Cosmic Bulk Flow and the Local Motion from Cosmicflows-2

Full sky surveys of peculiar velocity are arguably the best way to map the large scale structure out to distances of a few times 100 Mpc/h. Using the largest and most accurate ever catalog of galaxy peculiar velocities "Cosmicflows-2", the large scale structure has been reconstructed by means of the Wiener filter and constrained realizations assuming as a Bayesian prior model the LCDM model with the WMAP inferred cosmological parameters. The present paper focuses on studying the bulk flow of the local flow field, defined as the mean velocity of top-hat spheres with radii ranging out to R=500 Mpc/h. The estimated large scale structures, in general, and the bulk flow, in particular, are determined by the tension between the observational data and the assumed prior model. A prerequisite for such an analysis is the requirement that the estimated bulk flow is consistent with the prior model. Such a consistency is found here. At R=50(150) Mpc/h the estimated bulk velocity is 250+/-21 (239+/-38) km/s. The corresponding cosmic variance at these radii is 126(60)km/s, which implies that these estimated bulk flows are dominated by the data and not by the assumed prior model. The estimated bulk velocity is dominated by the data out to R~200 Mpc/h, where the cosmic variance on the individual Supergalactic Cartesian components (of the r.m.s. values) exceeds the variance of the Constrained Realizations by at least a factor of 2. The supergalactic SGX and SGY components of the CMB dipole velocity are recovered by the Wiener filter velocity field down to a very few km/s. The SGZ component of the estimated velocity, the one that is most affected by the Zone of Avoidance, is off by 126 km/s (an almost 2 sigma discrepancy).Comment: 10 pages, accepted for MNRA

The Mid-Infrared Tully-Fisher Relation: Calibration of the SNIa Scale and Ho

This paper builds on a calibration of the SNIa absolute distance scale begun with a core of distances based on the correlation between galaxy rotation rates and optical Ic band photometry. This new work extends the calibration through the use of mid-infrared photometry acquired at 3.6 microns with Spitzer Space Telescope. The great virtue of the satellite observations is constancy of the photometry at a level better than 1% across the sky. The new calibration is based on 39 individual galaxies and 8 clusters that have been the sites of well observed SNIa. The new 3.6 micron calibration is not yet as extensively based as the Ic band calibration but is already sufficient to justify a preliminary report. Distances based on the mid-infrared photometry are 2% greater in the mean than reported at Ic band. This difference is only marginally significant. The Ic band result is confirmed with only a small adjustment. Incorporating a 1% decrease in the LMC distance, the present study indicates Ho = 75.2 +/- 3.0 km/s/Mpc.Comment: Accepted for publication in The Astrophysical Journal Letters, 6 pages, 2 figure

Goodness-of-fit analysis of the Cosmicflows-2 database of velocities

The goodness-of-fit (GoF) of the Cosmicflows-2 (CF2) database of peculiar velocities with the LCDM standard model of cosmology is presented. Standard application of the Chi^2 statistics of the full database, of its 4,838 data points, is hampered by the small scale nonlinear dynamics which is not accounted for by the (linear regime) velocity power spectrum. The bulk velocity constitutes a highly compressed representation of the data which filters out the small scales non-linear modes. Hence the statistics of the bulk flow provides an efficient tool for assessing the GoF of the data given a model. The particular approach introduced here is to use the (spherical top-hat window) bulk velocity extracted from the Wiener filter reconstruction of the 3D velocity field as a linear low pass filtered highly compressed representation of the CF2 data. An ensemble 2250 random linear realizations of the WMAP/LCDM model has been used to calculate the bulk velocity auto-covariance matrix. We find that the CF2 data is consistent with the WMAP/LCDM model to better than the 2 sigma confidence limits. This provides a further validation that the CF2 database is consistent with the standard model of cosmology.Comment: submitted to MNRAS, V2 : solved page sizing proble

The Arrowhead Mini-Supercluster of Galaxies

Superclusters of galaxies can be defined kinematically from local evaluations of the velocity shear tensor. The location where the smallest eigenvalue of the shear is positive and maximal defines the center of a basin of attraction. Velocity and density fields are reconstructed with Wiener Filter techniques. Local velocities due to the density field in a restricted region can be separated from external tidal flows, permitting the identification of boundaries separating inward flows toward a basin of attraction and outward flows. This methodology was used to define the Laniakea Supercluster that includes the Milky Way. Large adjacent structures include Perseus-Pisces, Coma, Hercules, and Shapley but current kinematic data are insufficient to capture their full domains. However there is a small region trapped between Laniakea, Perseus-Pisces, and Coma that is close enough to be reliably characterized and that satisfies the kinematic definition of a supercluster. Because of its shape, it is given the name the Arrowhead Supercluster. This entity does not contain any major clusters. A characteristic dimension is ~25 Mpc and the contained mass is only ~10^15 Msun.Comment: Accepted for publication in The Astrophysical Journal. Video can be viewed at http://irfu.cea.fr/arrowhea

Bimodality of Galaxy Disk Central Surface Brightness Distribution in the Spitzer 3.6 micron band

We report on measurements of the disk central surface brightnesses (mu0) at 3.6 microns for 438 galaxies selected by distance and absolute magnitude cutoffs from the 2350+ galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G), one of the largest and deepest homogeneous mid-infrared datasets of nearby galaxies. Our sample contains nearly 3 times more galaxies than the most recent study of the mu0 distribution. We demonstrate that there is a bimodality in the distribution of mu0. Between the low and high surface brightness galaxy regimes there is a lack of intermediate surface brightness galaxies. Caveats invoked in the literature from small number statistics to the knowledge of the environmental influences, and possible biases from low signal to noise data or corrections for galaxy inclination are investigated. Analyses show that the bimodal distribution of mu0 cannot be due to any of these biases or statistical fluctuations. It is highly probable that galaxies settle in two stable modes: a dark matter dominated mode where the dark matter dominates at all radii - this gives birth to low surface brightness galaxies - and a baryonic matter dominated mode where the baryons dominate the dark matter in the central parts - this gives rise to the high surface brightness disks. The lack of intermediate surface brightness objects suggests that galaxies avoid (staying in) a mode where dark matter and baryons are co-dominant in the central parts of galaxies.Comment: Accepted for publication in MNRAS, 9 pages, 10 figures, 1 tabl

Planes of satellite galaxies and the cosmic web

Recent observational studies have demonstrated that the majority of satellite galaxies tend to orbit their hosts on highly flattened, vast, possibly co-rotating planes. Two nearly parallel planes of satellites have been confirmed around the M31 galaxy and around the Centaurus A galaxy, while the Milky Way also sports a plane of satellites. It has been argued that such an alignment of satellites on vast planes is unexpected in the standard ({\Lambda}CDM) model of cosmology if not even in contradiction to its generic predictions. Guided by {\Lambda}CDM numerical simulations, which suggest that satellites are channeled towards hosts along the axis of the slowest collapse as dictated by the ambient velocity shear tensor, we re-examine the planes of local satellites systems within the framework of the local shear tensor derived from the Cosmicflows-2 dataset. The analysis reveals that the Local Group and Centaurus A reside in a filament stretched by the Virgo cluster and compressed by the expansion of the Local Void. Four out of five thin planes of satellite galaxies are indeed closely aligned with the axis of compression induced by the Local Void. Being the less massive system, the moderate misalignment of the Milky Way's satellite plane can likely be ascribed to its greater susceptibility to tidal torques, as suggested by numerical simulations. The alignment of satellite systems in the local universe with the ambient shear field is thus in general agreement with predictions of the {\Lambda}CDM model.Comment: 9 pages, 3 figures, 3 tables. Accepted by MNRAS, 9 June 201