HST-COS OBSERVATIONS OF AGNs. II. EXTENDED SURVEY OF ULTRAVIOLET COMPOSITE SPECTRA FROM 159 ACTIVE GALACTIC NUCLEI

Abstract

The ionizing fluxes from quasars and other active galactic nuclei (AGN) are critical for interpreting their emission-line spectra and for photoionizing and heating the intergalactic medium (IGM). Using far-ultraviolet spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we directly measure the rest-frame ionizing continua and emission lines for 159 AGN at redshifts 0.001 < z AGN < 1.476 and construct a composite spectrum from 475-1875Å. We identify the underlying AGN continuum and strong EUV emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the H I Lyα forest, 7 Lyman-limit systems (N HI ≥ 10 17.2 cm −2 ) and 214 partial Lyman-limit systems (15.0 < log N HI < 17.2). The 159 AGN exhibit a wide range of FUV/EUV spectral shapes, F ν ∝ ν αν typically with −2 ≤ α ν ≤ 0 and no discernible continuum edges at 912Å (H I) or 504Å (He I). The composite rest-frame continuum shows a gradual break at λ br ≈ 1000Å, with mean spectral index α ν = −0.83 ± 0.09 in the FUV (1200-2000Å) steepening to α ν = −1.41±0.15 in the EUV (500-1000Å). We discuss the implications of the UV flux turnovers and lack of continuum edges for the structure of accretion disks, AGN mass inflow rates, and luminosities relative to Eddington values.

Statistical Detection of the He ii Transverse Proximity Effect: Evidence for Sustained Quasar Activity for >25 Million Years

“…The quasar spectral slope α beyond 912Å is not very well constrained since the frequencies between the extreme UV and soft X-rays are basically unobservable. We adopt a value of α = −1.7 as determined by Lusso et al (2015) (however with a large uncertainty of ±0.6), which is consistent with the independent measurement of Stevans et al (2014), as well as the slope between UV and X-ray regime (Lusso et al 2015), but differs from the value of α = −0.73 ± 0.26 reported by Tilton et al (2016). The uncertainty in α and the long range of extrapolation from 912Å beyond 228Å causes substantial uncertainty in the inferred photoionization rate, e.g.…”

Flickering AGN can explain the strong circumgalactic O viobserved by COS-Halos

“…This spectrum was derived for the average Type I unobscured quasar from a variety of observations available at that time. More recent work by Stevans et al (2014) found an EUV slope of ν −1.41 , which is even harder and would create an even greater PZF effect for high ions like O vi as discussed in OS13. Observing the EUV slope of Seyfert-like galaxies is critical for understanding the magnitude of the PZF effect around star-forming galaxies, but we note that our assumption of the Sazonov et al (2004) spectrum could be a conservative assumption for the PZF effect.…”

AGNfitter: A BAYESIAN MCMC APPROACH TO FITTING SPECTRAL ENERGY DISTRIBUTIONS OF AGNs

“…Therefore, we should consider this effect while modeling AGN SEDs. To correct for the intervening Lyα forest and continuum absorption, one possibility is to employ a set of IGM transmission functions (T λ ) calibrated from multiple quasar absorption-line observables (Lusso et al 2015, see also Stevans et al 2014;Tilton et al 2016 for a different approach) over a wide range of redshifts. Such functions can be estimated statistically given the stochasticity of Lyman limit systems (e.g., Worseck & Prochaska 2011;Prochaska et al 2014) and critically depends on the parametrization of the H 1 column density distribution (Madau 1995;Meiksin 2006;Inoue et al 2014;Prochaska et al 2014).…”