A search for signatures of quasar evolution

TY - JOUR

T1 - A search for signatures of quasar evolution

T2 - Comparison of the shapes of the rest-frame optical/ultraviolet continua of quasars at z > 3 and z ∼ 0.1

AU - Kuhn, Olga

AU - Elvis, Martin

AU - Bechtold, Jill

AU - Elston, Richard

PY - 2001/10

Y1 - 2001/10

N2 - For 15 bright (V < 17.5), high-redshift (z > 3) quasars, we have obtained infrared spectra and photometry, and optical spectrophotometry and photometry, which we use to construct their spectral energy distributions (SEDs) from λrest ∼ 1285-5100 Å. High-resolution spectroscopy for seven enable measurements of their continua shortward of Lyα, and L′ detections of four of these extend their SEDs redward to λrest ∼ 7500 Å. We examine the optical/UV continuum shapes and compare these to those of a set of 27 well-studied low-redshift (z ∼ 0.1) quasars which are matched to the high-redshift ones in evolved luminosity. Single power-law fits to the average fluxes within a set of narrow, line-free, windows between 1285 and 5100 Å, but excluding the 2000-4000 Å region of the Fe II + BaC "small bump," are adequate for most of the objects. For both the high- and low-redshift samples, the distributions of spectral indices, αouv (Fv ∼ vαouv) span a wide range, with δαouv ∼ 1. The cause of such diversity is investigated, and our analysis is consistent with the conclusion of Rowan-Robinson: that it arises from differences in both the emitted continua themselves and in the amounts of intrinsic extinction undergone. The mean (median) optical/UV spectral indices for the high- and low-redshift samples are -0.32 (-0.29) and -0.38 (-0.40), respectively. A Student's t-test indicates that these do not differ significantly, and a K-S test shows likewise for the distributions. Assuming the optical/UV continuum derives from accretion, the similarity of the spectral indices at high and low redshift is inconsistent with models which interpret the statistical evolution as resulting from a single generation of slowly dimming quasars and instead favors those involving multiple generations of short-lived quasars formed at successively lower luminosities. A clear difference between the high- and low-redshift samples occurs in the region of "small bump." The power-law fit residuals for the low-redshift sample show a systematic excess from ∼2200 to 3000 Å; but this feature is weak or absent in the high-redshift sample. Further study is needed to determine what is responsible for this contrast, but it could reflect differences in iron abundance or Fe II energy source, or alternatively, an intrinsic turnover in the continuum itself which is present at low but not at high redshift.

AB - For 15 bright (V < 17.5), high-redshift (z > 3) quasars, we have obtained infrared spectra and photometry, and optical spectrophotometry and photometry, which we use to construct their spectral energy distributions (SEDs) from λrest ∼ 1285-5100 Å. High-resolution spectroscopy for seven enable measurements of their continua shortward of Lyα, and L′ detections of four of these extend their SEDs redward to λrest ∼ 7500 Å. We examine the optical/UV continuum shapes and compare these to those of a set of 27 well-studied low-redshift (z ∼ 0.1) quasars which are matched to the high-redshift ones in evolved luminosity. Single power-law fits to the average fluxes within a set of narrow, line-free, windows between 1285 and 5100 Å, but excluding the 2000-4000 Å region of the Fe II + BaC "small bump," are adequate for most of the objects. For both the high- and low-redshift samples, the distributions of spectral indices, αouv (Fv ∼ vαouv) span a wide range, with δαouv ∼ 1. The cause of such diversity is investigated, and our analysis is consistent with the conclusion of Rowan-Robinson: that it arises from differences in both the emitted continua themselves and in the amounts of intrinsic extinction undergone. The mean (median) optical/UV spectral indices for the high- and low-redshift samples are -0.32 (-0.29) and -0.38 (-0.40), respectively. A Student's t-test indicates that these do not differ significantly, and a K-S test shows likewise for the distributions. Assuming the optical/UV continuum derives from accretion, the similarity of the spectral indices at high and low redshift is inconsistent with models which interpret the statistical evolution as resulting from a single generation of slowly dimming quasars and instead favors those involving multiple generations of short-lived quasars formed at successively lower luminosities. A clear difference between the high- and low-redshift samples occurs in the region of "small bump." The power-law fit residuals for the low-redshift sample show a systematic excess from ∼2200 to 3000 Å; but this feature is weak or absent in the high-redshift sample. Further study is needed to determine what is responsible for this contrast, but it could reflect differences in iron abundance or Fe II energy source, or alternatively, an intrinsic turnover in the continuum itself which is present at low but not at high redshift.

KW - Galaxies: active

KW - Galaxies: evolution

KW - Quasars: general

KW - Ultraviolet: galaxies

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U2 - 10.1086/322535

DO - 10.1086/322535

M3 - Article

VL - 136

SP - 225

EP - 264

JO - Astrophysical Journal, Supplement Series

JF - Astrophysical Journal, Supplement Series

SN - 0067-0049

IS - 2

ER -