We estimate the flux weighted acceleration on the Local Group (LG) from the
near-infrared Two Micron All Sky Redshift Survey (2MRS). The near-infrared flux
weighted dipoles are very robust because they closely approximate a mass
weighted dipole, bypassing the effects of redshift distortions and require no
preferred reference frame. We use this method with the redshift information to
determine the change in dipole with distance. The LG dipole seemingly converges
by 60 Mpc/h. Assuming convergence, the comparison of the 2MRS flux dipole and
the CMB dipole provides a value for the combination of the mass density and
luminosity bias parameters Omega_m^0.6/b_L= 0.40+/-0.09.Comment: 4 pages, 2 figures, Contribution to Rencontres de Moriond: Contents
  and Structures of the Universe, March 18-25, 2006, La Thuil

Colless, M.

Cutri, R. M.

Erdogdu, P.

Falco, E.

George, T.

Huchra, J. P.

Jarrett, T.

Jones, D. H.

Kochanek, C. S.

Lahav, O.

Macri, L.

Mader, J.

Martimbeau, N.

Pahre, M.

Parker, Q.

Rassat, A.

Saunders, W.

English

arXiv

We estimate the flux weighted acceleration on the Local Group (LG) from the near-infrared Two Micron All Sky Redshift Survey (2MRS). The near-infrared flux weighted dipoles are very robust because they closely approximate a mass weighted dipole, bypassing the effects of redshift distortions and require no preferred reference frame. We use this method with the redshift information to determine the change in dipole with distance. The LG dipole seemingly converges by 60 Mpc/h. Assuming convergence, the comparison of the 2MRS flux dipole and the CMB dipole provides a value for the combination of the mass density and luminosity bias parameters Omega_m^0.6/b_L= 0.40+/-0.09.postprin

Huchra, JP

Colless, M

Cutri, RM

Falco, E

George, T

Jarrett, T

Jones, DH

Kochanek, CS

Macri, L

Mader, J

Martimbeau, N

Pahre, M

Parker, QA

Rassat, A

Saunders, W

Erdogdu, P

Lahav, O

HKU Scholars Hub

Title The Dipole Anisotropy of the 2mass Redshift SurveyAuthor(s)Erdogdu, P; Huchra, JP; Lahav, O; Colless, M; Cutri, RM; Falco,E; George, T; Jarrett, T; Jones, DH; Kochanek, CS; Macri, L;Mader, J; Martimbeau, N; Pahre, M; Parker, QA; Rassat, A;Saunders, WCitation ArXiv Astrophysics e-prints, 2006, p. 5343-5343Issued Date 2006URL http://hdl.handle.net/10722/211298Rights Creative Commons: Attribution 3.0 Hong Kong LicensearXiv:astro-ph/0605343v1  14 May 2006THE DIPOLE ANISOTROPY OF THE 2MASS REDSHIFT SURVEYPI˙RI˙N ERDOG˘DU AND THE 2MRS TEAMaSchool of Physics & Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD,UKWe estimate the flux weighted acceleration on the Local Group (LG) from the near-infraredTwo Micron All Sky Redshift Survey (2MRS). The near-infrared flux weighted dipoles arevery robust because they closely approximate a mass weighted dipole, bypassing the effectsof redshift distortions and require no preferred reference frame. We use this method with theredshift information to determine the change in dipole with distance. The LG dipole seeminglyconverges by 60 h−1Mpc. Assuming convergence, the comparison of the 2MRS flux dipoleand the CMB dipole provides a value for the combination of the mass density and luminositybias parameters Ω0.6m /bL = 0.40 ± 0.09.1 IntroductionThe most popular mechanism for the formation of large-scale structure and motions in theUniverse is the gravitational growth of primordial density perturbations. According to thisparadigm, the peculiar acceleration vector g(r) is induced by the matter distribution aroundposition r and if the density perturbations are small enough to be approximated by a lineartheory, then the peculiar velocity field, v(r), is proportional to the peculiar acceleration:v(r) =H0f(Ωm)4piGρ¯g(r) =2f(Ωm)3H0Ωmg(r), (1)where H0 = 100 h kms−1Mpc−1 is the Hubble constant and f(Ωm) ≃ Ω0.6m is the logarithmicderivative of the amplitude of the growing mode of the perturbations in mass with respect tothe scale factor (Peebles 1980).It is now widely accepted that the dipole anisotropy of the cosmic microwave background(CMB) is a direct and accurate measurement of the LG peculiar velocity. The LG accelerationcan also be estimated using surveys of the galaxies tracing the density inhomogeneities respon-sible for the acceleration. If the mass can be related to light by a bias parameter, bL, thenby comparing the CMB velocity vector with the acceleration vector obtained from the galaxysurveys, it is possible to investigate the cause of the LG motion and its cosmological implications.aJ.P. Huchra (CfA), O. Lahav (UCL), M. Colless (AAO), R.M. Cutri (IPAC), E. Falco (Harvard-Smithsonian),T. George (CFTC), T. Jarrett (IPAC), D. H. Jones (AAO), C.S. Kochanek (Ohio State), L. Macri (NOAO), J.Mader (Keck), N. Martimbeau (CfA), M. Pahre (CfA), Q. Parker (Maquarie), A. Rassat (UCL), W. Saunders(AAO, Maquarie)Like peculiar acceleration, the flux of light received from a galaxy falls off inversely as thesquare of the distance. If the mass-to-light ratio is constant, we can relate the two byg(r) = G∑iMirˆir2i≃ G〈ML〉∑iLirˆir2i= 4piG〈ML〉∑iSirˆi, (2)where the sum is over all galaxies in the Universe, 〈M/L〉 is the average mass-to-light ratio andSi = Li/4pir2 is the flux of galaxy i. The peculiar velocity vector is derived by substitutingEquation 2 into the second line of Equation 1. For a flux limited catalogue the observed LGvelocity isv(r) =8piGf(Ωm)3H0ΩmbL〈ML〉∑iwLiSirˆi (3)where bL is the luminosity bias factor introduced to account for the dark matter haloes not fullyrepresented by 2MRS galaxies and 0 ≤ wLi ≤ 1 is the weight assigned to galaxy i to accountfor the luminosity that was not observed due to the flux limit of the survey. With the inclusionof redshift information, we can calculate Equation 3 within concentric spheres with increasingradii and thus estimate the distance at which most of the LG velocity is generated generated(the convergence depth).2 The Two Micron All-Sky Redshift SurveyThe Two Micron All-Sky Redshift Survey (Huchra et al. 2005, Erdog˘du et al. 2006) is thedensest all-sky redshift survey to date. The first phase of 2MRS is now complete. In this phasewe obtained redshifts for approximately 23,150 2MASS galaxies from a total sample of 24,773galaxies with extinction corrected magnitudes (Schlegel, Finkbeiner & Davis 1998) brighter thanKs = 11.25. This magnitude limit corresponds to a median redshift of z ≃ 0.02 (≃ 6000 km s−1).Figure 1 shows all the objects in 2MRS in Galactic Aitoff Projection. Galaxies with z ≤ 0.01are plotted in red, 0.01 < z ≤ 0.025 are plotted in blue, 0.025 < z < 0.05 are plotted in greenand z ≥ 0.05 are plotted in magenta. Galaxies without measured redshifts (around 1600) areplotted in black.The 2MRS sample has very good photometric uniformity and an unprecedented integral skycoverage. The photometric uniformity is better than 4% over the sky including the celestialpoles and the survey is essentially complete down to very low galactic latitudes (Huchra et al.2005). In order to account for incompleteness at these galactic latitudes we fill the plane withgalaxies sampled from adjacent longitude/distance bins.3 ResultsFigure 2 shows the three components and the magnitudes of the acceleration of the Local Groupdue to galaxies within a series of successively larger concentric spheres centred on the localgroup (top plot). The bottom plot shows convergence of the direction of the LG dipole wherethe misalignment angle is between the LG and the CMB dipoles (vLG = 627 ± 22 km s−1,towards lLG = 273◦ ± 3◦, bLG = 29◦ ± 3◦, Bennett et al. 2003, Courteau & Van Den Bergh,1999).It is evident in the top plot that the LG velocity is dominated by structures within a distanceof 6000 km s−1. The ‘tug of war’ between the Great Attractor and the Perseus-Pisces is clearlyevident. The dip in the velocity vector is an indication that the local flow towards the GreatFigure 1: All Objects in the 2MASS Redshift Catalogue in Galactic Aitoff Projection. Galaxies with z ≤ 0.01are plotted in red, 0.01 < z ≤ 0.025 are plotted in blue, 0.025 < z < 0.05 are plotted in green and z ≥ 0.05 areplotted in magenta. Galaxies without measured redshifts are plotted in black. The masked region is outlined bydashed lines.Figure 2: Top: Three components and the magnitudes of the acceleration of the Local Group due to galaxieswithin a series of successively larger concentric spheres centred on the local group. Bottom: Convergence of thedirection of the LG dipole where the misalignment angle is between the LG and the CMB dipoles. The dottedlines denote 1σ errors from shot noise.Attractorb is counteracted by the Perseus-Pisces complex in the opposite direction. The dipoleseems to converge by 6000 km s−1.The misalignment angle between the LG and the CMB dipole is smallest at 5000 km s−1whereit drops to 12◦±7◦ and increases slightly at larger distances presumably due to shot-noise. Thedirection of the flux dipole (l=251◦±12◦,b=37◦± 10 ◦) is in good agreement with the 2MASSdipole derived by Maller et al. (2003). The difference in results is probably due to the fact thatthey use a higher latitude cutoff in the mask (|b| < 7◦) and exclude all galaxies below this lati-tude. We confirm this by changing our treatment of the Zone of Avoidance to match theirs. Wefind that the flux dipole is very close to their dipole direction. Their limiting Kron magnitudeis Ks = 13.57 which corresponds to an effective depth of 200 h−1 Mpc. As their sample is deepenough to pick out galaxies in the Shapley Supercluster, the comparison of their dipole valuewith our values suggests that the contribution to the LG dipole from structure further awaythan the maximum distance of our analysis is not significant.Assuming convergence, we equate the velocity inferred from the CMB measurements withthe value derived from a galaxy survey and obtain a value for the combination of the massdensity and luminosity bias parameters: Ω0.6m /bL = 0.40± 0.09. If we adopt Ωm = 0.23 from theWMAP3 results (Spergel et al. 2006) we get bL ≃ 1. This suggest that the 2MRS galaxies areunbiased.A detailed description of the analysis and results outlined in this paper can be found inErdog˘du et al. (2006).AcknowledgmentsOL acknowledges a PPARC Senior Research Fellowship. JPH, LM, CSK, NM, and TJ aresupported by NSF grant AST-0406906, and EF’s research is partially supported by the Smith-sonian Institution. DHJ is supported as a Research Associate by Australian Research CouncilDiscovery-Projects Grant (DP-0208876), administered by the Australian National University.This publication makes use of data products from the Two Micron All Sky Survey, which isa joint project of the University of Massachusetts and the Infrared Processing and AnalysisCenter/California Institute of Technology, funded by the National Aeronautics and Space Ad-ministration and the National Science Foundation. This research has also made use of theNASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Labora-tory, California Institute of Technology, under contract with the National Aeronautics and SpaceAdministration and the SIMBAD database, operated at CDS, Strasbourg, France.References1. Bennett C.L., et al. , 2003, ApJS, 148, 12. Courteau S. & Van Den Bergh S., 1999, AJ, 118, 3373. Erdog˘du P., et al. , 2006, MNRAS, in press (astro-ph/0507166)4. Huchra J., Jarrett T.H., Skrutskie M., Cutri R., Schneider S., Macri L., Steining R., MaderJ., Martimbeau N., George T., 2005, in Nearby Large-Scale Structures and the Zone ofAvoidance ASP Conference Series, eds. K.P. Fairall, P.A. Woudt, 329, 1355. Maller A.H., McIntosh D.H., Katz N. & Weinberg M.D., 2003, ApJ, 598, L16. Peebles P.J.E., 1980, The Large-Scale Structure of the Universe, Princeton UniversityPress, Princeton7. Schlegel D.J., Finkbeiner D.P., Davis M., 1998, ApJ, 500, 5258. Spergel D.N., et al. , 2006, submitted to ApJbBy ‘Great Attractor’, it is meant the entire steradian on the sky centred at (l ∼ 310◦,b ∼ 20◦) covering adistance of 20 h−1 Mpc to 60 h−1 Mpc.

The Dipole Anisotropy of the 2mass Redshift Survey

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The Dipole Anisotropy of the 2mass Redshift Survey

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