eaa.iop.orgDOI: 10.1888/0333750888/2360 Quasistellar Objects: SurveysPaul Hewett FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 22:43:41 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsQuasistellar Objects: Surveys ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSCopyright © Nature Publishing Group 2001Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Quasistellar Objects: SurveysSince the discovery of QUASARS in 1963, surveys havebeen a prerequisite both for investigating models of thecentral regions of quasars themselves and to providesamples of quasars for use as cosmological probes. Theearly history of surveys for quasars was determined byour incomplete knowledge of their nature and of theirrelationship to galaxies. Discovered initially through thecampaign to identify the strong RADIO SOURCES in theThird Cambridge Radio Catalogue (3CR), the opticalcounterparts were found to be stellar, i.e. unresolvedpoint sources, and to possess blue optical colors.Following the initial spectroscopic identification of3CR273 as an object with the then unprecedentedREDSHIFT of z = 0.16 (see BRIGHT QUASAR 3C 273), byMaarten Schmidt, a significant number of quasars weresoon found by obtaining optical spectra of unresolvedblue objects within the positional error boxes of radiosources. A further major advance was made in 1965when Allan Sandage showed that there existed a muchlarger population of unresolved blue sources with verysimilar properties to ‘quasars’ except that the objectswere not detectable as radio sources. Some researcherscontinue to draw a distinction between quasi-stellar radiosources (quasars) and quasi-stellar objects (QSOs).However, many workers regard the distinction asprimarily historical and both ‘quasar’ and ‘QSO’ may beused to refer to objects irrespective of whether theypossess significant radio emission.An enormous observational effort at radio, infrared,optical and x-ray wavelengths in the three decadesfollowing the discovery of quasars has resulted in theidentification of some 12 000 quasars. Most recently, theability to undertake sensitive surveys over very largeareas of sky has marked the beginning of projects thatwill produce catalogs of hundreds of thousands ofquasars, allowing entirely new scientific investigations tobe undertaken.Identifying quasarsQuasars are rare objects that exhibit a very broad rangeof intrinsic properties. Normal stars and the majority ofgalaxies, whose spectral energy distributions aredominated by the radiation from their constituent stars,possess blackbody-like spectral energy distributions thatare strongly peaked. The characteristic temperature of thestellar photospheres determines the wavelength at whichthe bulk of the radiation is emitted. By contrast, theextreme physical conditions in the central regions ofquasars lead to the emission of radiation over manydecades of wavelength and quasars emit significantly inthe x-ray, ultraviolet, optical, infrared and radio portionsof the spectrum. With only a few exceptions, surveys forquasars rely on identifying objects whose spectral energydistributions differ from the blackbody-like spectra ofnormal stars and galaxies.The majority of known quasars have been identifiedusing data from the optical region of the electromagneticspectrum. At redshifts z ≤ 2.2 quasars exhibit an excessof near-ultraviolet light compared with common Galacticstars. The ‘ultraviolet-excess’ technique, involving theselection of objects with unusually blue broad-bandcolors derived from B and U band magnitudes, hasproduced several thousand quasars—figure 1. Thetechnique can be made both more efficient, by reducingthe number of contaminating hot stars included, and morecomplete, by increasing the sensitivity to a larger fractionof the quasar population, by extending the wavelengthinterval through the use of additional broad-bandmagnitudes (e.g. one or more of V, R, I). The so-called‘multicolor’ technique has proved very effective,allowing the identification of quasars over the redshiftrange 0 < z < 5.The second primary methodology employed in theoptical regime relies on the identification of theprominent emission lines, present in most quasars, visiblein low-resolution spectra. Slitless spectroscopy, atechnique that employs a thin prism (objective prism) ora combination of prism and transmission grating (grism)enables spectra of thousands of objects over large fieldsto be obtained. Prior to the development of the multi-color selection techniques virtually all quasars withredshifts z > 2.2 were located through the identification Figure 1. Spectra of a solar-like star (dashed curve) and atypical quasar of redshift z = 1.8 (solid curve). The measuredfluxes of both objects in an optical passband, such as the V bandrepresented by the hatched region to the right, are very similar.However, the flux of the quasar measured using a passband inthe near-ultraviolet, such as the U band represented by thehatched region to the left, is relatively much greater than for thestar. Employing this characteristic excess of ultraviolet flux isone of the simplest and most effective techniques foridentifying quasars with redshifts z ≤ 2.2. Another class ofdiscovery techniques focus on the detection of the strongemission lines which are visible in the quasar spectrum butabsent in the spectra of normal stars.Quasistellar Objects: Surveys ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSCopyright © Nature Publishing Group 2001Brunel Road, Houndmills Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK2of the strong hydrogen Lyman-α 121.6 nm and carbon CIV 154.9 nm emission lines which, for redshifts 1.5 < z <3.5, appear in the optical portion of the spectrum.While the basic methodologies of the color, slitless-spectroscopic and other discovery techniques have beenestablished for several decades, there has been alongstanding debate over the relative effectiveness ofquasar survey techniques. Advocates of particularmethodologies have often claimed to detect a greaterfraction of the quasar population compared with otherapproaches. Originally, the selection of candidate quasarswas performed by individual