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HST Images of Nearby Luminous Quasars II: Results for Eight Quasars and Tests of the Detection Sensitivity

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HST Images of Nearby Luminous Quasars II: Results for Eight Quasars and Tests of the Detection Sensitivity HST IMAGES OF NEARBY LUMINOUS QUASARS I I RESULTS FOR EIGHT QUASARS AND TESTS OF THE 1 DETECTION SENSITIVITY John N Bahcall Soa Kirhakos Institute for Advanced Study School of Natural Sciences Princeton NJ and Donald P Schneider Department of Astronomy and Astrophysics The Pennsylvania State University University Park PA and Institute for Advanced Study School of Natural Sciences Princeton NJ Received accepted astro-ph/9501018 6 Jan 95 Based on observations with the NASAESA Hubble Space Telescope obtained at the Space Telescope Science Institute which is op erated by the Asso ciation of Universities for Research in Astronomy Inc under NASA contract NAS ABSTRACT HST observations of eight intrinsically luminous quasars with redshifts b etween and are presented Seven companion galaxies brighter than M H kms Mp c lie within a pro jected V distance of kp c of the quasars three of the companions are lo cated closer 00 than kp c pro jected distance from the quasars well within the volume that would b e enclosed by a typical L host galaxy The observed asso ciation of quasars and companion galaxies is statistically signicant and may b e an imp ortant element in the luminousquasar phenomenon Evidence for candidate host galaxies is presented for the three most promising cases PG C and PG but additional observations are required b efore the characteristics of the candidate hosts can b e regarded as established Upp er limits are placed on the visualband brightnesses of representative galactic hosts for all of the quasars These limits are established by placing galaxy images obtained with HST underneath the quasars and measuring at what faintness level the known galaxies are detected On average the HST spirals would have b een detected if they were one magnitude fainter than L or brighter and the earlytype galaxies could have b een detected down to a brightness level of ab out L where L is the Schechter characteristic luminosity of eld galaxies Smo oth featureless galaxy mo dels exp onential disks or de Vaucouleurs proles are t to the residual light after a b esttting p oint source is subtracted from the quasar images The results show that smo oth host galaxies brighter than on average ab out L would have b een detected These upp er limits or p ossible detections are consistent with for example the eight luminous quasars studied in this pap er o ccurring in host galaxies that have a Schechter luminosity function with a lowercuto of in the range L to L Tests are p erformed to determine if our failure to detect luminous host galaxies could b e an artifact caused by our analysis pro cedures These tests include comparing the measured PSF for our HST observations with the PSFs used in previous groundbased studies of host galaxies measuring the uctuations in the sky signals that were subtracted from the quasar images evaluating empirically the eects of using dierent stellar p oint spread functions in the analysis carrying out the subtraction of the stellar nuclear source in dierent ways creating and analyzing articial AGNs with known surface brightnesses and tting the observed quasar light to an analytic mo del that includes a host galaxy Our analysis pro cedures successfully pass all of these tests Subject headings quasars individual PG PG PG PKS C PG PG C INTRODUCTION It has long b een recognized that quasars might reside in galaxies and that the quasar phenomenon might b e strongly inuenced by the presence of companion galaxies Dep ending up on your theoretical bias you may exp ect to nd quasars in old galaxies in young galaxies or in interacting galaxies all p ossibilities that have b een advocated in the published literature Very close companions might provide fuel for or initiate pro cesses that lead to the quasar phenomenon Over the past two decades there have b een a large number of groundbased observational programs in which the prop erties of host galaxies of quasars were investigated In fact a consensus view has b een developed that quasars reside in luminous galaxies with the intrinsically brightest quasars residing in the most luminous galaxies Moreover many authors have concluded that radio bright quasars reside in elliptical galaxies and radio faint quasars reside in spiral galaxies Some representative pap ers providing evidence for these views are Kristian Wycko Wehinger Gehren Hutchings et al Gehren et al Heckman et al Malkan Malkan Margon Chanan Boroson Persson Oke Smith et al Hutchings Sto ckton MacKenty Yee Hutchings Janson Ne Romanishin Hintzen VeronCetty Woltjer Hutchings Ne Dunlop et al and McLeo d Rieke ab The analyses presented in these pioneering studies are dicult b ecause of atmospheric seeing the light from the bright central nuclear sources may b e a few magnitudes brighter than the total emission from the host galaxies Some of the pap ers that have previously discussed the companions of lowredshift quasars include Bahcall Schmidt and Gunn Bahcall and Bahcall Gunn Robinson and Wampler Burbidge and ODell Sto ckton Hutchings et al Heckman et al Malkan Margon Chanan Green Yee and Yee The most dramatic results presented in the present pap er refer to very close galaxy companions that are not visible in previouslyobtained groundbased images The excellent optical characteristics of the repaired HST make p ossible improved observational studies of the hosts of smallredshift quasars The characterization of faint galactic material the host galaxy in the vicinity of a bright stellar source the quasar can b e done with increased reliability using the HST to minimize the p ossibility that quasar 00 00 light is mistaken for diuse galactic light Close galactic companions within or of the quasar nucleus may also b e most convincingly detected with HST We describ e in this pap er the systematic results that have b een obtained for the rst eight ob jects in a sample of of the intrinsically most luminous M for V H kms Mp c nearby z radioquiet and radioloud quasars selected from the VeronCetty Veron catalog Images of the rst four of the quasars PG PG PG GQ Com and PG were presented in Bahcall Kirhakos Schneider Paper I images of four additional quasars C PKS PG C have not previously b een discussed hereafter the new quasars Hutchings et al rep orted on observations of two AGNs imaged with the planetary camera of WFPC In the absence of an empirical stellar PSF they used a mo del PSF and image restoration to analyze their images and to compare with groundbased data Our principal results for all eight quasars and their hosts are summarized in Table which lists the following quantities for each ob ject the date observed the redshift and the apparent V magnitude from VeronCetty Veron the distance in kp c that corresp onds to an angular separation of one arcsec as seen from earth the absolute V magnitude and the range of galaxy magnitudes at which eight representative galaxies selected from HST WFC observations with the same lter could b e detected when articially placed underneath the quasar images The last column gives the average limiting absolute magnitude at which the eight representative galaxies could have b een detected 00 as hosts Accurate co ordinates and multicolor CCD photometry for six of the eight quasars are given in Kirhakos et al Three of the quasars C PKS and C are radio loud the other ve quasars are radio quiet Kellermann et al The eight quasars studied in this pap er have an average absolute visual magnitude of M approximately two magnitudes brighter than the brightest galaxies of V rich clusters Ho essel Schneider Postman Lauer and ab out magnitudes brighter than the characteristic Schechter magnitude for eld galaxies Schechter Kirshner et al Efstathiou Ellis Peterson The average apparent brightness for the quasars is V At the typical redshift of the quasars z an L galaxy would have an apparent magnitude of V Table Sample of Completed Cycle Quasars and Detectable Hosts a a a Ob ject Date z kp c p er V M QSO m host hM hosti V lim lim arcsec FW FW PG Feb b PG Feb PG Feb b C Jun PKS Jun PG Apr b PG Jun C Jun a Computed for and H km s Mp c In this cosmology brightest cluster galaxies have M and the characteristic Schechter magnitude for eld galaxies V is M V b Candidate host galaxy see x This pap er is organized as follows In x we describ e the observational pro cedures including the determination of the stellar p oint spread function PSF We outline in x the metho d by which we set limits on the brightnesses of representative HSTobserved host galaxies This section describ es how we subtract the stellar PSF x displays a catalog of observed HST galaxies that are used as simulated host galaxies x and discusses the determination of the faintest magnitudes at which we could detect host galaxies underneath each observed quasar x We discuss in x ts of the residual diuse light to smo oth featureless galaxy mo dels either an exp onential disk or a de Vaucouleurs prole We present in x the characteristics of the candidate host galaxies that we have found for PG C and PG The HST images reveal separate galaxies whose pro jected separations from the centersoflight of the quasar images are very small these companion galaxies are discussed in x and their prop erties are summarized in Table McLeo d Rieke b have rep orted H band detections of host galaxies
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