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Register a new userDiscovery of an Optically-Faint Quasar at z=5.70 and Implications for the Faint End of the Quasar Luminosity Function
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A. Mahabal
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We present observations of an optically-faint quasar, RD J114816.2+525339, discovered from deep multi-color observations of the field around the z = 6.42 quasar SDSS J1148+5251. The two quasars have a projected separation of 109 arcsec and both are outliers in r-z versus z-J color-color space. Keck spectroscopy reveals RD J114816.2+525339 to be a broad-absorption line quasar at z = 5.70. With z_AB = 23.0, RD J114816.2+525339 is 3.3 mag fainter than SDSS J1148+5251, making it the faintest quasar known at z>5.5. This object was identified in a survey of ~2.5 square degrees. The implied surface density of quasars at these redshifts and luminosities is broadly consistent with previous extrapolations of the faint end of the quasar luminosity function and supports the idea that active galaxies provide only a minor component of the reionizing ultraviolet flux at these redshifts.
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We have conducted a spectroscopic survey to find faint quasars (-26.0 < M_{1450} < -22.0) at redshifts z=3.8-5.2 in order to measure the faint end of the quasar luminosity function at these early times. Using available optical imaging data from portions of the NOAO Deep Wide-Field Survey and the Deep Lens Survey, we have color-selected quasar candidates in a total area of 3.76 deg^2. Thirty candidates have R <= 23 mags. We conducted spectroscopic followup for 28 of our candidates and found 23 QSOs, 21 of which are reported here for the first time, in the 3.74 < z <5.06 redshift range. We estimate our survey completeness through detailed Monte Carlo simulations and derive the first measurement of the density of quasars in this magnitude and redshift interval. We find that the binned luminosity function is somewhat affected by the K-correction used to compute the rest-frame absolute magnitude at 1450A. Considering only our R <= 23 sample, the best-fit single power-law (Phi propto L^beta) gives a faint-end slope beta = -1.6+/-0.2. If we consider our larger, but highly incomplete sample going one magnitude fainter, we measure a steeper faint-end slope -2 < beta < -2.5. In all cases, we consistently find faint-end slopes that are steeper than expected based on measurements at z ~ 3. We combine our sample with bright quasars from the Sloan Digital Sky Survey to derive parameters for a double-power-law luminosity function. Our best fit finds a bright-end slope, alpha = -2.4+/-0.2, and faint-end slope, beta = -2.3+/-0.2, without a well-constrained break luminosity. This is effectively a single power-law, with beta = -2.7+/-0.1. We use these results to place limits on the amount of ultraviolet radiation produced by quasars and find that quasars are able to ionize the intergalactic medium at these redshifts.
We present an updated determination of the z ~ 4 QSO luminosity function (QLF), improving the quality of the determination of the faint end of the QLF presented in Glikman et al. (2010). We have observed an additional 43 candidates from our survey sample, yielding one additional QSO at z = 4.23 and increasing the completeness of our spectroscopic follow-up to 48% for candidates brighter than R = 24 over our survey area of 3.76 deg2. We study the effect of using K-corrections to compute the rest-frame absolute magnitude at 1450A compared with measuring M1450 directly from the object spectra. We find a luminosity-dependent bias: template-based K-corrections overestimate the luminosity of low-luminosity QSOs, likely due to their reliance on templates derived from higher luminosity QSOs. Combining our sample with bright quasars from the Sloan Digital Sky Survey and using spectrum-based M1450 for all the quasars, we fit a double-power-law to the binned QLF. Our best fit has a bright-end slope, {alpha} = 3.3pm0.2, and faint-end slope, {beta} = 1.6(+0.8/-0.6). Our new data revise the faint-end slope of the QLF down to flatter values similar to those measured at z ~ 3. The break luminosity, though poorly constrained, is at M* = -24.1(+0.7/-1.9), approximately 1 - 1.5 mag fainter than at z ~ 3. This QLF implies that QSOs account for about half the radiation needed to ionize the IGM at these redshifts.
We searched for quasars that are ~ 3 mag fainter than the SDSS quasars in the redshift range 3.7 < z < 4.7 in the COSMOS field to constrain the faint end of the quasar luminosity function. Using optical photometric data, we selected 31 quasar candidates with 22 < i < 24 at z ~ 4. We obtained optical spectra for most of these candidates using FOCAS on the Subaru telescope, and identified 8 low-luminosity quasars at z ~ 4. In order to derive the quasar luminosity function (QLF) based on our spectroscopic follow-up campaign, we estimated the photometric completeness of our quasar survey through detailed Monte Carlo simulations. Our QLF at z ~ 4 has a much shallower faint-end slope beta = -1.67^{+0.11}_{-0.17} than that obtained by other recent surveys in the same redshift. Our result is consistent with the scenario of downsizing evolution of active galactic nuclei inferred by recent optical and X-ray quasar surveys at lower redshifts.
(Abridged) Based on numerical simulations of galaxy mergers that incorporate black hole (BH) growth, we predict the faint end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our simulations have yielded a new model for quasar lifetimes where the lifetime depends on both the instantaneous and peak quasar luminosities. This motivates a new interpretation of the QLF in which the bright end consists of quasars radiating at nearly their peak luminosities, but the faint end is mostly made up of quasars in less luminous phases of evolution. The faint-end QLF slope is then determined by the faint-end slope of the quasar lifetime for quasars with peak luminosities near the observed break. We determine this slope from the quasar lifetime as a function of peak luminosity, based on a large set of simulations spanning a wide variety of host galaxy, merger, BH, and ISM gas properties. Brighter peak luminosity (higher BH mass) systems undergo more violent evolution, and expel and heat gas more rapidly in the final stages of quasar evolution, resulting in a flatter faint-end slope (as these objects fall below the observed break in the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher luminosities with increasing redshift, implying a larger typical quasar peak luminosity, the faint-end QLF slope flattens. From the quasar lifetime as a function of peak luminosity and this interpretation of the QLF, we predict the faint-end QLF slope and its evolution with redshift in good agreement with observations. Although BHs grow anti-hierarchically (with lower-mass BHs formed primarily at lower redshifts), the observed change in slope and differential or luminosity dependent density evolution in the QLF is completely determined by the luminosity-dependent quasar lifetime and physics of quasar feedback.
Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars ($M_{1450}>-24$ mag) at $z gtrsim6$, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z~6 in a 12.5 deg$^{2}$ region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at $sim8443~rm{AA}$, with emission lines redshifted to $z=5.944 pm 0.002$ and rest-frame ultraviolet continuum magnitude $M_{1450}=-23.59pm0.10$ AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z~6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified low-luminosity quasars . This suggests that the number of $M_{1450}sim-23$ mag quasars at z~6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS.
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