Validating a novel angular power spectrum estimator using simulated low frequency radio-interferometric data [CEA]

The “Tapered Gridded Estimator” (TGE) is a novel way to directly estimate the angular power spectrum from radio-interferometric visibility data that reduces the computation by efficiently gridding the data, consistently removes the noise bias, and suppresses the foreground contamination to a large extent by tapering the primary beam response through an appropriate convolution in the visibility domain. Here we demonstrate the effectiveness of TGE in recovering the diffuse emission power spectrum through numerical simulations. We present details of the simulation used to generate low frequency visibility data for sky model with extragalactic compact radio sources and diffuse Galactic synchrotron emission. We then use different imaging strategies to identify the most effective option of point source subtraction and to study the underlying diffuse emission. Finally, we apply TGE to the residual data to measure the angular power spectrum, and assess the impact of incomplete point source subtraction in recovering the input power spectrum $C_{\ell}$ of the synchrotron emission. This estimator is found to successfully recovers the $C_{\ell}$ of input model from the residual visibility data. These results are relevant for measuring the diffuse emission like the Galactic synchrotron emission. It is also an important step towards characterizing and removing both diffuse and compact foreground emission in order to detect the redshifted $21\, {\rm cm}$ signal from the Epoch of Reionization.

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S. Choudhuri, N. Roy, S. Bharadwaj, et. al.
Thu, 29 Jun 17

Comments: 18 pages, 1 table, 9 figures, Accepted for publication in New Astronomy

Simulating the effect of high column density absorbers on the one-dimensional Lyman-alpha forest flux power spectrum [CEA]

We measure the effect of high column density absorbing systems of neutral hydrogen (HI) on the one-dimensional (1D) Lyman-alpha forest flux power spectrum using cosmological hydrodynamical simulations from the Illustris project. High column density absorbers (which we define to be those with HI column densities $N(\mathrm{HI}) > 1.6 \times 10^{17}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$) cause broadened absorption lines with characteristic damping wings. These damping wings bias the 1D Lyman-alpha forest flux power spectrum by causing absorption in quasar spectra away from the location of the absorber itself. We investigate the effect of high column density absorbers on the Lyman-alpha forest using hydrodynamical simulations for the first time. We provide templates as a function of column density and redshift, allowing the flexibility to accurately model residual contamination, i.e., if an analysis selectively clips out the largest damping wings. This flexibility will improve cosmological parameter estimation, e.g., allowing more accurate measurement of the shape of the power spectrum, with implications for cosmological models containing massive neutrinos or a running of the spectral index. We provide fitting functions to reproduce these results so that they can be incorporated straightforwardly into a data analysis pipeline.

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K. Rogers, S. Bird, H. Peiris, et. al.
Wed, 28 Jun 17

Comments: 11 pages, 5 figures. To be submitted to MNRAS

Intrinsic Alignments and Splashback Radius of Dark Matter Halos from Cosmic Density and Velocity Fields [CEA]

We investigate the effects of intrinsic alignments (IA) of dark-matter halo shapes on cosmic density and velocity fields from cluster to cosmic scales beyond 100 Mpc/h. Besides the density correlation function binned by the halo orientation angle which was used in the literature, we introduce, for the first time, the corresponding two velocity statistics, the angle-binned pairwise infall momentum and momentum correlation function. Using large-volume, high-resolution N-body simulations, we measure the alignment statistics of density and velocity, both in real and redshift space. We find that the alignment signal is not amplified by redshift-space distortions at linear scales. Behaviors of IA in the velocity statistics are similar to those in the density statistics, except that the halo orientations are aligned with the velocity field up to a scale larger than those with the density field, x>100 Mpc/h. On halo scales, x~ R_{200m} ~ 1 Mpc/h, we detect a sharp steepening in the momentum correlation associated with the physical halo boundary, or the splashback feature, which is found more prominent than in the density correlation. Our results indicate that observations of IA with the velocity field can provide additional information on cosmological models from large scales and on physical sizes of halos from small scales.

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T. Okumura, T. Nishimichi, K. Umetsu, et. al.
Wed, 28 Jun 17

Comments: 5 pages, 3 figures

Shot noise and biased tracers: a new look at the halo model [CEA]

Shot noise is an important ingredient to any measurement or theoretical modeling of discrete tracers of the large scale structure. Recent work has shown that the shot noise in the halo power spectrum becomes increasingly sub-Poissonian at high mass. Interestingly, while the halo model predicts a shot noise power spectrum in qualitative agreement with the data, it leads to an unphysical white noise in the cross halo-matter and matter power spectrum. In this work, we show that absorbing all the halo model sources of shot noise into the halo fluctuation field leads to meaningful predictions for the shot noise contributions to halo clustering statistics and remove the unphysical white noise from the cross halo-matter statistics. Our prescription straightforwardly maps onto the general bias expansion, so that the renormalized shot noise terms can be expressed as combinations of the halo model shot noises. Furthermore, we demonstrate that non-Poissonian contributions are related to volume integrals over correlation functions and their response to long-wavelength density perturbations. This leads to a new class of consistency relations for discrete tracers, which appear to be satisfied by our reformulation of the halo model. We test our theoretical predictions against measurements of halo shot noise bispectra extracted from a large suite of numerical simulations. Our model reproduces qualitatively the observed sub-Poissonian noise, although it underestimates the magnitude of this effect.

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D. Ginzburg, V. Desjacques and K. Chan
Wed, 28 Jun 17

Comments: 20 pages, 3 figures, to be submitted to PRD

On general features of warm dark matter with reduced relativistic gas [CEA]

We investigate warm dark matter (WDM) features in a model independent approach through the very simple approximation of the Reduced Relativistic Gas (RRG). Our only and generic supposition is a non-negligible velocity $v$ for dark matter particles which is parameterized by a free parameter $b$. We show that high values for WDM velocities would erase radiation dominated epoch. This would cause an early warm matter domination after inflation, unless $b^2\lesssim 10^{-6}$ (or $v\lesssim 300 km/s$). Also it is shown that RRG approach allows to quantify the lack of power in linear matter spectrum at small scales and in particular, reproduces the relative transfer function commonly used in context of WDM with accuracy of $\lesssim 1\%$. This result with such accuracy does not alter significantly the CMB power spectrum agreeing also with the background observational tests. This suggests that the RRG approximation can be used as a complementary approach to investigate consequences of warmness of dark matter and especially for deriving the main observational exponents for the WDM in a model-independent way in linear and non-linear regime.

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W. Hipolito-Ricaldi, R. Marttens, J. Fabris, et. al.
Wed, 28 Jun 17

Comments: 13 pages, 8 figures

Discerning Dark Energy Models with High-Redshift Standard Candles [CEA]

Following the success of type Ia supernovae in constraining cosmologies at lower redshift $(z\lesssim2)$, effort has been spent determining if a similarly useful standardisable candle can be found at higher redshift. {In this work we determine the largest possible magnitude discrepancy between a constant dark energy $\Lambda$CDM cosmology and a cosmology in which the equation of state $w(z)$ of dark energy is a function of redshift for high redshift standard candles $(z\gtrsim2)$}. We discuss a number of popular parametrisations of $w(z)$ with two free parameters, $w_z$CDM cosmologies, including the Chevallier-Polarski-Linder and generalisation thereof, $n$CPL, as well as the Jassal-Bagla-Padmanabhan parametrisation. For each of these parametrisations we calculate and find extrema of $\Delta \mu$, the difference between the distance modulus of a $w_z$CDM cosmology and a fiducial $\Lambda$CDM cosmology as a function of redshift, given 68\% likelihood constraints on the parameters $P=(\Omega_{m,0}, w_0, w_a)$. The parameters are constrained using cosmic microwave background, baryon acoustic oscillations, and type Ia supernovae data using CosmoMC. We find that none of the tested cosmologies can deviate more than 0.05 mag from the fiducial $\Lambda$CDM cosmology at high redshift, implying that high redshift standard candles will not aid in discerning between a $w_z$CDM cosmology and the fiducial $\Lambda$CDM cosmology. Conversely, this implies that if high redshift standard candles are found to be in disagreement with $\Lambda$CDM at high redshift, then this is a problem not only for $\Lambda$CDM but for the entire family of $w_z$CDM cosmologies.

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P. Andersen and J. Hjorth
Wed, 28 Jun 17

Comments: 9 pages, 4 figues. Submitted to MNRAS

A scale dependent bias on linear scales: the case for HI intensity mapping at z=1 [CEA]

Neutral hydrogen (HI) will soon be the dark matter tracer observed over the largest volumes of Universe thanks to the 21 cm intensity mapping technique. To unveil cosmological information it is indispensable to understand the HI distribution with respect to dark matter. Using a full one-loop derivation of the power spectrum of HI, we show that higher order corrections change the amplitude and shape of the power spectrum on typical cosmological (linear) scales. These effects go beyond the expected dark matter non-linear corrections and include non-linearities in the way the HI signal traces dark matter. We show that, on linear scales at z = 1, the HI bias drops by up to 15% in both real and redshift space, which results in underpredicting the mass of the halos in which HI lies. Non-linear corrections give rise to a significant scale dependence when redshift space distortions arise, in particular on the scale range of the baryonic acoustic oscillations (BAO). There is a factor of 5 difference between the linear and full HI power spectra over the full BAO scale range, which will modify the ratios between the peaks. This effect will also be seen in other types of survey and it will be essential to take it into account in future experiments in order to match the expectations of precision cosmology.

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A. Penin, O. Umeh and M. Santos
Wed, 28 Jun 17

Comments: 10 pages, 7 figures, Submitted to MNRAS