A High Yield of New Sightlines for the Study of Intergalactic Helium: Far-UV-Bright Quasars from SDSS, GALEX, and HST

Abstract

Investigations of He II Ly-alpha (304 A rest) absorption toward a half-dozen quasars at z~3-4 have demonstrated the great potential of helium studies of the IGM, but the current critically small sample size of clean sightlines for the He II Gunn-Peterson test limits confidence in cosmological inferences, and a larger sample is required. Although the unobscured quasar sightlines to high redshift are extremely rare, SDSS DR6 provides thousands of z>2.8 quasars. We have cross-correlated these SDSS quasars with GALEX GR2/GR3 to establish a catalog of 200 higher-confidence (~70% secure) cases of quasars at z=2.8-5.1 potentially having surviving far-UV (restframe) flux. We also catalog another 112 likely far-UV-bright quasars from GALEX cross-correlation with other (non-SDSS) quasar compilations. Reconnaissance UV prism observations with HST of 24 of our SDSS/GALEX candidates confirm 12 as detected in the far-UV, with at least 9 having flux extending to very near the He II break; with refinements our success rate is even higher. Our SDSS/GALEX selection approach is thereby confirmed to be an order of magnitude more efficient than previous He II quasar searches, more than doubles the number of spectroscopically confirmed clean sightlines to high redshift, and provides a resource list of hundreds of high-confidence sightlines for upcoming He II and other far-UV studies from HST. Our reconnaissance HST prism spectra suggest some far-UV diversity, confirming the need to obtain a large sample of independent quasar sightlines across a broad redshift range to assess such issues as the epoch(s) of helium reionization, while averaging over individual-object pathology and/or cosmic variance.

32 pages, 7 figures, accepted by ApJ

References in corpus (7)

• The 2.5 m Telescope of the Sloan Digital Sky Survey
• Observational constraints on Cosmic Reionization
• The Sloan Digital Sky Survey Quasar Catalog IV. Fifth Data Release
• A galaxy at a redshift z = 6.96
• The nature and evolution of the highly ionized near-zones in the absorption spectra of z~6 quasars
• Reionisation scenarios and the temperature of the IGM
• The UV Properties of SDSS Selected Quasars

Cited by in corpus (9)

• A New Calculation of the Ionizing Background Spectrum and the Effects of HeII Reionization
• The End of Helium Reionization at z~2.7 Inferred from Cosmic Variance in HST/COS HeII Lyman Alpha Absorption Spectra
• Constraining the era of helium reionization using fast radio bursts
• The evolution of the helium-ionizing background at z ~ 2-3
• Fluctuations in the Ionizing Background During and After Helium Reionization
• AGN-driven helium reionization and the incidence of extended HeIII regions at redshift z>3
• New Evidence for Extended HeII Reionization at z>3.5 from HeII Lyman Alpha and Beta Transmission Spikes
• Spatial Correlations in the Helium-Ionizing Background
• HeII Ly$\alpha$ Transmission Spikes and Absorption Troughs in Eight High-resolution Spectra Probing the End of HeII Reionization