NASA/IPAC EXTRAGALACTIC DATABASE
Date and Time of the Query: 2019-04-25 T12:10:52 PDT
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For refcode 1990A&A...231....1G:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1990A&A...231....1G The local anomaly of the extragalactic velocity field E. Giraud Pommiers, Saint-Jean de Valeriscles, F-30960 Les Mages, France Received December 26, 1988; accepted August 28, 1989 Abstract. Newly measured I-band CCD photometry of a nearly complete sample of Virgo spiral galaxies is combined with 21 cm line-widths to derive an unbiased distance modulus {mu}_0_=(m-M)^0^_Virgo_ ~ 30.9 for Virgo. This distance estimate is rather imprecise because it is based on only one intermediate calibrator with Cepheid variables: NGC 2403. In flux-limited samples, the distances depend on the accuracy of the indicator. Thus if the scatter of a distance indicator is not correctly estimated and if a sample is insufficiently corrected for bias, or overcorrected, erroneous values of H_0_ quoted with high accuracy can be generated. In this context we find an acceptable agreement in the value of H_0_ ~ 85 km s^-1^ Mpc^-1^ for the Virgo and Ursa Major Clusters, for field galaxies in the Local Supercluster, and for a previous work on the Hercules Supercluster. Because the age of globular cluster systems (13-14Gy), to which a period of galaxy formation must be added, is too long compared with the cosmological age t_0_= 11.5 Gy implied by H_0_ = 85 km s^-1^ Mpc^-1^, if {DELTA} ={LAMBDA} = 0, we are lead to introduce a non-zero cosmological constant. The velocity-distance relation in the first few Mpc from the Local Group is reexamined in this context assuming a cosmological age of 15 Gy. The deceleration of the very local velocity field due to the mass of the Local Group is compared with the Kahn-Woltjer orbital mass determination of the pair Galaxy - M31. The calculated deceleration due to the Local Group in the case H_0_ = 85 km s^-1^ Mpc^-1^, t_0_ = 15 Gy, is M_LG_ ~ 1.2 10^12^ 2M_sun_, which is smaller by a factor of ~ 3 than the Kahn-Woltjer orbital mass. The deceleration method is not precise enough to decide whether this discrepancy is significant. A natural explanation of the low local value of the expansion rate (i.e. H_local_ = 75 km s^-1^ Mpc^-1^ in the short distance scale) is a retardation of velocities due to the mass associated with the Local Cloud. If the mass-to-light ratio of the Local Cloud is of the order of 50 (M/L_B_)_sun_, the local field must be significantly perturbed. Our simple modelling suggests that there is enough mass within the Local Cloud to cause a variation of the observed expansion rate from the local value of H_0_ = 75 km s^-1^ Mpc^-1^ within the Local Cloud, to a global value of H_0_ = 85 km s^-1^ Mpc^-1^, without invoking an extreme mass-to-light ratio. Because the Local Cloud is highly anisotropic and the Local Group is at the periphery of the Local Cloud, a local dipole must be present. If bright galaxies trace well the mass distribution, the apex of the dipole is SGL = 88^deg^ +/- 6^deg^; SGB = -4^deg^ +/- 4^deg^. Its intensity depends on the M/L of the Cloud. If M/L ~ 90(M/L_B_), where L is the luminosity of bright galaxies of the Local Cloud, the retardation velocity of the Local Group is ~ 60 km s^-1^. It seems reasonable to argue that this local dipole has been detected but misinterpreted as being due to the Virgo pull, because its direction is at ~ 25^deg^ from the Virgo complex. The corrected retardation velocity of the Local group caused by the Virgo complex is found to be {DELTA}V^LG^_Virgo_ = 210 - 230 km s^-1^, and thereby the mass of the Virgo region is M = 3.9 - 4.5 10^14^M_sun_. Key words: cosmology - distances, distance scale
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