arXiv subjects

Han Aung

Publications and source records attributed to Han Aung.

24 records · Page 2Linked to original sources

arXiv2020

Instability of Supersonic Cold Streams Feeding Galaxies IV: Survival of Radiatively Cooling Streams

We study the effects of Kelvin Helmholtz Instability (KHI) on the cold streams that feed massive halos at high redshift, generalizing our earlier results to include the effects of radiative cooling and heating from a UV background, using analytic models and high resolution idealized simulations. We currently do not consider self-shielding, thermal conduction or gravity. A key parameter in determining the fate of the streams is the ratio of the cooling time in the turbulent mixing layer which forms between the stream and the background following the onset of the instability, $t_{\rm cool,mix}$, to the time in which the mixing layer expands to the width of the stream in the non-radiative case, $t_{\rm shear}$. This can be converted into a critical stream radius, $R_{\rm s,crit}$, such that $R_{\rm s}/R_{\rm s,crit}=t_{\rm shear}/t_{\rm cool,mix}$. If $R_{\rm s} R_{\rm s,crit}$, which we find to almost always be the case for astrophysical cold streams, the stream is not disrupted by KHI. Rather, background mass cools and condenses onto the stream, and can increase the mass of cold gas by a factor of $\sim 3$ within 10 stream sound crossing times. The mass entrainment induces thermal energy losses from the background and kinetic energy losses from the stream, which we model analytically. Roughly half of the dissipated energy is radiated away from gas with $T<5\times 10^4{\rm K}$, suggesting much of it will be emitted in Ly$α$.

astro-ph.GA↗

arXiv2019

Kelvin-Helmholtz Instability in Self-Gravitating Streams

Self-gravitating gaseous filaments exist on many astrophysical scales, from sub-pc filaments in the interstellar medium to Mpc scale streams feeding galaxies from the cosmic web. These filaments are often subject to Kelvin-Helmotz Instability (KHI) due to shearing against a confining background medium. We study the nonlinear evolution of KHI in pressure-confined self-gravitating gas streams initially in hydrostatic equilibrium, using analytic models and hydrodynamic simulations, not including radiative cooling. We derive a critical line-mass as a function of the stream Mach number and density contrast with respect to the background, $μ_{cr}(M_b,δ_c)\le 1$, where $μ=1$ is normalized to the maximal line mass for which initial hydrostatic equilibrium is possible. For $μ<μ_{cr}$, KHI dominates the stream evolution. A turbulent shear layer expands into the background and leads to stream deceleration at a similar rate to the non-gravitating case. However, with gravity, penetration of the shear layer into the stream is halted at roughly half the initial stream radius by stabilizing buoyancy forces, significantly delaying total stream disruption. Streams with $μ_{cr}<μ\le 1$ fragment and form round, long-lived clumps by gravitational instability (GI), with typical separations roughly 8 times the stream radius, similar to the case without KHI. When KHI is still somewhat effective, these clumps are below the spherical Jeans mass and are partially confined by external pressure, but they approach the Jeans mass as $μ\rightarrow 1$ and GI dominates. We discuss potential applications of our results to filaments in the ISM and dense streams feeding galaxies at high redshift.

astro-ph.GA↗

arXiv2019

Generating Synthetic Cosmological Data with GalSampler

As part of the effort to meet the needs of the Large Synoptic Survey Telescope Dark Energy Science Collaboration (LSST DESC) for accurate, realistically complex mock galaxy catalogs, we have developed GalSampler, an open-source python package that assists in generating large volumes of synthetic cosmological data. The key idea behind GalSampler is to recast hydrodynamical simulations and semi-analytic models as physically-motivated galaxy libraries. GalSampler populates a new, larger-volume halo catalog with galaxies drawn from the baseline library; by using weighted sampling guided by empirical modeling techniques, GalSampler inherits statistical accuracy from the empirical model and physically-motivated complexity from the baseline library. We have recently used GalSampler to produce the cosmoDC2 extragalactic catalog made for the LSST DESC Data Challenge 2. Using cosmoDC2 as a guiding example, we outline how GalSampler can continue to support ongoing and near-future galaxy surveys such as the Dark Energy Survey (DES), the Dark Energy Spectroscopic Instrument (DESI), WFIRST, and Euclid.

astro-ph.CO↗

arXiv2019

Halo Concentrations and the New Baseline X-ray Luminosity-Temperature and Mass Relations of Galaxy Clusters

The standard self-similar model of galaxy cluster formation predicts that the X-ray luminosity-temperature ($L_X$-$T_X$) relation of galaxy clusters should have been $L_X\propto T_X^2$ in absence of the baryonic physics, such as radiative cooling and feedback from stars and black holes. However, this baseline relation is predicted without considering the fact that the halo concentration and the characteristic density of clusters increases as their mass decreases, which is a consequence of hierarchical structure formation of the universe. Here, we show that the actual baseline relation should be $L_X\propto T_X^α$, where $α\sim 1.7$, instead of $α=2$, given the mass dependence of the concentration and the fundamental plane relation of galaxy clusters. Numerical simulations show that $α\sim 1.6$, which is consistent with the prediction. We also show that the baseline luminosity-mass ($L_X$-$M_Δ$) relation should have been $L_X\propto M_Δ^β$, where $β\sim 1.1$-1.2, in contrast with the conventional prediction ($β=4/3$). In addition, some of the scatter in the $L_X$-$M_Δ$ relation can be attributed to the scatter in the concentration-mass ($c$-$M$) relation. The confirmation of the shallow slope could be a proof of hierarchical clustering. As an example, we show that the new baseline relations could be checked by studying the temperature or mass dependence of gas mass fraction of clusters. Moreover, the highest-temperature clusters would follow the shallow baseline relations if the influences of cool cores and cluster mergers are properly removed.

astro-ph.GA↗

arXiv2019

Instability of Supersonic Cold Streams Feeding Galaxies III: Kelvin-Helmholtz Instability in Three Dimensions

We study the effects of Kelvin-Helmholtz instability (KHI) on the cold streams that feed high-redshift galaxies through their hot haloes, generalizing our earlier analyses of a 2D slab to a 3D cylinder, but still limiting our analysis to the adiabatic case with no gravity. We combine analytic modeling and numerical simulations in the linear and non-linear regimes. For subsonic or transonic streams with respect to the halo sound speed, the instability in 3D is qualitatively similar to 2D, but progresses at a faster pace. For supersonic streams, the instability grows much faster in 3D and can be qualitatively different due to azimuthal modes, which introduce a strong dependence on the initial width of the stream-background interface. Using analytic toy models and approximations supported by high-resolution simulations, we apply our idealized hydrodynamical analysis to the astrophysical scenario. The upper limit for the radius of a stream that disintegrates prior to reaching the central galaxy is ~70% larger than the 2D estimate; it is in the range (0.5-5)% of the halo virial radius, decreasing with increasing stream density and velocity. Stream disruption generates a turbulent mixing zone around the stream with velocities at the level of ~20% of the initial stream velocity. KHI can cause significant stream deceleration and energy dissipation in 3d, contrary to 2D estimates. For typical streams, up to (10-50)% of the gravitational energy gained by inflow down the dark-matter halo potential can be dissipated, capable of powering Lyman-alpha blobs if most of it is dissipated into radiation.

astro-ph.GA↗

arXiv2015

Mean maps for cosmic web structures in cosmological initial conditions

Halos, filaments, sheets and voids in the cosmic web can be defined in terms of the eigenvalues of the smoothed shear tensor and a threshold $λ_{\rm th}$. Using analytic methods, we construct mean maps centered on these types of structures for Gaussian random fields corresponding to cosmological initial conditions. Each map also requires a choice of shear at the origin; we consider three possibilities. We find characteristic sizes, shapes and other properties of the central objects in these mean maps and explore how these properties change with varying the threshold and smoothing scale, i.e. varying the separation of the cosmic web into different kinds of components. The mean maps become increasingly complex as the threshold $λ_{\rm th}$ decreases to zero. We also describe scatter around these mean maps, subtleties which can arise in their construction, and some comparisons between halos in the maps and collapsed halos at final times.

astro-ph.CO↗