نتایج جستجو برای: quasars
تعداد نتایج: 6042 فیلتر نتایج به سال:
In our second paper on long-term quasar variability, we employ a much larger database of quasars than in de Vries, Becker & White. This expanded sample, containing 35 165 quasars from the Sloan Digital Sky Survey Data Release 2, and 6 413 additional quasars in the same area of the sky taken from the 2dF QSO Redshift Survey, allows us to significantly improve on our earlier conclusions. As befor...
A search for distant radio-loud quasars in the CLASS survey: three new radio-selected quasars at z>4
We report on the search for distant radio-loud quasars in the Cosmic Lens All Sky Survey (CLASS) of flat spectrum radio sources with S5GHz > 30 mJy. Unresolved optical counterparts were selected from APM scans of POSS-I plates, with e < 19.0 and red o−e > 2.0 colours, in an effective area of ∼ 6400 deg. Four sources were found to be quasars with z > 4, of which one was previously known. This sa...
We present new Chandra observations of 21 z > 4 quasars, including 11 sources at z > 5. These observations double the number of X-ray detected quasars at z > 5, allowing investigation of the X-ray spectral properties of a substantial sample of quasars at the dawn of the modern Universe. By jointly fitting the spectra of 15 z > 5 radio-quiet quasars (RQQs), including sources from the Chandra arc...
We define a new complete sample of 13 optically-luminous radio quasars (MAB(1450Å)< −26. 9 and log P1.4GHz(WHz−1) > 25.7) with redshift 3.8 < z < 4.5, obtained by crosscorrelating the FIRST radio survey and the APM catalogue of POSS-I. We measure the space density to be (1.0 ± 0.3) Gpc−3, a factor 1.9±0.7 smaller than the space density of similar quasars at z ≈ 2 (FBQS). Using a new measurement...
We have performed a successful targeted search for a population of red, radio-quiet, and probably absorbed quasars. Radio-quiet, optically red ROSAT PSPC X-ray sources brighter than 1 ] 10~13 ergs cm~2 s~1 were searched for red (O[E [2.0, O1 20) counterparts in the Automated Plate-measuring Machine catalog of Palomar Sky Survey objects. Of 45 objects for which we obtained adequate followup opti...
We present new quasars discovered in the vicinity of the Andromeda and Triangulum galaxies with the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope, also named Guoshoujing Telescope) during the 2010 and 2011 observational seasons. Quasar candidates are selected based on the available Sloan Digital Sky Survey (SDSS), Kitt Peak National Observatory (KPNO) 4 m telescope, Xuyi Sch...
Context. For the first time spectroscopic galaxy redshift surveys are reaching the scales where galaxies can be studied together with the nearest quasars. This gives an opportunity to study the dependence between the activity of a quasar and its environment in a more extensive way than before. Aims. We study the spatial distribution of galaxies and groups of galaxies in the environments of low ...
We present near-IR, low resolution spectra of eight of the most distant quasars known, with redshifts in the range 4.9<z<6.4. Half of these quasars are characterized by deep, broad and blueshifted absorption features associated with both high and low ionization species (CIV, SiIV, AlIII, MgII), i.e. they belong to the class of Broad Absorption Line (BAL) quasars, which are associated with power...
We present short Chandra observations of twelve bright (i<18) zem∼1.5 quasars from the Sloan Digital Sky Survey chosen to have significantly redder optical colors than most quasars at the same redshift. Of the five quasars with optical properties most consistent with dust reddening at the quasar redshift, four show indirect evidence of moderate X-ray absorption (inferred NH∼ 10 22 cm) with a du...
We have compiled a sample of 21 low redshift (z . 0.3), luminous active galactic nuclei (AGN) with large Balmer decrements (Hα/Hβ > 7) using the galaxy and QSO catalogs of the Sloan Digital Sky Survey Early Data Release. Using this sample we attempt to determine the fraction of quasars with large internal absorption. We find that these AGN have Strong [O III] λ5007 and broad Hα emission, and th...
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