We measured the surface mass density of the Galactic disk at the solar
position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick
disk stars. The results match the expectations for the visible mass only, and
no dark matter is detected in the volume under analysis. The current models of
dark matter halo are excluded with a significance higher than 5sigma, unless a
highly prolate halo is assumed, very atypical in cold dark matter simulations.
The resulting lack of dark matter at the solar position challenges the current
models.Comment: Proceeding of the first binational Sochias-AAA meeting, held in San
  Juan, Argentin

Bidin, C. Moni

Carraro, G.

Mendez, R. A.

Smith, R.

English

arXiv

Moni Bidin, C.

Méndez, R. A.

CARRARO, GIOVANNI

Archivio istituzionale della ricerca - Università di Padova

Original Citation:No evidence of dark matter in the solar neighborhoodPublisher:Published version:DOI:Terms of use:Open Access(Article begins on next page)This article is made available under terms and conditions applicable to Open Access Guidelines, asdescribed at http://www.unipd.it/download/file/fid/55401 (Italian only)Availability:This version is available at: 11577/3212943 since: 2016-11-24T18:22:46ZUniversità degli Studi di PadovaPadua Research Archive - Institutional RepositoryAsociación Argentina de AstronomíaBAAA, Vol. 54, 2011J.J. Clariá, P. Benaglia, R. Barbá, A.E. Piatti & F.A. Bareill s, eds.PRESENTACIÓN ORALNo evidence of dark matter in the solar neighborhoodC. Moni Bidin1, G. Carraro2,3, R.A. Méndez4 & R. Smith1(1) Universidad de Concepción, Chile(2) ESO - Chile(3) Universitá di Padova, Italy(4) Universidad de Chile, ChileAbstract. We measured the surface mass density of the Galactic disk at thesolar position, up to 4 kpc from the plane, by means of the kinematics of∼400thick disk stars. The results match the expectations for thevisible mass only,and no dark matter is detected in the volume under analysis. The current modelsof dark matter halo are excluded with a significance higher than 5σ, unless ahighly prolate halo is assumed, very atypical in cold dark matter simulations.The resulting lack of dark matter at the solar position challenges the currentmodels.Resumen. La densidad superficial de masa del disco Galáctico a la posiciónsolar fue medida, hasta 4 kpc del plano, a través de la cinemática de∼400estrellas del disco grueso. Los resultados coinciden con loesperado para materiavisible solamente y no se detecta materia obscura en el volumen analizado. Losmodelos actuales de halo de materia oscura son excluidos a nivel mayor de5σ, a menos que se asuma un halo fuertemente prolado, muy atípico en lassimulaciones de materia obscura fría. La consiguiente falta de materia oscura ala posición solar desafía los modelos actuales.1. IntroductionMeasuring the matter density of the Galactic disk by means ofthe spatial distributionand kinematics of its stars is an old art, dating nearly a century (Kapteyn 1922, Oort1932). The comparison of the results with the expected amount of visible matter pro-vides an estimate of the dark matter (DM) density in the analyzed volume. So far, allbut few estimates converged to the conclusion that “there isno evidence for a signifi-cant amount of DM in the Galactic disk"(e.g., Kuijken & Gilmore 1989; Holmberg &Flynn 2004). Apart from this very general statement, whose int rpretation is not evenunique (see Garbari et al. 2011; for a discussion), only litte progress has been made onconstraining the fundamental properties of the DM halo, such as its flattening and localdensity. This is very unfortunate, because the shape of the dark halo bears informationabout the nature of the DM itself (Olling & Merrifield 2000). Moreover, the results ofthe experiments for direct detection of DM are degenerate beween the unknown in-teraction cross-section of the searched particles and their local density. Therefore, thelocal DM density of the Standard Halo Model (SHM,ρ⊙,DM =8·10−3 M⊙ pc−3, Jung-289290 C. Moni Bidin et al.man et al. 1996) have so-far been assumed in their interpretation. This density, however,is only a mean value compatible with indirect evidences suchas t e Milky Way rotationcurve.The strongest limitations on the measurements of the Galactic dynamical masscome from the great observational effort required to derivethe spatial distribution of astellar population and the variation of its three-dimentioal kinematics. For this reason,approximations have always been introduced in the calculations, whose validity, oftenquestioned (e.g., Siebert et al. 2008; Garbari et al. 2011),decreases with distance fromthe Galactic plane. As a consequence, all previous investigations have been limited to±1.1 kpc from the plane, but the amount of DM in this volume is small compared tothe observational errors, and firm conclusions are prevented.2. ResultsWe estimated the dynamical mass at the solar Galactocentricposition betweenZ=1.5and 4 kpc from the plane, as inferred by the variation of the kin matics of the Galacticthick disk withZ. This was measured by Moni Bidin et al. (2012), who analyzed asam-ple of ∼400 red giants with 2MASS photometry (Skrutsie et al. 2006),SPM3 propermotion (Girard et al. 2004), and radial velocity (Moni Bidin2009). Their kinematicalresults were inserted into Equation 2 of Moni Bidin et al. (2010), that was obtainedinserting the Jeans equations into the Poisson equation andintegrating. This equationis exact within the limits of validity of simple symmetry requirements, plus a set of ad-ditional assumptions, namely:i) steady state;ii) radial and vertical exponential decayof the density;iii) flat rotation curve;iv) no disk flare;v) constancy of the radial scalelenght with distance from the plane;vi) radial exponential decay of the dispersions,with the same scale length as the mass density. The three required input parameters(solar Galactocentric distance, thick disk scale height and le gth) were defined by theaverage of about 20 literature estimates (Moni Bidin et al. 2010).The results of our calculations are shown in the left panel ofFigure 1, where theyare compared to the known amount of visible matter, as estimated by Moni Bidin etal. (2010). The expectations of two spherical Navarro et al.(1997) models for the DMhalo, with the local density equal to the SHM (labeled SHM), and to the minimum den-sity extrapolated by the Galactic rotation curve (ρ⊙,DM =5·10−3 M⊙ pc−3, MIN model;Weber & de Boer 2010), are also overplotted. The derived surface densityΣ(Z) wellmatches the expectations for visible mass alone, and no DM isdetected in the volumeunder analysis. From the derived curve, a local DM density of0±1·10−3 M⊙ pc−3 isderived. The SHM model is therefore excluded at the 8σ level, and even the model withthe minimum density (MIN) is 5σ more massive than the detected dynamical mass. In-terestingly, very similar results are obtained if the kinematical results of Casetti-Dinescuet al. (2011) are assumed in the calculations. In fact, the curve thus derived matches theexpectations for the visible mass only, although with much lower significance becauseof the larger errors. Identical conclusions are drawn if theint gration of the Poissonequation is performed in the interval 1.5–4 kpc instead of 0–4 kpc (right panel of Fi-gure 1). This calculation is more reliable, because the kinematics is not extrapolated atZ ≤1.5 kpc, where it was not measured. Moreover, the uncertainty o the quantity ofvisible mass is also limited, because this range is above thethin layer of disk interstellarmedium, and it encloses only the tail of the Galactic stellardisk distribution.Dark matter in the solar neighborhood 291Figura 1. Calculated absolute (left panel) and incremental(right panel) surfacemass density, as a function of distance from the Galactic plane. The dotted and das-hed curves indicate the 1σ and 3σ strip, respectively. The expectations of the knownvisible mass (VIS), and of two models (SHM and MIN) comprising the visible massplus the DM halo described in the text.It can be shown, by means of extensive calculations, that altering one of the hy-pothesis or the value of one of the three parameters cannot solve the problem of themissing DM in the volume under analysis. The details of this analysis will be givenin a forthcoming paper (Moni Bidin et al. 2012). The derived solution can be forcedto match the expectations of the DM halo models only under an exotic combinationof unlikely hypothesis as, for example, a very thin thick disk ( cale height 0.7 kpc)either very extended in the radial direction (scale length 4.6 kpc) or strongly flared atthe solar position. On the contrary, the expected visible mass matches the observationswithout any effort, by use of the most probable assumptions.The models can reprodu-ce the observed curve assuming a highly prolate DM halo, becaus the local quantityof DM is inversely proportional to the flatteningq of the spheroidal distribution. Therequirement that the least massive model (MIN) agrees with the observations within 2σreturns the constraintq ≥ 2. Nevertheless, current cold DM simulations have problemsin reproducing such strongly prolate structures (e.g., Dubinski & Carlberg 1991), andthis solution would therefore require a revision of the models.3. ConclusionsThe observations point to a noticeable lack of DM at the solarGalactocentric position. Itis easy to see that the presence of a classical DM halo as thoses wn in Figure 1 wouldhave been unequivocally detected with our method even if, assuggested by Moni Bidinet al. (2010), it could generate only a small variation of thepotential. In fact, insertingthe derivative of the potential of any DM halo model into the integrated Poisson equa-tion, no mismatch arises between the resulting dynamical mass and the quantity of DMmass enclosed between±Z. Moreover, Sánchez-Salcedo et al. (2011) showed that thepresence of a DM halo affects the disk kinematics noticeably(compare their Figure 1and 2), and the difference is much higher than the observational errors of Moni Bidin292 C. Moni Bidin et al.et al. (2012). In conclusion, the interpretation of the observed lack of DM at the solarposition is not straightforward: DM is required to sustain the flat Galactic rotation cur-ve, but the observations point to a distribution very different to what today accepted. Inparticular, while numerous experiments seek to directly detect the elusive DM particlesour results suggest that their density may be negligible in the solar neighborhood.Agradecimientos. C.M.B. and R.A.M. acknowledge support from the Chilean Cen-tro de Astrofísica FONDAP No. 15010003, and the Chilean Centro de Excelencia enAstrofísica y Tecnologías Afines (CATA) BASAL PFB/06.ReferenciasCasetti-Dinescu, D.I., Girard, T.M., Korchagin, V.I., & van Altena, W.F. 2011, ApJ, 728, 7Dubinski, J., & Carlberg, R.G. 1991, ApJ, 378, 496Garbari, S., Read, J.I., & Lake, G. 2011, MNRAS, in press, arXiv:1105.6339Girard, T.M., Dinescu, D.I., van Altena, W.F., et al. 2004, AJ, 127, 3060Holmberg, J., & Flynn, C. 2004, MNRAS, 352, 440Jungman, G., Kamionkowski, M., & Griest, K. 1996, Physics Reports, 267, 195Kapteyn, J.C. 1922, ApJ, 588, 823Kuijken, K., & Gilmore, G. 1989, MNRAS, 239, 605Moni Bidin, C. 2009, PhD Thesis, Universidad de Chile, 1Moni Bidin, C., Carraro, G., Méndez, R.A., & van Altena, W.F.2010, ApJ, 724, L122Moni Bidin, C., Carraro, G., & Méndez, R.A. 2012, ApJ,in press, arXiv:1202.1799Navarro, J.F., Frenk, C.S., & White, S.D.M. 1997, ApJ, 490, 493Olling, R.P., & Merrifield, M.R. 2000, MNRAS, 311, 361Oort, J.H. 1932, BAN, 6, 249Sánchez-Salcedo, F.J., Flynn, C., & Hidalgo-Gámez, A.M. 2011, ApJ, 731, L35Siebert, A., Bienaymé, O., Binney, J., et al. 2008, MNRAS, 391, 793Skrutskie, M.F., Cutri, R.M., Stiening, R., et al. 2006, AJ,131, 1163Weber, M., & de Boer, W. 2010, A&A, 509, A25

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No evidence of dark matter in the solar neighborhood

No evidence of dark matter in the solar neighborhood

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