Likelihood Non-Gaussianity in Large-Scale Structure Analyses

Standard present day large-scale structure (LSS) analyses make a major assumption in their Bayesian parameter inference --- that the likelihood has a Gaussian form. For summary statistics currently used in LSS, this assumption, even if the underlying density field is Gaussian, cannot be correct in detail. We investigate the impact of this assumption on two recent LSS analyses: the Beutler et al. (2017) power spectrum multipole ($P_\ell$) analysis and the Sinha et al. (2017) group multiplicity function ($\zeta$) analysis. Using non-parametric divergence estimators on mock catalogs originally constructed for covariance matrix estimation, we identify significant non-Gaussianity in both the $P_\ell$ and $\zeta$ likelihoods. We then use Gaussian mixture density estimation and Independent Component Analysis on the same mocks to construct likelihood estimates that approximate the true likelihood better than the Gaussian $pseudo$-likelihood. Using these likelihood estimates, we accurately estimate the true posterior probability distribution of the Beutler et al. (2017) and Sinha et al. (2017) parameters. Likelihood non-Gaussianity shifts the $f\sigma_8$ constraint by $-0.44\sigma$, but otherwise, does not significantly impact the overall parameter constraints of Beutler et al. (2017). For the $\zeta$ analysis, using the pseudo-likelihood significantly underestimates the uncertainties and biases the constraints of Sinha et al. (2017) halo occupation parameters. For $\log M_1$ and $\alpha$, the posteriors are shifted by $+0.43\sigma$ and $-0.51\sigma$ and broadened by $42\%$ and $66\%$, respectively. The divergence and likelihood estimation methods we present provide a straightforward framework for quantifying the impact of likelihood non-Gaussianity and deriving more accurate parameter constraints.Comment: 33 pages, 7 figure

Milky Way Mass and Potential Recovery Using Tidal Streams in a Realistic Halo

We present a new method for determining the Galactic gravitational potential based on forward modeling of tidal stellar streams. We use this method to test the performance of smooth and static analytic potentials in representing realistic dark matter halos, which have substructure and are continually evolving by accretion. Our FAST-FORWARD method uses a Markov Chain Monte Carlo algorithm to compare, in 6D phase space, an "observed" stream to models created in trial analytic potentials. We analyze a large sample of streams evolved in the Via Lactea II (VL2) simulation, which represents a realistic Galactic halo potential. The recovered potential parameters are in agreement with the best fit to the global, present-day VL2 potential. However, merely assuming an analytic potential limits the dark matter halo mass measurement to an accuracy of 5 to 20%, depending on the choice of analytic parametrization. Collectively, mass estimates using streams from our sample reach this fundamental limit, but individually they can be highly biased. Individual streams can both under- and overestimate the mass, and the bias is progressively worse for those with smaller perigalacticons, motivating the search for tidal streams at galactocentric distances larger than 70 kpc. We estimate that the assumption of a static and smooth dark matter potential in modeling of the GD-1 and Pal5-like streams introduces an error of up to 50% in the Milky Way mass estimates.Comment: 12 pages, 6 figures, submitted to ApJ; more information on our stream sample and a movie of the potential recovery method used can be found at http://www.astro.yale.edu/abonaca/research/potential_recovery.htm

Foreground and Source of a Cluster of Ultra-high Energy Cosmic Rays

We investigate the origin of a nearly pointlike cluster of 5 ultrahigh energy cosmic rays at RA ~169.2deg and dec ~56.8deg, using Sloan Digital Sky Survey and other data. No particular source candidates are found near the estimated source direction, but the direction is exceptional in having a likely merging pair of galaxy clusters at 140/h Mpc, with an unusually low foreground density. Large scale shocks or another product of the merging galaxy clusters may accelerate the UHECRs, or the merging galaxy clusters may be coincidental and the UHECRs may be accelerated in a rare event of an unexceptional progenitor. Low magnetic deflections in the foreground void may explain why this is the only identified pointlike cluster of so many UHECRs.Comment: 5 p

Globular Cluster Streams as Galactic High-Precision Scales - The Poster Child Palomar 5

Using the example of the tidal stream of the Milky Way globular cluster Palomar 5 (Pal 5), we demonstrate how observational data on streams can be efficiently reduced in dimensionality and modeled in a Bayesian framework. Our approach combines detection of stream overdensities by a Difference-of-Gaussians process with fast streakline models, a continuous likelihood function built from these models, and inference with MCMC. By generating $\approx10^7$ model streams, we show that the geometry of the Pal 5 debris yields powerful constraints on the solar position and motion, the Milky Way and Pal 5 itself. All 10 model parameters were allowed to vary over large ranges without additional prior information. Using only SDSS data and a few radial velocities from the literature, we find that the distance of the Sun from the Galactic Center is $8.30\pm0.25$ kpc, and the transverse velocity is $253\pm16$ km/s. Both estimates are in excellent agreement with independent measurements of these quantities. Assuming a standard disk and bulge model, we determine the Galactic mass within Pal 5's apogalactic radius of 19 kpc to be $(2.1\pm0.4)\times10^{11}$ M$_\odot$. Moreover, we find the potential of the dark halo with a flattening of $q_z = 0.95^{+0.16}_{-0.12}$ to be essentially spherical within the radial range that is effectively probed by Pal 5. We also determine Pal 5's mass, distance and proper motion independently from other methods, which enables us to perform vital cross-checks. We conclude that with more observational data and by using additional prior information, the precision of this method can be significantly increased.Comment: 28 pages, 14 figures, submitted to ApJ (revised version), comments welcom

What triggers galaxy transformations? The environments of post-starburst galaxies

(abridged) There are good observational reasons to believe that the progenitors of red galaxies have undergone starbursts, followed by a post-starburst phase. We investigate the environments of post-starburst galaxies by measuring \textsl{(1)} number densities in $8 h^{-1} \mathrm{Mpc}$ radius comoving spheres, \textsl{(2)} transverse distances to nearest Virgo-like galaxy clusters, and \textsl{(3)} transverse distances to nearest luminous-galaxy neighbors. We compare the post-starburst galaxies to currently star-forming galaxies identified solely by A-star excess or \Halpha emission. We find that post-starburst galaxies are in the same kinds of environments as star-forming galaxies; this is our ``null hypothesis''. More importantly, we find that at each value of the A-star excess, the star-forming and post-starburst galaxies lie in very similar distributions of environment. The only deviations from our null hypothesis are barely significant: a slight deficit of post-starburst galaxies (relative to the star-forming population) in very low-density regions, a small excess inside the virial radii of clusters, and a slight excess with nearby neighbors. None of these effects is strong enough to make the post-starburst galaxies a high-density phenomenon, or to argue that the starburst events are primarily triggered by external tidal impulses (e.g., from close passages of massive galaxies). The small excess inside cluster virial radii suggests that some post-starbursts are triggered by interactions with the intracluster medium, but this represents a very small fraction of all post-starburst galaxies.Comment: ApJ in pres

Parylene-AlOx Stacks for Improved 3D Encapsulation Solutions

The demand for ultra-tight encapsulation solutions with excellent barrier and high conformality properties has increased in recent years. To meet these challenges, thin-film barrier coatings have emerged as a promising solution. In this study, we investigate well-established silicon-based plasma-enhanced chemical vapor deposition (PECVD) and metal oxide atomic layer deposition (ALD) barrier coatings deposited at low temperatures (≤100 °C) regarding their abilities to address high-level 3D encapsulation applications. Various combinations of such layers are evaluated by measuring the water vapor transmission rate (WVTR) and considering the conformality properties. The impact and the benefits of the organic film integration, namely parylene VT4 grade, on the barrier performances is assessed. Among these combinations, parylene-AlOx stack emerges as one of the most effective solutions, obtaining a WVTR of 3.1 × 10^−4 g m^−2 day^−1 at 38°C and 90% relative humidity conditions

Development of a Water Transmission Rate (WTR) Measurement System for Implantable Barrier Coatings.

While water vapor transmission rate (WVTR) measurement is standardly used to assess material permeability, a system able to quantify liquid water transmission rate (WTR) measurement is highly desirable for implantable thin film barrier coatings. Indeed, since implantable devices are in contact or immersed in body fluids, liquid WTR was carried out to obtain a more realistic measurement of the barrier performance. Parylene is a well-established polymer which is often the material of choice for biomedical encapsulation applications due to its flexibility, biocompatibility, and attractive barrier properties. Four grades of parylene coatings were tested with a newly developed permeation measurement system based on a quadrupole mass spectrometer (QMS) detection method. Successful measurements of gas and water vapor and the water transmission rates of thin parylene films were performed and validated, comparing the results with a standardized method. In addition, the WTR results allowed for the extraction of an acceleration transmission rate factor from the vapor-to-liquid water measurement mode, which varies from 4 to 4.8 between WVTR and WTR. With a WTR of 72.5 µm g m-2 day-1, parylene C displayed the most effective barrier performance