6548}$). However, the used values for fixed intensity ratio are slightly different through literature. There are several theoretical calculations of the transition probabilities which can be used for the line ratio estimation, but there are no experimental measurements of this ratio, since the [N II] lines are extremely weak in laboratory plasma. Therefore, the intensity ratio of [N II] lines can be measured only in the spectra of astrophysical objects. However, precise and systematic measurements have not be done so far, because of difficulties in measurement of the [N II] ratio in various spectra (overlapping with H$\alpha$, weak intensity of [N II], influence of the continuum noise and outflow contribution, etc.). Here we present the measurements of the flux ratio of the [N II]$\lambda\lambda$ 6548, 6583 A emission lines for a sample of 250 Type 2 AGNs spectra taken form Sloan Digital Sky Survey (SDSS) data base. The spectra are chosen to have high signal-to-noise ratio and to [N II] and H$\alpha$ lines do not overlap. The obtained mean flux ratio from measurements is 3.049 $\pm$ 0.021. Our result is in agreement with theoretical result obtained by taking into account the relativistic corrections to the magnetic dipole operator.