119082603
Metal absorption lines as probes of the intergalactic medium prior to the reionization epoch
Winds from star-forming galaxies provide the most promising explanation for the enrichment of the intergalactic medium with heavy elements. Theoretical and observational arguments indicate that the pollution may have occurred at z 6; however, direct observational tests of such a scenario are needed. We model starburst winds in the high-redshift universe and find that the fraction of space filled by enriched material varies strongly with the assumed star formation efficiency f* and the fraction of supernova energy powering each wind fesc. We show that metals carried by these winds can be seen in absorption against bright background sources, such as quasars or gamma-ray bursts, in narrow lines with characteristic equivalent widths ~0.5 A W 5 A. We argue that a substantial fraction of the metals in high-redshift winds are likely to reside in low-ionization states (C II, O I, Si II, and Fe II), but higher ionization states (C IV and Si IV) could also provide useful probes of the winds. The number of such lines can constrain both f* and fesc. Statistics of metal absorption lines can also be used to identify whether H2 is an efficient coolant in the early universe and to study the initial mass function of stars at high redshifts.
16179092
Early formation and late merging of the giant galaxies
The most luminous galaxies in the present universe are found at the centers of the most massive dark matter halos: rich galaxy clusters. In the ΛCDM cosmology, such massive halo cores are present at redshift z = 6 with a comoving number density (as a function of mass interior to ~10 kpc) that is comparable to today's value. The identity of the matter in these central regions is, however, predicted to change as major mergers bring together stars and dark matter from initially well-separated subunits. We use N-body simulations to investigate how these mergers push preexisting matter outward in the dominant galaxy while preserving the inner density profile of collisionless matter. It appears that the central regions of large galaxies end up dominated by stars formed in a number of dense cores, well before the last major mergers. The density profile of collisionless matter (stars and dark matter combined) in these central regions appears to be stable and to have attractor-like behavior under merging. This suggests that the baryon loading associated with dissipative contraction and star formation may be erased as subsequent mergers drive the mass distribution back to a universal profile. Such suppression of the effects of baryon loading, along with the early assembly of mass concentrations, may help resolve some apparent challenges to the CDM model for structure formation.
118271059
21 cm Intensity Mapping
Using the 21 cm line, observed all-sky and across the redshift range from 0 to 5, the large scale structure of the Universe can be mapped in three dimensions. This can be accomplished by studying specific intensity with resolution ~ 10 Mpc, rather than via the usual galaxy redshift survey. The data set can be analyzed to determine Baryon Acoustic Oscillation wavelengths, in order to address the question: 'What is the nature of Dark Energy?' In addition, the study of Large Scale Structure across this range addresses the questions: 'How does Gravity effect very large objects?' and 'What is the composition our Universe?' The same data set can be used to search for and catalog time variable and transient radio sources.
17566037
Signatures of Relativistic Helical Motion in the Rotation Measures of AGN Jets
Polarization has proved an invaluable tool for probing magnetic fields in relativistic jets. Maps of the intrinsic polarization vectors have provided the best evidence to date for uniform, toroidally dominated magnetic fields within jets. More recently, maps of the rotation measure (RM) in jets have for the first time probed the field geometry of the cool, moderately relativistic surrounding material. In most cases, clear signatures of toroidal magnetic field are detected, corresponding to gradients in RM profiles transverse to the jet. However, in many objects these profiles also display marked asymmetries which are difficult to explain in simple helical jet models. Furthermore, in some cases the RM profiles are strongly frequency and/or time dependent. Here we show that these features may be naturally accounted for by including relativistic helical motion in the jet model. In particular, we are able to reproduce bent RM profiles observed in a variety of jets, frequency dependent RM profile morphologies and even the time dependence of the RM profiles of knots in 3C 273. Finally, we predict that some sources may show reversals in their RM profiles at sufficiently high frequencies, depending upon the the ratio of the components of jet sheath velocity transverse and parallel to the jet. Thus, multi-frequency RM maps promise a novel way in which to probe the velocity structure of relativistic outflows.
4688516
Constraints on the Nature of CID-42: Recoil Kick or Supermassive Black Hole Pair?
The galaxy CXOC J100043.1+020637, also known as CID-42, is a highly unusual object. An apparent galaxy merger remnant, it displays signatures of both an inspiraling, kiloparsecscale active galactic nucleus (AGN) pair and of a recoiling AGN with a kick velocity > 1300 km s −1 . Among recoiling AGN candidates, CID-42 alone has both spatial offsets (in optical and X-ray bands) and spectroscopic offsets. In order to constrain the relative likelihood of both scenarios, we develop models using hydrodynamic galaxy merger simulations coupled with radiative transfer calculations. Our gas-rich, major merger models are generally well matched to the galactic morphology and to the inferred stellar mass and star formation rate. We show that a recoiling supermassive black hole (SMBH) in CID-42 should be observable as an AGN at the time of observation. However, in order for the recoiling AGN to produce narrow-line emission, it must be observed shortly after the kick while it still inhabits a dense gaseous region, implying a large total kick velocity (vk > 2000 km s −1 ). For the dual AGN scenario, an unusually large broad-line offset is required, and the best match to the observed morphology requires a galaxy that is less luminous than CID-42. Further, the lack of X-ray emission from one of the two optical nuclei is not easily attributed to an intrinsically quiescent SMBH or to a Compton-thick galactic environment. While the current data do not allow either the recoiling or the dual AGN scenario for CID-42 to be excluded, our models highlight the most relevant parameters for distinguishing these possibilities with future observations. In particular, high-quality, spatially-resolved spectra that can pinpoint the origin of the broad and narrow line features will be critical for determining the nature of this unique source.
119094218
In the beginning: the first sources of light and the reionization of the universe
Abstract The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at a redshift z =30 and reionized most of the hydrogen in the universe by z =7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.
2038251
Time-dependent Ionization in Radiatively Cooling Gas
We present new computations of the equilibrium and nonequilibrium cooling efficiencies and ionization states for low-density radiatively cooling gas containing the elements H, He, C, N, O, Ne, Mg, Si, S, and Fe. We present results for gas temperatures between 10^4 and 10^8 K, assuming dust-free and optically thin conditions, and no external radiation. For nonequilibrium cooling we solve the coupled time-dependent ionization and energy loss equations for a radiating gas cooling from an initially hot, ≳5 × 10^6 K, equilibrium state, down to 10^4 K. We present results for heavy element compositions ranging from 10^(-3) to 2 times the elemental abundances in the Sun. We consider gas cooling at constant density (isochoric) and at constant pressure (isobaric). We calculate the critical column densities and temperatures at which radiatively cooling clouds make the dynamical transition from isobaric to isochoric evolution. We construct ion ratio diagnostics for the temperature and metallicity in radiatively cooling gas. We provide numerical estimates for the maximal cloud column densities for which the gas remains optically thin to the cooling radiation. We present our computational results in convenient online figures and tables.
31215380
X-ray orbital modulations in intermediate polars
We present an analysis of 30 archival ASCA and RXTE X-ray observations of 16 intermediate polars to investigate the nature of their orbital modulation. We show that X-ray orbital modulation is widespread amongst these systems, but not ubiquitous as indicated by previous studies that included fewer objects. Only seven of the sixteen systems show a clearly statistically significant modulation depth whose amplitude decreases with increasing X-ray energy. Interpreting this as due to photoelectric absorption in material at the edge of an accretion disc would imply that such modulations are visible for all system inclination angles in excess of 60 degrees. However, it is also apparent that the presence of an X-ray orbital modulation can appear and disappear on a timescale of ∼years or months in an individual system. This may be evidence for the presence of a precessing, tilted accretion disc, as inferred in some low mass X-ray binaries.
15136851
Broad-band X-ray measurements of the black hole candidate XTE J1908+094
XTE J1908+094 is an X-ray transient that went into outburst in February 2002. After two months it reached a 2- 250 keV peak fl ux of 1t o 2 10 8 erg cm 2 s 1 . Circumstantial evidence points to an accreting galactic black hole as the origin of the X-radiation: pulsations nor thermonuclear flashes were detected that would identify a neutron star and the spectrum was unusually hard for a neutron star at the outburst onset. We report on BeppoSAX and RXTE All Sky Monitor observations of the broad-band spectrum of XTE J1908+094. The spectrum is consistent with a model consisting of a Comptonization component by a40 keV plasma, a multicolor accretion disk blackbody component with a temperature just below 1 keV and a broad emission line at about 6 keV. The spectrum is heavily absorbed by cold interstellar matter with an equivalent hydrogen column density of 2:5 10 22 cm 2 , which makes it dicult to study the black body component in detail. The black body component exhibits strong evolution about 6 weeks into the outburst. Two weeks later this is followed by a swift decay of the power law component. The broadness of the 6 keV feature may be due to relativistic broadening or Compton scattering of a narrow Fe-K line.
73563531
Metastability of the false vacuum in a Higgs-seesaw model of dark energy
In a recently proposed Higgs-Seesaw model the observed scale of dark energy results from a metastable false vacuum energy associated with mixing of the standard model Higgs particle and a scalar associated with new physics at the GUT or Planck scale. Here we address the issue of how to ensure metastability of this state over cosmological time. We consider new tree-level operators, the presence of a thermal bath of hidden sector particles, and quantum corrections to the effective potential. We find that in the thermal scenario many additional light degrees of freedom are typically required unless coupling constants are somewhat fine-tuned. However quantum corrections arising from as few as one additional light scalar field can provide the requisite support. We also briefly consider implications of late-time vacuum decay for the perdurance of observed structures in the universe in this model.
119095544
Final state interactions and new physics in B ---> pi K decays
Within the standard model, and if one assumes that soft rescattering effects are negligible, the $\mathrm{CP}$ asymmetry ${\mathcal{A}}_{\mathrm{CP}}^{\mathrm{dir}}{(B}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}K)$ is predicted to be very small and the ratio ${R=B(B}_{d}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{\mp}}{K}^{\ifmmode\pm\else\textpm\fi{}}{)/B(B}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}K)$ provides a bound on the angle $\ensuremath{\gamma}$ of the unitarity triangle, ${\mathrm{sin}}^{2}\ensuremath{\gamma}l~R.$ We estimate the corrections from soft rescattering effects using an approach based on Regge phenomenology, and find effects of order $10%$ with large uncertainties. In particular, we conclude that ${\mathcal{A}}_{\mathrm{CP}}^{\mathrm{dir}}\ensuremath{\sim}0.2$ and ${\mathrm{sin}}^{2}\ensuremath{\gamma}\ensuremath{\sim}1.2R$ could not be taken unambiguously to signal new physics. Using SU(3) relations, we suggest experimental tests that could constrain the size of the soft rescattering effects thus reducing the related uncertainty. Finally, we study the effect of various models of new physics on ${\mathcal{A}}_{\mathrm{CP}}^{\mathrm{dir}}$ and on $R.$