The ionizing continuum from active galactic nuclei is fundamental for interpreting their broad emission lines and understanding their impact on the surrounding gas. Furthermore, it provides hints on ...how matter accretes on to supermassive black holes. Using Hubble Space Telescope's Wide Field Camera 3, we have constructed the first stacked ultraviolet (rest-frame wavelengths 600–2500 Å) spectrum of 53 luminous quasars at z ≃ 2.4, with a state-of-the-art correction for the intervening Lyman forest and Lyman continuum absorption. The continuum slope (
$f_\nu \propto \nu ^{\alpha _\nu }$
) of the full sample shows a break at ∼912 Å with spectral index αν = −0.61 ± 0.01 at λ > 912 Å and a softening at shorter wavelengths (αν = −1.70 ± 0.61 at λ ≤ 912 Å). Our analysis proves that a proper intergalactic medium absorption correction is required to establish the intrinsic continuum emission of quasars. We interpret our average ultraviolet spectrum in the context of photoionization, accretion disc models, and quasar contribution to the ultraviolet background. We find that observed broad line ratios are consistent with those predicted assuming an ionizing slope of αion = −2.0, similar to the observed ionizing spectrum in the same wavelength range. The continuum break and softening are consistent with accretion disc plus X-ray corona models when black hole spin is taken into account. Our spectral energy distribution yields a 30 per cent increase to previous estimates of the specific quasar emissivity, such that quasars may contribute significantly to the total specific Lyman limit emissivity estimated from the Lyα forest at z < 3.2.
More than three-quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to detect, with only a small fraction directly observed in galaxies and galaxy ...clusters
. Censuses of the nearby Universe have used absorption line spectroscopy
to observe the 'invisible' baryons, but these measurements rely on large and uncertain corrections and are insensitive to most of the Universe's volume and probably most of its mass. In particular, quasar spectroscopy is sensitive either to the very small amounts of hydrogen that exist in the atomic state, or to highly ionized and enriched gas
in denser regions near galaxies
. Other techniques to observe these invisible baryons also have limitations; Sunyaev-Zel'dovich analyses
can provide evidence from gas within filamentary structures, and studies of X-ray emission are most sensitive to gas near galaxy clusters
. Here we report a measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts; this technique determines the electron column density along each line of sight and accounts for every ionized baryon
. We augment the sample of reported arcsecond-localized
fast radio bursts with four new localizations in host galaxies that have measured redshifts of 0.291, 0.118, 0.378 and 0.522. This completes a sample sufficiently large to account for dispersion variations along the lines of sight and in the host-galaxy environments
, and we derive a cosmic baryon density of Formula: see text (95 per cent confidence; h
= H
/(70 km s
Mpc
) and H
is Hubble's constant). This independent measurement is consistent with values derived from the cosmic microwave background and from Big Bang nucleosynthesis
.
ABSTRACT
Early star-forming galaxies produced copious ionizing photons. A fraction of these photons escaped gas within galaxies to reionize the entire Universe. This escape fraction is crucial for ...determining how the Universe became reionized, but the neutral intergalactic medium precludes direct measurement of the escape fraction at high redshifts. Indirect estimates of the escape fraction must describe how the Universe was reionized. Here, we present new Keck Cosmic Web Imager spatially resolved spectroscopy of the resonant Mg ii 2800 Å doublet from a redshift 0.36 galaxy, J1503+3644, with a previously observed escape fraction of 6 per cent. The Mg ii emission has a similar spatial extent as the stellar continuum, and each of the Mg ii doublet lines are well fitted by single Gaussians. The Mg ii is optically thin. The intrinsic flux ratio of the red and blue Mg ii emission line doublet, $R=F_{2796}/F_{2803}$, is set by atomic physics to be two, but Mg$^+$ gas along the line of sight decreases R proportional to the Mg ii optical depth. Combined with the metallicity, R estimates the neutral gas column density. The observed R ranges across the galaxy from 0.8 to 2.7, implying a factor of 2 spatial variation of the relative escape fraction. All of the ionizing photons that escape J1503+3644 pass through regions of high R. We combine the Mg ii emission and dust attenuation to accurately estimate the absolute escape fractions for 10 local Lyman Continuum emitting galaxies and suggest that Mg ii can predict escape fraction within the epoch of reionization.
Abstract
Observational evidence shows that low-redshift galaxies are surrounded by extended haloes of multiphase gas, the so-called circumgalactic medium (CGM). To study the survival of relatively ...cool gas (T < 105 K) in the CGM, we performed a set of hydrodynamical simulations of cold (T = 104 K) neutral gas clouds travelling through a hot (T = 2 × 106 K) and low-density (n = 10−4 cm−3) coronal medium, typical of Milky Way-like galaxies at large galactocentric distances (∼50–150 kpc). We explored the effects of different initial values of relative velocity and radius of the clouds. Our simulations were performed on a two-dimensional grid with constant mesh size (2 pc), and they include radiative cooling, photoionization heating and thermal conduction. We found that for large clouds (radii larger than 250 pc), the cool gas survives for very long time (larger than 250 Myr): despite that they are partially destroyed and fragmented into smaller cloudlets during their trajectory, the total mass of cool gas decreases at very low rates. We found that thermal conduction plays a significant role: its effect is to hinder formation of hydrodynamical instabilities at the cloud–corona interface, keeping the cloud compact and therefore more difficult to destroy. The distribution of column densities extracted from our simulations is compatible with those observed for low-temperature ions (e.g. Si ii and Si iii) and for high-temperature ions (O vi) once we take into account that O vi covers much more extended regions than the cool gas and, therefore, it is more likely to be detected along a generic line of sight.
On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves (GWs) emanating from a binary neutron star merger, ...GW170817. Nearly simultaneously, the Fermi and INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) telescopes detected a gamma-ray transient, GRB 170817A. At 10.9 hours after the GW trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and GW observations.
The z–DM distribution of fast radio bursts James, C W; Prochaska, J X; Macquart, J-P ...
Monthly notices of the Royal Astronomical Society,
02/2022, Letnik:
509, Številka:
4
Journal Article
Recenzirano
ABSTRACT
We develop a sophisticated model of fast radio burst (FRB) observations, accounting for the intrinsic cosmological gas distribution and host galaxy contributions, and give the most detailed ...account yet of observational biases due to burst width, dispersion measure, and the exact telescope beamshape. Our results offer a significant increase in both accuracy and precision beyond those previously obtained. Using results from ASKAP and Parkes, we present our best-fitting FRB population parameters in a companion paper. Here, we consider in detail the expected and fitted distributions in redshift, dispersion measure, and signal to noise. We estimate that the unlocalized ASKAP FRBs arise from z < 0.5, with between a third and a half within z < 0.1. Our predicted source-counts (‘logN–logS’) distribution confirms previous indications of a steepening index near the Parkes detection threshold of 1 Jy ms. We find no evidence for a minimum FRB energy, and rule out Emin > 1039.0 erg at 90 per cent C.L. Importantly, we find that above a certain DM, observational biases cause the Macquart (DM–z) relation to become inverted, implying that the highest-DM events detected in the unlocalized Parkes and ASKAP samples are unlikely to be the most distant. More localized FRBs will be required to quantitatively estimate this effect, though its cause is a well-understood observational bias. Works assuming a 1–1 DM–z relation may therefore derive erroneous results. Our analysis of errors suggests that limiting factors in our analysis are understanding of FRB spectral behaviour, sensitivity response of search experiments, and the treatment of the repeating population and luminosity function.
The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by ...observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kiloparsec) halos of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data indicate that it is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution.
The merging neutron star gravitational-wave event GW170817 has been observed throughout the entire electromagnetic spectrum from radio waves to γ-rays. The resulting energetics, variability, and ...light curves are shown to be consistent with GW170817 originating from the merger of two neutron stars, in all likelihood followed by the prompt gravitational collapse of the massive remnant. The available γ-ray, X-ray, and radio data provide a clear probe for the nature of the relativistic ejecta and the non-thermal processes occurring within, while the ultraviolet, optical, and infrared emission are shown to probe material torn during the merger and subsequently heated by the decay of freshly synthesized r-process material. The simplest hypothesis, that the non-thermal emission is due to a low-luminosity short γ-ray burst (sGRB), seems to agree with the present data. While low-luminosity sGRBs might be common, we show here that the collective prompt and multi-wavelength observations are also consistent with a typical, powerful sGRB seen off-axis. Detailed follow-up observations are thus essential before we can place stringent constraints on the nature of the relativistic ejecta in GW170817.
We present the first results from our NASA Keck Observatory Database of Ionized Absorbers toward Quasars (KODIAQ) survey which aims to characterize the properties of the highly ionized gas of ...galaxies and their circumgalactic medium (CGM) at 2 < z < 4. We found 20 absorbers that satisfy these rules: 7 Lyman limit systems (LLSs), 8 super-LLSs (SLLSs) and 5 damped Ly alpha (DLAs). The LLSs and SLLSs with no O VI absorption have properties consistent with those seen in cosmological simulations tracing cold streams feeding galaxies. When the highly ionized (Si IV and O VI) gas is taken into account, we determine that the tau sub(LL) > 1 absorbers could contain as much as 3%-14% of the cosmic baryon budget at z ~ 2-3, only second to the Ly alpha forest. We conservatively show that 5%-20% of the metals ever produced at z ~ 2-3 are in form of highly ionized metals ejected in the CGM of galaxies.
Fast radio bursts (FRBs) are brief radio emissions from distant astronomical sources. Some are known to repeat, but most are single bursts. Nonrepeating FRB observations have had insufficient ...positional accuracy to localize them to an individual host galaxy. We report the interferometric localization of the single-pulse FRB 180924 to a position 4 kiloparsecs from the center of a luminous galaxy at redshift 0.3214. The burst has not been observed to repeat. The properties of the burst and its host are markedly different from those of the only other accurately localized FRB source. The integrated electron column density along the line of sight closely matches models of the intergalactic medium, indicating that some FRBs are clean probes of the baryonic component of the cosmic web.
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