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Identifying Doppelganger Active Galactic Nuclei across redshifts from spectroscopic surveys

Shreya Sareen, Swayamtrupta Panda

TL;DR

The study tackles AGN evolution by testing whether low-$z$ AGNs can serve as proxies for high-$z$ counterparts through spectral doppelgängers. It analyzes SDSS DR16 spectra focusing on Mg II, Hβ, C IV, Fe II, N V, and continuum luminosities, and employs PCA to reduce dimensionality followed by a nearest-neighbors search to identify cross-redshift matches. The authors report the discovery of roughly 600 cross-redshift doppelgänger pairs with highly similar emission-line properties, including Mg II, Hβ, Fe II, and C IV profiles, distributed across diverse sky regions. These findings suggest a degree of universality in certain AGN spectral characteristics across cosmic time and demonstrate a viable methodology for cross-epoch AGN comparison, with future work expanding the dataset via DESI and JWST and enhancing the pipeline with non-linear methods and supervision.

Abstract

Active Galactic Nuclei (AGNs) are among the most luminous objects in the universe, making them valuable probes for studying galaxy evolution. However, understanding how AGN properties evolve over cosmic time remains a fundamental challenge. This study investigates whether AGNs at low redshift (nearby) can serve as proxies for their high-redshift (distant) counterparts by identifying spectral 'doppelgängers', AGNs with remarkably similar emission line properties despite being separated by vast cosmic distances. We analyze key spectral features of bona fide AGNs using the Sloan Digital Sky Survey's Data Release 16, including continuum and emission lines: Nitrogen (N V), Carbon (C IV), Magnesium (Mg II), Hydrogen-beta (H$β$), and Iron (Fe II - optical and UV) emission lines. We incorporated properties such as equivalent width, velocity dispersion in the form of full width at half maximum (FWHM), and continuum luminosities (135nm, 300nm, and 510nm) closest to these prominent lines. Our initial findings suggest the existence of multiple AGNs with highly similar spectra, hinting at the possibility that local AGNs may indeed share intrinsic properties with high-redshift ones. We showcase here one of the better candidate pairs of AGNs resulting from our analyses.

Identifying Doppelganger Active Galactic Nuclei across redshifts from spectroscopic surveys

TL;DR

The study tackles AGN evolution by testing whether low- AGNs can serve as proxies for high- counterparts through spectral doppelgängers. It analyzes SDSS DR16 spectra focusing on Mg II, Hβ, C IV, Fe II, N V, and continuum luminosities, and employs PCA to reduce dimensionality followed by a nearest-neighbors search to identify cross-redshift matches. The authors report the discovery of roughly 600 cross-redshift doppelgänger pairs with highly similar emission-line properties, including Mg II, Hβ, Fe II, and C IV profiles, distributed across diverse sky regions. These findings suggest a degree of universality in certain AGN spectral characteristics across cosmic time and demonstrate a viable methodology for cross-epoch AGN comparison, with future work expanding the dataset via DESI and JWST and enhancing the pipeline with non-linear methods and supervision.

Abstract

Active Galactic Nuclei (AGNs) are among the most luminous objects in the universe, making them valuable probes for studying galaxy evolution. However, understanding how AGN properties evolve over cosmic time remains a fundamental challenge. This study investigates whether AGNs at low redshift (nearby) can serve as proxies for their high-redshift (distant) counterparts by identifying spectral 'doppelgängers', AGNs with remarkably similar emission line properties despite being separated by vast cosmic distances. We analyze key spectral features of bona fide AGNs using the Sloan Digital Sky Survey's Data Release 16, including continuum and emission lines: Nitrogen (N V), Carbon (C IV), Magnesium (Mg II), Hydrogen-beta (H), and Iron (Fe II - optical and UV) emission lines. We incorporated properties such as equivalent width, velocity dispersion in the form of full width at half maximum (FWHM), and continuum luminosities (135nm, 300nm, and 510nm) closest to these prominent lines. Our initial findings suggest the existence of multiple AGNs with highly similar spectra, hinting at the possibility that local AGNs may indeed share intrinsic properties with high-redshift ones. We showcase here one of the better candidate pairs of AGNs resulting from our analyses.
Paper Structure (8 sections, 1 figure)

This paper contains 8 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Methodology
  3. low- and high-z SDSS sample
  4. Doppelganger detection pipeline
  5. Results and Discussions
  6. PCA
  7. Nearest-neighbors algorithm and tweaks
  8. Summary and Future Work

Figures (1)

  • Figure 1: Flowchart for an end-to-end pipeline to retrieve doppelgängers across redshift using SDSS DR16 Spectroscopic Database SDSS16. On the bottom left panels, we are highlighting pairs of AGN spectra for the low-redshift sample and the high-redshift sample. On the extreme right, we highlight one pair of AGN spectra resulting from the analysis for the merged sample. We note that the detection of these doppelgängers is independent of the sky region.