Direct Abundance Determination of Neon

Sep 16, 2021 · 3 min read
projects

Read the full published paper here

This research bridges the gap between optical and infrared observational datasets. By leveraging multi-wavelength emission lines, this project bypasses traditional temperature-dependent optical limitations, allowing for a more robust and direct measurement of Neon abundances within the complex environments of Seyfert 2 AGNs.

Interactive Neon Abundance Simulator

Explore the temperature dependence of optical derivations versus infrared derivations.

Interactive Figure 5: Nebular Structure Profiles

The interactive plot below reproduces the cloudy photoionization models from our 2021 publication. It compares the internal gas structure of a cloud ionized by an AGN (red solid line, hard X-ray power-law) versus a star-forming H ii region (blue dashed line, stellar SED) across a normalized radius ($R/R_{\rm e}$).

Because the axes are normalized to the total radius of the cloud ($R_{\rm e}$), the physical differences between a Strömgren sphere (H ii) and an extended Partially Ionized Zone (AGN) become starkly apparent. By plotting the fractional abundances and electron temperature, this figure demonstrates exactly why traditional H ii region assumptions fail when applied to AGNs:

  • Bottom Panel (Electron Temperature, $T_{\rm e}$): The AGN model exhibits a very distinct temperature distribution compared to the H ii region, showing a stronger decrease with the radius. The AGN temperature curve doesn’t just rise at the inner radius. The AGN temperature actually starts from a peak of nearly $1.4 \times 10^4$ K at the inner radius, dips down in the middle zone and intersects the H ii region temperature model curve at $\sim$$0.88 \times 10^4$ K, and finally drops to $\sim$$0.4 \times 10^4$ K where it intersects once again with the H ii region temperature model curve at the outer region. In stark contrast, the H ii temperature remains a completely flat $\sim$$0.85 \times 10^4$ K.
  • Middle & Top Panels (O²⁺/O and Ne²⁺/Ne): In the H ii region model, both ionic ratios show similar distributions along the radius, confirming the standard assumption that $T_{\rm e}$(O iii) $\approx$ $T_{\rm e}$(Ne iii). In the AGN model, however, the Ne²⁺/Ne ionic abundance extends significantly further into the outer nebular radius (where the temperature is lower) in comparison to the O²⁺/O abundance. While O²⁺ drops off rapidly, Ne²⁺ persists throughout the cooler regions of the cloud.
  • The Critical AGN Deviation: In the AGN model, the Ne²⁺/Ne ionic abundance extends to an outer nebular radius (where the temperature is lower) in comparison to the O²⁺/O abundance. Because Ne²⁺ exists in a distinct, cooler outer region compared to O²⁺, the assumption that $T_{\rm e}$(O iii) $\approx$ $T_{\rm e}$(Ne iii) is strictly invalid for AGNs. Furthermore, this structural difference clearly indicates that the standard assumption used in H ii regions, (Ne²⁺/O²⁺) = (Ne/O), cannot be applied to AGNs.
Seyfert Galaxies Infrared Astronomy Extragalactic Astrophysics
Authors
Mark Armah (He/Him)
Postdoctoral Fellow
I am an extragalactic astrophysicist specializing in the chemical evolution of Active Galactic Nuclei (AGNs) and Seyfert galaxies. My research addresses a fundamental question in galaxy evolution: how do actively accreting supermassive black holes regulate the chemical and physical properties of their host environments? By bridging the gap between intricate multi-dimensional spectroscopic datasets and advanced theoretical photoionization models, I develop custom computational pipelines to untangle AGN radiation from true gas-phase abundances. Ultimately, my work pioneers robust new metallicity calibrations and diagnostic tools that allow wider applications to decode complex AGN feedback mechanisms.