1,180 research outputs found

    The dusty environment of Quasars. Far-IR properties of Optical Quasars

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    We present the ISO far-IR photometry of a complete sub-sample of optically selected bright quasars belonging to two complete surveys selected through multicolour (U,B,V,R,I) techniques. The ISOPHOT camera on board of the ISO Satellite was used to target these quasars at wavelengths of 7.3, 11.5, 60, 100 and 160 micron. Almost two thirds of the objects were detected at least in one ISOPHOT band. The detection rate is independent of the source redshift, very likely due to the negative K-correction of the far-IR thermal emission. More than a half of the optically selected QSOs show significant emission between 4 and 100 micron in the quasar rest-frame. These fluxes have a very likely thermal origin, although in a few objects an additional contribution from a non-thermal component is plausible in the long wavelength bands. In a colour-colour diagram these objects span a wide range of properties from AGN-dominated to ULIRG-like. The far-IR composite spectrum of the quasar population presents a broad far-IR bump between 10 and 30 micron and a sharp drop at wavelengths greater than 100 micron in the quasar restframe. The amount of energy emitted in the far-IR, is on average a few times larger than that emitted in the blue and the ratio L(FIR)/L(B) increases with the bolometric luminosity. Objects with fainter blue magnitudes have larger ratios between the far-IR (wavelengths > 60 micron) fluxes and the blue band flux, which is attributed to extinction by dust around the central source. No relation between the blue absolute magnitude and the dust colour temperature is seen, suggesting that the dominant source of FIR energy could be linked to a concurrent starburst rather than to gravitational energy produced by the central engine.Comment: Astronomical Journal, in pres

    Clustering at high redshift

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    The addition of deep near infrared images to the database provided by the HDF-S WFPC2 is essential to monitor the SEDs of the objects on a wide baseline and address a number of key issues including the total stellar content of baryonic mass, the effects of dust extinction, the dependence of morphology on the rest frame wavelength, the photometric redshifts, the detection and nature of extremely red objects (EROs). For these reasons deep near infrared images were obtained with the ISAAC instrument at the ESO VLT in the Js, H and Ks bands reaching, respectively, 23.5, 22.0, 22.0 limiting Vega-magnitude. A multi-color (F300, F450, F606, F814, Js, H, Ks) photometric catalog of the HDF-S has been produced. Photometric redshifts have been generated both fitting templates to the observed SEDs and with neural network techniques. Spectroscopic observations of the 9 candidates with I_AB <24.25 have confirmed all of them to be galaxies with 2<z<3.5. The photometric redshifts for all the galaxies brighter than I_AB< 27.5 have been used to study the evolution of galaxy clustering in the interval 0<z<4.5.Comment: 2 pages Latex, To appear in the proceedings of "The mass of galaxies at low and high redshift", Venice, Oct 24-26, 2001,eds. R. Bender and A. Renzini (ESO Astrophysics Symposia, Springer-Verlag

    The Variance of QSO Counts in Cells

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    {}From three quasar samples with a total of 1038 objects in the redshift range 1.0÷2.21.0 \div 2.2 we measure the variance σ2\sigma^2 of counts in cells of volume VuV_u. By a maximum likelihood analysis applied separately on these samples we obtain estimates of σ2(ℓ)\sigma^2(\ell), with ℓ≡Vu1/3\ell \equiv V_u^{1/3}. The analysis from a single catalog for ℓ= 40 h−1\ell = ~40~h^{-1} Mpc and from a suitable average over the three catalogs for ℓ= 60, 80\ell = ~60,~80 and 100 h−1100~h^{-1} Mpc, gives σ2(ℓ)=0.46−0.27+0.27\sigma^2(\ell) = 0.46^{+0.27}_{-0.27}, 0.18−0.15+0.140.18^{+0.14}_{-0.15}, 0.05−0.05+0.140.05^{+0.14}_{-0.05} and 0.12−0.12+0.130.12^{+0.13}_{-0.12}, respectively, where the 70%70\% confidence ranges account for both sampling errors and statistical fluctuations in the counts. This allows a comparison of QSO clustering on large scales with analogous data recently obtained both for optical and IRAS galaxies: QSOs seem to be more clustered than these galaxies by a biasing factor bQSO/bgal∼1.4−2.3b_{QSO}/b_{gal} \sim 1.4 - 2.3.Comment: 13 pages in plain Tex, 5 figures available in postscript in a separate file, submitted to ApJ, DAPD-33

    Testing Cosmological Models With A \lya Forest Statistic: The High End Of The Optical Depth Distribution

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    We pay particular attention to the high end of the \lya optical depth distribution of a quasar spectrum. Based on the flux distribution (Miralda-Escud\'e et al 1996), a simple yet seemingly cosmological model -differentiating statistic, Δτ0\Delta_{\tau_0} -- the cumulative probability of a quasar spectrum with \lya optical depth greater than a high value τ0\tau_0 -- is emphasized. It is shown that two different models -- the cold dark matter model with a cosmological constant and the mixed hot and cold dark matter model, both normalized to COBE and local galaxy cluster abundance -- yield quite different values of Δτ0\Delta_{\tau_0}: 0.13 of the former versus 0.058 of the latter for τ0=3.0\tau_0=3.0 at z=3z=3. Moreover, it is argued that Δτ0\Delta_{\tau_0} may be fairly robust to compute theoretically because it does not seem to depend sensitively on small variations of simulations parameters such as radiation field, cooling, feedback process, radiative transfer, resolution and simulation volume within the plausible ranges of the concerned quantities. Furthermore, it is illustrated that Δτ0\Delta_{\tau_0} can be obtained sufficiently accurately from currently available observed quasar spectra for τ0∼3.0−4.0\tau_0\sim 3.0-4.0, when observational noise is properly taken into account. We anticipate that analyses of observations of quasar \lya absorption spectra over a range of redshift may be able to constrain the redshift evolution of the amplitude of the density fluctuations on small-to-intermediate scales, therefore providing an independent constraint on Ω0\Omega_0, Ω0,HDM\Omega_{0,HDM} and Λ0\Lambda_0.Comment: ApJ Letters, in press, substantial changes have been made from the last versio

    The Optical-UV Continuum of a Sample of QSOs

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    The average optical-UV continuum shape of QSOs has been investigated using spectra of 62 QSOs having good relative photometric calibrations. The QSO spectra were extracted from two complete color selected samples in the magnitude intervals B ~ 16-20. The analysis was performed fitting power-law continua (f proportional to nu^(alpha)) in well defined rest-frame wavelength intervals after removing regions of the spectrum affected by strong emission lines or weak emission bumps. The average slope in the rest-frame optical-UV region 1200 - 5500 A shows a rapid change around the 3000 A emission bump with alpha=0.15 longward of it and alpha=-0.65 at shorter wavelengths. Although these average slopes have been obtained using spectra of QSOs with different luminosities and redshifts, there are no significant correlations of the average spectral index with these quantities. For a few QSOs in the sample we were able to measure the same softening of the spectral shape within the individual spectrum. These results have significant consequences on the estimate of the cosmological evolution of the optically selected QSOs as they affect, for instance, the k-corrections. New k-corrections in the B, V, R and Gr bands were computed. The derived average spectral shape in the optical-UV band puts interesting constraints on the expected emission mechanisms.Comment: 12 pages. To appear in the Astronomical Journa

    Probing dark energy beyond z=2z=2 with CODEX

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    Precision measurements of nature's fundamental couplings and a first measurement of the cosmological redshift drift are two of the key targets for future high-resolution ultra-stable spectrographs such as CODEX. Being able to do both gives CODEX a unique advantage, allowing it to probe dynamical dark energy models (by measuring the behavior of their equation of state) deep in the matter era and thereby testing classes of models that would otherwise be difficult to distinguish from the standard Λ\LambdaCDM paradigm. We illustrate this point with two simple case studies.Comment: 4 pages, 4 figures; submitted to Phys. Rev.
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