نتایج جستجو برای: quasars
تعداد نتایج: 6042 فیلتر نتایج به سال:
We investigate the correlation between the accretion disk (UV) luminosity and the radio core emission of a quasar sample, containing all PG quasars, also deriving empirical conversion factors from emission line luminosities to disk luminosities. This method allows us to investigate the radio properties of AGN on the absolute scale set by the accretion power. In a radio vs. Ldisk plot we find th...
Utilizing 21 new Chandra observations as well as archival Chandra, ROSAT, and XMM-Newton data, we study the X-ray properties of a representative sample of 59 of the most optically luminous quasars in the Universe (Mi ≈ −29.3 to −30.2) spanning a redshift range of z ≈ 1.5–4.5. Our full sample consists of 32 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 3 (DR3) quasar catalog, two...
Although the population of luminous quasars rises and falls over a period of ∼ 109 years, the typical lifetime of individual quasars is uncertain by several orders of magnitude. We show that quasar clustering measurements can substantially narrow the range of possible lifetimes with the assumption that luminous quasars reside in the most massive host halos. If quasars are long-lived, then they ...
Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or 2H), 3He, 4He and 7Li during the first few minutes of the universe. In this review we concentrate on recent data which give the primordial deuterium (D) abundance. We have measured the primordial D/H in gas with very nearly primordial abundances. We use the Lyman series absorption lines seen in the spectra of q...
We present the results of a pilot survey to find dust-reddened quasars by matching the FIRST radio catalog to the UKIDSS near-infrared survey, and using optical data from SDSS to select objects with very red colors. The deep K-band limit provided by UKIDSS allows for finding more heavily-reddened quasars at higher redshifts as compared with previous work using FIRST and 2MASS. We selected 87 ca...
We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars (νL6μm ≥ 6× 1044 erg s−1) at redshift z ≈ 1–3, identified through detailed spectral energy distribution analyses of distant star-forming galaxies, using the deepest IR data from Spitzer and Herschel in the GOODS–Herschel fields. The aim is to constrain the fraction of obscured, and Compton-thick (CT,NH >...
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