arXiv subjects

Jaehong Park

Publications and source records attributed to Jaehong Park.

34 records · Page 2Linked to original sources

arXiv2016

The spatial distribution of neutral hydrogen as traced by low HI mass galaxies

The formation and evolution of galaxies with low neutral atomic hydrogen (HI) masses, M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$, are affected by host dark matter halo mass and photoionisation feedback from the UV background after the end of reionization. We study how the physical processes governing the formation of galaxies with low HI mass are imprinted on the distribution of neutral hydrogen in the Universe using the hierarchical galaxy formation model, GALFORM. We calculate the effect on the correlation function of changing the HI mass detection threshold at redshifts $0 \le z \le 0.5$. We parameterize the clustering as $ξ(r)=(r/r_{0})^{-γ}$ and we find that including galaxies with M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$ increases the clustering amplitude $r_{0}$ and slope $γ$ compared to samples of higher HI masses. This is due to these galaxies with low HI masses typically being hosted by haloes with masses greater than 10$^{12}{h}^{-1}$M$_{\odot}$, and is in contrast to optically selected surveys for which the inclusion of faint, blue galaxies lowers the clustering amplitude. We show the HI mass function for different host dark matter halo masses and galaxy types (central or satellite) to interpret the values of $r_{0}$ and $γ$ of the clustering of HI-selected galaxies. We also predict the contribution of low HI mass galaxies to the 21cm intensity mapping signal. We calculate that a dark matter halo mass resolution better than $\sim$10$^{10}{h}^{-1}$M$_{\odot}$ at redshifts higher than 0.5 is required in order to predict converged 21cm brightness temperature fluctuations.

astro-ph.GA↗

arXiv2016

DeepSpark: A Spark-Based Distributed Deep Learning Framework for Commodity Clusters

The increasing complexity of deep neural networks (DNNs) has made it challenging to exploit existing large-scale data processing pipelines for handling massive data and parameters involved in DNN training. Distributed computing platforms and GPGPU-based acceleration provide a mainstream solution to this computational challenge. In this paper, we propose DeepSpark, a distributed and parallel deep learning framework that exploits Apache Spark on commodity clusters. To support parallel operations, DeepSpark automatically distributes workloads and parameters to Caffe/Tensorflow-running nodes using Spark, and iteratively aggregates training results by a novel lock-free asynchronous variant of the popular elastic averaging stochastic gradient descent based update scheme, effectively complementing the synchronized processing capabilities of Spark. DeepSpark is an on-going project, and the current release is available at http://deepspark.snu.ac.kr.

cs.LG↗

arXiv2016

The clustering and halo occupation distribution of Lyman-break galaxies at $z\sim4$

We investigate the clustering of Lyman-break galaxies (LBGs) at $z\sim4$. Using the hierarchical galaxy formation model GALFORM, we predict, for the first time using a semi-analytical model with feedback from active galactic nuclei (AGN), the angular correlation function (ACF) of LBGs and find agreement within $3\,σ$ with new measurements of the ACF from surveys including the Hubble eXtreme Deep Field (XDF) and CANDELS field. Our simulations confirm the conclusion reached using independent models that although the predicted ACFs reproduce the trend of increased clustering with luminosity, the dependence is less strong than observed. We find that for the detection limits of the XDF field central LBGs at $z\sim 4$ predominantly reside in haloes of mass $\sim 10^{11}-10^{12}h^{-1}M_{\rm \odot}$ and that satellites reside in larger haloes of mass $\sim 10^{12}-10^{13}h^{-1}M_{\rm \odot}$. The model predicts fewer bright satellite LBGs at $z\sim4$ than is inferred from measurements of the ACF at small scales. By analysing the halo occupation distribution (HOD) predicted by the model, we find evidence that AGN feedback affects the HOD of central LBGs in massive haloes. This is a new high-redshift test of this important feedback mechanism. We investigate the effect of photometric errors in the observations on the ACF predictions. We find that the observational uncertainty in the galaxy luminosity reduces the clustering amplitude and that this effect increases towards faint galaxies, particularly on small scales. To compare properties of model with observed LBGs this uncertainty must be considered.

astro-ph.GA↗

arXiv2015

A hybrid multi resolution scheme to efficiently model the structure of reionization on the largest scales

Redshifted 21cm measurements of the structure of ionised regions that grow during reionization promise to provide a new probe of early galaxy and structure formation. One of the challenges of modelling reionization is to account both for the sub-halo scale physics of galaxy formation and the regions of ionization on scales that are many orders of magnitude larger. To bridge this gap we first calculate the statistical relationship between ionizing luminosity and Mpc-scale overdensity using detailed models of galaxy formation computed using relatively small volume - ($\sim$100Mpc/$h$)$^{3}$, high resolution dark matter simulations. We then use a Monte-Carlo technique to apply this relationship to reionization of the intergalactic medium within large volume dark matter simulations - ($>$1Gpc/$h$)$^{3}$. The resulting simulations can be used to address the contribution of very large scale clustering of galaxies to the structure of reionization, and show that volumes larger than 500Mpc/$h$ are required to probe the largest reionization features mid-way through reionization. As an example application of our technique, we demonstrate that the predicted 21cm power spectrum amplitude and gradient could be used to determine the importance of supernovae feedback for early galaxy formation.

astro-ph.GA↗

arXiv2015

The HI mass function as a probe of photoionisation feedback on low mass galaxy formation

We explore the galaxy formation physics governing the low mass end of the HI mass function in the local Universe. Specifically, we predict the effects on the HI mass function of varying i) the strength of photoionisation feedback and the redshift of the end of the epoch of reionization, ii) the cosmology, iii) the supernovae feedback prescription, and iv) the efficiency of star formation. We find that the shape of the low-mass end of the HI mass function is most affected by the critical halo mass below which galaxy formation is suppressed by photoionisation heating of the intergalactic medium. We model the redshift dependence of this critical dark matter halo mass by requiring a match to the low-mass end of the HI mass function. The best fitting critical dark matter halo mass decreases as redshift increases in this model, corresponding to a circular velocity of $\sim 50 \, {\rm km \,s}^{-1}$ at $z=0$, $\sim 30 \, {\rm km\, s}^{-1}$ at $z \sim 1$ and $\sim 12 \, {\rm km \, s}^{-1}$ at $z=6$. We find that an evolving critical halo mass is required to explain both the shape and abundance of galaxies in the HI mass function below $M_{\rm HI} \sim 10^{8} h^{-2} {\rm M_{\odot}}$. The model makes specific predictions for the clustering strength of HI-selected galaxies with HI masses > $10^{6} h^{-2} {\rm M_{\odot}}$ and $> 10^{7} h^{-2} {\rm M_{\odot}}$ and for the relation between the HI and stellar mass contents of galaxies which will be testable with upcoming surveys with the Square Kilometre Array and its pathfinders. We conclude that measurements of the HI mass function at $z \ge 0$ will lead to an improvement in our understanding of the net effect of photoionisation feedback on galaxy formation and evolution.

astro-ph.GA↗

arXiv2015

Simultaneous Acceleration of Protons and Electrons at Nonrelativistic Quasiparallel Collisionless Shocks

We study diffusive shock acceleration (DSA) of protons and electrons at nonrelativistic, high Mach number, quasiparallel, collisionless shocks by means of self-consistent 1D particle-in-cell simulations. For the first time, both species are found to develop power-law distributions with the universal spectral index $-4$ in momentum space, in agreement with the prediction of DSA. We find that scattering of both protons and electrons is mediated by right-handed circularly polarized waves excited by the current of energetic protons via non-resonant hybrid (Bell) instability. Protons are injected into DSA after a few gyro-cycles of shock drift acceleration (SDA), while electrons are first pre-heated via SDA, then energized via a hybrid acceleration process that involves both SDA and Fermi-like acceleration mediated by Bell waves, before eventual injection into DSA. Using the simulations we can measure the electron/proton ratio in accelerated particles, which is of paramount importance for explaining the cosmic ray fluxes measured at Earth and the multi-wavelength emission of astrophysical objects such as supernova remnants, radio supernovae, and galaxy clusters. We find the normalization of electron power-law is $\lesssim 10^{-2}$ that of the protons for strong nonrelativistic shocks.

astro-ph.HE↗

arXiv2013

The cross-power spectrum between 21cm emission and galaxies in hierarchical galaxy formation models

The correlation between 21cm fluctuations and galaxies is sensitive to the astrophysical properties of the galaxies that drove reionization. Thus, detailed measurements of the cross-power spectrum and its evolution could provide a powerful measurement both of the properties of early galaxies and the process of reionization. In this paper, we study the evolution of the cross-power spectrum between 21cm emission and galaxies using a model which combines the hierarchical galaxy formation model GALFORM implemented within the Millennium-II dark matter simulation, with a semi-numerical scheme to describe the resulting ionization structure. We find that inclusion of different feedback processes changes the cross-power spectrum shape and amplitude. In particular, the feature in the cross-power spectrum corresponding to the size of ionized regions is significantly affected by supernovae feedback. We calculate predicted observational uncertainties of the cross-correlation coefficient based on specifications of the Murchison Widefield Array (MWA) combined with galaxy surveys of varying area and depth. We find that the cross-power spectrum could be detected over several square degrees of galaxy survey with galaxy redshift errors less than 0.1.

astro-ph.CO↗

arXiv2013

Variation in the escape fraction of ionising photons from galaxies and the redshifted 21-cm power spectrum during reionization

The observed power spectrum of redshifted 21cm fluctuations is known to be sensitive to the astrophysical properties of the galaxies that drove reionization. Thus, detailed measurements of the 21cm power spectrum and its evolution could lead to measurements of the properties of early galaxies that are otherwise inaccessible. In this paper, we study the effect of mass and redshift dependent escape fractions of ionizing radiation on the ability of forthcoming experiments to constrain galaxy formation via the redshifted 21cm power spectrum. We use a model for reionization which combines the hierarchical galaxy formation model GALFORM implemented within the Millennium-II dark matter simulation, with a semi-numerical scheme to describe the resulting ionization structure. Using this model we show that the structure and distribution of ionised regions at fixed neutral fraction, and hence the slope and amplitude of the 21 cm power spectrum, is dependent on the variation of ionising photon escape fraction with galaxy mass and redshift. However, we find that the influence of the unknown escape fraction and its evolution is smaller than the dominant astrophysical effect provided by SNe feedback strength in high redshift galaxies. The unknown escape fraction of ionizing radiation from galaxies is therefore unlikely to prevent measurement of the properties of high redshift star formation using observations of the 21cm power spectrum.

astro-ph.CO↗

arXiv2013

Particle-in-cell simulations of particle energization via shock drift acceleration from low Mach number quasi-perpendicular shocks in solar flares

Low Mach number, high beta fast mode shocks can occur in the magnetic reconnection outflows of solar flares. These shocks, which occur above flare loop tops, may provide the electron energization responsible for some of the observed hard X-rays and contemporaneous radio emission. Here we present new 2D particle-in-cell simulations of low Mach number/high beta quasi-perpendicular shocks. The simulations show that electrons above a certain energy threshold experience shock-drift-acceleration. The transition energy between the thermal and non-thermal spectrum and the spectral index from the simulations are consistent with some of the X-ray spectra from RHESSI in the energy regime of $E\lesssim 40\sim 100$ keV. Plasma instabilities associated with the shock structure such as the modified-two-stream and the electron whistler instabilities are identified using numerical solutions of the kinetic dispersion relations. We also show that the results from PIC simulations with reduced ion/electron mass ratio can be scaled to those with the realistic mass ratio.

astro-ph.SR↗

arXiv2012

Particle-in-cell simulations of particle energization from low Mach number fast mode shocks

Astrophysical shocks are often studied in the high Mach number limit but weakly compressive fast shocks can occur in magnetic reconnection outflows and are considered to be a site of particle energization in solar flares. Here we study the microphysics of such perpendicular, low Mach number collisionless shocks using two-dimensional particle-in-cell (PIC) simulations with a reduced ion/electron mass ratio and employ a moving wall boundary method for initial generation of the shock. This moving wall method allows for more control of the shock speed, smaller simulation box sizes, and longer simulation times than the commonly used fixed wall, reflection method of shock formation. Our results, which are independent of the shock formation method, reveal the prevalence shock drift acceleration (SDA) of both electron and ions in a purely perpendicular shock with Alfvén Mach number $M_A=6.8$ and ratio of thermal to magnetic pressure $β=8$. We determine the respective minimum energies required for electrons and ions to incur SDA. We derive a theoretical electron distribution via SDA that compares to the simulation results. We also show that a modified two-stream instability due to the incoming and reflecting ions in the shock transition region acts as the mechanism to generate collisionless plasma turbulence that sustains the shock.

astro-ph.SR↗

arXiv2011

Analysis of recent type Ia supernova data based on evolving dark energy models

We study characters of recent type Ia supernova (SNIa) data using evolving dark energy models with changing equation of state parameter w. We consider sudden-jump approximation of w for some chosen redshift spans with double transitions, and constrain these models based on Markov Chain Monte Carlo (MCMC) method using the SNIa data (Constitution, Union, Union2) together with baryon acoustic oscillation A parameter and cosmic microwave background shift parameter in a flat background. In the double-transition model the Constitution data shows deviation outside 1 sigma from LCDM model at low (z < 0.2) and middle (0.2 < z < 0.4) redshift bins whereas no such deviations are noticeable in the Union and Union2 data. By analyzing the Union members in the Constitution set, however, we show that the same difference is actually due to different calibration of the same Union sample in the Constitution set, and is not due to new data added in the Constitution set. All detected deviations are within 2 sigma from the LCDM world model. From the LCDM mock data analysis, we quantify biases in the dark energy equation of state parameters induced by insufficient data with inhomogeneous distribution of data points in the redshift space and distance modulus errors. We demonstrate that location of peak in the distribution of arithmetic means (computed from the MCMC chain for each mock data) behaves as an unbiased estimator for the average bias, which is valid even for non-symmetric likelihood distributions.

astro-ph.CO↗

arXiv2009

Observational constraints on a unified dark matter and dark energy model based on generalized Chaplygin gas

We study a generalized version of Chaplygin gas as unified model of dark matter and dark energy. Using realistic theoretical models and the currently available observational data from the age of the universe, the expansion history based on the type Ia supernovae, the matter power spectrum, the cosmic microwave background radiation anisotropy power spectra, and the perturbation growth factor we put the unified model under observational test. As the model has only two free parameters in the flat Friedmann background [$Λ$CDM (cold dark matter) model has only one free parameter] we show that the model is already tightly constrained by currently available observations. The only parameter space extremely close to the $Λ$CDM model is allowed in this unified model.

astro-ph.CO↗

arXiv2008

An Infrared Survey of Brightest Cluster Galaxies. II: Why are Some Brightest Cluster Galaxies Forming Stars?

Quillen et al.(2007) presented an imaging survey with the {\it Spitzer Space Telescope} of 62 brightest cluster galaxies with optical line emission located in the cores of X-ray luminous clusters. They found that at least half of these sources have signs of excess infrared emission. Here we discuss the nature of the IR emission and its implications for cool core clusters. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding the four systems dominated by an AGN, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star forming galaxies. The gas depletion time scale is about 1 Gyr. The physical extent of the infrared excess is consistent with that of the optical emission line nebulae. This supports the hypothesis that the star formation occurs in molecular gas associated with the emission line nebulae and with evidence that the emission line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates (${\dot M}_X$) estimated from the X-ray emission and the star formation rate estimated from the infrared luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates suggesting that the re-heating of the ICM is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely.

astro-ph↗

arXiv2007

An infrared survey of brightest cluster galaxies: Paper I

We report on an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission. These galaxies are located in the cores of X-ray luminous clusters selected from the ROSAT All-Sky Survey. We find that about half of these sources have a sign of excess infrared emission; 22 objects out of 62 are detected at 70 microns, 18 have 8 to 5.8 micron flux ratios above 1.0 and 28 have 24 to 8 micron flux ratios above 1.0. Altogether 35 of 62 objects in our survey exhibit at least one of these signs of infrared excess. Four galaxies with infrared excesses have a 4.5/3.6 micron flux ratio indicating the presence of hot dust, and/or an unresolved nucleus at 8 microns. Three of these have high measured [OIII](5007A)/Hbeta flux ratios suggesting that these four, Abell 1068, Abell 2146, and Zwicky 2089, and R0821+07, host dusty active galactic nuclei (AGNs). 9 objects (including the four hosting dusty AGNs) have infrared luminosities greater than 10^11 L_sol and so can be classified as luminous infrared galaxies (LIRGs). Excluding the four systems hosting dusty AGNs, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely related to star formation.

astro-ph↗

arXiv2007

The Minimum Gap-opening Planet Mass in an Irradiated Circumstellar Accretion Disk

We consider the minimum mass planet, as a function of radius, that is capable of opening a gap in an $α$-accretion disk. We estimate that a half Jupiter mass planet can open a gap in a disk with accretion rate $\dot M \la 10^{-8} M_\odot$/yr for viscosity parameter $α=0.01$, and Solar mass and luminosity. The minimum mass is approximately proportional to $\dot M^{0.48} α^{0.8} M_*^{0.42} L_*^{-0.08}$. This estimate can be used to rule out the presence of massive planets in gapless accretion disks. We identify two radii at which an inwardly migrating planet may become able to open a gap and so slow its migration; the radius at which the heating from viscous dissipation is similar to that from stellar radiation in a flared disk, and the radius at which the disk has optical depth order 1 in a self-shadowed disk. If a gap opening planet cuts off disk accretion allowing the formation of a central hole or clearing in the disk then we would estimate that the clearing radius would approximately be proportional to the stellar mass.

astro-ph↗

arXiv2004

Two Dimensional Poynting Flux Dominated Flow onto a Schwarzschild Black Hole

We discuss the dynamics of the accretion flow onto a black hole driven by Poynting flux in a simplified model of a two-dimensional accretion disk. In this simplified model, the condition of the stationary accretion flow is found to impose a nontrivial constraint on the magnetic field configuration. The effect of the magnetic field on the accretion flow is discussed in detail using the paraboloidal and hyperboloidal type configuration for the poloidal structure suggested by Blandford in 1976. It is demonstrated explicitly that the angular velocity of the disk, $Ω_D$, deviates from the Keplerian angular velocity. The angular velocity of the rigidly-rotating magnetic surface, $Ω_F$, does not have to be the same as the angular velocity of the disk for the paraboloidal type configuration. But for the hyperboloidal type configuration, it is found that we can set $Ω_F = Ω_D$, which corresponds to an accretion disk of perfect conductor. We discuss the numerical solutions of the stream equation for stationary accretion flow in the Schwarzschild background using a paraboloidal type configuration. The dynamics of the accretion disk is found to depend strongly on the ratio of the accretion rate to the magnetic field strength.

astro-ph↗