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Quasar-Galaxy Associations

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Mon Not R Astron So c ? Quasargalaxy asso ciations P A Thomas R L Webster and M J Drinkwater Astronomy Centre MAPS University of Sussex Brighton BN QH School of Physics University of Melbourne Parkvil le Victoria Australia AngloAustralian Observatory Coonabarabran NSW Australia August ABSTRACT There is controversy ab out the measurement of statistical asso ciations b etween bright quasars and faint presumably foreground galaxies We lo ok at the distribution of galax ies around an unbiased sample of bright mo derate redshift quasars using a new statistic based on the separation of the quasar and its nearest neighbour galaxy We nd a signicant excess of close neighbours at separations less than ab out arcsec which we attribute to the magnication by gravitational lensing of quasars which would otherwise b e to o faint to b e included in our sample Ab out one quarter to one third of the quasars are so aected although the allowed error in this fraction is large Key words quasars general gravitational lensing signicant excess of galaxies near mo derately high red INTRODUCTION shift quasars Tyson Webster et al Fugmann In this pap er we lo ok at the asso ciation b etween quasars and and Hintzen Romanishin Valdes How foreground galaxies We nd a signicant excess over that ever some more recent studies eg Crampton McClure exp ected from a random distribution which we interpret as Fletcher and Yee Filipp enko Tang claim no b eing due to gravitational lensing ie many of the quasars statistical excess though quantitative results have yet to b e in our sample are intrinsically faint but are magnied into published Comparison of dierent samples is dicult since the sample by gravitational lensing dierent metho dologies have b een used and the samples of The physical signicance of quasargalaxy asso ciations quasars observed vary markedly in redshift and magnitude dep ends on the redshifts of the quasars and galaxies In the In addition the algorithm for galaxy detection varies A quasars we might exp ect to de case of lowredshift z discussion of the observational problems is given by Hewett tect asso ciations due to galaxies lying in clusters asso ciated astro-ph/9408088 29 Aug 1994 Harding Webster In addition dierent groups at with the quasar Several instances of lowredshift clustering tribute the excess to dierent physical reasons That issue are rep orted in the literature eg Yee Green Smith will only b e clearly resolved when a substantial number of Heckman and Ellingson Green Yee but in redshifts are obtained for the asso ciated galaxies the present study we are not interested in galaxies at the quasar redshift Because of the hints of gravitational lensing of various quasars is much The situation for highredshift z degrees of signicance in previous work we set out to design less clear Asso ciations have b een detected in recent observa an observing programme which would b e optimal for observ tions but the cause has not b een unequivocal l y established ing lensing if it were present The choice of quasar sample If the galaxies are at lower redshifts than the quasars there is describ ed at length in Drinkwater Webster Thomas is evidence that these are biased linesofsight The ampli Paper but the relevant p oints are reiterated b e cation of the background sources may b e due either to low We chose to lo ok at bright m mo derate V macrolensing by the galaxy p otential or to microlensing by quasars the bright apparent mag z redshift compact ob jects asso ciated with the galaxy Alternatively nitude enhances the fraction of lensed quasars while the the galaxies may b e in groups at the quasar redshift If this high redshift gives a reasonable crosssection for lensing It is the case the galaxies must b e much brighter than at the is of crucial imp ortance that the sample b e unbiased with present ep o ch in order to have b een detected at such a high resp ect to the presence of galaxies near quasars the au redshift thors of pap ers on MgI I samples claim the quasar selection Until recently most studies detected a statistically was random To test the p ossibili ty that there has b een an unintentional favouring of quasars whose sp ectra show MgI I absorption lines we analysed the subsample of quasars with out detected MgI I separately These show a similar asso ci Data obtained at lObservatoire du Mont Megantic Quebec 2 Thomas Webster and Drinkwater i m ation of quasars and foreground galaxies see Section V ii z Preliminary results from essentially the same sample were q so iii no bias with resp ect to nearby galaxies or absorption presented by Drinkwater et al but we now apply a iv Declination degrees more rigorous statistical analysis The outline of the pap er is as follows The selection cri v Right Ascension determined by the allo cated observing times teria and data analysis are briey describ ed in Section for a fuller account we refer the reader to Paper In Section The full list of quasars observed is given in Table we describ e the statistics of quasargalaxy asso ciations We None of the samples we used were strictly magnitude rst discuss the use of the sparameter as a measure of limited but all are of bright quasars The ma jority of quasar quasargalaxy separation and then show that it is signi magnitudes in our nal sample are in the range m R cantly biased to low values The number of elds which show asso ciated galaxies is then quantied Section assesses the validity of our results we can think of no reasonable expla nation other than gravitational lensing and compares them Image Catalogues with previous work We then go on to lo ok at the prop er The CCD images were pro cessed and calibrated using the ties of individua l lensing galaxies and of the p opulation as a y whole The signicance for cosmology is discussed Finally IRAF image analysis software The FOCAS faint ob ject Section presents a summary of the most imp ortant features classicatio n and analysis system package Valdes of our results was then used to detect and classify images in each eld We used the DAOPHOT package to detect and measure any images very close to the quasar which were then added into the FOCAS catalogues The magnitudes of all the ob jects DATA were then measured with the ap erture photometry routines in IRAF All these pro cedures are describ ed in more detail in Our data consist of Rband KronCousins CCD images of Paper apart from the following mo dications which were bright quasars observed at the Observatoire astronomique made to avoid in bias in our catalogues du Mont Megantic Quebec Most of these were part of the study of MgI I absorption systems describ ed in Paper al i We use a more conservative magnitude limit now de though some ob jects are excluded from the present sample ned as the magnitude of the brightest ob ject with a b ecause we apply stricter selection criteria We also include photometry error m mag in each frame This re additional ob jects not in Paper Our observing pro cedure sults in limits a few tenths of a magnitude brighter than and data reduction are essentially the same as describ ed in the previous denition the magnitude of the faintest Paper with some mo dications that we describ e in this ob ject with a photometry error m mag In fact Section the results are not sensitive to the exact value of the magnitude error chosen It can b e reduced to mag nitudes b efore the signal from galaxies asso ciated with Selection Criteria quasars b egins to decrease ii We include only those ob jects which lie more than To measure quasargalaxy asso ciations we require a sam pixels from the edge of the eld This is to avoid incom ple of bright quasars that is unbiased with resp ect to the pleteness which might otherwise o ccur for bright ob jects presence of galaxies near the quasars This condition means which extend over the eld b oundary that we must exclude ob jects from the sample in Paper iii We have checked the extra ob jects included from the and DAOPHOT analysis to ensure they all have signicant which were taken from references that only listed detections detections all were in fact clearly visible in the raw of MgI I absorption and may therefore b e biased We also images excluded the quasar since it was added to the iv A new set of control stars has b een dened using an Young Sargent Boksenberg compilation b ecause ob jective algorithm to avoid any bias These satisfy the it was known to b e gravitationally lensed and therefore was following criteria distance from the edge arcsec biased towards having a close galaxy the lens in this case distance from the quasar arcsec and magnitude To this list we add more quasars of these are range m If more than one star p er eld R from the Kiso survey see Wegner Swanson b They satised these criteria one of them was chosen at ran were selected by colour and morphological criteria from op dom In some cases no star satised these criteria so tical photographic plates and have no bias to the presence only elds have control stars of galaxies or absorption The remaining ob jects were se lected at random from the Hewitt Burbidge cat Table lists all detected ob jects within arcsec of alogue sub ject to the magnitude and redshift constraints each quasar The eect of the DAOPHOT analysis is to describ ed b elow to ll a gap in our observing programme add an extra galaxy close to the quasar in elds Two of them were excluded from our statistical analysis as they are b elow our redshift cuto see Section the oth ers
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