Genuine Retrieval of the AGN Host Stellar Population (GRAHSP)

Abstract

The assembly and co-evolution of supermassive black holes (SMBH) and their host galaxy stellar population is a key open questions in galaxy evolution. Stellar mass ($M_\star$) and star formation rate (SFR), are inferred by modeling the spectral energy distribution (SED). For galaxies triggering SMBH activity, the active galactic nucleus (AGN) contaminates the light at all wavelengths, hampering the inference of galaxy parameters. Incomplete AGN templates can lead to systematic overestimates of the stellar mass, biasing our understanding of AGN-galaxy co-evolution. This challenge has gained further impetus with the advent of sensitive wide-area surveys with millions of luminous AGN, including by eROSITA, Euclid and LSST. We aim to estimate the accuracy and bias of AGN host galaxy parameters and improve upon existing techniques. This work makes two contributions: 1) a new SED fitting code, GRAHSP, with a flexible, empirically motivated AGN model including a power law continuum emission lines, a FeII forest and a flexible infrared torus. We verify that our model reproduces published X-ray to infrared SEDs of AGN to better than 20\% accuracy. A fully Bayesian fit with nested sampling includes uncertainties in the model and the data, making the inference highly robust. 2) we created a benchmark photometric dataset where pure quasars are merged with non-AGN pure galaxies into a hybrid (Chimera) object but with known galaxy and AGN properties. Comparing the true and retrieved $M_\star$, SFR and AGN luminosities shows that previous codes systematically over-estimate $M_\star$ and SFR by 0.5 dex with a wide scatter of 0.7 dex, at AGN luminosities above 10^44 erg/s. In contrast, GRAHSP shows no bias on $M_\star$ and SFR. GRAHSP also estimates more realistic uncertainties. GRAHSP enables characterization of the environmental conditions conducive to black hole growth. (abridged)

Figures (35)

• Figure 1: Overview of how the individual model components contribute to the summed emission (black). The AGN power-law continuum (blue; accretion disk, or BBB) is enhanced by emission lines including an iron forest (red). The disk model is normalised at the monochromatic luminosity at 5100$\,\angstrom$ (blue square), here $L_\mathrm{AGN}^{5100\angstrom}=\qty{e44}{\erg\per\second}=\qty{e37}{\watt}$. The torus (yellow dashed curve) typically dominates the disk continuum above approximately $1µm$. The torus is normalised by the luminosity ratio 5100$\,\angstrom$ to 12$µm$ (yellow diamond). The galaxy components include a stellar population (purple), nebular emission lines (gray at the bottom), and galaxy dust emission (green at the bottom right).
• Figure 2: Detailed view of the optical continuum and emission lines. The model continuum bending power-law (yellow dashed line) reproduces the polarization measurements of Kishimoto2008 (dark blue data points). The full model including a torus and emission lines is shown in solid lines. Variations of the power-law slope (light grey to black) reproduce SDSS steep and flat unabsorbed spectra from Richards2006 and Selsing2016XShooter (red, blue and pink lines). Towards the infrared, the torus component dominates the continuum.
• Figure 3: Overview of the AGN model parameters and how they configure the spectrum, shown here in $L_\lambda$ with arbitrary units. The power law (blue) is normalised at $5100\unit{\angstrom}$, where it has a power law slope of $\beta$. The power law bends over at $\lambda_\mathrm{bend}$ towards the UV, where it has slope $\beta_{UV}$. The width of this transition is set by $W_\mathrm{bend}$. Emission lines with FWHM $W_\mathrm{lines}$ and an FeII template are added and can be further scaled by $A_\mathrm{lines}$ and $A_\mathrm{FeII}$, respectively. The torus component (dark yellow) is normalised by the ratio of $12\mu{}m$ and $5100\unit{\angstrom}$$\lambda L_\lambda$ luminosities, $f_\mathrm{cov}$ (see \ref{['eq:fcov']}). It consists of the sum of two log-quadratic curves, with width $W_\mathrm{cool}$ and $W_\mathrm{hot}$ and location $\lambda_\mathrm{cool}$ and $\lambda_\mathrm{hot}$. The peak-to-peak $\lambda L_\lambda$ ratio is set by $f_\mathrm{hot}$ (see \ref{['eq:fhot']}). The depth of the Si feature, in emission if positive or in absorption if negative (here: -1), is set by Si. The flexibility of these 15 parameters is restricted in \ref{['subsec:calibration']}.
• Figure 4: Example galaxy SEDs of stellar populations with different star formation histories. The star formation rate (see inset) rises linearly towards the present (right), with an exponential cutoff timescale $\tau$. At low $\tau$, the yellow SFR curve truncates quickly, i.e., is dominated by old stars. The corresponding yellow SED in the main panel peaks between $0.3-3\unit{\micro\meter}$. The blue curve in the inset corresponds to continuously rising star formation. The corresponding blue SED in the main panel is dominated by luminous young stars, nebular emission lines and infrared dust emission. Minimal attenuation, E(B-V)=0.01, is applied.
• Figure 5: Effect of attenuation on the galaxy model. Models are shown from intrinsic (dark blue) to strongly attenuated (dark red). For illustration, the extremely attenuated local low-metallicity star-bursting galaxy Haro 11 from Lyu2016 is overplotted as a dashed red curve.