Composite Quasar Spectra from the Sloan Digital Sky Survey

Abstract

We have created a variety of composite quasar spectra using a homogeneous data set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). The quasar sample spans a redshift range of 0.044 ¹ z ¹ 4.789 and an absolute r@ magnitude range of [18.0 to [26.5. The input spectra cover an observed wavelength range of 3800È9200 at a resolution of 1800. The median composite covers a rest-A wavelength range from 800 to 8555 and reaches a peak signal-to-noise ratio of over 300 per 1 A A resolution element in the rest frame. We have identiÐed over 80 emission-line features in the spectrum. Emission-line shifts relative to nominal laboratory wavelengths are seen for many of the ionic species. Peak shifts of the broad permitted and semiforbidden lines are strongly correlated with ionization energy, as previously suggested, but we Ðnd that the narrow forbidden lines are also shifted by amounts that are strongly correlated with ionization energy. The magnitude of the forbidden line shifts is [100 km s~1, compared with shifts of up to 550 km s~1 for some of the permitted and semiforbidden lines. At wavelengths longer than the Lya emission, the continuum of the geometric mean composite is well Ðtted by two power laws, with a break at B5000 The frequency power-law index, is [0.44 from B1300 A. a l , to 5000 and [2.45 redward of B5000 The abrupt change in slope can be accounted for partly by A A. host-galaxy contamination at low redshift. Stellar absorption lines, including higher order Balmer lines, seen in the composites suggest that young or intermediate-age stars make a signiÐcant contribution to the light of the host galaxies. Most of the spectrum is populated by blended emission lines, especially in the range 1500È3500 which can make the estimation of quasar continua highly uncertain unless large A , ranges in wavelength are observed. An electronic table of the median quasar template is available.

The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3

“…The clear stellar light bump sampled by the IRAC bands ( Figure S8a) indicates insignificant AGN contribution. Assuming that AGN activity is responsible for the broad Hα lines, we can scale the composite AGN SED 73,98,99 to the observed Hα flux and compare it to the observed SED. As shown by the blue dotted curve in Figure S8b, the AGN's hot dust bump would start to dominate the SED above ∼5 µm and produce a red [5.8]−[8.0] color, contradicting the observations.…”

Metallicity and Quasar Outflows

“…With increased S/N, composite spectra are useful for studying weak lines and the overall properties of quasars by averaging out object-to-object variations (Vanden Berk et al 2001, hereafter VB01; Nagao et al 2006, hereafter N06). Since we are interested in the relative intensities of BELs, the composites are constructed using the arithmetic mean method.…”

HST-COS OBSERVATIONS OF AGNs. III. SPECTRAL CONSTRAINTS IN THE LYMAN CONTINUUM FROM COMPOSITE COS/G140L DATA*

“…Though such a technique cannot address the dramatic variations among AGN spectra, it can constrain the consistent mean behavior of their spectral energy distributions while also allowing the detection of spectral features that may be too weak to detect in any one observation, such as faint emission lines or absorption edges. The rest-frame FUV continuum has been well-studied in composites that draw from samples of moderate-to-highredshift AGNs, where their rest-frame UV is accessible to ground-based telescopes (Francis et al 1991;Carballo et al 1999;Brotherton et al 2001;Vanden Berk et al 2001;Selsing et al 2015;Xie et al 2015). Probing deeper into the rest-frame EUV from the ground, however, is complicated by the increased density of IGM absorbers at higher redshifts.…”