Day 906

Wednesday.


1506.04151
Please of satellite galaxies: when exceptions are the rule
Cautun, et al

The detection of planar structures within the satellite systems of both MW and M31 has been reported as being in stark contradiction to the predictions of LCDM.  Given the ambiguity in defining a planar configurations, it is unclear how to interpret the low incidence of the MW and M31 planes in LCDM.  Investigate the prevalence of satellite planes around galactic mass haloes identified in high resolution cosmo sims.  Find that planar structures are very common, and that ~10% of LCDM haloes have even more prominent planes than those present in the Local Group.  While ubiquitous, the planes of satellite galaxies show a large diversity in their properties.  This precludes using one or two systems as small scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation.  This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the LG and LCDM predictions.  In fact, ~10% of LCDM galactic halos have planes of satellites that are as infrequent as the MW and M31 planes.  The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and the M31 systems, respectively.


1506.04156
The radial variation of HI velocity dispersions in dwarfs and spirals
Ianjamasimanana, et al

Gas velocity dispersions provide important diagnostics of the forces counteracting gravity to prevent collapse of the gas.  Use the 21 cm line of neutral HI to study HI velocity dispersion and HI phases as a function of galaxy morphology in 22 galaxies from the THINGS survey.  Stack individual HI velocity profiles and decompose them into broad and narrow gaussian components.  Study the HI velocity dispersion and the HI surface density, as a function of radius.  For spirals, the velocity dispersions of the narrow and broad components decline with radius and their radial profiles are well described by an exponential function.  For dwarfs, however, the profiles are much flatter.  The single Gaussian dispersions profiles are, in general, flatter than those of the narrow and broad components.  In most cases, the dispersion profiles in the outer disks do not drop as fast fast the SF profiles, derived in the literature.  This indicates the importance of other energy sources in driving HI velocity dispersion in the outer disks.  The radial surface density profiles of spirals and dwarfs are similar.  The surface density profiles of the narrow component decline more steeply than those of the broad component, but not as steep as what was found previously for the molecular component.  As a consequence, the surface density ratio between the narrow and broad components, an estimate of the mass ratio between cold HI and warm HI, tends to decrease with radius.  On average, this ratio is lower in dwarfs than in spirals.  This lack of a narrow, cold HI component in dwarfs may explain their low star formation activity.


1506.04166
Impact of the geo-synchronous orbit radiation environment on the design of astronomical observatories
Kruk, ... Hirata, et al

Geosynchronous orbits are appealing for Solar or astrophysical observatories because they permit continuous data downlink at high rats.  The radiation environment in these orbits presents unique challenges, however. This paper describes the characteristics of the radiation environment in Geo-synchronous orbit and the implications for instrument design.  Radiation-induced BG event rates are given for some simplified shielding models, and for a detailed model of the proposed WFIRST.


1506.04746
Accelerating the 2-point and 3-point galaxy correlation functions using Fourier transforms
Slepian, Eisenstein

Though FTs are a common technique for finding correlation functions, they are not typically used in computations of the anisotropy of the 2PCF about the LoS in wide-angle surveys because the LoS direction is not constant on the Cartesian grid.  Show how FTs can be used to compute the multipole moments of the anisotropic 2PCF.  Also show how FTs can be used to accelerate the 3PCF algorithm of Slepian & Eisenstein 2015.  In both cases, these FT methods allow one to avoid the computational cost of pair counting, which scales as the square of the number density of objects in the survey.  With the upcoming large datasets of DESI, Euclid, and LSST, FT techniques will therefore offer an important complement to simple pair or triplet counts.