Primordial Galaxy Formation and IGM Reionization

In this talk I will present a model for primordial galaxy formation. In particular, I will review the feedback effects that regulate the process: (i) radiative (i.e. ionizing and H₂-photodissociating photons) and (ii) stellar (i.e. SN explosions) feedback produced by massive stars. I will also address how the IGM reionization can be influenced by this population of primordial galaxies and describe a Monte Carlo method for the radiative transfer of ionizing photons through the IGM. This revised version was published online in September 2006 with corrections to the Cover Date.     

Reionization and structure formation with ARCADE

Structure formation in the early universe is expected to produce a cosmological background of free–free emission. The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) will measure the spectrum of the cosmic microwave background at centimeter wavelengths to detect this signature of structure formation. I describe the ARCADE instrument and the cryogenic engineering required to achieve millikelvin accuracy at such long wavelengths.     

Inhomogeneous Reionization and the Polarization of the Cosmic Microwave Background

In a universe with inhomogeneous reionization, the ionized patches create a second-order signal in the cosmic microwave background polarization anisotropy. This signal originates in the coupling of the free-electron fluctuation to the quadruple moment of the temperature anisotropy. We examine the contribution from a simple inhomogeneous reionization model and find that the signal from such a process is below the detectable limits of the Planck Surveyor mission. However, the signal is above the fundamental uncertainty limit from cosmic variance, so that a future detection with a high-accuracy experiment on subarcminute scales is possible.     

宇宙再电离时期的kSZ效应和X射线背景与21 cm信号的互相关研究

正宇宙再电离是宇宙从黑暗时期到完全电离过渡的重要阶段,也是宇宙学研究的一个非常重要的课题,但是目前为止人们对宇宙再电离仍然缺乏足够精确的观测,其中最大的问题是微弱的有效信号往往淹没于巨大的前景噪声中因而很难提取出来.本工作中研究了宇宙再电离时代的动力学苏尼阿耶夫-泽尔多维奇效应(Kinetic Sunyaev-Zel’dovich, k SZ)、X射线背景以及与中性氢的21 cm信息的互相     

Reionization and the Cosmic Dawn with the Square Kilometre Array

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21 cm line from the earliest phases of star and galaxy formation in the Universe. This 21 cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.     

Reionization and the large-scale 21-cm cosmic microwave background cross-correlation

Of the many probes of reionization, the 21-cm line and the cosmic microwave background (CMB) are among the most effective. We examine how the cross-correlation of the 21-cm brightness and the CMB Doppler fluctuations on large angular scales can be used to study this epoch. We employ a new model of the growth of large-scale fluctuations of the ionized fraction as reionization proceeds. We take into account the peculiar velocity field of baryons and show that its effect on the cross-correlation can be interpreted as a mixing of Fourier modes. We find that the cross-correlation signal is strongly peaked towards the end of reionization and that the sign of the correlation should be positive because of the inhomogeneity inherent to reionization. The signal peaks at degree scales (ℓ∼ 100) and comes almost entirely from large physical scales ( k ∼ 10⁻² Mpc). Since many of the foregrounds and noise that plague low-frequency radio observations will not correlate with CMB measurements, the cross-correlation might appear to provide a robust diagnostic of the cosmological origin of the 21-cm radiation around the epoch of reionization. Unfortunately, we show that these signals are actually only weakly correlated and that cosmic variance dominates the error budget of any attempted detection. We conclude that the detection of a cross-correlation peak at degree-size angular scales is unlikely even with ideal experiments.     

The Effect of AGN and SNe Feedback on Star Formation, Reionization and the Near Infrared Background

Feedback from supernovae (SNe) and from active galactic nuclei (AGN) accom-panies the history of star formation and galaxy evolution. We present an analytic model to explain how and when the SNe and AGN exert their feedback effects on the star formation and galaxy evolution processes. By using SNe and AGN kinetic feedback mechanisms based on the Lambda Cold Dark Matter (LCDM) model, we explore how these feedback mecha-nisms affect the star formation history (SFH), the Near-Infrared Background (NIRB) flux and the cosmological reionization. We find the values of the feedback strengths, ∈AGN =1.0+0.50.3and ∈SN=0.04+0.02-0.02, can provide a reasonable explanation of most of the observational re-suits, and that the AGN feedback effect on star formation history is quite different from the SNe feedback at high redshifts. Our conclusions manifest quantitatively that these feedback effects decrease star formation rate density (SFRD) and the NIRB flux (in 1.4 - 4.0μm), and postpone the time of completion of the cosmological reionization.     

Zero-Metallicity Stars and the Effects of the First Stars on Reionization

We present a model that relates the width of the broad emission lines of active galactic nuclei (AGNs) to the Keplerian velocity of an accretion disk at a critical distance from the central black hole. This critical distance falls in a region bounded on the inward side by the transition radius between the radiation pressure- and the gas pressure-dominated region of the accretion disk and on the outward side by the maximum radius below which a stabilizing, radially accreting and vertically outflowing corona exists. We show that in the framework of this picture, the observed range of Hbeta FWHMs from broad-line to narrow-line type 1 AGNs is well reproduced as a function of the accretion rate. This interval of velocities is the only permitted range and goes from approximately 20,000 km s-1 for sub-Eddington accretion rates to approximately 1000 km s-1 for Eddington accretion rates.     

Modelling the 21-cm Signal from the Epoch of Reionization and Cosmic Dawn

Studying the cosmic dawn and the epoch of reionization through the redshifted 21-cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the Universe. Interpreting the upcoming data would require detailed modelling of the relevant physical processes. In this article, we focus on the theoretical models of reionization that have been worked out by various groups working in India with the upcoming SKA in mind. These models include purely analytical and semi-numerical calculations as well as fully numerical radiative transfer simulations. The predictions of the 21-cm signal from these models would be useful in constraining the properties of the early galaxies using the SKA data.     

Line-of-Sight Anisotropies in the Cosmic Dawn and Epoch of Reionization 21-cm Power Spectrum

The line-of-sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reionization is quite unique in many ways compared to any other cosmological signal. Different unique effects, such as the evolution history of the signal, non-linear peculiar velocities of the matter etc. will imprint their signature along the line-of-sight axis of the observed signal. One of the major goals of the future SKA-LOW radio interferometer is to observe the cosmic dawn and the epoch of reionization through this 21-cm signal. It is thus important to understand how these various effects affect the signal for its actual detection and proper interpretation. For more than one and half decades, various groups in India have been actively trying to understand and quantify the different line-of-sight effects that are present in this signal through analytical models and simulations. In many ways the importance of this sub-field under 21-cm cosmology have been identified, highlighted and pushed forward by the Indian community. In this article, we briefly describe their contribution and implication of these effects in the context of the future surveys of the cosmic dawn and the epoch of reionization that will be conducted by the SKA-LOW.     

Probing Individual Sources during Reionization and Cosmic Dawn using Square Kilometre Array HI 21-cm Observations

Detection of individual luminous sources during the reionization epoch and cosmic dawn through their signatures in the HI 21-cm signal is one of the direct approaches to probe the epoch. Here, we summarize our previous works on this and present preliminary results on the prospects of detecting such sources using the SKA1-low experiment. We first discuss the expected HI 21-cm signal around luminous sources at different stages of reionization and cosmic dawn. We then introduce two visibility based estimators for detecting such signals: one based on the matched filtering technique and the other relies on simply combing the visibility signal from different baselines and frequency channels. We find that the SKA1-low should be able to detect ionized bubbles of radius \(R_{\mathrm {b}} \gtrsim 10\) Mpc with ～100 h of observations at redshift z～8 provided that the mean outside neutral hydrogen fraction \(\mathrm {x}_{\text {HI}} \gtrsim 0.5\). We also investigate the possibility of detecting HII regions around known bright QSOs such as around ULASJ1120+0641 discovered by Mortlock et al. (Nature 474, 7353 (2011)). We find that a 5σ detection is possible with 600 h of SKA1-low observations if the QSO age and the outside x_HI are at least ～2×10⁷ Myr and ～0.2 respectively. Finally, we investigate the possibility of detecting the very first X-ray and Ly- α sources during the cosmic dawn. We consider mini-QSOs like sources which emits in X-ray frequency band. We find that with a total ～ 1000 h of observations, SKA1-low should be able to detect those sources individually with a ～ 9σ significance at redshift z=15. We summarize how the SNR changes with various parameters related to the source properties.     

Cosmological parameters and cosmic topology

Geometry constrains but does not dictate the topology of the three-dimensional space. In a locally spatially homogeneous and isotropic universe, however, the topology of its spatial section dictates its geometry. We show that, besides determining the geometry, the knowledge of the spatial topology through the circles-in-the-sky offers an effective way of setting constraints on the density parameters associated with dark matter (Ω_m) and dark energy  (Ω_Λ)  . By assuming the Poincaré dodecahedral space as the circles-in-the-sky detectable topology of the spatial sections of the Universe, we re-analyse the constraints on the density parametric plane  Ω_m–Ω_Λ  from the current Type Ia supernova plus X-ray gas mass fraction data, and show that a circles-in-the sky detection of the dodecahedral space topology gives rise to strong and complementary constraints on the region of the density parameter plane currently allowed by these observational data sets.     

Relationship between electric currents,photospheric motions,chromospheric activity,and magnetic field topology

Through coordinated observations made during the Max'91 campaign in June 1989 in Potsdam (magnetograms), Debrecen (white light and H), and Meudon (MSDP), we follow the evolution of the sunspot group in active region NOAA 5555 for 6 days. The topology of the coronal magnetic field is investigated by using a method based on the concept of separatrices - applied previously (Mandriniet al., 1991) to a magnetic region slightly distorted by field-aligned currents. The present active region differs by having significant magnetic shear. We find that the H flare kernels and the main photospheric electric current cells are located close to the intersection of the separatrices with the chromosphere, in a linear force-free field configuration adapted to the observed shear. Sunspot motions, strong currents, isolated polarities, or intersecting separatrices are not in themselves sufficient to produce a flare. A combination of them all is required. This supports the idea that flares are due to magnetic reconnection, when flux tubes with field-aligned currents move towards the separatrix locations.     

Solar flares and magnetic topology

This article is a very brief review and comparison of the observational properties of flares and theoretical concepts of models of flares, especially the concepts of magnetic topology and its evolution. We examine the environmental aspects of flare behavior. Some of these aspects must be consequences of unknown processes occurring below the photosphere. Other aspects involve structures--such as filaments--that are closely related to flares. We then examine properties of flares to try to distinguish the different phases of energy release that can occur in the course of a flare. Finally we offer a schematic scenario and attempt to interpret these phases in terms of this scenario.     

Solar flares and magnetic topology

Solar Physics - This article is a very brief review and comparison of the observational properties of flares and theoretical concepts of models of flares, especially the concepts of magnetic...     

Relativistic reference frames including time scales: Questions and answers

The subject of relativistic reference frames in astronomy is discussed with respect to the problems and needs of the various user groups. For didactical reasons the discussion is presented in form of a sequence of questions and answers.     

宇宙黎明和再电离时期低频全天总功率探测实验现状

宇宙黎明和再电离时期探测是目前宇宙学最前沿科学研究方向之一.对这一时期的直接探测只能依赖于观测来自这一时期红移后的中性氢21 cm信号,其3种主要探测方式之一是21 cm信号全天总功率测量.在此回顾已有和正在计划中的探测宇宙黎明和再电离时期的低频全天总功率测量实验及其进展,包括地面射电望远镜如BIGHORNS¹、EDGES²、LACE³、LEDA⁴、MIST⁵、REACH⁶、SARAS⁷ 3、SCI-HI⁸、PRI^ZM⁹以及空间低频总功率相关实验如DARE¹⁰、DAPPER¹¹、FARSIDE¹²、鸿蒙计划.其中, EDGES实验是目前唯一声称观测到疑似宇宙黎明信号的实验,然而其实验结果与标准宇宙学模型(Λ Cold Dark Matter,ΛCDM)有不符之处.如果该探测结果被证实,那么这将是人类第1次...