arXiv subjects

Sungwook E. Hong

Publications and source records attributed to Sungwook E. Hong.

At least 55 records · Page 3Linked to original sources

arXiv2018

DOTIFS: spectrograph optical and opto-mechanical design

Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS) is a new multi-Integral Field Unit (IFU) instrument, planned to be mounted on the 3.6m Devasthal optical telescope in Nainital, India. It has eight identical, fiber-fed spectrographs to disperse light coming from 16 IFUs. The spectrographs produce 2,304 spectra over a 370-740nm wavelength range simultaneously with a spectral resolution of R=1200-2400. It is composed of all-refractive, all spherical optics designed to achieve on average 26.0% throughput from the telescope to the CCD with the help of high transmission spectrograph optics, volume phase holographic grating, and graded coated e2v 2K by 4K CCD. We present the optical and opto-mechanical design of the spectrograph as well as current development status. Optics and optomechanical components for the spectrographs are being fabricated.

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arXiv2018

Noise Characterization of IUCAA Digital Sampling Array Controller (IDSAC)

IUCAA Digital Sampling Array Controller (IDSAC) is a flexible and generic yet powerful CCD controller which can handle a wide range of scientific detectors. Based on an easily scalable modular backplane architecture consisting of Single Board Controllers (SBC), IDSAC can control large detector arrays and mosaics. Each of the SBCs offers the full functionality required to control a CCD independently. The SBCs can be cold swapped without the need to reconfigure them. Each SBC can handle data from up to four video channels with or without dummy outputs at speeds up to 500 kilo Pixels Per Second (kPPS) Per Channel with a resolution of 16 bits. Communication with Linux based host computer is through a USB3.0 interface, with the option of using copper or optical fibers. A Field Programmable Gate Array (FPGA) is used as the master controller in each SBC which allows great flexibility in optimizing performance by adjusting gain, timing signals, bias levels, etc. using user-editable configuration files without altering the circuit topology. Elimination of thermal kTC noise is achieved via Digital Correlated Double Sampling (DCDS). We present the results of noise performance characterization of IDSAC through simulation, theoretical modeling, and actual measurements. The contribution of different types of noise sources is modeled using a tool to predict noise of a generic DCDS signal chain analytically. The analytical model predicts the net input referenced noise of the signal chain to be 5 electrons for 200k pixels per second per channel readout rate with 3 samples per pixel. Using a cryogenic test set up in the lab, the noise is measured to be 5.4 e (24.3 \muV), for the same readout configuration.

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arXiv2018

Multiple rooks of chess - a generic integral field unit deployment technique

A new field re-configuration technique, Multiple Rooks of Chess (MRC), for multiple deployable Integral Field Spectrographs has been developed. The method involves mechanical geometry as well as an optimized deployment algorithm. The geometry is found to be simple for mechanical implementation. The algorithm initially assigns the IFUs to the target objects and then devises the movement sequence based on the current and the desired IFU positions. The reconfiguration time using the suitable actuators which runs at 20 cm/s is found to be a maximum of 25 seconds for the circular DOTIFS focal plane (180 mm diameter). The Geometry Algorithm Combination (GAC) has been tested on several million mock target configurations with object-to-IFU (τ ) ratio varying from 0.25 to 16. The MRC method is found to-be efficient in target acquisition in terms of field revisit and deployment time without any collision or entanglement of the fiber bundles. The efficiency of the technique does not get affected by the increase in number density of target objects. The technique is compared with other available methods based on sky coverage, flexibility and overhead time. The proposed geometry and algorithm combination is found to have an advantage in all of the aspects.

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arXiv2018

Minkowski Tensors in Two Dimensions - Probing the Morphology and Isotropy of the Matter and Galaxy Density Fields

We apply the Minkowski Tensor statistics to two dimensional slices of the three dimensional density field. The Minkowski Tensors are a set of functions that are sensitive to directionally dependent signals in the data, and furthermore can be used to quantify the mean shape of density peaks. We begin by introducing our algorithm for constructing bounding perimeters around subsets of a two dimensional field, and reviewing the definition of Minkowski Tensors. Focusing on the translational invariant statistic $W^{1,1}_{2}$ - a $2 \times 2$ matrix - we calculate its eigenvalues for both the entire excursion set ($Λ_{1},Λ_{2}$) and for individual connected regions and holes within the set ($λ_{1},λ_{2}$). The ratio of eigenvalues $Λ_{2}/Λ_{1}$ informs us of the presence of global anisotropies in the data, and $\langle λ_{2}/λ_{1} \rangle$ is a measure of the mean shape of peaks and troughs in the density field. We study these quantities for a Gaussian field, then consider how they are modified by the effect of gravitational collapse using the latest Horizon Run 4 cosmological simulation. We find $Λ_{1,2}$ are essentially independent of gravitational collapse, as the process maintains statistical isotropy. However, the mean shape of peaks is modified significantly - overdensities become relatively more circular compared to underdensities of the same area. When applying the statistic to a redshift space distorted density field, we find a significant signal in the eigenvalues $Λ_{1,2}$, suggesting that they can be used to probe the large-scale velocity field.

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arXiv2018

Cosmological constraints from the redshift dependence of the Alcock-Paczynski effect: Dynamical dark energy

We perform an anisotropic clustering analysis of 1,133,326 galaxies from the Sloan Digital Sky Survey (SDSS-III) Baryon Oscillation Spectroscopic Survey (BOSS) Data Release (DR) 12 covering the redshift range $0.15<z<0.69$. The geometrical distortions of the galaxy positions, caused by incorrect cosmological model assumptions, are captured in the anisotropic two-point correlation function on scales 6 -- 40 $h^{-1}\rm Mpc$. The redshift evolution of this anisotropic clustering is used to place constraints on the cosmological parameters. We improve the methodology of Li et al. 2016, to enable efficient exploration of high dimensional cosmological parameter spaces, and apply it to the Chevallier-Polarski-Linder parametrization of dark energy, $w=w_0+w_a{z}/({1+z})$. In combination with the CMB, BAO, SNIa and $H_0$ from Cepheid data, we obtain $Ω_m = 0.301 \pm 0.008,\ w_0 = -1.042 \pm 0.067,\ $ and $w_a = -0.07 \pm 0.29$ (68.3\% CL). Adding our new AP measurements to the aforementioned results reduces the error bars by $\sim$30 -- 40\% and improves the dark energy figure of merit by a factor of $\sim$2. We check the robustness of the results using realistic mock galaxy catalogues.

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arXiv2018

Cosmological Parameter Estimation Using the Genus Amplitude - Application to Mock Galaxy Catalogs

We study the topology of the matter density field in two dimensional slices, and consider how we can use the amplitude $A$ of the genus for cosmological parameter estimation. Using the latest Horizon Run 4 simulation data, we calculate the genus of the smoothed density field constructed from lightcone mock galaxy catalogs. Information can be extracted from the amplitude of the genus by considering both its redshift evolution and magnitude. The constancy of the genus amplitude with redshift can be used as a standard population, from which we derive constraints on the equation of state of dark energy $w_{\rm de}$ - by measuring $A$ at $z \sim 0.1$ and $z \sim 1$, we can place an order $Δw_{\rm de} \sim {\cal O}(15\%)$ constraint on $w_{\rm de}$. By comparing $A$ to its Gaussian expectation value we can potentially derive an additional stringent constraint on the matter density $ΔΩ_{\rm mat} \sim 0.01$. We discuss the primary sources of contamination associated with the two measurements - redshift space distortion and shot noise. With accurate knowledge of galaxy bias, we can successfully remove the effect of redshift space distortion, and the combined effect of shot noise and non-linear gravitational evolution is suppressed by smoothing over suitably large scales $R_{\rm G} \ge 15 {\rm Mpc}/h$. Without knowledge of the bias, we discuss how joint measurements of the two and three dimensional genus can be used to constrain the growth factor $β= f/b$. The method can be applied optimally to redshift slices of a galaxy distribution generated using the drop-off technique.

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arXiv2017

Small-scale Effects of Thermal Inflation on Halo Abundance at High-$z$, Galaxy Substructure Abundance and 21-cm Power Spectrum

We study the impact of thermal inflation on the formation of cosmological structures and present astrophysical observables which can be used to constrain and possibly probe the thermal inflation scenario. These are dark matter halo abundance at high redshifts, satellite galaxy abundance in the Milky Way, and fluctuation in the 21-cm radiation background before the epoch of reionization. The thermal inflation scenario leaves a characteristic signature on the matter power spectrum by boosting the amplitude at a specific wavenumber determined by the number of e-foldings during thermal inflation ($N_{\rm bc}$), and strongly suppressing the amplitude for modes at smaller scales. For a reasonable range of parameter space, one of the consequences is the suppression of minihalo formation at high redshifts and that of satellite galaxies in the Milky Way. While this effect is substantial, it is degenerate with other cosmological or astrophysical effects. The power spectrum of the 21-cm background probes this impact more directly, and its observation may be the best way to constrain the thermal inflation scenario due to the characteristic signature in the power spectrum. The Square Kilometre Array (SKA) in phase 1 (SKA1) has sensitivity large enough to achieve this goal for models with $N_{\rm bc}\gtrsim 26$ if a 10000-hr observation is performed. The final phase SKA, with anticipated sensitivity about an order of magnitude higher, seems more promising and will cover a wider parameter space.

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arXiv2017

CMB Spectral Distortion Constraints on Thermal Inflation

Thermal inflation is a second epoch of exponential expansion at typical energy scales $V^{1/4} \sim 10^{6 \sim 8} \mathrm{GeV}$. If the usual primordial inflation is followed by thermal inflation, the primordial power spectrum is only modestly redshifted on large scales, but strongly suppressed on scales smaller than the horizon size at the beginning of thermal inflation, $k > k_{\rm b} = a_{\rm b} H_{\rm b}$. We calculate the spectral distortion of the cosmic microwave background generated by the dissipation of acoustic waves in this context. For $k_{\rm b} \ll 10^3 \mathrm{Mpc}^{-1}$, thermal inflation results in a large suppression of the $μ$-distortion amplitude, predicting that it falls well below the standard value of $μ\simeq 2\times 10^{-8}$. Thus, future spectral distortion experiments, similar to PIXIE, can place new limits on the thermal inflation scenario, constraining $k_{\rm b} \gtrsim 10^3 \mathrm{Mpc}^{-1}$ if $μ\simeq 2\times 10^{-8}$ were found.

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arXiv2017

Cosmological constraints from the redshift dependence of the volume effect using the galaxy 2-point correlation function across the line-of-sight

We develop a methodology to use the redshift dependence of the galaxy 2-point correlation function (2pCF) across the line-of-sight, $ξ(r_{\bot})$, as a probe of cosmological parameters. The positions of galaxies in comoving Cartesian space varies under different cosmological parameter choices, inducing a {\it redshift-dependent scaling} in the galaxy distribution. This geometrical distortion can be observed as a redshift-dependent rescaling in the measured $ξ(r_{\bot})$. We test this methodology using a sample of 1.75 billion mock galaxies at redshifts 0, 0.5, 1, 1.5, 2, drawn from the Horizon Run 4 N-body simulation. The shape of $ξ(r_{\bot})$ can exhibit a significant redshift evolution when the galaxy sample is analyzed under a cosmology differing from the true, simulated one. Other contributions, including the gravitational growth of structure, galaxy bias, and the redshift space distortions, do not produce large redshift evolution in the shape. We show that one can make use of this geometrical distortion to constrain the values of cosmological parameters governing the expansion history of the universe. This method could be applicable to future large scale structure surveys, especially photometric surveys such as DES, LSST, to derive tight cosmological constraints. This work is a continuation of our previous works as a strategy to constrain cosmological parameters using redshift-invariant physical quantities.

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arXiv2017

A question of separation: disentangling tracer bias and gravitational nonlinearity with counts-in-cells statistics

Starting from a very accurate model for density-in-cells statistics of dark matter based on large deviation theory, a bias model for the tracer density in spheres is formulated. It adopts a mean bias relation based on a quadratic bias model to relate the log-densities of dark matter to those of mass-weighted dark haloes in real and redshift space. The validity of the parametrised bias model is established using a parametrisation-independent extraction of the bias function. This average bias model is then combined with the dark matter PDF, neglecting any scatter around it: it nevertheless yields an excellent model for densities-in-cells statistics of mass tracers that is parametrised in terms of the underlying dark matter variance and three bias parameters. The procedure is validated on measurements of both the one and two point statistics of subhalo densities in the state-of-the-art Horizon Run 4 simulation showing excellent agreement for measured dark matter variance and bias parameters. Finally, it is demonstrated that this formalism allows for a joint estimation of the nonlinear dark matter variance and the bias parameters using solely the statistics of subhaloes. Having verified that galaxy counts in hydrodynamical simulations sampled on a scale of 10 Mpc/h closely resemble those of subhaloes, this work provides important steps towards making theoretical predictions for density-in-cells statistics applicable to upcoming galaxy surveys like Euclid or WFIRST.

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arXiv2017

Topology of Large-Scale Structures of Galaxies in Two Dimensions - Systematic Effects

We study the two-dimensional topology of the galactic distribution when projected onto two-dimensional spherical shells. Using the latest Horizon Run 4 simulation data, we construct the genus of the two-dimensional field and consider how this statistic is affected by late-time nonlinear effects -- principally gravitational collapse and redshift space distortion (RSD). We also consider systematic and numerical artifacts such as shot noise, galaxy bias, and finite pixel effects. We model the systematics using a Hermite polynomial expansion and perform a comprehensive analysis of known effects on the two-dimensional genus, with a view toward using the statistic for cosmological parameter estimation. We find that the finite pixel effect is dominated by an amplitude drop and can be made less than $1\%$ by adopting pixels smaller than $1/3$ of the angular smoothing length. Nonlinear gravitational evolution introduces time-dependent coefficients of the zeroth, first, and second Hermite polynomials, but the genus amplitude changes by less than $1\%$ between $z=1$ and $z=0$ for smoothing scales $R_{\rm G} > 9 {\rm Mpc/h}$. Non-zero terms are measured up to third order in the Hermite polynomial expansion when studying RSD. Differences in shapes of the genus curves in real and redshift space are small when we adopt thick redshift shells, but the amplitude change remains a significant $\sim {\cal O}(10\%)$ effect. The combined effects of galaxy biasing and shot noise produce systematic effects up to the second Hermite polynomial. It is shown that, when sampling, the use of galaxy mass cuts significantly reduces the effect of shot noise relative to random sampling.

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arXiv2017

Effects of thermal inflation on small scale density perturbations

In cosmological scenarios with thermal inflation, extra eras of moduli matter domination, thermal inflation and flaton matter domination exist between primordial inflation and the radiation domination of Big Bang nucleosynthesis. During these eras, cosmological perturbations on small scales can enter and re-exit the horizon, modifying the power spectrum on those scales. The largest modified scale, $k_\mathrm{b}$, touches the horizon size when the expansion changes from deflation to inflation at the transition from moduli domination to thermal inflation. We analytically calculate the evolution of perturbations from moduli domination through thermal inflation and evaluate the curvature perturbation on the constant radiation density hypersurface at the end of thermal inflation to determine the late time curvature perturbation. Our resulting transfer function suppresses the power spectrum by a factor $\sim 50$ at $k \gg k_\mathrm{b}$, with $k_\mathrm{b}$ corresponding to anywhere from megaparsec to subparsec scales depending on the parameters of thermal inflation. Thus, thermal inflation might be constrained or detected by small scale observations such as CMB distortions or 21cm hydrogen line observations.

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arXiv2016

Cosmological constraints from the redshift dependence of the Alcock-Paczynski effect: application to the SDSS-III BOSS DR12 galaxies

We apply the methodology developed in \cite{Li2014,Li2015} to BOSS DR12 galaxies and derive cosmological constraints from the redshift dependence of the Alcock-Paczynski (AP) effect. The apparent anisotropy in the distribution of observed galaxies arise from two main sources, the redshift-space distortion (RSD) effect due to the galaxy peculiar velocities, and the geometric distortion when incorrect cosmological models are assumed for transforming redshift to comoving distance, known as the AP effect. Anisotropies produced by the RSD effect are, although large, maintaining a nearly uniform magnitude over a large range of redshift, while the degree of anisotropies from the AP effect varies with redshift by much larger magnitude. We split the DR12 galaxies into six redshift bins, measure the 2-point correlation function in each bin, and assess the redshift evolution of anisotropies. We obtain constraints of $Ω_m=0.290 \pm 0.053,\ \ w = -1.07 \pm 0.15$, which are comparable with the current constraints from other cosmological probes such as type Ia supernovae, cosmic microwave background, and baryon acoustic oscillation (BAO). Combining these cosmological probes with our method yield tight constraints of $ Ω_m = 0.301 \pm 0.006,\ w=-1.054 \pm 0.025$. Our method is complementary to the other large scale structure probes like BAO and topology. We expect this technique will play an important role in deriving cosmological constraints from large scale structure surveys.

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arXiv2016

AAO Starbugs: software control and associated algorithms

The Australian Astronomical Observatory's TAIPAN instrument deploys 150 Starbug robots to position optical fibres to accuracies of 0.3 arcsec, on a 32 cm glass field plate on the focal plane of the 1.2 m UK-Schmidt telescope. This paper describes the software system developed to control and monitor the Starbugs, with particular emphasis on the automated path-finding algorithms, and the metrology software which keeps track of the position and motion of individual Starbugs as they independently move in a crowded field. The software employs a tiered approach to find a collision-free path for every Starbug, from its current position to its target location. This consists of three path-finding stages of increasing complexity and computational cost. For each Starbug a path is attempted using a simple method. If unsuccessful, subsequently more complex (and expensive) methods are tried until a valid path is found or the target is flagged as unreachable.

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arXiv2016

The Most Bound Halo Particle-Galaxy Correspondence Model: Comparison between Models with Different Merger Timescales

We develop a galaxy assignment scheme that populates dark matter halos with galaxies by tracing the most bound member particles (MBPs) of simulated halos. Several merger-timescale models based on analytic calculations and numerical simulations are adopted as the survival time of mock satellite galaxies. We build mock galaxy samples from halo merger data of the Horizon Run 4 $N$-body simulation from $z = 12-0$. We compare group properties and two-point correlation functions (2pCFs) of mock galaxies with those of volume-limited SDSS galaxies, with $r$-band absolute magnitudes of $\mathcal{M}_r - 5 \log h < -21$ and $-20$ at $z=0$. It is found that the MBP-galaxy correspondence scheme reproduces the observed population of SDSS galaxies in massive galaxy groups ($M > 10^{14} h^{-1} M_{\odot}$) and the small-scale 2pCF ($r_{\rm p} < 10 h^{-1} {\rm Mpc}$) quite well for the majority of the merger timescale models adopted. The new scheme outperforms the previous subhalo-galaxy correspondence scheme by more than $2σ$.

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arXiv2015

Radio and X-ray Shocks in Clusters of galaxies

Radio relics detected in the outskirts of galaxy clusters are thought to trace radio-emitting relativistic electrons accelerated at cosmological shocks. In this study, using the cosmological hydrodynamic simulation data for the large-scale structure formation and adopting a diffusive shock acceleration (DSA) model for the production of cosmic-ray (CR) electrons, we construct mock radio and X-ray maps of simulated galaxy clusters that are projected in the sky plane. Various properties of shocks and radio relics, including the shock Mach number, radio spectral index and luminosity are extracted from the synthetic maps and compared with observations. A substantial fraction of radio and X-ray shocks identified in these maps involve multiple shock surfaces along line of sights (LoSs), and the morphology of shock distributions in the maps depends on the projection direction. Among multiple shocks in a given LoS, radio observations tend to pick up stronger shocks with flatter radio spectra, while X-ray observations preferentially select weaker shocks with larger kinetic energy flux. As a result, the shock Mach numbers and locations derived from radio and X-ray observations could differ from each other in some cases. We also find that the distributions of the spectral index and radio power of the synthetic radio relics are somewhat inconsistent with those of observed real relics; a bit more radio relics have been observed closer to the cluster core and with steeper spectral indices. We suggest the inconsistency could be explained, if very weak shocks with $M_s \la 2$ accelerate CR electrons more efficiently, compared to the DSA model adopted here.

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arXiv2015

Horizon Run 4 Simulation: Coupled Evolution of Galaxies and Large-scale Structures of the Universe

The Horizon Run 4 is a cosmological $N$-body simulation designed for the study of coupled evolution between galaxies and large-scale structures of the Universe, and for the test of galaxy formation models. Using $6300^3$ gravitating particles in a cubic box of $L_{\rm box} = 3150 ~h^{-1}{\rm Mpc}$, we build a dense forest of halo merger trees to trace the halo merger history with a halo mass resolution scale down to $M_s = 2.7 \times 10^{11} h^{-1}{\rm M_\odot}$. We build a set of particle and halo data, which can serve as testbeds for comparison of cosmological models and gravitational theories with observations. We find that the FoF halo mass function shows a substantial deviation from the universal form with tangible redshift evolution of amplitude and shape. At higher redshifts, the amplitude of the mass function is lower, and the functional form is shifted toward larger values of $\ln (1/σ)$. We also find that the baryonic acoustic oscillation feature in the two-point correlation function of mock galaxies becomes broader with a peak position moving to smaller scales and the peak amplitude decreasing for increasing directional cosine $μ$ compared to the linear predictions. From the halo merger trees built from halo data at 75 redshifts, we measure the half-mass epoch of halos and find that less massive halos tend to reach half of their current mass at higher redshifts. Simulation outputs including snapshot data, past lightcone space data, and halo merger data are available at http://sdss.kias.re.kr/astro/Horizon-Run4/.

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arXiv2015

March of the Starbugs: Configuring Fibre-bearing Robots on the UK-Schmidt Optical Plane

The TAIPAN instrument, currently being developed for the Australian Astronomical Observatory's UK Schmidt telescope at Siding Spring Observatory, makes use of the AAO's Starbug technology to deploy 150 science fibres to target positions on the optical plane. This paper describes the software system for controlling and deploying the fibre-bearing Starbug robots. The TAIPAN software is responsible for allocating each Starbug to its next target position based on its current position and the distribution of targets, finding a collision-free path for each Starbug, and then simultaneously controlling the Starbug hardware in a closed loop, with a metrology camera used to determine the position of each Starbug in the field during reconfiguration. The software is written in C++ and Java and employs a DRAMA middleware layer (Farrell et al. 1995).

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