The First Galaxies

We review our current understanding of how the first galaxies formed at the end of the cosmic dark ages, a few 100 million years after the Big Bang. Modern large telescopes discovered galaxies at redshifts greater than seven, whereas theoretical studies have just reached the degree of sophistication necessary to make meaningful predictions. A crucial ingredient is the feedback exerted by the first generation of stars, through UV radiation, supernova blast waves, and chemical enrichment. The key goal is to derive the signature of the first galaxies to be observed with upcoming or planned next-generation facilities, such as the James Webb Space Telescope or Atacama Large Millimeter Array. From the observational side, ongoing deep-field searches for very high-redshift galaxies begin to provide us with empirical constraints on the nature of the first galaxies.Comment: 75 pages, 14 figures, draft version for 2011 Annual Reviews of Astronomy and Astrophysic

Active regulator of SIRT1 is required for cancer cell survival but not for SIRT1 activity

The NAD+-dependent deacetylase SIRT1 is involved in diverse cellular processes, and has also been linked with multiple disease states. Among these, SIRT1 expression negatively correlates with cancer survival in both laboratory and clinical studies. Active regulator of SIRT1 (AROS) was the first reported post-transcriptional regulator of SIRT1 activity, enhancing SIRT1-mediated deacetylation and downregulation of the SIRT1 target p53. However, little is known regarding the role of AROS in regulation of SIRT1 during disease. Here, we report the cellular and molecular effects of RNAi-mediated AROS suppression, comparing this with the role of SIRT1 in a panel of human cell lines of both cancerous and non-cancerous origins. Unexpectedly, AROS is found to vary in its modulation of p53 acetylation according to cell context. AROS suppresses p53 acetylation only following the application of cell damaging stress, whereas SIRT1 suppresses p53 under all conditions analysed. This supplements the original characterization of AROS but indicates that SIRT1 activity can persist following suppression of AROS. We also demonstrate that knockdown of AROS induces apoptosis in three cancer cell lines, independent of p53 activation. Importantly, AROS is not required for the viability of three non-cancer cell lines indicating a putative role for AROS in specifically promoting cancer cell survival

Symmetry protected topological order at nonzero temperature

We address the question of whether symmetry-protected topological (SPT) order can persist at nonzero temperature, with a focus on understanding the thermal stability of several models studied in the theory of quantum computation. We present three results in this direction. First, we prove that nontrivial SPT order protected by a global on-site symmetry cannot persist at nonzero temperature, demonstrating that several quantum computational structures protected by such on-site symmetries are not thermally stable. Second, we prove that the 3D cluster state model used in the formulation of topological measurement-based quantum computation possesses a nontrivial SPT-ordered thermal phase when protected by a global generalized (1-form) symmetry. The SPT order in this model is detected by long-range localizable entanglement in the thermal state, which compares with related results characterizing SPT order at zero temperature in spin chains using localizable entanglement as an order parameter. Our third result is to demonstrate that the high error tolerance of this 3D cluster state model for quantum computation, even without a protecting symmetry, can be understood as an application of quantum error correction to effectively enforce a 1-form symmetry.Comment: 42 pages, 10 figures, comments welcome; v2 published versio

On the Formation Age of the First Planetary System

Recently, it has been observed the extreme metal-poor stars in the Galactic halo, which must be formed just after Pop III objects. On the other hand, the first gas clouds of mass $\sim 10^6 M_{\odot}$ are supposed to be formed at $z \sim$ 10, 20, and 30 for the $1\sigma$, $2\sigma$ and $3\sigma$, where the density perturbations are assumed of the standard $\Lambda$CDM cosmology. If we could apply this gaussian distribution to the extreme small probability, the gas clouds would be formed at $z \sim$40, 60, and 80 for the $4\sigma$, $6\sigma$, and $8\sigma$. The first gas clouds within our galaxy must be formed around $z\sim 40$. Even if the gas cloud is metal poor, there is a lot of possibility to form the planets around such stars. The first planetary systems could be formed within $\sim 6\times 10^7$ years after the Big Bang in the universe. Even in our galaxies, it could be formed within $\sim 1.7\times 10^8$ years. It is interesting to wait the observations of planets around metal-poor stars. For the panspermia theory, the origin of life could be expected in such systems.Comment: 5 pages,Proceedings IAU Symposium No. 249, 2007, Exoplanets:Y-S. Sun, S. Ferraz-Mello and J.-L, Zhou, eds. (p325

The faintest galaxies

We investigate the nature of Ultra Faint dwarf spheroidal galaxies (UF dSphs) in a general cosmological context, simultaneously accounting for various "classical" dSphs and Milky Way (MW) properties, including their Metallicity Distribution Function (MDF). The model successfully reproduces both the observed [Fe/H]-Luminosity relation and the mean MDF of UFs. According to our results UFs are the living fossils of H2-cooling minihaloes formed at z>8.5, i.e. before the end of reionization. They are the oldest and the most dark matter-dominated (M/L > 100) dSphs in the MW system, with a total mass of M = 10^(7-8) Msun. The model allows to interpret the different shape of UFs and classical dSphs MDF, along with the frequency of extremely metal-poor stars in these objects. We discuss the "missing satellites problem" by comparing the UF star formation efficiencies with those derived for minihaloes in the Via Lactea simulation.Comment: To appear in the conference proceeding: "First Stars and Galaxies: Challenges in the Next Decade" . Publisher: American Institute of Physics. Editors: V. Bromm, D. Whalen, N. Yoshid

Weakly Self-Interacting Dark Matter and the Structure of Dark Halos

We study the formation of dark halos in a $\Lambda$CDM universe under the assumption that Cold Dark Matter particles have a finite cross-section for elastic collisions. We compare evolution when CDM mean free paths are comparable to halo sizes with the collisionless and fluid limits. We show that a few collisions per particle per Hubble time at halo centre can substantially affect the central density profile. Cross-sections an order of magnitude larger produce sufficient relaxation for rich clusters to develop core radii in the range 100-200 $h^{-1}$kpc. The structural evolution of halos is a competition between collisional relaxation caused by individual particle interactions and violent relaxation resulting from the infall and merging processes by which clusters grow. Although our simulations concentrate on systems of cluster size, we can scale our results to address the halo structure expected for dwarf galaxies. We find that collision cross-sections sufficiently large to significantly modify the cores of such galaxies produce cluster cores which are too large and/or too round to be consistent with observation. Thus the simplest model for self-interacting dark matter is unable to improve fits to published dwarf galaxy rotation curves without violating other observational constraints.Comment: Revised, accepted for publication in ApJ Letters. Figure1 replace

Statistical Analysis of Spectral Line Candidates in Gamma-Ray Burst GRB870303

The Ginga data for the gamma-ray burst GRB870303 exhibit low-energy dips in two temporally distinct spectra, denoted S1 and S2. S1, spanning 4 s, exhibits a single line candidate at ~ 20 keV, while S2, spanning 9 s, exhibits apparently harmonically spaced line candidates at ~ 20 and 40 keV. We evaluate the statistical evidence for these lines, using phenomenological continuum and line models which in their details are independent of the distance scale to gamma-ray bursts. We employ the methodologies based on both frequentist and Bayesian statistical inference that we develop in Freeman et al. (1999b). These methodologies utilize the information present in the data to select the simplest model that adequately describes the data from among a wide range of continuum and continuum-plus-line(s) models. This ensures that the chosen model does not include free parameters that the data deem unnecessary and that would act to reduce the frequentist significance and Bayesian odds of the continuum-plus-line(s) model. We calculate the significance of the continuum-plus-line(s) models using the Chi-Square Maximum Likelihood Ratio test. We describe a parametrization of the exponentiated Gaussian absorption line shape that makes the probability surface in parameter space better-behaved, allowing us to estimate analytically the Bayesian odds. The significance of the continuum-plus-line models requested by the S1 and S2 data are 3.6 x 10^-5 and 1.7 x 10^-4 respectively, with the odds favoring them being 114:1 and 7:1. We also apply our methodology to the combined (S1+S2) data. The significance of the continuum-plus-lines model requested by the combined data is 4.2 x 10^-8, with the odds favoring it being 40,300:1.Comment: LaTeX2e (aastex.cls included); 41 pages text, 10 figures (on 11 pages); accepted by ApJ (to be published 1 Nov 1999, v. 525

Tuning the spin dynamics of kagome systems

Despite the conceptional importance of realizing spin liquids in solid states only few compounds are known. On the other side the effect of lattice distortions and anisotropies on the magnetic exchange topology and the fluctuation spectrum are an interesting problem. We compare the excitation spectra of the two s=1/2 kagome lattice compounds volborthite and vesignieite using Raman scattering. We demonstrate that even small modifications of the crystal structure may have a huge effect on the phonon spectrum and low temperature properties.Comment: 3 pages, 2 figure

Gas cooling in simulations of the formation of the galaxy population

We compare two techniques for following the cooling of gas and its condensation into galaxies within high resolution simulations of cosmologically representative regions. Both techniques treat the dark matter using N-body methods. One follows the gas using smoothed particle hydrodynamics (SPH) while the other uses simplified recipes from semi-analytic (SA) models. We compare the masses and locations predicted for dense knots of cold gas (the `galaxies') when the two techniques are applied to evolution from the same initial conditions and when the additional complications of star formation and feedback are ignored. We find that above the effective resolution limit of the two techniques, they give very similar results both for global quantities such as the total amount of cooled gas and for the properties of individual `galaxies'. The SA technique has systematic uncertainties arising from the simplified cooling model adopted, while details of the SPH implementation can produce substantial systematic variations in the galaxy masses it predicts. Nevertheless, for the best current SPH methods and the standard assumptions of the SA model, systematic differences between the two techniques are remarkably small. The SA technique gives adequate predictions for the condensation of gas into `galaxies' at less than one percent of the computational cost of obtaining similar results at comparable resolution using SPH.Comment: Revised, Figure 7 added. To appear in MNRA