Decline of the Space Density of Quasars between<i>z</i>= 2 and<i>z</i>= 4

Vigotti, M.; Carballo, R.; Benn, Chris; Zotti, G. de; Fanti, R.; Gonzalez-Serrano, J. I.; Mack, K.‐H.; Holt, J.

doi:10.1086/375266

Cited by 18 publications

(56 citation statements)

References 43 publications

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“…A better global fit was achieved allowing z top to vary with luminosity as z top,0 and δ z top being free parameters. The fit gave positive values for both of them, consistent with earlier indications that the high‐ z decline of the space density is more pronounced and starts at lower redshifts for less powerful sources (Hook, Shaver & McMahon 1998; Waddington et al 2001; Vigotti et al 2003; Cirasuolo, Magliocchetti & Celotti 2005; Cirasuolo et al 2006). The luminosity dependence of the high‐ z decline is qualitatively similar to the downsizing observed for galaxies and optically and X‐ray selected quasars (Cowie et al 1996; Barger et al 2005; Pérez‐González et al 2008).…”

Section: Agn‐powered Sourcessupporting

confidence: 87%

A model for the cosmological evolution of low-frequency radio sources

Massardi

Bonaldi

Negrello

et al. 2010

Monthly Notices of the Royal Astronomical Society

150

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We present a new evolutionary model that describes the population properties of radio sources at frequencies ≲5 GHz, thus complementing the De Zotti et al. model, holding at higher frequencies. We find that simple analytic luminosity evolution is still sufficient to fit the wealth of available data on local luminosity functions, multifrequency source counts and redshift distributions. However, the fit requires a luminosity‐dependent decline of source luminosities at high redshifts, at least for steep‐spectrum sources, thus confirming earlier indications of a ‘downsizing’ also for radio sources. The upturn of source counts at sub‐mJy levels is accounted for by a straightforward extrapolation, using the empirical far‐infrared (far‐IR)/radio correlation, of evolutionary models matching the far‐IR counts and redshift distributions of star‐forming galaxies. We also discuss the implications of the new model for the interpretation of data on large‐scale clustering of radio sources and on the integrated Sachs–Wolfe (ISW) effect, and for the investigation of the contribution of discrete sources to the extragalactic background. As for the ISW effect, a new analysis, exploiting a very clean cosmic microwave background map, yields at a substantially higher significance than reported before.

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Section: Agn‐powered Sourcessupporting

confidence: 87%

A model for the cosmological evolution of low-frequency radio sources

Massardi

Bonaldi

Negrello

et al. 2010

Monthly Notices of the Royal Astronomical Society

150

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“…The 5 GHz luminosity functions, computed using the full 1 Jy sample, while confirming that the radio luminosity density peaks at z peak 2.5 (Dunlop & Peacock 1990;Shaver et al 1996Shaver et al , 1999 do not provide evidence for deviations from pure luminosity evolution, reported by other analyses (Hook et al 1998;Vigotti et al 2003), perhaps due to the poor statistics. In particular we notice that the derived epoch-dependent 5 GHz luminosity function is fairly well represented by the model by De Zotti et al (2005).…”

Section: Discussionsupporting

confidence: 69%

High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars

Ricci¹,

Prandoni

Gruppioni

et al. 2005

A&A

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We discuss our ATCA 18.5 and 22 GHz flux density measurements of Southern extragalactic sources in the complete 5 GHz sample of Kühr et al. (1981, A&AS, 45, 367). The high frequency (5-18.5 GHz) spectral indices of steep-spectrum sources for which we have 18.5 GHz data (66% of the complete sample) are systematically steeper than the low frequency (2.7-5 GHz) ones, with median α with luminosity. The completeness of 18.5 GHz data is much higher (89%) for flat-spectrum sources (mostly quasars), which also exhibit a spectral steepening: median α 5 2.7 = −0.14, median α 18.5 5 = 0.16 (S ν ∝ ν −α ), and median ∆α = 0.19. Taking advantage of the almost complete redshift information on flat-spectrum quasars, we have estimated their 5 GHz luminosity function in several redshift bins. The results confirm that their radio luminosity density peaks at z peak 2.5 but do not provide evidence for deviations from pure luminosity evolution as hinted at by other data sets. A comparison of our 22 GHz flux densities with WMAP K-band data for flat-spectrum sources suggests that WMAP flux densities may be low by a median factor of 1.2. The extrapolations of 5 GHz counts and luminosity functions of flat-spectrum radio quasars using the observed distribution of the 5-18.5 GHz spectral indices match those derived directly from WMAP data, indicating that the high frequency WMAP survey does not detect any large population of FSRQs with anomalous spectra.

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“…At lower frequencies the steep‐spectrum sources dominate the population at all luminosities and only contain the rare extremely luminous flat‐spectrum sources. Since it is the flat‐spectrum population that has a high quasar identification rate, this is the sample used to investigate evolution of flat‐spectrum quasars (Dunlop & Peacock 1990; Shaver et al 1996; Jarvis & Rawlings 2000; Vigotti et al 2003; Wall et al 2005). Our frequency selection provides a much larger fraction of these sources without introducing a different class of sources; thus, we reduce the bias by having a high‐frequency‐selected sample (e.g.…”

Section: The Redshift Cut‐off and The Effect Of Spectral Curvaturementioning

confidence: 99%

“…The space density has been shown to decline beyond the redshift of 3 in flat‐spectrum radio sources (e.g. Peacock 1985; Dunlop & Peacock 1990; Shaver et al 1996; Wall et al 2005), X‐ray‐selected quasars (Hasinger, Miyaji & Schmidt 2005) and optically selected quasars (Schmidt, Schneider & Gunn 1991; Vigotti et al 2003). The details of whether such an evolution is purely due to space density evolution or due to luminosity evolution is yet to be resolved.…”

Section: Introductionmentioning

confidence: 99%

Spectral properties and the effect on redshift cut-off of compact active galactic nuclei from the AT20G survey

Chhetri

Ekers

Mahony

et al. 2012

Monthly Notices of the Royal Astronomical Society

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Spectral index has been traditionally used to separate extragalactic radio sources into compact and extended populations, with the spectral transition placed variably between −0.4 and −0.6. We use high angular resolution data, measured from visibility of sources at the longest baseline of 4500 m of the Australia Telescope Compact Array (ATCA), for the Australia Telescope 20 GHz (AT20G) survey to obtain angular size information for over 94 per cent of AT20G sources. We confirm the previous AT20G result that due to the high survey frequency of 20 GHz, the source population is strongly dominated by compact sources (79 per cent). At 0.15‐arcsec angular resolution limit, we show a very strong correlation between the compact and extended sources with flat and steep‐spectrum sources respectively for spectral indices obtained between 1 and 5 GHz. Thus, we provide a firm physical basis for the traditional spectral classification into flat and steep‐spectrum sources to select compact and extended sources. We find that for spectral indices between 1 and 5 GHz, the cut‐offs at −0.4 and −0.5 are quite similar to the optimum cut‐off of −0.46 and, hence, recommend the continued use of −0.5 for future studies. We use the recently published redshift data to study the effect of spectral curvature on the redshift cut‐off of compact active galactic nuclei (AGNs). Using spectral indices at different frequencies, we correct for the redshift effect and also produce rest‐frame frequency spectra for compact sources for redshifts up to z∼ 5. We show that the spectra of most compact sources are flat to ∼30 GHz and then start to steepen. At higher frequencies, the spectra of both compact and extended sources are steep, so the use of spectral index does not separate the compact and extended source populations as well as in lower frequencies. We also find that due to spectral steepening at high frequencies, surveys of compact sources at higher frequencies (ν > 5 GHz) will have redshift cut‐off due to spectral curvature but at frequencies below a few gigahertz, the surveys are not significantly affected by spectral curvature and, thus, the evidence for a strong redshift cut‐off in AGNs found in lower frequency surveys is a real cut‐off and not a result of K‐correction.

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Decline of the Space Density of Quasars betweenz= 2 andz= 4

Cited by 18 publications

References 43 publications

A model for the cosmological evolution of low-frequency radio sources

A model for the cosmological evolution of low-frequency radio sources

High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars

Spectral properties and the effect on redshift cut-off of compact active galactic nuclei from the AT20G survey

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