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Register a new userHigh redshift quasars in the COSMOS survey: the space density of z>3 X-ray selected QSOs
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M. Brusa
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We present a new measurement of the space density of high redshift (3.0<z<4.5), X-ray selected QSOs obtained by exploiting the deep and uniform multiwavelength coverage of the COSMOS survey. We have assembled a statistically large (40 objects), X-ray selected (F_{0.5-2 keV} >10^{-15} cgs), homogeneous sample of z>3 QSOs for which spectroscopic (22) or photometric (18) redshifts are available. We present the optical (color-color diagrams) and X-ray properties, the number counts and space densities of the z>3 X-ray selected quasars population and compare our findings with previous works and model predictions. We find that the optical properties of X-ray selected quasars are not significantly different from those of optically selected samples. There is evidence for substantial X-ray absorption (logN_H>23 cm^{-2}) in about 20% of the sources in the sample. The comoving space density of luminous (L_X >10^{44} erg s^-1) QSOs declines exponentially (by an e--folding per unit redshift) in the z=3.0-4.5 range, with a behavior similar to that observed for optically bright unobscured QSOs selected in large area optical surveys. Prospects for future, large and deep X-ray surveys are also discussed.
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We use a simple optical/infrared (IR) photometric selection of high-redshift QSOs that identifies a Lyman Break in the optical photometry and requires a red IR color to distinguish QSOs from common interlopers. The search yields 100 z~3 (U-dropout) QSO candidates with 19<r<22 over 11.7 deg^2 in the ELAIS-N1 (EN1) and ELAIS-N2 (EN2) fields of the Spitzer Wide-area Infrared Extragalactic (SWIRE) Legacy Survey. The z~3 selection is reliable, with spectroscopic follow-up of 10 candidates confirming they are all QSOs at 2.83<z<3.44. We find that our z~4$ (g-dropout) sample suffers from both unreliability and incompleteness but present 7 previously unidentified QSOs at 3.50<z<3.89. Detailed simulations show our z~3 completeness to be ~80-90% from 3.0<z<3.5, significantly better than the ~30-80% completeness of the SDSS at these redshifts. The resulting luminosity function extends two magnitudes fainter than SDSS and has a faint end slope of beta=-1.42 +- 0.15, consistent with values measured at lower redshift. Therefore, we see no evidence for evolution of the faint end slope of the QSO luminosity function. Including the SDSS QSO sample, we have now directly measured the space density of QSOs responsible for ~70% of the QSO UV luminosity density at z~3. We derive a maximum rate of HI photoionization from QSOs at z~3.2, Gamma = 4.8x10^-13 s^-1, about half of the total rate inferred through studies of the Ly-alpha forest. Therefore, star-forming galaxies and QSOs must contribute comparably to the photoionization of HI in the intergalactic medium at z~3.
We present a new X-ray selected high redshift quasar CXOCY J125304.0-090737 at z=4.179, discovered by the Calan-Yale Deep Extragalactic Research (CYDER) Survey. This quasar is the fifth X-ray selected high redshift radio quiet quasar ($z>4$) found so far. Here, we present its observed properties which are characterized by its relative optical and X-ray faintness, its X-ray hardness and its X-ray strength compared to optically selected quasars at high redshift. We also compare the X-ray selected high redshift radio quiet quasars to their optically selected counterparts. We find that the optical to X-ray spectral slope, $alpha_{ox}$, is statistically harder (more X-ray luminous) for the X-ray selected radio quiet quasars than for the optically selected ones. This result, given the different range of rest frame ultraviolet luminosities studied and the selection of the samples, is consistent with the previously found correlation between X-ray and rest frame ultraviolet luminosities and would extend that result to a much wider luminosity range at high redshift. Finally, we discuss the prospects of unveiling the quasar luminosity function at high redshifts using X-ray surveys. The discovery of a high redshift object in the first field of our survey program provides suggestive evidence that X-ray selected surveys may identify more such objects than would be expected from an extrapolation of the optical luminosity function.
We present a detailed study of a X -ray selected sample of 5 submillimeter bright QSOs at $zsim2$, where the highest rates of star formation (SF) and further growth of black holes (BH) occur. Therefore, this sample is a great laboratory to investigate the co-evolution of star formation and AGN. We present here the analysis of the spectral energy distributions (SED) of the 5 QSOS, including new data from Herschel PACS and SPIRE. Both AGN components (direct and reprocessed) and like Star Formation (SF) are needed to model its SED. From the SED and their UV-optical spectra we have estimated the mass of the black hole ($M_{BH} = 10^9 - 10^{10} M_{SUN}$) and bolometric luminosities of AGN ($L_{BOL} = (0.8-20) times 10^{13} L_{SUN}$). These objects show very high luminosities in the far infrared range (at the H/ULIRG levels) and very high rates of SF (SFR = 400-1400 $M_{SUN}$/y). Known their current SFR and their BH masses, we deduce that their host galaxies must be already very massive, or would not have time to get to the local relation between BH mass and bulge. Finally, we found evidence of a possible correlation between the column density of ionized gas detected in X-rays ($NH_{ion}$) and SFR, which would provide a link between AGN and SF processes.
We present the measurement of the projected and redshift space 2-point correlation function (2pcf) of the new catalog of Chandra COSMOS-Legacy AGN at 2.9$leq$z$leq$5.5 ($langle L_{bol} rangle sim$10$^{46}$ erg/s) using the generalized clustering estimator based on phot-z probability distribution functions (Pdfs) in addition to any available spec-z. We model the projected 2pcf estimated using $pi_{max}$ = 200 h$^{-1}$ Mpc with the 2-halo term and we derive a bias at z$sim$3.4 equal to b = 6.6$^{+0.60}_{-0.55}$, which corresponds to a typical mass of the hosting halos of log M$_h$ = 12.83$^{+0.12}_{-0.11}$ h$^{-1}$ M$_{odot}$. A similar bias is derived using the redshift-space 2pcf, modelled including the typical phot-z error $sigma_z$ = 0.052 of our sample at z$geq$2.9. Once we integrate the projected 2pcf up to $pi_{max}$ = 200 h$^{-1}$ Mpc, the bias of XMM and textit{Chandra} COSMOS at z=2.8 used in Allevato et al. (2014) is consistent with our results at higher redshift. The results suggest only a slight increase of the bias factor of COSMOS AGN at z$gtrsim$3 with the typical hosting halo mass of moderate luminosity AGN almost constant with redshift and equal to logM$_h$ = 12.92$^{+0.13}_{-0.18}$ at z=2.8 and log M$_h$ = 12.83$^{+0.12}_{-0.11}$ at z$sim$3.4, respectively. The observed redshift evolution of the bias of COSMOS AGN implies that moderate luminosity AGN still inhabit group-sized halos at z$gtrsim$3, but slightly less massive than observed in different independent studies using X-ray AGN at z$leq2$.
The space density of the various classes of cataclysmic variables (CVs) could only be weakly constrained in the past. Reasons were the small number of objects in complete X-ray flux-limited samples and the difficulty to derive precise distances to CVs. The former limitation still exists. Here the impact of Gaia parallaxes and implied distances on the space density of X-ray selected complete, flux-limited samples is studied. The samples are described in the literature, those of non-magnetic CVs are based on ROSAT (RBS - ROSAT Bright Survey & NEP -- North Ecliptic Pole), that of the Intermediate Polars stems from Swift/BAT. All CVs appear to be rarer than previously thought, although the new values are all within the errors of past studies. Upper limits at 90% confidence for the space densities of non-magnetic CVs are $rho_{rm RBS} < 1.1 times 10^{-6}$ pc$^{-3}$, and $rho_{rm RBS+NEP} < 5.1 times 10^{-6}$ p$^{-3}$, for an assumed scale height of $h=260$ pc and $rho_{rm IPs} < 1.3 times 10^{-7}$ p$^{-3}$ for the long-period Intermediate Polars at a scale height of 120 pc. Most of the distances to the IPs were under-estimated in the past. The upper limits to the space densities are only valid in the case where CVs do not have lower X-ray luminosities than the lowest-luminosity member of the sample. These results need consolidation by larger sample sizes, soon to be established through sensitive X-ray all-sky surveys to be performed with eROSITA on the Spektrum-X-Gamma mission.
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