Distinctive 21 cm structures of the first stars, galaxies, and quasars

Observations of the redshifted 21 cm line with upcoming radio telescopes promise to transform our understanding of the cosmic reionization. To unravel the underlying physical process, we investigate the 21 cm structures of three different ionizing sources, Population (Pop) III stars, the first galaxies, and the first quasars, by using radiative transfer simulations that include both ionization of neutral hydrogen and resonant scattering of Lya photons. We find that Pop III stars and quasars produce a smooth transition from an ionized and hot state to a neutral and cold one, owing to their hard spectral energy distribution with abundant ionizing photons, in contrast to the sharp transition in galaxies. Furthermore, Lya scattering plays a dominant role in producing the 21 cm signal as it determines the relation between hydrogen spin temperature and gas kinetic temperature. This effect, also called Wouthuysen-Field coupling, depends strongly on the ionizing source. It is the strongest around galaxies, where the spin temperature is highly coupled to that of the gas, resulting in extended absorption troughs in the 21 cm brightness temperature. On the other hand, in the case of Pop III stars, the 21 cm signal shows both emission and absorption regions around a small HII bubble. For quasars, a large emission region in the 21 cm signal is produced, and the absorption region decreases as the size of the HII bubble becomes large due to the limited traveling time of photons. We predict that future surveys from large radio arrays such as MWA, LOFAR and SKA may be able to detect the 21 cm signals of primordial galaxies and quasars, but not likely Pop III stars due to its small angular diameter.Comment: 12 pages, 11 figures, accepted for publication in MNRA

The Formation of a Milky Way-sized Disk Galaxy 1. A Comparison of Numerical Methods

The long-standing challenge of creating a Milky Way-like disk galaxy from cosmological simulations has motivated significant developments in both numerical methods and physical models in recent years. We investigate these two fundamental aspects in a new comparison project using a set of cosmological hydrodynamic simulations of the formation and evolution of a Milky Way-size galaxy. In this study, we focus on the comparison of two particle-based hydrodynamics methods: the improved smoothed particle hydrodynamics (SPH) code Gadget, and the Lagrangian Meshless Finite-Mass (MFM) code GIZMO. All the simulations in this paper use the same initial conditions and physical models, which include physics of both dark matter and baryons, star formation, "energy-driven" outflow, metal-dependent cooling, stellar evolution and metal enrichment from supernovae. We find that both numerical schemes produce a late-type galaxy with extended gaseous and stellar disks. However, notable differences are present in a wide range of galaxy properties and their evolution, including star formation history, gas content, disk structure and kinematics. In particular, there is significant difference in gas properties and their evolution between the two simulations. Compared to GIZMO, Gadget simulation produces a larger fraction of cold, dense gas at high redshift which fuels rapid star formation and results in a higher stellar mass by $20\%$ and a lower gas fraction by $10\%$ at $z = 0$, and the resulting gas disk is smoother and more coherent in rotation due to damping of turbulent motion by the numerical viscosity in SPH, in contrast to the GIZMO simulation which shows more prominent spiral structure. Given its better convergence properties and lower computational cost, we argue that MFM method is a promising alternative to the widely used SPH in cosmological hydrodynamic simulations.Comment: 22 pages, accepted for publication in Ap

Numerical Convergence in Smoothed Particle Hydrodynamics

We study the convergence properties of smoothed particle hydrodynamics (SPH) using numerical tests and simple analytic considerations. Our analysis shows that formal numerical convergence is possible in SPH only in the joint limit $N \rightarrow \infty$, $h \rightarrow 0$, and $N_{nb} \rightarrow \infty$, where $N$ is the total number of particles, $h$ is the smoothing length, and $N_{nb}$ is the number of neighbor particles within the smoothing volume used to compute smoothed estimates. Previous work has generally assumed that the conditions $N \rightarrow \infty$ and $h \rightarrow 0$ are sufficient to achieve convergence, while holding $N_{nb}$ fixed. We demonstrate that if $N_{nb}$ is held fixed as the resolution is increased, there will be a residual source of error that does not vanish as $N \rightarrow \infty$ and $h \rightarrow 0$. Formal numerical convergence in SPH is possible only if $N_{nb}$ is increased systematically as the resolution is improved. Using analytic arguments, we derive an optimal compromise scaling for $N_{nb}$ by requiring that this source of error balance that present in the smoothing procedure. For typical choices of the smoothing kernel, we find $N_{nb} \propto N^{1/2}$. This means that if SPH is to be used as a numerically convergent method, the required computational cost does not scale with particle number as $O(N)$, but rather as $O(N^{1+\delta})$, where $\delta \approx 1/2$, with a weak dependence on the form of the smoothing kernel.Comment: The revised version accepted in ApJ, with typos corrected and references adde

Accretion onto Intermediate-mass Seed Black Holes in Primordial Galaxies

The origin of the supermassive black holes that power the most distant quasars observed is largely unknown. One hypothesis is that they grew rapidly from intermediate-mass seeds (~100 M_sun) left by the first stars. However, some previous studies argued that accretion onto these black holes was too low to build up the mass due to strong suppression by radiative feedback. Here, we re-exam the accretion process of such a black hole embedded in a primordial gas cloud, by considering a wide range of physical and numerical parameters not explored before. We find that, while radiative heating and pressure indeed suppress accretion effectively, self-gravity of the gas eventually overcomes the feedback effects and boosts the accretion to the Eddington rate after one free-fall timescale of the cloud. Moreover, for a given black hole mass, there exists a critical density above which the accretion can reach Eddington limit. Furthermore, we find a universal correlation between black hole accretion rate and ambient gas density, which may serve as a realistic recipe for black hole growth in simulations.Comment: 8 pages, 4 figures. Submitted to Ap

Scaling Relations Between Low-mass Black Holes and Their Host Galaxies

It is well established that supermassive black holes in nearby elliptical galaxies correlate tightly with the kinematic property (\mbhsigma correlation) and stellar mass (\mbhhost correlation) of their host spheroids. However, it is not clear what the relations would be at the low-mass end, and how they evolve. Here, we investigate these relations in low-mass systems (\MBH \sim \rm{10^{6}- 10^{8}}\, \Msun) using the Aquila Simulation, a high-resolution cosmological hydrodynamic simulation which follows the formation and evolution of stars and black holes in a Milky Way-size galaxy and its substructures. We find a number of interesting results on the origin and evolution of the scaling relations in these systems: (1) there is a strong redshift evolution in the \mbhsigma relation, but a much weaker one in the \mbhhost relation; (2) there is a close link between the \mbhsigma relation and the dynamical state of the system -- the galaxies that fall on the observed correlation appear to have reached virial equilibrium. (3) the star formation and black hole growth are self-regulated in galaxies -- the ratio between black hole accretion rate and star formation rate remains nearly constant in a wide redshift span $z = 0-6$. These findings suggest that the observed correlations have different origins: the \mbhsigma relation may be the result of virial equilibrium, while the \mbhhost relation may the result of self-regulated star formation and black hole growth in galaxies.Comment: 12 pages, 7 figures, in emulateapj style, submitted to Ap

The Co-evolution of Cosmic Entropy and Structures in the Universe

According to the second law of thermodynamics, the arrow of time points to an ever increasing entropy of the Universe. However, exactly how the entropy evolves with time and what drives the growth remain largely unknown. Here, for the first time, we quantify the evolving entropy of cosmic structures using a large-scale cosmological hydrodynamical simulation. Our simulation starts from initial conditions predicted by the leading LambdaCDM cosmology, self-consistently evolves the dynamics of both dark and baryonic matter, star formation, black hole growth and feedback processes, from the cosmic dawn to the present day. Tracing the entropy contributions of these distinct components in the simulation, we find a strong link between entropy growth and structure formation. The entropy is dominated by that of the black holes in all epochs, and its evolution follows the same path as that of galaxies: it increases rapidly from a low-entropy state at high redshift until z~2, then transits to a slower growth. Our results suggest that cosmic entropy may co-evolve with cosmic structure, and that its growth may be driven mainly by the formation of black holes in galaxies. We predict that the entropy will continue to increase in the near future, but likely at a constant rate.Comment: 5 pages, 4 figures, submitted to ApJ Letter

Cold accretion in early galaxy formation and its Lyman-alpha signatures

The Lyman-alpha (Lya) emission has played an important role in detecting high-redshift galaxies, including recently distant ones at redshift z > 7. It may also contain important information on the origin of these galaxies. Here, we investigate the formation of a typical L* galaxy and its observational signatures at the earliest stage, by combining a cosmological hydrodynamic simulation with three-dimensional radiative transfer calculations using the newly improved ART^2 code. Our cosmological simulation uses the Aquila initial condition which zooms in onto a Milky Way-like halo with high resolutions, and our radiative transfer couples multi-wavelength continuum, Lya line, and ionization of hydrogen. We find that the modeled galaxy starts to form at redshift z ~ 24 through efficient accretion of cold gas, which produces a strong Lya line with a luminosity of L(Lya) ~ 10^42 erg/s as early as z ~ 14. The Lya emission appears to trace the cold, dense gas. The lines exhibit asymmetric, single-peak profiles, and are shifted to the blue wing, a characteristic feature of gas inflow. Moreover, the contribution to the total Lya luminosity by excitation cooling increases with redshift, and it becomes dominant at z >~ 6. We predict that L* galaxies such as the modeled one may be detected at z <~ 8 by JWST and ALMA with a reasonable integration time. Beyond redshift 12, however, only Lya line may be observable by spectroscopic surveys. Our results suggest that Lya line is one of the most powerful tools to detect the first generation of galaxies, and to decipher their formation mechanism.Comment: 10 pages, 11 figures, accepted for publication in Ap

The Effects of Local Primordial Non-Gaussianity on the Formation and Evolution of Galaxies

Thanks to the rapid progress in precision cosmology in the last few years, we now have access to physical observables that may constrain the theory of inflation through the non-Gaussianity (NG) signatures in the cosmic microwave background radiation and the distribution of large-scale structure. Numerical modeling of the NG signals from different inflation models is essential to correctly interpret current and near future data from large-scale structure surveys. In this study, we use high-resolution cosmological hydrodynamical simulations to investigate the effects of primordial NG on the formation and evolution of galaxies from the cosmic dawn to the present day. Focusing on the local type primordial NG, we find that it may affect the formation history of stars and black holes in galaxies, and their distribution. Compared to the Gaussian case, large non-Gaussian potential with $f_{NL} \gtrsim 10^3$ leads to earlier collapse of the first structures, more massive galaxies especially at high redshifts, stronger clustering of galaxies, and higher halo bias. However, for smaller NG with $f_{NL} \lesssim 10^2$, the effect is significantly weaker. Observations of the distribution and properties of high-redshift, rare objects such as the first galaxies and quasars may provide further constraints on the primordial NG.Comment: 15 pages, 15 figures, submitted to MNRA

Gravoturbulent Star Cluster Formation

Stars form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in Galactic molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Collapse sets in to build up stars and star clusters. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. Stellar birth is thus intimately linked to the dynamical behavior of parental gas cloud, which determines when and where protostellar cores form, and how they contract and grow in mass via accretion from the surrounding cloud material to build up stars. Slow, inefficient, isolated star formation is a hallmark of turbulent support, whereas fast, efficient, clustered star formation occurs in its absence. The fact that Galactic molecular clouds are highly filamentary can be explained by a combination of compressional flows and shear. The dynamical evolution of nascent star clusters is very complex. This strongly influences the stellar mass spectrum. The equation of state (EOS) plays a pivotal role in the fragmentation process. Under typical cloud conditions, massive stars form as part of dense clusters. However, for gas with effective polytropic index greater than unity star formation becomes biased towards isolated massive stars, which may be of relevance for understanding Pop III stars.Comment: 8 pages, including 3 figures; to appear in the proceedings of the conference "The Formation and Evolution of Massive Young Star Clusters" held in Cancun, Mexico, November 17-21, 200

The formation and evolution of star clusters in interacting galaxies

Observations of globular clusters show that they have universal lognormal mass functions with a characteristic peak at $\sim 2\times 10^{5}\, {\rm{M_{\odot}}}$, but the origin of this peaked distribution is highly debated. Here we investigate the formation and evolution of star clusters in interacting galaxies using high-resolution hydrodynamical simulations performed with two different codes in order to mitigate numerical artifacts. We find that massive star clusters in the range of $\sim 10^{5.5} - 10^{7.5}\, {\rm{M_{\odot}}}$ form preferentially in the highly-shocked regions produced by galaxy interactions. The nascent cluster-forming clouds have high gas pressures in the range of $P/k \sim 10^8 - 10^{12}\, \rm{K}\,\rm{cm^{-3}}$, which is $\sim 10^4 - 10^8$ times higher than the typical pressure of the interstellar medium but consistent with recent observations of a pre-super star cluster cloud in the Antennae Galaxies. Furthermore, these massive star clusters have quasi-lognormal initial mass functions with a peak around $\sim 10^{6}\, {\rm{M_{\odot}}}$. The number of clusters declines with time due to destructive processes, but the shape and the peak of the mass functions do not change significantly during the course of galaxy collisions. Our results suggest that gas-rich galaxy mergers may provide a favorable environment for the formation of massive star clusters such as globular clusters, and that the lognormal mass functions and the unique peak may originate from the extreme high-pressure conditions of the birth clouds and may survive the dynamical evolution.Comment: Accepted in ApJ. 16 pages, 9 figure