Constrained simulations of the local universe – II. The nature of the local Hubble flow

Monthly Notices of the Royal Astronomical Society, Aug 2009

Using a suite of N-body simulations in different cold dark matter (CDM) scenarios, with cosmological constant (ΛCDM) and without (OCDM, SCDM), we study the Hubble flow (σH) in Local Volumes (LV) around Local Group (LG) like objects found in these simulations, and compare the numerical results with the most recent observations. We show that ΛCDM and OCDM models exhibit the same behaviour of σH. Hence, we demonstrate that the observed coldness of the Hubble flow is not likely to be a manifestation of the dark energy, contrary to previous claims. The coldness does not constitute a problem by itself but it poses a problem to the standard ΛCDM model only if the mean density within the LV is greater than twice the mean matter cosmic density. The lack of blueshifted galaxies in the LV, outside of the LG can be considered as another manifestation of the coldness of the flow. Finally, we show that the main dynamical parameter that affects the coldness of the flow is the relative isolation of the LG, and the absence of nearby Milky Way like objects within a distance of about 3 Mpc.

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Constrained simulations of the local universe – II. The nature of the local Hubble flow

C 2009 The Authors. Journal compilation C 2009 RAS Constrained simulations of the local universe - II. The nature of the local Hubble flow Luis A. Martinez-Vaquero,1 and Mira Sivan2 0 Astrophysikalisches Institut Potsdam , An der Sternwarte 16, 14482 Potsdam , Germany 1 Racah Institute of Physics, Hebrew University , Jerusalem 91904 , Israel 2 Grupo de Astrof ́ısica, Universidad Auto ́noma de Madrid , Madrid E-280049 , Spain A B S T R A C T Using a suite of N-body simulations in different cold dark matter (CDM) scenarios, with cosmological constant ( CDM) and without (OCDM, SCDM), we study the Hubble flow (σ H) in Local Volumes (LV) around Local Group (LG) like objects found in these simulations, and compare the numerical results with the most recent observations. We show that CDM and OCDM models exhibit the same behaviour of σ H. Hence, we demonstrate that the observed coldness of the Hubble flow is not likely to be a manifestation of the dark energy, contrary to previous claims. The coldness does not constitute a problem by itself but it poses a problem to the standard CDM model only if the mean density within the LV is greater than twice the mean matter cosmic density. The lack of blueshifted galaxies in the LV, outside of the LG can be considered as another manifestation of the coldness of the flow. Finally, we show that the main dynamical parameter that affects the coldness of the flow is the relative isolation of the LG, and the absence of nearby Milky Way like objects within a distance of about 3 Mpc. methods; N-body simulations - methods; numerical - Local Group - dark matter - 1 I N T R O D U C T I O N The neighbourhood of the Local Group (LG) is often described as being cold. This attribute implies that the dispersion of the radial velocities of galaxies from a pure Hubble flow is small, and the ‘smallness’ amounts to less than 100 km s−1. Sandage, Tammann & Hardy (1972 ) studied local departures from a uniform Hubble flow and could only put upper limits on such departures. This led Sandage et al. to conclude that q0 0 (where q0 is the deceleration parameter). Later on Sandage & Tammann (1975) estimated that the upper limit to the mean random motion of field galaxies is 50 km s−1. These early findings of Sandage, Tammann and their collaborators have been corroborated and vigorously improved by many others. Karachentsev et al. (2003) estimated the radial peculiar velocity dispersion of all galaxies within 5.5 Mpc to be 85 km s−1. This value drops down to 41 km s−1 if members of galaxy groups are removed and distance errors are taken into account. Using a newer set of Karachentsev’s data, Tikhonov & Klypin (2009) found a velocity dispersion of 97 km s−1, within 7 Mpc, which reduces after correction for apex motion and distance errors only slightly to 84 km s−1. Maccio`, Governato & Horellou (2005) compiled the data from three different sources: the Cepheid-based distance measurements of the Hubble Space Telescope Key Project (Freedman et al. 2001), distance estimates based on the surface brightness fluctuations method (SBF; Tonry et al. 2001) and Tully–Fisher distances (Tully, Shaya & Pierce 1992). They fitted the data by σ H = 88 ± 20 km s−1 × (R/7 Mpc), where σ H is a measure of the dispersion of the radial velocities around a pure Hubble flow of galaxies within a sphere of radius R (a thorough discussion of the various estimates of σ H is given below). The observational evidences for a local cold Hubble flow seem to be indisputable. Yet, the question arises as to why a σ H of the order of a few tens of km s−1 is labelled as ‘cold’. Namely, by what standard it is cold? Rich clusters of galaxies provide the first and the most robust evidence for a departure from a pure Hubble flow, with a dispersion of peculiar velocities of up to ≈103 km s−1. Compared with the rich clusters, the neighbourhood of the LG is definitely cold. A statistical estimate is also given by the pair weighted velocity dispersion (σ 12) which was measured from the Center of Astrophysics redshift survey to be σ 12(r = 1 h−1 Mpc) = 340 ± 40 km s−1 (Davis & Peebles 1983) . Another more robust measure of the deviation from the Hubble flow is provided by the σ 1 statistics which measures the one-dimensional root mean square (rms) peculiar-velocity dispersion of galaxies relative to their neighbours within a projected radius of 2 h−1 Mpc (Davis, Miller & White 1997) . These authors found σ 1 = 95 ± 16 km s−1 (for the IRAS survey) and 130 ± 15 km s−1 for the UGC catalogue. The measured σ 1 is indeed much ‘hotter’ than the σ H = 25 km s−1 within R = 3.0 Mpc (Karachentsev et al. 2009) . So, with regard to the other measure of the dispersion of peculiar velocities the immediate neighbourhood of the LG is indeed very cold. However, one should recall that the σ 1 and σ 12 measures consider all galaxies in a given survey. The σ H considered here, on the other hand, refers to one particular object, namely the LG, that re (...truncated)


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Luis A. Martinez-Vaquero, Gustavo Yepes, Yehuda Hoffman, Stefan Gottlöber, Mira Sivan. Constrained simulations of the local universe – II. The nature of the local Hubble flow, Monthly Notices of the Royal Astronomical Society, 2009, pp. 2070-2080, 397/4, DOI: 10.1111/j.1365-2966.2009.15093.x