In the current cosmological model, only the three lightest elements were created in the first few minutes after the Big Bang; all other elements were produced later in stars. To date, however, heavy ...elements have been observed in all astro physical environments. We report the detection of two gas clouds with no discernible elements heavier than hydrogen. These systems exhibit the lowest heavy-element abundance in the early universe, and thus are potential fuel for the most metal-poor halo stars. The detection of deuterium in one system at the level predicted by primordial nucleosynthesis provides a direct confirmation of the standard cosmological model. The composition of these clouds further implies that the transport of heavy elements from galaxies to their surroundings is highly inhomogeneous.
We present Keck NIRSPEC and Keck NIRES spectroscopy of sixteen metal-poor galaxies that have pre-existing optical observations. The near-infrared (NIR) spectroscopy specifically targets the He i ...λ10830 emission line, due to its sensitivity to the physical conditions of the gas in H ii regions. We use these NIR observations, combined with optical spectroscopy, to determine the helium abundance of sixteen galaxies across a metallicity range = 7.13-8.00. This data set is combined with two other samples where metallicity and helium abundance measurements can be secured: star-forming galaxies selected from the Sloan Digital Sky Survey spectroscopic database, and existing low-metallicity systems in the literature. We calculate a linear fit to these measurements, accounting for intrinsic scatter, and report a new determination of the primordial helium number abundance, , which corresponds to a primordial helium mass fraction . Using our determination of the primordial helium abundance in combination with the latest primordial deuterium measurement, , we place a bound on the baryon density and the effective number of neutrino species . These values are in 1.3 agreement with those deduced from the Planck satellite observations of the temperature fluctuations imprinted on the cosmic microwave background.
Correcting C iv-based virial black hole masses Coatman, Liam; Hewett, Paul C; Banerji, Manda ...
Monthly Notices of the Royal Astronomical Society,
02/2017, Volume:
465, Issue:
2
Journal Article
Peer reviewed
Open access
The C IV...1498,1501 broad emission line is visible in optical spectra to redshifts exceeding z ~ 5. C IV has long been known to exhibit significant displacements to the blue and these 'blueshifts' ...almost certainly signal the presence of strong outflows. As a consequence, single-epoch virial black hole (BH) mass estimates derived from C IV velocity widths are known to be systematically biased compared to masses from the hydrogen Balmer lines. Using a large sample of 230 high-luminosity (LBol = 10...-10... erg s...), redshift 1.5 < z < 4.0 quasars with both C IV and Balmer line spectra, we have quantified the bias in C IV BH masses as a function of the C IV blueshift. C IV BH masses are shown to be a factor of 5 larger than the corresponding Balmer-line masses at C IV blueshifts of 3000 km s... and are overestimated by almost an order of magnitude at the most extreme blueshifts, ...5000 km s... Using the monotonically increasing relationship between the C IV blueshift and the mass ratio BH(C IV)/BH(Ha), we derive an empirical correction to all C IV BH masses. The scatter between the corrected C IV masses and the Balmer masses is 0.24 dex at low C IV blueshifts (~0 km s...) and just 0.10 dex at high blueshifts (~3000 km s...), compared to 0.40 dex before the correction. The correction depends only on the C IV line properties -- i.e. full width at half-maximum and blueshift -- and can therefore be applied to all quasars where C IV emission line properties have been measured, enabling the derivation of unbiased virial BH-mass estimates for the majority of high-luminosity, high-redshift, spectroscopically confirmed quasars in the literature. (ProQuest: ... denotes formulae/symbols omitted.)
All galaxies once passed through a hyperluminous quasar phase powered by accretion onto a supermassive black hole. But because these episodes are brief, quasars are rare objects typically separated ...by cosmological distances. In a survey for Lyman-α emission at redshift z ≈ 2, we discovered a physical association of four quasars embedded in a giant nebula. Located within a substantial overdensity of galaxies, this system is probably the progenitor of a massive galaxy cluster. The chance probability of finding a quadruple quasar is estimated to be ∼10–7, implying a physical connection between Lyman-α nebulae and the locations of rare protoclusters. Our findings imply that the most massive structures in the distant universe have a tremendous supply (≃1011 solar masses) of cool dense (volume density ≃ 1 cm–3) gas, which is in conflict with current cosmological simulations.
We study the physical properties of a homogeneous sample of 157 optically thick absorption line systems at redshifts ∼1.8–4.4, selected from a high-dispersion spectroscopic survey of Lyman limit ...systems (LLSs). By means of multiple ionization models and Bayesian techniques, we derive the posterior probability distribution functions for the density, metallicity, temperature and dust content of the absorbing gas. We find that z > 2 LLSs are highly ionized with ionization parameters between −3 ≲ log U ≲ −2, depending on the H i column density. LLSs are characterized by low temperatures (T < 5 × 104K) and reside in dust-poor environments. Between z ∼ 2.5–3.5, ∼80 per cent of the LLSs have physical densities between n
H ∼ 10− 3.5–10− 2 cm− 3 for the assumed UV background, but we caution that a degeneracy between the ionization parameter and the intensity of the radiation field prevents robust inference on the density and sizes of LLSs. Conversely, metallicity estimates are less sensitive to the assumptions behind ionization corrections. LLSs at z > 2 are characterized by a broad unimodal distribution over > 4 orders of magnitude, with a peak at log Z/Z⊙ ∼ −2. LLSs are metal poor, significantly less enriched than DLAs, with ∼70 per cent of the metallicity PDF below log Z/Z⊙ ≤ −1.5. The median metallicity of super LLSs with
$\log N_{\rm H\,\small {I}}\ge 19$
rapidly evolves with redshift, with a 10-fold increase between z ∼ 2.1–3.6 (∼1.5 Gyr). Based on this sample, we find that LLSs at z = 2.5–3.5 account for ∼15 per cent of all the metals produced by UV-selected galaxies. The implications for theories of cold gas accretion and metal ejection from galaxies are also discussed.
Abstract
We present the localization and host galaxies of one repeating and two apparently nonrepeating fast radio bursts (FRBs). FRB 20180301A was detected and localized with the Karl G. Jansky Very ...Large Array to a star-forming galaxy at
z
= 0.3304. FRB20191228A and FRB20200906A were detected and localized by the Australian Square Kilometre Array Pathfinder to host galaxies at
z
= 0.2430 and
z
= 0.3688, respectively. We combine these with 13 other well-localized FRBs in the literature, and analyze the host galaxy properties. We find no significant differences in the host properties of repeating and apparently nonrepeating FRBs. FRB hosts are moderately star forming, with masses slightly offset from the star-forming main sequence. Star formation and low-ionization nuclear emission-line region emission are major sources of ionization in FRB host galaxies, with the former dominant in repeating FRB hosts. FRB hosts do not track stellar mass and star formation as seen in field galaxies (more than 95% confidence). FRBs are rare in massive red galaxies, suggesting that progenitor formation channels are not solely dominated by delayed channels which lag star formation by gigayears. The global properties of FRB hosts are indistinguishable from core-collapse supernovae and short gamma-ray bursts hosts, and the spatial offset (from galaxy centers) of FRBs is mostly inconsistent with that of the Galactic neutron star population (95% confidence). The spatial offsets of FRBs (normalized to the galaxy effective radius) also differ from those of globular clusters in late- and early-type galaxies with 95% confidence.
We measure the effective optical depth of He II Lyalpha absorption tau sub(eff,HeII) at 2.3 < z< 3.5 in 17 UV-transmitting quasars observed with UV spectrographs on the Hubble Space Telescope. The ...median tau sub(eff,HeII) values increase gradually from 1.95 at z= 2.7 to 5.17 at z= 3.4, but with a strong sightline-to-sightline variance. Many Asymptotically = to35 comoving Mpc regions of the z> 3 intergalactic medium (IGM) remain transmissive (tau sub(eff,HeII)< 4), and the gradual trend with redshift appears consistent with density evolution of a fully reionized IGM. These modest optical depths imply average He II fractions of x sub(eff,HeII)< 0.01 and He II ionizing photon mean free paths of Asymptotically = to50 comoving Mpc at zAsymptotically = to 3.4, thus requiring that a substantial volume of the helium in the universe was already doubly ionized at early times; this stands in conflict with current models of He II reionization driven by luminous quasars. Along 10 sightlines we measure the coeval H I Lyalpha effective optical depths, allowing us to study the density dependence of tau sub(eff,HeII) at z~ 3. We establish that the dependence of tau sub(eff,HeII) on increasing tau sub(eff,HeI) is significantly shallower than expected from simple models of an IGM reionized in He ii. This requires higher He ii photoionization rates in overdense regions or underdense regions being not in photoionization equilibrium. Moreover, there are very large fluctuations in tau sub(eff,HeII) at all tau sub(eff,HeI) which greatly exceed the expectations from these simple models. These data present a distinct challenge to scenarios of He II reionization-an IGM where He ii appears to be predominantly ionized at zAsymptotically = to 3.4, and with a radiation field strength that may be correlated with the density field, but exhibits large fluctuations at all densities.
ABSTRACT
By now, tens of gravitational-wave (GW) events have been detected by the LIGO and Virgo detectors. These GWs have all been emitted by compact binary coalescence, for which we have excellent ...predictive models. However, there might be other sources for which we do not have reliable models. Some are expected to exist but to be very rare (e.g. supernovae), while others may be totally unanticipated. So far, no unmodelled sources have been discovered, but the lack of models makes the search for such sources much more difficult and less sensitive. We present here a search for unmodelled GW signals using semisupervised machine learning. We apply deep learning and outlier detection algorithms to labelled spectrograms of GW strain data, and then search for spectrograms with anomalous patterns in public LIGO data. We searched ${\sim}13{{\ \rm per\ cent}}$ of the coincident data from the first two observing runs. No candidates of GW signals were detected in the data analyzed. We evaluate the sensitivity of the search using simulated signals, we show that this search can detect spectrograms containing unusual or unexpected GW patterns, and we report the waveforms and amplitudes for which a $50{{\ \rm per\ cent}}$ detection rate is achieved.
We present the equivalent width and column density measurements for low and intermediate ionization states of the circumgalactic medium (CGM) surrounding 44 low-z, L approximate L* galaxies drawn ...from the COS-Halos survey. These measurements are derived from far-UV transitions observed in HST/COS and Keck/HIRES spectra of background quasars within an impact parameter R < 160 kpc to the targeted galaxies. The data show significant metal-line absorption for 33 of the 44 galaxies, including quiescent systems, revealing the common occurrence of a cool (T approximate 10 super(4)-10 super(5) K), metal-enriched CGM. The detection rates and column densities derived for these metal lines decrease with increasing impact parameter, a trend we interpret as a declining metal surface density profile for the CGM. A comparison of the relative column densities of adjacent ionization states indicates that the gas is predominantly ionized. The large surface density in metals demands a large reservoir of metals and gas in the cool CGM (very conservatively, (ProQuest: Formulae and/or non-USASCII text omitted)), which likely traces a distinct density and/or temperature regime from the highly ionized CGM traced by O super(+5) absorption. The large dispersion in absorption strengths (including non-detections) suggests that the cool CGM traces a wide range of densities or a mix of local ionizing conditions. Lastly, the kinematics inferred from the metal-line profiles are consistent with the cool CGM being bound to the dark matter halos hosting the galaxies; this gas may serve as fuel for future star formation. Future work will leverage this data set to provide estimates on the mass, metallicity, dynamics, and origin of the cool CGM in low-z, L* galaxies.
ABSTRACT The total contribution of diffuse halo gas to the galaxy baryon budget strongly depends on its dominant ionization state. In this paper, we address the physical conditions in the highly ...ionized circumgalactic medium (CGM) traced by absorption lines observed in COS-Halos spectra. We analyze the observed ionic column densities, absorption-line widths and relative velocities, along with the ratios of / for 39 fitted Voigt profile components of O vi. We compare these quantities with the predictions given by a wide range of ionization models. Photoionization models that include only extragalactic UV background radiation are ruled out; conservatively, the upper limits to / and measurements of imply unphysically large path lengths 100 kpc. Furthermore, very broad absorption (b > 40 km s−1) is a defining characteristic of the CGM of star-forming L* galaxies. We highlight two possible origins for the bulk of the observed : (1) highly structured gas clouds photoionized primarily by local high-energy sources or (2) gas radiatively cooling on large scales behind a supersonic wind. Approximately 20% of circumgalactic O vi does not align with any low-ionization state gas within 50 km s−1 and is found only in halos with < 1012 . We suggest that this type of unmatched O vi absorption traces the hot corona itself at a characteristic temperature of K. We discuss the implications of these very distinct physical origins for the dynamical state, gas cooling rates, and total baryonic content of L* gaseous halos.
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