FORMATION OF PROTO-GLOBULAR CLUSTER CLOUDS BY THERMAL INSTABILITY

KANG HYESUNG,LAKE GEORGE,RYU DONGSU The Korean Astronomical Society 2000 Journal of The Korean Astronomical Society Vol.33 No.2

Many models of globular cluster formation assume the presence of cold dense clouds in early universe. Here we re-examine the Fall & Rees (1985) model for formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in a protogalactic halo. We first argue, based on the previous study of two-dimensional numerical simulations of thermally unstable clouds in a stratified halo of galaxy clusters by Real et al. (1991), that under the protogalactic environments only nonlinear (${\delta}{\ge}1$) density inhomogeneities can condense into PGCCs without being disrupted by the buoyancy-driven dynamical instabilities. We then carry out numerical simulations of the collapse of overdense douds in one-dimensional spherical geometry, including self-gravity and radiative cooling down to T = $10^4$ K. Since imprinting of Jeans mass at $10^4$ K is essential to this model, here we focus on the cases where external UV background radiation prevents the formation of $H_2$ molecules and so prevent the cloud from cooling below $10^4$ K. The quantitative results from these simulations can be summarized as follows: 1) Perturbations smaller than $M_{min}\~(10^{5.6}\;M{\bigodot})(nh/0.05cm^{-3})^{-2}$ cool isobarically, where nh is the unperturbed halo density, while perturbations larger than $M_{min}\~(10^8\;M{\bigodot})(nh/0.05cm^{-3})^{-2}$ cool isochorically and thermal instabilities do not operate. On the other hand, intermediate size perturbations ($M_{min} < M_{pgcc} < M_{max}$) are compressed supersonically, accompanied by strong accretion shocks. 2) For supersonically collapsing clouds, the density compression factor after they cool to $T_c = 10^4$ K range $10^{2.5} - 10^6$, while the isobaric compression factor is only $10^{2.5}$. 3) Isobarically collapsed clouds ($M < M_{min}$) are too small to be gravitationally bound. For supersonically collapsing clouds, however, the Jeans mass can be reduced to as small as $10^{5.5}\;M_{\bigodot}(nh/0.05cm^{-3})^{-1/2}$ at the maximum compression owing to the increased density compression. 4) The density profile of simulated PGCCs can be approximated by a constant core with a halo of $p{\infty} r^{-2}$ rather than a singular isothermal sphere.

COSMIC RAYS ACCELERATED AT SHOCK WAVES IN LARGE SCALE STRUCTURE

RYU DONGSU,KANG HYESUNG The Korean Astronomical Society 2004 Journal of The Korean Astronomical Society Vol.37 No.5

Shock waves form in the intergalactic space as an ubiquitous consequence of cosmic structure formation. Using N-body/hydrodynamic simulation data of a ACDM universe, we examined the properties of cosmological shock waves including their morphological distribution. Adopting a diffusive shock acceleration model, we then calculated the amount of cosmic ray energy as well as that of gas thermal energy dissipated at the shocks. Finally, the dynamical consequence of those cosmic rays on cluster properties is discussed.

COSMIC SHOCK WAVES ON LARGE SCALES OF THE UNIVERSE

RYU DONGSU,KANG HYESUNG The Korean Astronomical Society 1996 Journal of The Korean Astronomical Society Vol.29 No.suppl1

In the standard theory of the large scale structure formation, matter accretes onto high density perturbations via gravitational instability. Collision less dark matter forms caustics around such structures, while collisional baryonic matter forms accretion shocks which then halt and heat the infalling gas. Here we discuss the characteristics. roles, and observational consequences of these accretion shocks.

CURVED RADIO SPECTRA OF WEAK CLUSTER SHOCKS

Kang, Hyesung,Ryu, Dongsu IOP Publishing 2015 The Astrophysical journal Vol.809 No.2

<P>In order to understand certain observed features of arc-like giant radio relics such as the rareness, uniform surface brightness, and curved integrated spectra, we explore a diffusive shock acceleration (DSA) model for radio relics in which a spherical shock impinges on a magnetized cloud containing fossil relativistic electrons. Toward this end, we perform DSA simulations of spherical shocks with the parameters relevant for the Sausage radio relic in cluster CIZA J2242.8+ 5301, and calculate the ensuing radio synchrotron emission from re-accelerated electrons. Three types of fossil electron populations are considered: a delta-function like population with the shock injection momentum, a power-law distribution, and a power law with an exponential cutoff. The surface brightness profile of the radio-emitting postshock region and the volume-integrated radio spectrum are calculated and compared with observations. We find that the observed width of the Sausage relic can be explained reasonably well by shocks with speed u(s) similar to 3 x 10(3) km s(-1) and sonic Mach number M-s similar to 3. These shocks produce curved radio spectra that steepen gradually over (0.1-10)v(br) with a break frequency v(br) similar to 1GHz if the duration of electron acceleration is similar to 60-80 Myr. However, the abrupt increase in the spectral index above similar to 1.5 GHz observed in the Sausage relic seems to indicate that additional physical processes, other than radiative losses, operate for electrons with gamma(e) greater than or similar to 10(4).</P>

DIFFUSIVE SHOCK ACCELERATION SIMULATIONS OF RADIO RELICS

Kang, Hyesung,Ryu, Dongsu,Jones, T. W. IOP Publishing 2012 The Astrophysical journal Vol.756 No.1

<P>Recent radio observations have identified a class of structures, so-called radio relics, in clusters of galaxies. The radio emission from these sources is interpreted as synchrotron radiation from GeV electrons gyrating in mu G-level magnetic fields. Radio relics, located mostly in the outskirts of clusters, seem to associate with shock waves, especially those developed during mergers. In fact, they seem to be good structures to identify and probe such shocks in intracluster media (ICMs), provided we understand the electron acceleration and re-acceleration at those shocks. In this paper, we describe time-dependent simulations for diffusive shock acceleration at weak shocks that are expected to be found in ICMs. Freshly injected as well as pre-existing populations of cosmic-ray (CR) electrons are considered, and energy losses via synchrotron and inverse Compton are included. We then compare the synchrotron flux and spectral distributions estimated from the simulations with those in two well-observed radio relics in CIZA J2242.8+5301 and ZwCl0008.8+5215. Considering that CR electron injection is expected to be rather inefficient at weak shocks with Mach number M less than or similar to a few, the existence of radio relics could indicate the pre-existing population of low-energy CR electrons in ICMs. The implication of our results on the merger shock scenario of radio relics is discussed.</P>

CLUSTERS OF GALAXIES: SHOCK WAVES AND COSMIC RAYS

RYU DONGSU,KANG HYESUNG The Korean Astronomical Society 2003 Journal of The Korean Astronomical Society Vol.36 No.3

Recent observations of galaxy clusters in radio and X-ray indicate that cosmic rays and magnetic fields may be energetically important in the intracluster medium. According to the estimates based on theses observational studies, the combined pressure of these two components of the intracluster medium may range between $10\%{\~}100\%$ of gas pressure, although their total energy is probably time dependent. Hence, these non-thermal components may have influenced the formation and evolution of cosmic structures, and may provide unique and vital diagnostic information through various radiations emitted via their interactions with surrounding matter and cosmic background photons. We suggest that shock waves associated with cosmic structures, along with individual sources such as active galactic nuclei and radio galaxies, supply the cosmic rays and magnetic fields to the intracluster medium and to surrounding large scale structures. In order to study 1) the properties of cosmic shock waves emerging during the large scale structure formation of the universe, and 2) the dynamical influence of cosmic rays, which were ejected by AGN-like sources into the intracluster medium, on structure formation, we have performed two sets of N-body /hydrodynamic simulations of cosmic structure formation. In this contribution, we report the preliminary results of these simulations.

Shock‐heated Gas in the Large‐Scale Structure of the Universe

Kang, Hyesung,Ryu, Dongsu,Cen, Renyue,Song, Doojong IOP Publishing 2005 The Astrophysical journal Vol.620 No.1

RE-ACCELERATION MODEL FOR RADIO RELICS WITH SPECTRAL CURVATURE

Kang, Hyesung,Ryu, Dongsu American Astronomical Society 2016 The Astrophysical journal Vol.823 No.1

<P>Most of the observed features of radio gischt relics, such as spectral steepening across the relic width and a powerlaw- like integrated spectrum, can be adequately explained by a diffusive shock acceleration (DSA) model in which relativistic electrons are (re-) accelerated at shock waves induced in the intracluster medium. However, the steep spectral curvature in the integrated spectrum above similar to 2 GHz detected in some radio relics, such as the Sausage relic in cluster CIZA J2242.8+ 5301, may not be interpreted by the simple radiative cooling of postshock electrons. In order to understand such steepening, we consider here a model in which a spherical shock sweeps through and then exits out of a finite-size cloud with fossil relativistic electrons. The ensuing integrated radio spectrum is expected to steepen much more than predicted for aging postshock electrons, since the re-acceleration stops after the cloudcrossing time. Using DSA simulations that are intended to reproduce radio observations of the Sausage relic, we show that both the integrated radio spectrum and the surface brightness profile can be fitted reasonably well, if a shock of speed u(s) similar to 2.5-2.8 x 10(3)km s(-1)and a sonic Mach number M-s similar to 2.7-3.0 traverses a fossil cloud for similar to 45 Myr, and the postshock electrons cool further for another similar to 10 Myr. This attempt illustrates that steep curved spectra of some radio gischt relics could be modeled by adjusting the shape of the fossil electron spectrum and adopting the specific configuration of the fossil cloud.</P>

INTERGALACTIC MAGNETIC FIELD AND ARRIVAL DIRECTION OF ULTRA-HIGH-ENERGY PROTONS

Ryu, Dongsu,Das, Santabrata,Kang, Hyesung IOP Publishing 2010 The Astrophysical journal Vol.710 No.2

<P>We studied how the intergalactic magnetic field (IGMF) affects the propagation of super-Greisen-Zatsepin-Kuz'min (GZK) protons that originate from extragalactic sources within the local GZK sphere. To this end, we set up hypothetical sources of ultra-high-energy cosmic rays (UHECRs), virtual observers, and the magnetized cosmic web in a model universe constructed from cosmological structure formation simulations. We then arranged a set of reference objects mimicking active galactic nuclei (AGNs) in the local universe, with which correlations of simulated UHECR events are analyzed. With our model IGMF, the deflection angle between the arrival direction of super-GZK protons and the sky position of their actual sources is quite large with a mean value of <theta > similar to 15 degrees and a median value of (0) over tilde similar to 7 degrees-10 degrees. On the other hand, the separation angle between the arrival direction and the sky position of nearest reference objects is substantially smaller with < S > similar to 3 degrees.5-4 degrees, which is similar to the mean angular distance in the sky to nearest neighbors among the reference objects. This is a direct consequence of our model that the sources, observers, reference objects, and the IGMF all trace the matter distribution of the universe. The result implies that extragalactic objects lying closest to the arrival direction of UHECRs are not necessarily their actual sources. With our model for the distribution of reference objects, the fraction of super-GZK proton events, whose closest AGNs are true sources, is less than 1/3. We discussed implications of our findings for correlation studies of real UHECR events.</P>

DIFFUSIVE SHOCK ACCELERATION IN TEST-PARTICLE REGIME

Kang, Hyesung,Ryu, Dongsu IOP Publishing 2010 The Astrophysical journal Vol.721 No.1