The Index Diagram as a Tool to Disentangle Host Galaxy Contributions in Quasar Spectra

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 active galactic nuclei (AGN). After identifying SDSS-DR19 spectra with broad emission lines, we constructed the Index diagram, a novel, model-independent diagnostic based on redshift-corrected spectra, designed to efficiently identify sources with significant stellar HG contributions and to classify AGN spectra into three categories: HG-dominated (HGD), intermediate (INT), and AGN-dominated (AGND) sources.

Our QMS analysis shows that HGD sources predominantly occupy the Population B region, typically exhibiting negligible Fe II emission, while INT and AGND sources display similar distributions in the Population A region. Cross-matching with radio, infrared, and X-ray catalogues reveals that the strongest radio emitters are preferentially associated with HGD and INT sources, whereas strong X-ray emitters are found among INT and AGND objects, which also populate the AGN region in the WISE colour diagram. The applicability of the Index diagram is also demonstrated by its implementation in large spectroscopic datasets, including SDSS DR20 and DESI, enabling systematic studies of AGN-HG decomposition across big data samples.

Fecha y lugar: 09/06/2026 – 12:30 | Salón de Actos
Alenka Negrete
Instituto de Astronomía – UNAM