How Similar are the Properties of Quasars with Nearly Identical Ultraviolet Spectra?

TL;DR

This study quantifies how similar ultraviolet spectra of quasars relate to their physical properties, revealing that quasars with nearly identical spectra, called doppelgängers, have similar luminosities and masses, with implications for cosmology.

Contribution

The paper introduces a quantitative method using principal component analysis to measure spectral similarity and explores the relationship between spectral similarity and quasar properties, including the concept of spectral doppelgängers.

Findings

Spectral similarity correlates with similar luminosity, mass, and Eddington ratio.

Quasars with nearly identical spectra show luminosity differences with 57% uncertainty.

Highly accreting quasars are not better standard candles than others.

Abstract

The spectrum of a quasar contains important information about its properties. Thus, it can be expected that two quasars with similar spectra will have similar properties, but just how similar has not before been quantified. Here we compare the ultraviolet spectra of a sample of 5553 quasars from Data Release 7 of the Sloan Digital Sky Survey, focusing on the $1350$ \AA \ $\leq \lambda \leq 2900$ \AA \ rest-frame region which contains prominent emission lines from \SiIV, O IV], \CIV, \CIII, and \MgII\ species. We use principal component analysis to determine the dominant components of spectral variation, as well as to quantitatively measure spectral similarity. As suggested by both the Baldwin effect and modified Baldwin effect, quasars with similar spectra have similar properties: bolometric luminosity, Eddington fraction, and black hole mass. The latter two quantities are calculated from the luminosity in conjunction with spectral features, and the variation between quasars with virtually identical spectra (which we call doppelg\"angers) is driven by the variance in the luminosity plus measurement uncertainties. In the doppelgangers the luminosity differences show 1$\sigma$ uncertainties of 57\% (or 0.63 magnitudes) and $\sim$70\% 1$\sigma$ uncertainties for mass and Eddington fraction. Much of the difference in luminosities may be attributable to time lags between the spectral lines and the continuum. Furthermore, we find that suggestions that the mostly highly accreting quasars should be better standard candles than other quasars are not bourne out for doppelgangers. Finally, we discuss the implications for using quasars as cosmological probes and the nature of the first two spectral principal components.