Day 339

Sunday.  Catching up Thanksgiving week.

1211.4874

Kelvin-Helmholtz instabilities at the sloshing cold fronts in the Virgo cluster as a measure for the effective ICM viscosity

Roediger, Kraft, Ofrman, Nulsen, Churazov

Sloshing cold fronts (CFs) arise from minor merger triggered gas sloshing.  Their detailed structure depends on the properties of the ICM: hydrosims predict the CFs to be distorted by KH instabilities (KHIs), but aligned B-fields, viscosity, or thermal conduction can suppress the KHIs. Thus, observing the detailed structure of sloshing CFs can be used to constrain these ICM properties.  Both smooth and distorted sloshing CFs have been observed, indicating that the KHI is suppressed in some clusters, but not in all.  Need to address at least some sloshing clusters individually before drawing general conclusions about the ICM properties.  Present first detailed attempt to constrain the ICM properties in a specific cluster from the structure of its sloshing CF.  Proximity and brightness make the Virgo cluster and ideal target.  Combine observations and Virgo-specific hydrodynamical sloshing simulations.  Focus on a Spitzer-like temperature dependent viscosity as a mechanism to suppress the KHI, but discuss the alternative mechanisms in detail.  For viscosities >~ 10% of Spitzer values KHIs, CF are suppressed.  Describe in detail the observable signatures at low and high viscosities, i.e., in the presence or absence of KHIs.  Find indications for a low ICM viscosity in archival XMM-Newton data and demonstrate the detectability of the predicted features in deep Chandra observations.

1211.4875

The extensive age gradient of the Carina dwarf galaxy

Battaglia, Irwin, Tolstoy, de Boer, Mateo

The evolution of small systems such as dSph's is likely to have been a balance between external environmental effects and internal processes within their own relatively shallow potential wells.  Assessing how strong such environmental interactions may have been is therefore an important element in understanding the baryonic evolution of dSphs and their derived DM distribution.  Present results from a wide-area CTIO/MOSAIC II photometric survey of Carina dSph, reaching down to ~2 mags below the oldest MS turn-off (MSTO).  This dataset enables tracing of Carina structure in detail out to very large distances from its center, and as a function of stellar age.  Observe the precense of an extended structure made up primarily of ancient MSTO stars, at distances between 25-60 arcmin from Carina's center, confirming that Carina extends well beyond its nominal tidal radius.  The large number statistics of survey reveals features such as isophote twists and tails that were undetected in other shallower surveys.  Unambiguous signs of tidal disruption, other than Sagittarius dwarf. Demonstrate the presence of a negative age gradient in Carina from its MSTOs, and trace it out to very large distances from the center.  Signs of interaction with MW make it unclear whether the age gradient was already in place before Carina underwent tidal disruption.

1211.5025

On the shear estimation bias induced by the spatial variation of colour across galaxy profiles

Semboloni, Hoekstra, ... Joachimi, Kitching, Kuijken, ... Mellier, Miller, Rhodes, .. Schrabback, Velander et al

Spatial variation of the color of a galaxy may introduce a bias in the measurement of its shape if the PSF profile depends on wavelength.  Study how this bias depends on the properties of the PSF and the galaxies themselves.  The bias depends on the scales used to estimate the shape, which may be used to optimise methods to reduce the bias.  here we develop a general approach to quantify the bias.  Although applicable to any WL survey, focus on the implications for Euclid.  Based on study of synthetic galaxies, find that the bias is a few x 1e-3 for a typical galaxy observed by Euclid.  Consequently, it cannot be neglected and needs to be accounted for.  Demonstrate how one can do so using spatially resolved observations of galaxies in two filters.  Show that HST observations in the F606W and F814W filters allow modeling and reduce the bias by an order of magnitude, sufficient to meet Euclid's scientific requirements.  The precision of the correction is ultimately determined by the number of galaxies for which spatially-resolved observations in at least 2 filters are available.  Use results from the Millennium Simulation to demonstrate that archival HST data will be sufficient for the tomographic cosmic shear analysis with the Euclid dataset.  

1211.4864

THe universe at extreme scale: Multi-petaflop sky simulation on the BG/Q

To explore the 'dark universe' requires a corresponding extreme-scale simulation capability the HACC (Hybrid/Hardware accelerated cosmology code) framework has been designed to deliver this level of performance.  HACC attains unprecedented scalable performance: 14 PFlops at 70% of peak, and 90% parallel efficiency on 1M cores and equal number of MPI ranks, and a concurrency of 6.3 million.  Benchmark run with >3.6 trillion particles, significantly larger than any cosmo simulation yet performed.

1211.4579

He II optical depth and UV escape fraction of galaxies

Khaire, Srianand

Study the effect of H I ionizing photons escaping from high-z galaxies on He II ionizing UVB.  Show that, while these photons do not directly interact with He II ions, they play an important role through radiative transport in modifying the shape of He II ionizing part of UVB spectrum . Within the observed range of UV escape from galaxies, show the rapid increase in He II Lyman alpha effective optical depth at z~2.7 can be naturally explained without resorting to pre-overlap area of He II reionization.  A well measured He II Lyman alpha effective optical depth vs z can be used to constrain the z evolution of UV escape from high-z galaxies.  Study also stresses the importance of including galaxy contribution even in the fluctuating UV BG calculations.

1211.4585

Toward a direct measurement of the cosmic acceleration

Darling

Precise HI 21cm absorption line z observed in multiple epochs to directly constrain the secular redshift drift dz/dt_o or the cosmic acceleration dv/dt_o; use Green Bank telescope.  10 objects from 0.09<z<0.69 observed over 13.5 years show the rates.  Measurements are 3 orders of magnitude larger than the theoretically expected acceleration at z=0.5, but they demonstrate the lack of peculiar acceleration in absorption line systems and the long-term frequency stability of modern radio telescopes.  A comparison of UV metal absorption lines to the 21 cm line improves constraints on the cosmic variation of physical constants.  Linear evolution is consistent with no variation.  The cosmic acceleration could be directly measured in ~125 years using current telescopes, or in ~5 years using SKA, but systematic effects will arise at the 1cm/s/yr level.

1211.4590

The imminent detection of gravitational waves from massive black-hole binaries with pulsar timing arrays

McWilliams, Ostriker, Pretorius

Observations: massive balaxies double in mass and quintuple in size between z=1 and present, despite undergoing very little SF, suggesting that galaxy mergers drive the evolution.  Since these galaxies will contain SMBH, this suggests a larger BH merger rate, and tehrefore a larger gravitational-wave signal.  Calculate the merger-driven evolution of the mass function, and find taht merger rates are 10 to 30 times higher and gravitational waves are 3 to 5 times stronger than previously estimated, so that the gravitational-wave signal may already be detectable with existing data from pulsar timing arrays.  Also provide an explanation for the disagreement with past estimates that were based on DM halo simulations.

1211.4610

21-cm lensing and the cold spot in the cosmic microwave background

Kovetz, Kamionkowski

Test CMB large cold spot hypothesis, (i) extremely large void and (ii) cosmic texture, with WL of 21-cm fluctuations from EoR with SKA.  Void can be tested with SKA, texture requires prolonged observations at the highest frequencies that correspond to EoR, over the field of view containing the cold spot.

1211.4612

The ring nebula around the blue supergiant SBW1: pre-explosion snapshot of a SN 1987A twin

Smith (Nathan), ... Filippenko

[The double-polar ring, as also seen in SN 1987, in another star]  Suggest model: does not require a fast wind colliding with a previous red supergiant wind.  Observation also show diffuse emission filling the interior of the ring in H-alpha and thermal-IR emission: 190K dust with 1e-5 Msun.  Dense inner ring causes the two polar rings (model).

1211.4799

Origin of galactic cosmic rays

Blasi

From the energetic point of view, SNRs remain the most plausible sources of CRs: CRs are accelerated through diffusive shock acceleration in SNRs and propagate diffusively in the galaxy in an energy-dependent way.  Qualitative confirmation of the SNR acceleration scenario has recently been provided by gamma ray and X-ray observations.  Diffusive propagation in the Galaxy is probed observationally through measurement of the secondary to primary nuclei flux ratios.  There are some weak points in the paradigm, which suggest missing physical ingredients in the models: Predicted spectra of accelerated particles are systematically too hard compared to gamma-ray observations, and hard injection spectra indirectly imply a steep energy dependence of the diffusion coefficient in the galaxy, which in turn leads to anisotropy larger than the observed one.  Recent measurements of the flux of nuclei suggest that the spectra have a break at some rigidity that does not agree with acceleration and propagation.  

1211.4834

Comments on "A huge reservoir of ionized gas around the Milky Way: accounting for the missing mass?" (2012 ApJL, 756, 8) and "The warm-hot gaseous halo of the Milky Way" (arXiv1211.3137)

Wang, Yao

These papers are seriously flowed in many aspects (analysis, assumptions) as well as mis-reading and mis-interpreting earlier studies. Show examples of such flaws.

1211.4847

A simple fitting method (gfit) for galaxy shape measurement in weak lensing surveys

Gentile, Courbin, Meylan

A simple forward deconvolution method to measure galaxy shapes with PSF and noise.  2d elliptical Sersic profile fit to data, convolved with the PSF.  Apply to GREAT10; lowest additive bias and the second lowest multiplicative bias.  Fitting method, so will be affected by noise bias.  Competitive performances for a relatively low computing time.

1211.4979

Finding the first cosmic explosions I: pair-instability supernovae

Whalen , ... Holz, .. et al

Properties of Pop III stars: direct observations are still decades away, but can probe its properties via their SNe.  JWST can detect such SNe out to z>30; WFIRST in the all-sky survey out to z=15~20, and LSST and Pan-STARRS at z~7-8.  The discovery of these ancient explosions will probe the first stellar populations and reveal the existence of primitive galaxies that might not otherwise have been detected.