Helene Courtois

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 corotating 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 alignmentofsatellitesonvastplanesisunexpectedinthestandardcolddarkmatter(� CDM) model of cosmology if not even in contradiction to its generic predictions. Guided byCDM numerical simulations, which suggest that satellites are channelled 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 data set. 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 Ways satellite plane can likely be ascribed to its greater susceptibilitytotidaltorques,assuggestedbynumericalsimulations.Thealignmentofsatellite systems in the local Universe with the ambient shear field is thus in general agreement with predictions of theCDM model.

From Spitzer Galaxy photometry to Tully–Fisher distances

This paper involves a data release of the observational campaign: Cosmicflows with Spitzer (CFS). Surface photometry of the 1270 galaxies constituting the survey is presented. An additional ∼400 galaxies from various other Spitzer surveys are also analysed. CFS complements the Spitzer Survey of Stellar Structure in Galaxies, that provides photometry for an additional 2352 galaxies, by extending observations to low galactic latitudes (|b| < 30°). Among these galaxies are calibrators, selected in the K band, of the Tully–Fisher relation. The addition of new calibrators demonstrates the robustness of the previously released calibration. Our estimate of the Hubble constant using supernova host galaxies is unchanged, H_0 = 75.2 ± 3.3 km s^(−1) Mpc^(−1). Distance-derived radial peculiar velocities, for the 1935 galaxies with all the available parameters, will be incorporated into a new data release of the Cosmicflows project. The size of the previous catalogue will be increased by 20 peru2009cent, including spatial regions close to the Zone of Avoidance.

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 (LSS) out to distances of a few x 100 h(-1) Mpc. Using the largest and most accurate ever catalogue of galaxy peculiar velocities Cosmicflows-2, the LSS has been reconstructed by means of the Wiener filter (WF) and constrained realizations (CRs) assuming as a Bayesian prior model the Lambda cold dark matter model with the WMAP inferred cosmological parameters. This 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 h(-1) Mpc. The estimated LSS, in general, and the bulk flow, in particular, are determined by the tension between the observational data and the assumed prior model. A pre-requisite 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) h(-1) Mpc, the estimated bulk velocity is 250 +/- 21 (239 +/- 38) km s(-1). The corresponding cosmic variance at these radii is 126 (60) km s(-1), 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 approximate to 200 h(-1) Mpc, where the cosmic variance on the individual supergalactic Cartesian components (of the rms values) exceeds the variance of the CRs by at least a factor of 2. The SGX and SGY components of the cosmic microwave background dipole velocity are recovered by the WF velocity field down to a very few km s(-1). The SGZ component of the estimated velocity, the one that is most affected by the zone of avoidance, is off by 126 km s(-1) (an almost 2 sigma discrepancy). The bulk velocity analysis reported here is virtually unaffected by the Malmquist bias and very similar results are obtained for the data with and without the bias correction.

Simulations of the Local Universe Constrained by Observational Peculiar Velocities

Peculiar velocities, obtained from direct distance measur ements, are data of choice to achieve constrained simulations of the Local Universe reliable down to a scale of a few Megaparsecs. Unlike redshift surveys, peculiar velocities are direct tracers o f the underlying gravitational field as they trace both baryonic and dark matter. This paper presents the first attempt to use solely observational peculiar velocities to constrain cosmological simulation s of the nearby universe. In order to set up Initial Conditions, a Reverse Zel’dovich Approximation (R ZA) is used to displace constraints from their positions at z=0 to their precursors’ locations at higher redshifts. An add itional new feature replaces original observed radial peculiar velocity vecto rs by their full 3D reconstructions provided by the Wiener-Filter (WF) estimator. Subsequently, the Constrained Realization of Gaussian fields technique (CR) is applied to build various realizations of t he Initial Conditions. The WF/RZA/CR method is first tested on realistic mock catalogs built from a reference simulation similar to the Local Universe. These mocks include errors on peculiar velocities, on data-point positions and a large continuous zone devoid of data in order to mimic galactic extinction. Large scale structures are recovered with a typical accuracy of 5 h −1 Megaparsecs in position, the best realizations reaching a 2-3 h −1 Mpc precision, the limit imposed by the RZA linear theory. Then, the method is applied to the first observational radial peculiar velocity catalog of the project Cosmicflows. This paper is a proof of concept that the WF/RZA/CR method can be applied to observational peculiar velocities to successfully build constrained Initial Conditions.

SOFIA: A flexible source finder for 3D spectral line data

We introduce SOFIA, a flexible software application for the detection and parametrization of sources in 3D spectral line data sets. SOFIA combines for the first time in a single piece of software a set of new source-finding and parametrization algorithms developed on the way to future H I surveys with ASKAP (WALLABY, DINGO) and APERTIF. It is designed to enable the general use of these new algorithms by the community on a broad range of data sets. The key advantages of SOFIA are the ability to: search for line emission on multiple scales to detect 3D sources in a complete and reliable way, taking into account noise level variations and the presence of artefacts in a data cube; estimate the reliability of individual detections; look for signal in arbitrarily large data cubes using a catalogue of 3D coordinates as a prior; provide a wide range of source parameters and output products which facilitate further analysis by the user. We highlight the modularity of SOFIA, which makes it a flexible package allowing users to select and apply only the algorithms useful for their data and science questions. This modularity makes it also possible to easily expand SOFIA in order to include additional methods as they become available. The full SOFIA distribution, including a dedicated graphical user interface, is publicly available for download.

Reconstructing cosmological initial conditions from galaxy peculiar velocities – I. Reverse Zeldovich Approximation

We propose a new method to recover the cosmological initial conditions of the presently observed galaxy distribution, which can serve to run constrained simulations of the Local Universe. Our method, the Reverse Zeldovich Approximation (RZA), can be applied to radial galaxy peculiar velocity data and extends the previously used Constrained Realizations (CR) method by adding a Lagrangian reconstruction step. The RZA method consists of applying the Zeldovich approximation in reverse to galaxy peculiar velocities to estimate the cosmic displacement field and the initial linear matter distributi on from which the present-day Local Universe evolved. We test our method with a mock survey taken from a cosmological simulation. We show that the halo peculiar velocities at z = 0 are close to the linear prediction of the Zeldovich approximation, if a grouping is applied to the data to remove virial motions. We find that the addition of RZA to the CR method significantly imp roves the reconstruction of the initial conditions. The RZA is able to recover the correct initial positions of the velocity tracers with a median error of only 1.36 Mpc/h in our test simulation. For realistic sparse and noisy data, this median increases to 5 Mpc/h. This is a significant improvement over the previous approach of neglecting the displacement field, which i ntroduces errors on a scale of 10 Mpc/h or even higher. Applying the RZA method to the upcoming high-quality observational peculiar velocity catalogues will generate much more precise constrained simulations of the Local Universe.

The alignment of galaxy spin with the shear field in observations

Tidal torque theory suggests that galaxies gain angular momentum in the linear stage of structure formation. Such a theory predicts alignments between the spin of haloes and tidal shear field. However, non-linear evolution and angular momentum acquisition may alter this prediction significantly. In this paper, we use a reconstruction of the cosmic shear field from observed peculiar velocities combined with spin axes extracted from galaxies within

Filaments from the galaxy distribution and from the velocity field in the local universe

115, mathrm{Mpc}

Anatomy of Ursa Majoris

(

Reconstructing cosmological initial conditions from galaxy peculiar velocities – II. The effect of observational errors

sim8000 , {mathrm {km}}{mathrm s}^{-1}