The first ultraviolet quasar-stacked spectrum at z ≃ 2.4 from WFC3

Abstract

The ionising continuum from active galactic nuclei (AGN) is fundamental for interpreting their broad emission lines and understanding their impact on the surrounding gas. Furthermore, it provides hints on how matter accretes onto supermassive black holes. Using HST's Wide Field Camera 3 we have constructed the first stacked ultraviolet (rest-frame wavelengths 600-2500Å) spectrum of 53 luminous quasars at z 2.4, with a state-of-the-art correction for the intervening Lyman forest and Lyman continuum absorption. The continuum slope (f ν ∝ ν αν ) of the full sample shows a break at ∼912Å with spectral index α ν = −0.61 ± 0.01 at λ > 912Å and a softening at shorter wavelengths (α ν = −1.70 ± 0.61 at λ 912Å). Our analysis proves that a proper intergalactic medium absorption correction is required to establish the intrinsic continuum emission of quasars. We interpret our average ultraviolet spectrum in the context of photoionisation, accretion disk models, and quasar contribution to the ultraviolet background. We find that observed broad line ratios are consistent with those predicted assuming an ionising slope of α ion =−2.0, similar to the observed ionising spectrum in the same wavelength range. The continuum break and softening are consistent with accretion disk plus X-ray corona models when black hole spin is taken into account. Our spectral energy distribution yields a 30% increase to previous estimates of the specific quasar emissivity, such that quasars may contribute significantly to the total specific Lyman limit emissivity estimated from the Lyα forest at z < 3.2.

An X-Shooter composite of bright 1 <z< 2 quasars from UV to infrared

“…The physical scales locally are expected to be larger (Bentz et al 2013), which will make variability timescales longer and variability magnitudes smaller (Vanden Berk et al 2004;Schmidt et al 2012). Blackhole masses as determined from Mg  increases with luminosity (Wu et al 2015) which in turn affects the temperature of the accretion disc (Shakura & Sunyaev 1973;Pereyra et al 2006) and thereby the position of the "big blue bump" and the degree to which the continuum is well modeled by a single power law (see also Lusso et al 2015, for a discussion). As can be seen from Overplotted in dark green is the corresponding composite generated from the full sample of SDSS quasars fulfilling the selection criteria and general agreement is observed, albeit with a brighter Balmer continuum in the SDSS-constructed composite.…”

“…The composite by Vanden Berk et al (2001) consists of 2204 SDSS quasars at z median = 1.253 with absolute M r -magnitudes between −18.0 and −26.5 and therefore intrinsically fainter and lower redshift objects. Lusso et al (2015) calculated the M i (z = 2) mag for the 184 constituent objects in Telfer et al (2002) observed with FOS, GHRS and STIS onboard the Hubble Space Telescope and find an average M i (z = 2) ∼ −27.5 at z ∼ 1.2, which yields a composite of fainter, more nearby sources. For construction of a composite, Glikman et al (2006) used 27 objects at z ∼ 0.25 with an average absolute i-band magnitude M i ∼ −24 observed with IRTF, therefore constituting a Notes.…”

“…Lastly, Lusso et al (2015) used 53 quasars observed with WFC3 at z ∼ 2.4 with absolute i-band magnitudes at redshift 2, M i (z = 2) ∼ −28.5, so of comparable brightness, but at slightly larger distances. Regardless of the variation across selection criteria, luminosities and redshifts, a remarkable similarity in the overall shape is visible over a wide wavelength range.…”

“…Examples of such composite spectra include template spectra of various classes of galaxies (Mannucci et al 2001;Shapley et al 2003;Dobos et al 2012), quasi-stellar object (QSO; Cristiani & Vio 1990;Boyle 1990;Francis et al 1991;Zheng et al 1997;Brotherton et al 2001;Vanden Berk et al 2001;Telfer et al 2002;Richards et al 2006a;Glikman et al 2006;Lusso et al 2015) and gamma-ray burst (GRB) afterglows (Christensen et al 2011). When investigating dust in QSOs or in galaxies along QSO sightlines, template spectra can be used to determine the amount of extinction by artificially reddening the template to match an observed spectrum (e.g., Glikman et al 2007;Urrutia et al 2009;Wang et al 2012;Fynbo et al 2013;Krogager et al 2015).…”

Reaching the peak of the quasar spectral energy distribution – II. Exploring the accretion disc, dusty torus and host galaxy

“…Continuum recovery in this region requires high-resolution UV spectra, e.g. from HST (Finn et al 2014;Lusso et al 2015).…”