Composite Quasar Spectra From the Sloan Digital Sky Survey

TL;DR

The paper builds high-S/N composite quasar spectra from over 2200 SDSS quasars, producing both a median and a geometric-mean template to capture emission-line richness and continuum shape. It uses an [O III] 5007-based template to align redshifts and then analyzes continuum fits, emission/absorption features, and systematic line shifts, identifying over 80 emission features (notably Fe II/Fe III blends) and host-galaxy absorption signatures. The study reveals ionization-energy–dependent line shifts in both broad-line and narrow-line regions, and finds a two-slope continuum with a break near 5000 Å, partly due to host contamination and young stellar populations. The resulting median template is released for cross-correlation and photometric applications, with future plans to explore sub-samples and extended wavelength coverage for a more complete line census and quasar physics insight.

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 <= z <= 4.789, and an absolute r' magnitude range of -18.0 to -26.5. The median composite covers a rest wavelength range from 800 - 8555A, and reaches a peak signal-to-noise ratio of over 300 per 1A resolution element in the rest frame. We have identified over 80 emission line features in the spectrum. Emission line peak velocity shifts of the broad permitted and semi-forbidden lines are strongly correlated with ionization energy, as previously suggested, but we find that the narrow forbidden lines are also shifted by amounts which are strongly correlated with ionization energy. The magnitude of the forbidden line shifts is < 100 km/s, compared to shifts of up to 550 km/s for some of the permitted and semi-forbidden lines. At wavelengths longer than the Ly-a emission, the continuum of the geometric mean composite is well-fit by two power-laws, with a break at approximately 5000A. The frequency power law index, alpha_nu, is -0.44 from 1300 - 5000A, and -2.45 redward of about 5000A. The abrupt change in slope can be accounted for partly by 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 significant contribution to the light of the host galaxies. An electronic table of the median quasar template is available.

Figures (11)

• Figure 1: Redshift distribution of the 2204 quasars used for the composite spectra (top), and the absolute $r'$ magnitude, $M_{r'}$, vs. redshift (bottom). The median redshift is $z=1.253$.
• Figure 2: Number of quasar spectra combined in each 1 Å bin of the composite as a function of rest wavelength.
• Figure 3: Composite quasar spectrum using median combining. Power-law fits to the estimated continuum flux are shown. The resolution of the input spectra is $\approx 1800$, which gives a wavelength resolution of about $1$Å in the rest frame.
• Figure 4: Signal to noise ratio per $1$Å bin for the median composite quasar spectrum. The peak reaches almost 330 at $2800$ Å.
• Figure 5: Composite quasar spectrum generated using the geometric mean of the input spectra. Power-law fits to the estimated continuum flux are shown. The geometric mean is a better estimator than the arithmetic mean (or median) for power-law distributions. The resolution of the input spectra is $\approx 1800$ in the observed frame, which gives a wavelength resolution of about $1$Å in the rest frame.