1806.10141
A distance of 13 Mpc resolves the claimed anomalies of the galaxy lacking dark matter
Trujillo et al
The claimed detection of a diffuse galaxy lacking DM represents a possible challenge to our understanding of the properties of these galaxies and galaxy formation in general. The galaxy, already identified in photographic plates taken in the summer of 1976 at the UK 48-in Schmidt telescope, presents normal distance-independent properties (e.g. color, velocity dispersion of its globular clusters). However, distance-dependent quantities are at odds with those of other similar galaxies, namely the luminosity function and sizes of its globular clusters, mass-to-light ratio and DM content. Carry out a careful analysis of all extant data and show that they consistently indicate a much shorter distance (13 Mpc) than previously indicated (20 Mpc). With this revised distance, the galaxy appears to be a rather ordinary low surface brightness galaxy (Re=1.4±0.1 Kpc; M*=6.0±3.6e7 Msun) with plenty of room for DM (the fraction of DM inside the half mass radius is >75% and M_halo/M*>20) corresponding to a minimum halo mass >1e9 Msun. At 13 Mpc, the luminosity and structural properties of the globular clusters around the object are the same as the ones found in other galaxies.
Pieter van Dokkum replies:
We fully agree that a distance of 13 Mpc would resolve many of the unusual properties of NGC1052-DF2, although it would imply a peculiar velocity of ~900 km/s. We had carefully considered the possibility that the galaxy is in the foreground of NGC1052, but we believe the proposed distance can be ruled out as it is incompatible with the appearance of the galaxy in the HST images. The key tests and demonstrations are now on the web page that we are maintaining: https://www.pietervandokkum.com/ngc1052-df2 . The central issue is that we do not detect stars below the tip of the red giant branch, as demonstrated in this image: http://www.astro.yale.edu/dokkum/outgoing/models_outskirts.jpg . On the webpage we show that the authors likely confused surface brightness fluctuations for stars, and the AGB for the RGB, in their TRGB analysis. We also show that our SBF measurement is largely independent of calibration uncertainties as it can be tied to galaxies with a secure distance at ~10 Mpc. I apologize for not making the material on the webpage available earlier – we should have provided this as supplementary information in the Nature paper. A small unrelated comment: we do not limit the luminosity function analysis in vD18b to confirmed clusters, as stated here, but include all candidates without a spectrum. We also apply a correction for interlopers using the 3D-HST data set.
1806.11113
The role of images and priors in measuring $H_0$ from supernova Refsdal in galaxy cluster MACS J1149.5+2223
Williams, Liesenborgs
MACS J1149 which lenses SN Refsdal in a quad S1-S4, and two other widely separated mages, SX and SY, with its measured time delays, provide a multiple image gravitational lensing systems that can determine H_0. The remaining uncertainty arises from the mass distribution in the lens, which often has prior assumptions --- such as the mass of the galaxy splitting S1-S4, magnification of SX, and prior assumptions on the mass distribution. To better understand what affects the measurement of H_0, separate out the uncertainties associated with these constraints. Using images alone yields ~1000% uncertainty, despite the fact that the position of SX is recovered to within ~0.04 arcsec (rms~0.26 arcsec) by Grale lens inversion. Fixing the mass of the galaxy that spits S1-S4 would reduce 1 sigma uncertainties to 20-33%, depending on the mass, while fixing the magnification of SX would reduce 1 sigma uncertainties to 35%. Smaller uncertainties, of order few percent, are a consequence of imposing assumptions on the shapes of the galaxy and cluster mass distributions, which may or may not apply in a highly non-equilibrium environment of a merging cluster. Propose that if a measurement of H_0 is to be considered reliable, it must be supported by a wide range of lens inversion methods.
1806.11417
Ghost busting: introducing a new, robust galaxy finder algorithm for simulations
Cañas, et al
Identifying galaxies in hydro sims is a difficult task, particularly in regions of high density such as galaxy groups and clusters. Present a new scale-free shape-independent algorithm to robustly and accurately identify galaxies in simulation, implemented within the phase-space halo-finder code VELOCIraptor. This is achieved by using the full phase-space dispersion tensor for particle assignment and an iterative adjustment of search parameters, which help overcome common structure finding problems. Apply the improved method to theHorizon-AGN simulations and compare galaxy stellar masses (M*), SFR and sizes with the elaborate configuration-space halo finder, HaloMaker. Galaxies living in haloes with >1 galaxy are the most affected by the shortcomings of real-space finders, with their mass, SFR, and sizes being >2 times larger (smaller) in the case of host (satellite) galaxies. Thus, the ability to measure minor/major merger rates and disentangle environmental effects in simulations can be generally hindered if the identification of galaxies is not treated carefully. Though large systematic differences are obtained on a one-to-one basis, the overall Galaxy Stellar Mass Function, the SFR function and mass-size relations are not greatly affected. This is due to isolated galaxies being the most abundant population, dominating broad statistics.
1807.00040
Precision modeling of the matter power spectrum in a Planck-like Universe
Smith, Angulo
Use a new suite of high-resolution N-body sims to improve the calibration of the nonlinear matter power spectrum in the LCDM model, specifically centered on the Planck mission best-fit parameters. On large scales, k<~0.07 h/Mpc, the semi-analytic model evaluates a modified version of the 2-loop Multi-point Propagator Theory of Bernardeau+2012 to predict the matter spectrum. On smaller scales, k>~0.07 h/Mpc, transition to a smoothing-spline-fit model that characterizes the differences between the Takahashi+2012 recalibration of halo fit and the simulations. Then use an additional suite of high-resolution sims to explore the response of the PS to variations in the cosmo params around the Planck best-fit. In particular, examine variations in: the time evolution of DE, the matter density, the physical densities of CDM and baryons, the primordial PS amplitude and spectral index, and its running. Use a further suite of simulations to construct a set of correction functions, which is used to improve halofit's dependence on cosmo parameters. The newly calibrated model reproduces all the data with ~1% precision. Including the possibility of various systematic errors, such as choice of N-body code, resolution, etc., and based on inspection of the scaled second order derivatives, estimate the accuracy of the new code to be <~3% over a well defined parameter hyper-cube, up to k=9.0 h/Mpc and out to z=3. Outside of this hyper-cube the model reverts back to halo fit. Release all of the numerical power spectra data along with the C-code NGenHalofit at https://CosmologyCode@bitbucket.org/ngenhalofitteam/ngenhalofitpublic.git.
1807.00016
Exploring the distance-redshift relation with gravitational wave standard sirens and tomographic weak lensing
Osato
Explore how cosmo params can be constrained with gravitational wave standard sirens and WL. In addition to auto-correlations of these observables, take into account cross-correlations of the projected number density of gravitational wave sources and WL convergence field. For WL, use tomography technique to efficiently obtain information of LSS at wide ranges of z. Combining all these correlations, present a forecast of constraints on 4 cosmo parameters, i.e., Hubble parameter, matter density, the equation of state parameter of DE, and the amplitude of matter fluctuation. In the case of the upcoming surveys such as Euclid for WL and Einstein Telescope for gravitational waves, can place a tight constraint on these parameters.
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