Spectrum of AGN

The AGN spectrum model consists of four components, each corresponding to a physical origin:

  • Broad Line Region (BLR),

  • Narrow Line Region (NLR),

  • Power-law continuum,

  • Fe II emission blends.

These components are combined to construct the final AGN spectrum.

Broad Line Region (BLR)

The BLR model generates broad Balmer emission lines (Hε to Hα) based on fixed line ratios from Ilić et al. (2006). The key parameters are:

  • hbeta_flux: total flux of the Hβ broad line,

  • hbeta_fwhm: FWHM of the broad line profile (Gaussian),

  • vel: systemic velocity for redshift,

  • ebv: dust extinction using the Calzetti law.

The emission lines are constructed on a fine rest-frame wavelength grid, broadened using Gaussian profiles, scaled by the total Hβ flux, and attenuated by dust extinction. A redshift is applied based on input velocity.

Narrow Line Region (NLR)

The NLR model uses precomputed flux ratios from photoionization models of Feltre et al. (2016), calculated with Cloudy.

Key parameters:

  • halpha: total flux of narrow Hα line,

  • logz: gas-phase metallicity in \(\log(Z/Z_\odot)\) used to select flux ratios,

  • vdisp: velocity dispersion of ionized gas for line broadening,

  • vel: systemic velocity for redshift,

  • ebv: dust extinction.

The emission line spectrum is scaled by Hα flux, broadened by convolution with Gaussian profiles, and corrected for extinction and redshift.

Power-law Continuum

The AGN continuum is modeled as a power-law:

\[F_{\rm PL}(\lambda) \propto \lambda^{\alpha}\]

where:

  • alpha: the power-law slope (default: -1.5),

  • m5100: AB magnitude at rest-frame 5100 Å for normalization,

  • vel: systemic velocity,

  • ebv: dust extinction.

The rest-frame spectrum is normalized using the SDSS-like narrow filter centered at 5100 Å, and then redshifted and dust-corrected.

Fe II Emission Blends

The Fe II pseudo-continuum is generated using the empirical template from Park et al. (2022), covering the range:

\[\lambda \in [4000\,\text{\AA}, 5600\,\text{\AA}]\]

The Fe II strength is set by the ratio:

\[R_{4570} = \frac{F_{\rm FeII}(4334{-}4684)}{F_{\rm H\beta}}\]

where:

  • hbeta_broad: total flux of broad Hβ,

  • r4570: input Fe II/Hβ ratio (default: 0.4),

  • vel: redshift velocity,

  • ebv: dust extinction.

The template is scaled by the desired Fe II flux, then dust-corrected and shifted to the observed frame.

Output

The final AGN spectrum is generated by summing the fluxes from the BLR, NLR, power-law continuum, and Fe II components, all interpolated to the target output wavelength grid defined by config.wave.

Each component can be independently toggled or scaled to simulate different AGN types (e.g., type 1 vs. type 2).