SDSS-V Black Hole Mapper: The Index Diagram as a tool to disentangle the influence of the Host Galaxy in Quasar spectra

TL;DR

This work develops an Index Diagram to disentangle host-galaxy stellar contamination from AGN emission in SDSS-V BHM quasar spectra, enabling accurate recovery of Quasar Main Sequence parameters. By combining a color–redshift diagnostic with the H-Hβ index diagram, the authors classify spectra into HG-dominated, intermediate, and AGN-dominated groups, then perform stellar subtraction and emission-line modeling to extract QMS components. The results show HG-dominated sources preferentially occupy Population B with broad Hβ and weak Fe II, while INT and AGND span the QMS more broadly, and crosswavelength analyses reveal systematic trends in radio, X-ray, and infrared properties across the groups. The Index Diagram, supported by multiwavelength diagnostics, provides a practical framework to correct for HG influence in large AGN samples, with implications for using QMS-based proxies in cosmology and AGN demographics.

Abstract

We revisit the Quasar Main Sequence (QMS) by investigating the impact of the stellar component from the host galaxy (HG) on the emission line spectra of the active galactic nuclei (AGN). We first detect spectra with broad emission lines using a line ratio method for a sample of $\sim$3000 high SNR ($>$20) Black Hole Mapper objects (part of the fifth phase of the Sloan Digital Sky Survey). We then built the Index diagram, a novel diagnostic tool using the $z$-corrected spectra, model-free, designed to easily identify spectra with significant stellar HG contributions and to classify the AGN spectra into three categories based on AGN-HG dominance: HG-dominated (HGD), Intermediate (INT), and AGN-dominated (AGND) sources. A colour-$z$ diagram was used to refine the AGN-HG classification. We subtract the stellar contributions from the HGD and INT spectra before modeling the AGN spectrum to extract the QMS parameters. Our QMS reveals that HGD galaxies predominantly occupy the Population B region with no \rfe, %FWHM$\gtrsim$4000 \kms, with outliers exhibiting \rfe\ $>$ 1, likely due to HG subtraction residuals and a faint contribution of \hbbc. INT and AGND spectra show similar distributions in the Population A %FWHM(\hbbc)$<$4000 \kms\ region, while in Population B, %For broader lines, a tail of AGND sources becomes apparent. Cross-matching with radio, infrared, and X-ray catalogs, we find that the strongest radio emitters are associated with HGD and INT groups. Strong X-ray emitters are found in INT and AGND sources, also occupying the AGN region in the WISE colour diagram.

Figures (10)

• Figure 1: Zoom in flux ratio distributions for objects with $z \leq$ 0.5. Solid lines are the distributions in bins of 0.001. Dashed lines are the fitted Gaussians (see text for details). Vertical black lines indicate a flux ratio equal to one, while blue and green vertical lines are the limits for F$_{\rm B,H\alpha}$/F$_{\rm C,H\alpha}$ and F$_{\rm H\beta Sel}$/F$_{\rm C,H\beta}$, respectively (see Table \ref{['tab:histograms']}). Top panel: Blue lines: F$_{\rm B,H\alpha}$/F$_{\rm C,H\alpha}$. Orange lines: F$_{\rm R,H\alpha}$/F$_{\rm C,H\alpha}$. Lower Panel: Purple lines: F$_{\rm H\beta Tot}$/F$_{\rm C,H\beta}$. Green lines: F$_{\rm H\beta Sel}$/F$_{\rm C,H\beta}$. The vertical axis is the number of objects in each bin.
• Figure 2: $\Delta z = z_{SDSS} - z_{corr}$ (upper panel) and $z_{SDSS}$ for objects with $z <$ 1 (lower panel) as functions of the corrected redshift distribution ($z_{corr}$) for objects with wrong $z$ assignment.
• Figure 3: Upper panel: Colour--$z$ diagram using the CoSa. The ordinate axis is the redshift, while the abscissa is the $g-r$ colour of the PSF aperture in magnitudes. Solid red line represents the lower limit of the red spectra. Solid blue line is the upper limit of the bluer spectra. Red, Green, and Blue dots are the corresponding objects of each group. Over-plotted black isocontours are meant to enhance the density distribution of a sample without a S/N restriction (see text for details). Lower panel: H-H$\beta$ index diagram using the CoSa. Dashed black vertical and horizontal lines delimit the AGND, HGD, and INT groups described in the text. The colour points, their isocontours, and the histograms correspond to the points delimited by $g-r$ in the figure of the upper panel. Solid black lines are the correlations given in eqs. \ref{['eq:index_AGND']} and \ref{['eq:index_HGD-INT']} for AGND and HGD+INT groups, respectively.
• Figure 4: (Upper panel) Average spectra of the selected groups. The shaded region is the variance interval, following the colour code for each spectrum, where blue represents AGND, red represents HGD, and green represents INT sources. Vertical dashed lines are the rest frames for H$\alpha$ and H$\beta$. (Lower panel) Zoom-in plot to visualize the H and H$\beta$ indices regions shown in pink bands.
• Figure 5: Examples of stellar-AGN decomposition for HGD (left) and INT (right) groups. The solid black line is the observed spectrum, the yellow line is the stellar contribution from the HG, the green line is the AGN power law, and the pink line is the AGN emission line spectrum.