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Binary Galaxies and Alternative Physics

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Binary Galaxies and Alternative Physics AA manuscript no ASTRONOMY will b e inserted by hand later AND Your thesaurus co des are ASTROPHYSICS Binary galaxies and alternative physics I A qualitative application of MOND and MannheimKazanas gravity DSL Soares Departamento de FsicaICEX UFMG CP Belo Horizonte MG Brazil Email dsoaressicaufmgbr Received accepted Abstract Binary galaxies are mo deled as p ointmasses ob eying the nonNewtonian MOND and MannheimKazanas MKG theories of gravity Random samples of such systems are generated by means of Monte Carlo simulations of binary orbits Mo del pairs have total masses and mass ratios similar to pairs in the cataloged sample used in the analysis General features of synthetic samples are derived from a comparison with observed data of galaxy pairs in R V L L space Both MOND and MannheimKazanas binaries either on circular or loweccentricity orbits cannot b e the source of observations b ecause they require extremely high M L values solar units Both MOND and astro-ph/9505014 19 Mar 96 MKG binaries on higheccentricity orbits and reasonable M L values solar units pro duce envelopes of R V L L consistent with the observations but the distribution of separations is inconsistent with the observed data unless strong selection eects are at work A denite answer to the issue whether one or another mo del is suitable to explain real binary galaxy dynamics will b e only p ossible when a large sample containing a signicant fraction of wide pairs determined with velocityblind selection procedures is investigated Send oprint requests to DSL Soares under a rigorous statistical analysis taking into account sample selection biases and contamination by nonphysical pairs Key words binary galaxies alternative physics Monte Carlo simulations Introduction The dynamics p ertaining to Newtonian gravity are unable of avoiding mass discrepancies in astronomical systems unless some kind of yet unknown dark particles are invoked Faber and Gallagher van Albada and Sancisi Trimble Sanders Ashman For example the MACHO and EROS searches for dark matter in the form of compact subluminous ob jects in our Galaxy halo Alco ck et al Bennett et al Aub ourg et al and references therein have not detected sucient microlensing events to explain the kinematics of the Milky Way with a spheroidal halo p opulated by brown dwarfs Furthermore direct optical searches of the Hubble Space Telescope have not also b een able to detect the predicted amount of matter in the form of faint stars in the halo of the Milky Way Bahcall Another much explored approach to the problem has b een the suggestion that Newton laws are not adequate and that the solution might b e found in alternatives to classical gravity and dynamical laws Many of such alternatives are empirically motivated eg Milgrom a b c Sanders Kuhn and Kruglyak by the observations of at rotation curves but that is not always the case Mannheim and Kazanas have derived an alternative theory of gravity whose main motivation was the search for the correct covariant general theory of gravity Sp ecically they found the exterior solution to conformal Weyl gravity asso ciated with a static spherically symmetric gravitational source The most successful of the empirical alternatives to Newtons laws has b een Milgroms MOND Modied Newtonian Dynamics which has b een sub jected to successive exp eri mental tests eg Gerhard but see Milgrom The theory has b een strongly and successfully defended by Milgrom in many pap ers in recent years see also Bege man Bro eils and Sanders Sanders Nevertheless a most serious setback is that there is no covariant theory of gravity that in the weak eld limit reduces itself to MOND Some attempts have b een made otherwise in this direction Bekenstein and Mil grom Sanders but yet MOND remains as a phenomenological ad hoc theory b eing this up to now its main source of criticism Other consequences dynamical and cosmological of MOND are discussed by Felten Mannheim has applied the Mannheim and Kazanas theory of gravity hereafter MKG to mo del the circular velocity proles of four spiral galaxies with fairly reasonable ts without the requirement of dark matter He claims that EinsteinNewton gravity proves to b e inadequate in comparison with fourth order conformal Weyl gravity represented by the MKG solution In spite of that and of having on its foundation a general theory of gravity with the status of a full covariant one MKG has b een ques tioned on its ability of explaining exceptionally extended at rotation curves Sanders and Begeman Moreover even conformal Weyl gravity has b een criticized on cos mological grounds Elizondo and Yepes presented exact solutions to the conformal Weyl gravity cosmological equations that failed to yield primordial nucleosynthesis abun dances consistent with present observational constraints They conclude that conformal cosmological mo dels are very unlikely to give a realistic description of the Universe This is intended to b e the rst one of a series of pap ers devoted to binary galaxy dynamics under the premises of alternative theories of gravity Here it is investigated what binary galaxy data might suggest one ab out MOND and MKG Simple p oint mass mo dels are used to represent b ound pairs of galaxies under MOND and MKG prescriptions for the gravitational mutual galaxy interaction Monte Carlo simulations of synthetic samples are p erformed by means of numerical calculations of pair orbits They are subsequently pro jected on the sky and the relevant observed quantities derived A qualitative comparison b etween a sample of real pairs and simulated samples is then made The second pap er of the series will present a rigorous statistical analysis of observed samples of binaries in the light of MOND and the third one will fo cus on MKG Biases introduced by selection eects on sample determinations and contamination by non physical pairs must b e considered separately for each theory of gravity It is obvious for example that the denition of what a nonphysical pair is dep ends up on the theory that describ es the interactions and the dynamics of galaxies in pairs Section presents the data which come as a list of disk binary galaxies extracted from the Catalogue of Multiple Galaxies A ducial Keplerian mo del determined in the App endix is qualitatively tted to the data Such a mo del is meant to b e confronted with MOND and MKG simulated samples In section the MOND gravitational p otential for p ointmass binaries and technical details of the simulations are describ ed The same is done in section for MKG Section is devoted to Monte Carlo simulations of MOND and MKG synthetic samples In section results of the qualitative comparison b etween observed and synthetic binaries are discussed and the main conclusions presented Binary galaxy data Much eort has b een dedicated in the last years to the compilation of binary galaxy lists Karachentsev Turner Peterson Schweizer Soares Chengalur Salp eter Terzian Soares et al etc For the sake of unifor mity only one list of pairs is considered namely that extracted from the Catalogue of Multiple Galaxies CMG van Mo orsel Oosterlo o Soares The selection of pairs was done by applying a surface density enhancement pro cedure to the Uppsala General Catalogue of Galaxies Nilson a metho d devised by TS van Albada A description of the metho d can b e found in van Mo orsel Soares chapter by van Albada and Soares and Soares et al The metho d do es not require any redshift information and this is a feature that makes it particularly suitable for investigations concerning alternative physics The concept of a b ound pair is dep endent on what gravity law rules the dynamics of the pair and cannot b e relied up on simple upp er limits on lineofsight velocity dierences which is a typical characteristic of velocitydependent selection pro cedures Still on uniformity from the whole list of binaries in the CMG a homogeneous subsample of pairs is considered see Soares which is formed by rotationally supp orted galaxies ranging from lenticulars to late spirals The main reason for avoiding pairs with elliptical galaxies is that b oth MOND and MKG have gathered their status of likely alternative theories after b eing applied on mo deling circular velocity radial proles of spiral galaxies The whole sample is available on machine readable form up on request dsoaresfisicaufmgbr The lineofsight helio centric velocity dierences are normalized by the square ro ot of the total blue lu minosities of the pairs in units of L Such a normalization allows one to scale the envelopes given by eqs A and A Figure App endix by a prop er M L value It has b een shown in many recent binary galaxy investigations based on conventional physics that there might b e an extended dark halo either around the individual galaxies or embedding the whole pair White et al Schweizer Soares Charlton and Salp eter Bartlett and Charlton Although the presence of optical ie nonphysical pairs in a sample could bias the analysis against p ointmass mo dels as may have happ ened with the study by White et al as p ointed out by Pichio and TanzellaNitti and Pacheco and Junqueira there is comp elling evidence that Keplerian mo dels are far from b eing a go o d description of binary dynamics Nevertheless Keplerian approximations serve as reliable upp er limits to V L for a sample of binary galaxies provided one not b e interested in the detailed distribution of pairs in that space In the App endix upp er b ounds
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