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Cool core galaxy clusters are considered to be dynamically relaxed clusters with regular morphology and highly X-ray luminous central region. However, cool core clusters can also be sites for merging events that exhibit cold fronts in X-ray and minihalos in radio. We present recent radio/X-ray observations of the Phoenix Cluster or South Pole Telescope (SPT)-CL J2344-4243 at the redshift of z = 0.596. Using archival Chandra X-ray observations, we detect spiraling cool gas around the cluster core as well as discover two cold fronts near the core. This is perhaps the most distant galaxy cluster to date known to host cold fronts. Also, we present JVLA8 1.52 GHz observations of the minihalo, previously discovered at 610 MHz with GMRT9 observations in the center of the Phoenix galaxy cluster. The minihalo flux density at 1.52 GHz is 9.65 ± 0.97 mJy with the spectral index between 610 MHz and 1.52 GHz being −0.98 ± 0.16.10 A possible origin of these radio sources is turbulence induced by sloshing of the gas in the cluster core. [ABSTRACT FROM AUTHOR]

Detecting the cosmological sky-averaged (global) 21 cm signal as a function of observed frequency will provide a powerful tool to study the ionization and thermal history of the intergalactic medium (IGM) in the early universe (∼400 million years after the big bang). The greatest challenge in conventional total-power global 21 cm experiments is the removal of the foreground synchrotron emission (∼103–104 K) to uncover the weak cosmological signal (tens to hundreds of millikelvin), especially because the intrinsic smoothness of the foreground spectrum is corrupted by instrumental effects. Although the EDGES (Experiment to Detect the Global EoR Signature) team has recently reported an absorption profile at 78 MHz in the sky-averaged spectrum, it is necessary to confirm this detection with an independent approach. The projection effect from observing anisotropic foreground source emission with a wide-view antenna pointing at the North Celestial Pole can induce a net polarization, referred to as the projection-induced polarization effect (PIPE). Due to Earth's rotation, observations centered at the circumpolar region will impose a dynamic sky modulation on the net polarization's waveforms that is unique to the foreground component. In this study, we review the implementation practicality and underlying instrumental effects of this new polarimetry-based technique with detailed numerical simulations and a test-bed instrument, the Cosmic Twilight Polarimeter. In addition, we explore a singular value decomposition–based analysis approach for separating the foreground and instrumental effects from the background global 21 cm signal using the sky-modulated PIPE. [ABSTRACT FROM AUTHOR]

Most of the current Global 21 cm experiments include ground screens that help moderate effects from the Earth. In this paper, we report on a possible systematic artifact within the ground plane that may produce broad absorption features in the spectra observed by these experiments. Using analytical approximations and numerical modeling, the origin of the artifact and its impact on the sky-averaged spectrum are described. The publicly released EDGES data set, from which a 78 MHz absorption feature was recently suggested, is used to probe for the potential presence of ground plane resonances. While the lack of a noise level for the EDGES spectrum makes traditional goodness-of-fit statistics unattainable, the rms residual can be used to assess the relative goodness of fits performed under similar circumstances. The fit to the EDGES spectrum using a model with a simple two-term foreground and three cavity-mode resonances is compared to a fit to the same spectrum with a model used by the EDGES team consisting of a five-term foreground and a flattened-Gaussian signal. The fits with the physically motivated resonance and empirical flattened-Gaussian models have rms residuals of 20.8 mK (11 parameters) and 24.5 mK (9 parameters), respectively, allowing us to conclude that ground plane resonances constitute another plausible explanation for the EDGES data. [ABSTRACT FROM AUTHOR]

We present results from an X-ray and radio study of the merging galaxy cluster Abell 115. We use the full set of five Chandra observations taken of A115 to date (360 ks total integration) to construct high-fidelity temperature and surface brightness maps. We also examine radio data from the Very Large Array at 1.5 GHz and the Giant Metrewave Radio Telescope at 0.6 GHz. We propose that the high X-ray spectral temperature between the subclusters results from the interaction of the bow shocks driven into the intracluster medium by the motion of the subclusters relative to one another. We have identified morphologically similar scenarios in Enzo numerical N-body/hydrodynamic simulations of galaxy clusters in a cosmological context. In addition, the giant radio relic feature in A115, with an arc-like structure and a relatively flat spectral index, is likely consistent with other shock-associated giant radio relics seen in other massive galaxy clusters. We suggest a dynamical scenario that is consistent with the structure of the X-ray gas, the hot region between the clusters, and the radio relic feature. [ABSTRACT FROM AUTHOR]

The sky-averaged (global) highly redshifted 21 cm spectrum from neutral hydrogen is expected to appear in the VHF range of ∼20–200 MHz and its spectral shape and strength are determined by the heating properties of the first stars and black holes, by the nature and duration of reionization, and by the presence or absence of exotic physics. Measurements of the global signal would therefore provide us with a wealth of astrophysical and cosmological knowledge. However, the signal has not yet been detected because it must be seen through strong foregrounds weighted by a large beam, instrumental calibration errors, and ionospheric, ground, and radio-frequency-interference effects, which we collectively refer to as “systematics.” Here, we present a signal extraction method for global signal experiments which uses Singular Value Decomposition of “training sets” to produce systematics basis functions specifically suited to each observation. Instead of requiring precise absolute knowledge of the systematics, our method effectively requires precise knowledge of how the systematics can vary. After calculating eigenmodes for the signal and systematics, we perform a weighted least square fit of the corresponding coefficients and select the number of modes to include by minimizing an information criterion. We compare the performance of the signal extraction when minimizing various information criteria and find that minimizing the Deviance Information Criterion most consistently yields unbiased fits. The methods used here are built into our widely applicable, publicly available Python package, pylinex, which analytically calculates constraints on signals and systematics from given data, errors, and training sets. [ABSTRACT FROM AUTHOR]

The redshifted 21 cm monopole is expected to be a powerful probe of the epoch of the first stars and galaxies (). The global 21 cm signal is sensitive to the thermal and ionization state of hydrogen gas and thus provides a tracer of sources of energetic photons—primarily hot stars and accreting black holes—which ionize and heat the high redshift intergalactic medium (IGM). This paper presents a strategy for observations of the global spectrum with a realizable instrument placed in a low-altitude lunar orbit, performing night-time 40–120 MHz spectral observations, while on the farside to avoid terrestrial radio frequency interference, ionospheric corruption, and solar radio emissions. The frequency structure, uniformity over large scales, and unpolarized state of the redshifted 21 cm spectrum are distinct from the spectrally featureless, spatially varying, and polarized emission from the bright foregrounds. This allows a clean separation between the primordial signal and foregrounds. For signal extraction, we model the foreground, instrument, and 21 cm spectrum with eigenmodes calculated via Singular Value Decomposition analyses. Using a Markov Chain Monte Carlo algorithm to explore the parameter space defined by the coefficients associated with these modes, we illustrate how the spectrum can be measured and how astrophysical parameters (e.g., IGM properties, first star characteristics) can be constrained in the presence of foregrounds using the Dark Ages Radio Explorer (DARE). [ABSTRACT FROM AUTHOR]

The velocity distribution of galaxies in clusters is not universal; rather, galaxies are segregated according to their spectral type and relative luminosity. We examine the velocity distributions of different populations of galaxies within 89 Sunyaev Zel’dovich (SZ) selected galaxy clusters spanning . Our sample is primarily draw from the SPT-GMOS spectroscopic survey, supplemented by additional published spectroscopy, resulting in a final spectroscopic sample of 4148 galaxy spectra—2868 cluster members. The velocity dispersion of star-forming cluster galaxies is 17 ± 4% greater than that of passive cluster galaxies, and the velocity dispersion of bright () cluster galaxies is 11 ± 4% lower than the velocity dispersion of our total member population. We find good agreement with simulations regarding the shape of the relationship between the measured velocity dispersion and the fraction of passive versus star-forming galaxies used to measure it, but we find a small offset between this relationship as measured in data and simulations, which suggests that our dispersions are systematically low by as much as 3% relative to simulations. We argue that this offset could be interpreted as a measurement of the effective velocity bias that describes the ratio of our observed velocity dispersions and the intrinsic velocity dispersion of dark matter particles in a published simulation result. Measuring velocity bias in this way suggests that large spectroscopic surveys can improve dispersion-based mass-observable scaling relations for cosmology even in the face of velocity biases, by quantifying and ultimately calibrating them out. [ABSTRACT FROM AUTHOR]