Detailed modelling of the 21-cm Forest

The 21-cm forest is a promising probe of the Epoch of Reionization. The local state of the intergalactic medium (IGM) is encoded in the spectrum of a background source (radio-loud quasars or gamma ray burst afterglow) by absorption at the local 21-cm wavelength, resulting in a continuous and fluctuating absorption level. Small-scale structures (filaments and minihaloes) in the IGM are responsible for the strongest absorption features. The absorption can also be modulated on large scales by inhomogeneous heating and Wouthuysen-Field coupling. We present the results from a simulation that attempts to preserve the cosmological environment while resolving some of the small-scale structures (a few kpc resolution in a 50 Mpc/h box). The simulation couples the dynamics and the ionizing radiative transfer and includes X-ray and Lyman lines radiative transfer for a detailed physical modelling. As a result we find that soft X-ray self-shielding, Lyman-alpha self-shielding and shock heating all have an impact on the predicted values of the 21-cm optical depth of moderately overdense structures like filaments. An correct treatment of the peculiar velocities is also critical. Modelling these processes seems necessary for accurate predictions and can be done only at high enough resolution. As a result, based on our fiducial model, we estimate that LOFAR should be able to detect a few (strong) absorptions features in a frequency range of a few tens of MHz for a 20 mJy source located at z=10, while the SKA would extract a large fraction of the absorption information for the same source.Comment: 13 pages, accepted for publication in MNRA

The impacts of ultraviolet radiation feedback on galaxies during the epoch of reionization

We explore the impacts of ultraviolet (UV) radiation feedback on galaxies during the epoch of reionisation by cosmological simulations in which hydrodynamics and the transfer of the H and He ionising photons are consistently coupled. Moreover we take into account H_2 non-equilibrium chemistry, including photo-dissociation. The most striking feature of the simulations is a high spatial resolution for the radiative transfer (RT) calculation which enables us to start considering not only external UV feedback processes but also internal UV feedback processes in each galaxy. We find that the star formation is significantly suppressed due to the internal UV and supernova (SN) feedback. In low mass galaxies with M<10^9Msun, a large amount of gas is evacuated by photo-evaporation as previous studies have shown, which results in the suppression of star formation. Surprisingly, star formation in massive halos is also strongly suppressed despite the fact that these halos hardly lose any gas by photo-evaporation. The suppression of star formation in massive halos is mainly caused by following two factors; (i) small scale clumpy structures in the galaxies are smoothened by the internal feedback, (ii) although the dense gas in the galaxies is mostly neutral, the H_2 formation and cooling processes are disturbed by mild photo-heating. Photo-dissociating radiations actually suppress star formation, but the magnitude of the effect is not so large in massive galaxies. Even though our simulation volume is too small to be a representative patch of the Universe during reionisation, we find that our simulated star formation rate densities and HI fractions at z=6-7 are consistent with those found in observations.Comment: 14 pages, 13 figures, 1 table, accepted for publication in MNRA

Formation and Radiative Feedback of First Objects and First Galaxies

First, the formation of first objects driven by dark matter is revisited by high-resolution hydrodynamic simulations. It is revealed that dark matter haloes of ~10^4M_sun can produce first luminous objects with the aid of dark matter cusps. Therefore, the mass of first objects is smaller by roughly two orders of magnitude than in the previous prediction. This implies that the number of Pop III stars formed in the early universe could be significantly larger than hitherto thought. Secondly, the feedback by photo-ionization and photo-dissociation photons in the first objects is explored with radiation hydrodynamic simulations, and it is demonstrated that multiple stars can form in a 10^5M_sun halo. Thirdly, the fragmentation of an accretion disk around a primordial protostar is explored with photo-dissociation feedback. As a result, it is found that the photo-dissociation can reduce the mass accretion rate onto protostars. Also, protostars as small as 0.8M_sun may be ejected and evolve with keeping their mass, which might be detected as "real first stars" in the Galactic halo. Finally, state-of-the-art radiation hydrodynamic simulations are performed to investigate the internal ionization of first galaxies and the escape of ionizing photons. We find that UV feedback by forming massive stars enhances the escape fraction even in a halo as massive as > 6* 10^9M_sun, while it reduces the star formation rate significantly. This may have a momentous impact on the cosmic reionization.Comment: 26 pages, 14 figure

Gas inflows, star formation and metallicity evolution in galaxy pairs

It has been known since many decades that galaxy interactions can induce star formation (hereafter SF) enhancements and that one of the driving mechanisms of this enhancement is related to gas inflows into the central galaxy regions, induced by asymmetries in the stellar component, like bars. In the last years many evidences have been accumulating, showing that interacting pairs have central gas-phase metallicities lower than those of field galaxies, by {\sim} 0.2-0.3 dex on average. These diluted ISM metallicities have been explained as the result of inflows of metal-poor gas from the outer disk to the galaxy central regions. A number of questions arises: What's the timing and the duration of this dilution? How and when does the SF induced by the gas inflow enrich the circumnuclear gas with re-processed material? Is there any correlation between the timing and strength of the dilution and the timing and intensity of the SF? By means of Tree-SPH simulations of galaxy major interactions, we have studied the effect that gas inflows have on the ISM dilution, and the effect that the induced SF has, subsequently, in re-enriching the nuclear gas. In this contribution, we present the main results of this study.Comment: Proceedings of the IAU Symposium 277 "Tracing the Ancestry of Galaxies", 4 pages, 2 figure

Criticality and convergence in Newtonian collapse

We study through numerical simulation the spherical collapse of isothermal gas in Newtonian gravity. We observe a critical behavior which occurs at the threshold of gravitational instability leading to core formation. For a given initial density profile, we find a critical temperature, which is of the same order as the virial temperature of the initial configuration. For the exact critical temperature, the collapse converges to a self-similar form, the first member in Hunter's family of self-similar solutions. For a temperature close to the critical value, the collapse first approaches this critical solution. Later on, in the supercritical case, the collapse converges to another self-similar solution, which is called the Larson-Penston solution. In the subcritical case, the gas bounces and disperses to infinity. We find two scaling laws: one for the collapsed mass in the supercritical case and the other for the maximum density reached before dispersal in the subcritical case. The value of the critical exponent is measured to be $\simeq 0.11$ in the supercritical case, which agrees well with the predicted value $\simeq 0.10567$. These critical properties are quite similar to those observed in the collapse of a radiation fluid in general relativity. We study the response of the system to temperature fluctuation and discuss astrophysical implications for the insterstellar medium structure and for the star formation process. Newtonian critical behavior is important not only because it provides a simple model for general relativity but also because it is relevant for astrophysical systems such as molecular clouds.Comment: 15 pages, 8 figures, accepted for publication in PRD, figures 1 and 3 at lower resolution than in journal version, typos correcte

seurat: SPH scheme extended with ultraviolet line radiative transfer

We present a novel Lyman alpha (Ly α) radiative transfer code, seurat (SPH scheme Extended with Ultraviolet line RAdiative Transfer), where line scatterings are solved adaptively with the resolution of the smoothed particle hydrodynamics (SPH). The radiative transfer method implemented in seurat is based on a Monte Carlo algorithm in which the scattering and absorption by dust are also incorporated. We perform standard test calculations to verify the validity of the code; (i) emergent spectra from a static uniform sphere, (ii) emergent spectra from an expanding uniform sphere, and (iii) escape fraction from a dusty slab. Thereby, we demonstrate that our code solves the Lyα radiative transfer with sufficient accuracy. We emphasize that seurat can treat the transfer of Lyαphotons even in highly complex systems that have significantly inhomogeneous density fields. The high adaptivity of seurat is desirable to solve the propagation of Lyα photons in the interstellar medium of young star-forming galaxies likeLyα emitters (LAEs). Thus, seurat provides a powerful tool to model the emergent spectra of Lyα emission, which can be compared to the observations of LAEs.Lyα radiative transfer with sufficient accuracy. We emphasize that seurat can treat the transfer of Lyα photons even in highly complex systems that have significantly inhomogeneous density fields. The high adaptivity of seurat is desirable to solve the propagation of Lyα photons in the interstellar medium of young star-forming galaxies like Lyα emitters (LAEs). Thus, seurat provides a powerful tool to model the emergent spectra of Lyα emission, which can be compared to the observations of LAEs

Cosmological radiative transfer comparison project - II. The radiation-hydrodynamic tests

The development of radiation hydrodynamical methods that are able to follow gas dynamics and radiative transfer (RT) self-consistently is key to the solution of many problems in numerical astrophysics. Such fluid flows are highly complex, rarely allowing even for approximate analytical solutions against which numerical codes can be tested. An alternative validation procedure is to compare different methods against each other on common problems, in order to assess the robustness of the results and establish a range of validity for the methods. Previously, we presented such a comparison for a set of pure RT tests (i.e. for fixed, non-evolving density fields). This is the second paper of the Cosmological Radiative Transfer Comparison Project, in which we compare nine independent RT codes directly coupled to gas dynamics on three relatively simple astrophysical hydrodynamics problems: (i) the expansion of an H ii region in a uniform medium, (ii) an ionization front in a 1/r2 density profile with a flat core and (iii) the photoevaporation of a uniform dense clump. Results show a broad agreement between the different methods and no big failures, indicating that the participating codes have reached a certain level of maturity and reliability. However, many details still do differ, and virtually every code has showed some shortcomings and has disagreed, in one respect or another, with the majority of the results. This underscores the fact that no method is universal and all require careful testing of the particular features which are most relevant to the specific problem at han

Peering into the dark (ages) with low-frequency space interferometers: Using the 21-cm signal of neutral hydrogen from the infant universe to probe fundamental (Astro)physics.

The Dark Ages and Cosmic Dawn are largely unexplored windows on the infant Universe (z ~ 200-10). Observations of the redshifted 21-cm line of neutral hydrogen can provide valuable new insight into fundamental physics and astrophysics during these eras that no other probe can provide, and drives the design of many future ground-based instruments such as the Square Kilometre Array (SKA) and the Hydrogen Epoch of Reionization Array (HERA). We review progress in the field of high-redshift 21-cm Cosmology, in particular focussing on what questions can be addressed by probing the Dark Ages at z > 30. We conclude that only a space- or lunar-based radio telescope, shielded from the Earth's radio-frequency interference (RFI) signals and its ionosphere, enable the 21-cm signal from the Dark Ages to be detected. We suggest a generic mission design concept, CoDEX, that will enable this in the coming decades

A bubble size distribution model for the Epoch of Reionization

The bubble size distribution is a summary statistics that can be computed from the observed 21-cm signal from the Epoch of Reionization. As it depends only on the ionization field and is not limited to gaussian information, it is an interesting probe, complementary to the power spectrum of the full 21-cm signal. Devising a flexible and reliable theoretical model for the bubble size distribution paves the way for using it for astrophysical parameters inference. The proposed model is built from the excursion set theory and a functional relation between the bubble volume and the collapsed mass in the bubble. Unlike previous models it accommodates any functional relation or distributions. Using parameterized relations allows us to test the predictive power of the model by performing a minimization best-fit to the bubble size distribution obtained from a high resolution, fully coupled radiative hydrodynamics simulations, HIRRAH-21. Our model is able to provide a better fit to the numerical bubble size distribution at ionization fraction of $x_{\text{H}_{\text{II}}} \sim 1\%$ and $3\%$ than other existing models. Moreover, the bubble volume to collapsed mass relation corresponding to the best-fit parameters, which is not an observable, is compared to numerical simulation data. A good match is obtained, confirming the possibility to infer this relation from an observed bubble size distribution using our model. Finally we present a simple algorithm that empirically implements the process of percolation. We show that it extends the usability of our bubble size distribution model up to $x_{\text{H}_{\text{II}}} \sim 30\%$