It has been known for decades that the observed number of baryons in the local Universe falls about 30-40 per cent short
of the total number of baryons predicted
by Big Bang nucleosynthesis, as ...inferred
from density fluctuations of the cosmic microwave background and seen during the first 2-3 billion years of the Universe in the so-called 'Lyman α forest'
(a dense series of intervening H I Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas-hydrogen-is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios
. Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far
. Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The O VII systems lie in regions characterized by large (four times larger than average
) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium. We conclude that the missing baryons have been found.
When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation
. This magneto-optical Kerr effect (MOKE) has been intensively ...studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties
as well as for various applications including magneto-optical recording
. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts
. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure
. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of
~0.002 µ
/Mn, the non-collinear AF metal Mn
Sn
exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Néel state
may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.
We present a full data analysis of the pure-parallel Hubble Space Telescope (HST) imaging observations in the Brightest of Reionizing Galaxies Survey (BoRGz9) in Cycle 22. The medium-deep exposures ...with five HST/WFC3IR+UVIS filter bands from 79 independent sightlines (∼370 arcmin2) provide the least biased determination of number density for z 9 bright galaxies against cosmic variance. After a strict two-step selection for candidate galaxies, including dropout color and photometric redshift analyses, and revision of previous BoRG candidates, we identify one source at z ∼ 10 and two sources at z ∼ 9. The z ∼ 10 candidate shows evidence of line-of-sight lens magnification ( ∼ 1.5), yet it appears surprisingly luminous ( mag), making it one of the brightest candidates at known (∼0.3 mag brighter than the z = 8.68 galaxy EGSY8p7, spectroscopically confirmed by Zitrin and collaborators). For z ∼ 9 candidates, we include previous data points at fainter magnitudes and find that the data are well fitted by a Schechter luminosity function with , mag, and Mpc−3 mag−1, for the first time without fixing any parameters. The inferred cosmic star formation rate density is consistent with unaccelerated evolution from lower redshift.
ABSTRACT
We revisit the reliability of metallicity estimates of high velocity clouds with the help of hydrodynamical simulations. We quantify the effect of accretion and viewing angle on metallicity ...estimates derived from absorption lines. Model parameters are chosen to provide strong lower limits on cloud contamination by ambient gas. Consistent with previous results, a cloud traveling through a stratified halo is contaminated by ambient material to the point that <10 per cent of its mass in neutral hydrogen consists of original cloud material. Contamination progresses nearly linearly with time, and it increases from head to tail. Therefore, metallicity estimates will depend on the evolutionary state of the cloud, and on position. While metallicities change with time by more than a factor of 10, well beyond observational uncertainties, most lines-of-sight range only within those uncertainties at any given time over all positions. Metallicity estimates vary with the cloud’s inclination angle within observational uncertainties. The cloud survives the infall through the halo because ambient gas continuously condenses and cools in the cloud’s wake and thus appears in the neutral phase. Therefore, the cloud observed at any fixed time is not a well-defined structure across time, since material gets constantly replaced. The thermal phases of the cloud are largely determined by the ambient pressure. Internal cloud dynamics evolve from drag gradients caused by shear instabilities, to complex patterns due to ram-pressure shielding, leading to a peloton effect, in which initially lagging gas can catch up to and even overtake the head of the cloud.
We report the discovery of 33 Lyman-break galaxy candidates at z ~ 8 detected in Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging as part of the Brightest of Reionizing Galaxies (BoRG) ...pure-parallel survey. Our sample of 33 relatively bright Y sub(098)-dropout galaxies have J sub(125)-band magnitudes between 25.5 and 27.4 mag. This is the largest sample of bright (J sub(125) lap 27.4) z ~ 8 galaxy candidates presented to date. Combining our data set with the Hubble Ultra-Deep Field data set, we constrain the rest-frame ultraviolet galaxy luminosity function at z ~ 8 over the widest dynamic range currently available. With a future expansion of the BoRG survey, combined with planned ultradeep WFC3/IR observations, it will be possible to further reduce this uncertainty and clearly demonstrate the steepening of the faint-end slope compared to measurements at lower redshift, thereby confirming the key role played by small galaxies in the reionization of the universe.
We present the first results on the search for very bright (M AB --21) galaxies at redshift z ~ 8 from the Brightest of Reionizing Galaxies (BoRG) survey. BoRG is a Hubble Space Telescope Wide Field ...Camera 3 (WFC3) pure-parallel survey that is obtaining images on random lines of sight at high Galactic latitudes in four filters (F606W, F098M, F125W, and F160W), with integration times optimized to identify galaxies at z 7.5 as F098M dropouts. We discuss here results from a search area of approximately 130 arcmin2 over 23 BoRG fields, complemented by six other pure-parallel WFC3 fields with similar filters. This new search area is more than two times wider than previous WFC3 observations at z ~ 8. We identify four F098M-dropout candidates with high statistical confidence (detected at greater than 8 Delta *s confidence in F125W). These sources are among the brightest candidates currently known at z ~ 8 and approximately 10 times brighter than the z = 8.56 galaxy UDFy-38135539. They thus represent ideal targets for spectroscopic follow-up observations and could potentially lead to a redshift record, as our color selection includes objects up to z ~ 9. However, the expected contamination rate of our sample is about 30% higher than typical searches for dropout galaxies in legacy fields, such as the GOODS and HUDF, where deeper data and additional optical filters are available to reject contaminants.
ABSTRACT Long-duration gamma-ray bursts (GRBs) allow us to pinpoint and study star-forming galaxies in the early universe, thanks to their orders of magnitude brighter peak luminosities compared to ...other astrophysical sources, and their association with the deaths of massive stars. We present Hubble Space Telescope Wide Field Camera 3 detections of three Swift GRB host galaxies lying at redshifts z = 5.913 (GRB 130606A), z = 6.295 (GRB 050904), and z = 6.327 (GRB 140515A) in the F140W (wide-JH band, ) filter. The hosts have magnitudes (corrected for Galactic extinction) of and , respectively. In all three cases, the probability of chance coincidence of lower redshift galaxies is , indicating that the detected galaxies are most likely the GRB hosts. These are the first detections of high-redshift ( ) GRB host galaxies in emission. The galaxies have luminosities in the range 0.1-0.6 (with ) and half-light radii in the range 0.6-0.9 . Both their half-light radii and luminosities are consistent with existing samples of Lyman-break galaxies at . Spectroscopic analysis of the GRB afterglows indicate low metallicities ( ) and low dust extinction ( ) along the line of sight. Using stellar population synthesis models, we explore the implications of each galaxy's luminosity for its possible star-formation history and consider the potential for emission line metallicity determination with the upcoming James Webb Space Telescope.
The new Wide Field Camera 3/IR observations on the Hubble Ultra-Deep Field (HUDF) started investigating the properties of galaxies during the reionization epoch. To interpret these observations, we ...present a novel approach inspired by the conditional luminosity function method. We calibrate our model to observations at z = 6 and assume a non-evolving galaxy luminosity versus halo mass relation. We first compare model predictions against the luminosity function (LF) measured at z = 5 and z = 4. We then predict the LF at z {>=} 7 under the sole assumption of evolution in the underlying dark-matter halo mass function. Our model is consistent with the observed z {approx_gt} 7 galaxy number counts in the HUDF survey and suggests a possible steepening of the faint-end slope of the LF: {alpha}(z {approx_gt} 8) {approx_lt} -1.9 compared to {alpha} = -1.74 at z = 6. Although we currently see only the brightest galaxies, a hidden population of lower luminosity objects (L/L {sub *} {approx_gt} 10{sup -4}) might provide {approx_gt}75% of the total reionizing flux. Assuming escape fraction f {sub esc} {approx} 0.2, clumping factor C {approx} 5, top-heavy initial mass function (IMF), and low metallicity, galaxies below the detection limit produce complete reionization at z {approx_gt} 8. For solar metallicity and normal stellar IMF, reionization finishes at z {approx_gt} 6, but a smaller C/f {sub esc} is required for an optical depth consistent with the Wilkinson Microwave Anisotropy Probe measurement. Our model highlights that the star formation rate in sub-L {sub *} galaxies has a quasi-linear relation to dark-matter halo mass, suggesting that radiative and mechanical feedback were less effective at z {>=} 6 than today.
Abstract
We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line spectroscopy and integral-field unit (IFU) observations of the intragroup medium in Stephan’s Quintet (SQ). SQ ...hosts a 30 kpc long shocked ridge triggered by a galaxy collision at a relative velocity of 1000 km s
−1
, where large amounts of molecular gas coexist with a hot, X-ray-emitting, plasma. COS spectroscopy at five positions sampling the diverse environments of the SQ intragroup medium reveals very broad (≈2000 km s
−1
) Ly
α
line emission with complex line shapes. The Ly
α
line profiles are similar to or much broader than those of H
β
, C
ii
157.7
μ
m, and CO (1–0) emission. The extreme breadth of the Ly
α
emission, compared with H
β
, implies resonance scattering within the observed structure. Scattering indicates that the neutral gas of the intragroup medium is clumpy, with a significant surface covering factor. We observe significant variations in the Ly
α
/H
β
flux ratio between positions and velocity components. From the mean line ratio averaged over positions and velocities, we estimate the effective escape fraction of Ly
α
photons to be ≈10%–30%. Remarkably, over more than four orders of magnitude in temperature, the powers radiated by X-rays, Ly
α
, H
2
, and C
ii
are comparable within a factor of a few, assuming that the ratio of the Ly
α
to H
2
fluxes over the whole shocked intragroup medium stay in line with those observed at those five positions. Both shocks and mixing layers could contribute to the energy dissipation associated with a turbulent energy cascade. Our results may be relevant for the cooling of gas at high redshifts, where the metal content is lower than in this local system, and a high amplitude of turbulence is more common.
ABSTRACT We present the first results and design from the redshift z ∼ 9-10 Brightest of the Reionizing Galaxies Hubble Space Telescope survey BoRGz9-10, aimed at searching for intrinsically luminous ...unlensed galaxies during the first 700 Myr after the Big Bang. BoRGz9-10 is the continuation of a multi-year pure-parallel near-IR and optical imaging campaign with the Wide Field Camera 3. The ongoing survey uses five filters, optimized for detecting the most distant objects and offering continuous wavelength coverage from λ = 0.35 m to λ = 1.7 m. We analyze the initial ∼130 arcmin2 of area over 28 independent lines of sight (∼25% of the total planned) to search for galaxies using a combination of Lyman-break and photometric redshift selections. From an effective comoving volume of (5-25) × 105 Mpc3 for magnitudes brighter than in the -band respectively, we find five galaxy candidates at 8.3-10 detected at high confidence ( ), including a source at 8.4 with ( ), which, if confirmed, would be the brightest galaxy identified at such early times ( ). In addition, BoRGz9-10 data yield four galaxies with . These new Lyman-break galaxies with are ideal targets for follow-up observations from ground and space-based observatories to help investigate the complex interplay between dark matter growth, galaxy assembly, and reionization.
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