We investigate the origin and physical properties of OVI absorbers at low redshift (z = 0.25) using a subset of cosmological, hydrodynamical simulations from the OverWhelmingly Large Simulations (OWLS) project. Intervening OVI absorbers are believed to trace shock-heated gas in the Warm-Hot Intergalactic Medium (WHIM) and may thus play a key role in the search for the missing baryons in the present-day Universe. When compared to observations, the predicted distributions of the different OVI line parameters (column density, Doppler parameter, rest equivalent width) from our simulations exhibit a lack of strong OVI absorbers. This suggests that physical processes on sub-grid scales (e.g. turbulence) may strongly influence the observed properties of OVI systems. We find that the intervening OVI absorption arises mainly in highly metal-enriched (0.1 << Z/Z_sun < 1) gas at typical overdensities of 1 << rho/<rho> < 100. One third of the OVI absorbers in our simulation are found to trace gas at temperatures T < 10^5 K, while the rest arises in gas at higher temperatures around T =10^5.3 K. The OVI resides in a similar region of (rho,T)-space as much of the shock-heated baryonic matter, but the vast majority of this gas has a lower metal content and does not give rise to detectable OVI absorption As a consequence of the patchy metal distribution, OVI absorbers in our simulations trace only a very small fraction of the cosmic baryons (<2 percent) and the cosmic metals. Instead, these systems presumably trace previously shock-heated, metal-rich material from galactic winds that is now cooling. The common approach of comparing OVI and HI column densities to estimate the physical conditions in intervening absorbers from QSO observations may be misleading, as most of the HI (and most of the gas mass) is not physically connected with the high-metallicity patches that give rise to the OVI absorption.
We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium. The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at …
The current observational status of the hot (log T(K) > 5.5) warm-hot intergalactic medium (WHIM) remains incomplete. While recent observations from stacking large numbers of Cosmic Web filaments have yielded statistically significant detections, direct measurements of single objects remain scarce. The lack of such a sample currently prevents a robust analysis of the cosmic baryon content composed of the hot WHIM, which could help solve the cosmological missing baryons problem. To improve the search for the missing baryons, we used the EAGLE simulation. Our aim is to understand the metal enric…
With high resolution (0.46kpc/h), adaptive mesh-refinement Eulerian cosmological hydrodynamic simulations we compute properties of O VI and O VII absorbers from the warm-hot intergalactic medium (WHIM). Our new simulations are in broad agreement with previous simulations, with ~40% of the intergalactic medium being in the WHIM at z=0. It is found (1) The amount of gas in the WHIM at temperature below and above 10^6K is about equal within uncertainties. (1) Our simulations are in excellent agreement with observed properties of O VI absorbers, with respect to the line incidence rate and Doppler …
Known as the "Missing Baryon Problem", about one-third of baryons in the local universe remain unaccounted for. The missing baryons are thought to reside in the warm-hot intergalactic medium (WHIM) of the cosmic web filaments, which are challenging to detect. Recent Chandra X-ray observations used a novel stacking analysis and detected an OVII absorption line toward the sightline of a luminous quasar, hinting that the missing baryons may reside in the WHIM. To explore how the properties of the OVII absorption line depend on feedback physics, we compare the observational results with prediction…
We present a detailed study of the ionised environment of the Seyfert 1 galaxy Mrk 704 using medium and high resolution X-ray spectra obtained with a long XMM-Newton observation. The 0.3-10 kev continuum, well described by a power-law (Gamma ~ 1.86) and two blackbodies (kT ~ 0.085 and 0.22 kev), is found to be affected by a neutral partial covering absorption (N_H ~ 10^{23} per cm square, covering fraction ~ 0.22) and two warm absorber components. We identify a low ionisation, xi ~ 20 erg-cm/sec, and high outflow velocity, v ~ 1350 km/s, phase producing the OVI and Fe M-shell unresolved-transi…
Thermally broadened Ly alpha absorbers (BLAs) offer an alternative method to highly-ionized metal lines for tracing the WHIM. We compile a catalog of reliable BLA candidates along seven AGN sight lines from a larger set of Lya absorbers observed by HST/STIS. We compare our measurements based on independent reduction and analysis of the data to those published by other research groups. Purported BLAs are grouped into probable (15), possible (48) and non-BLA (56) categories. We infer a line frequency (dN/dz)_BLA=18+-11, comparable to observed OVI absorbers. There is significant overlap between B…
Thermally-broadened Lya absorbers (BLAs) offer an alternate method to using highly-ionized metal absorbers (OVI, OVII, etc.) to probe the warm-hot intergalactic medium (WHIM, T=10^5-10^7 K). Until now, WHIM surveys via BLAs have been no less ambiguous than those via far-UV and X-ray metal-ion probes. Detecting these weak, broad features requires background sources with a well-characterized far-UV continuum and data of very high quality. However, a recent HST/COS observation of the z=0.03 blazar Mrk421 allows us to perform a metal-independent search for WHIM gas with unprecedented precision. Th…
The warm-hot intergalactic medium (WHIM) at temperatures 1E5-1E7 K is believed to contain 30-50% of the baryons in the local universe. However, all current X-ray detections of the WHIM at redshifts z>0 are of low statistical significance (<=3sigma) and/or controversial. In this work, we aim to establish the detection limits of current X-ray observatories and explore requirements for next-generation X-ray telescopes for studying the WHIM through X-ray absorption lines. We analyze all available grating observations of Mrk 421 and obtain spectra with signal-to-noise ratio (S/N) of \sim90 and 190 …
Using hydrodynamic simulations we compute the metal enrichment history of the intergalactic medium (IGM). We show that galactic superwind (GSW) feedback can transport metals to the IGM and that the properties of simulated metal absorbers match observations. The distance of influence of GSW is typically limited to >0.5Mpc and within regions of overdensity >10. Most CIV and OVI absorbers are located within shocked regions of elevated temperature (T>2x10^4K), overdensity (>10), and metallicity ([-2.5,-0.5]). OVI absorbers have typically higher metallicity, lower density and higher temperature tha…
The cosmological missing baryons at z<1 most likely hide in the hot (T$\gtrsim10^{5.5}$ K) phase of the Warm Hot Intergalactic Medium (WHIM). While the hot WHIM is hard to detect due to its high ionisation level, the warm (T$\lesssim10^{5.5}$ K) phase of the WHIM has been very robustly detected in the FUV band. We adopted the assumption that the hot and warm WHIM phases are co-located and thus used the FUV-detected warm WHIM as a tracer for the cosmologically interesting hot WHIM. We utilised the assumption by performing an X-ray follow-up in the sight line of a blazar PKS 2155-304 at the reds…
We study the components of cool and warm/hot gas in the circumgalactic medium (CGM) of simulated galaxies and address the relative production of OVI by photoionization versus collisional ionization, as a function of halo mass, redshift, and distance from the galaxy halo center. This is done utilizing two different suites of zoom-in hydro-cosmological simulations, VELA (6 halos; $z>1$) and NIHAO (18 halos; to $z=0$), which provide a broad theoretical basis because they use different codes and physical recipes for star formation and feedback. In all halos studied in this work, we find that colli…
We explore the high spectral resolution X-ray data towards the quasar 3C273 to search for signals of hot ($\sim10^{6-7}$ K) X-ray-absorbing gas co-located with two established intergalactic FUV OVI absorbers. We analyze the soft X-ray band grating data of all XMM-Newton and Chandra instruments to search for the hot phase absorption lines at the FUV predicted redshifts. The viability of potential line detections is examined by adopting the constraints of a physically justified absorption model. The WHIM hypothesis is investigated with a complementary 3D galaxy distribution analysis, and by comp…
The circumgalactic medium (CGM) around galaxies is believed to record various forms of galaxy feedback and contain a significant portion of the "missing baryons" of individual dark matter halos. However, clear observational evidence for the existence of the hot CGM is still absent. We use intervening galaxies along 12 background AGNs as tracers to search for X-ray absorption lines produced in the corresponding CGM. Stacking Chandra grating observations with respect to galaxy groups and different luminosities of these intervening galaxies, we obtain spectra with signal-to-noise ratios of 46-72 …
Modern analyses of structure formation predict a universe tangled in a 'cosmic web' of dark matter and diffuse baryons. These theories further predict that at low-z, a significant fraction of the baryons will be shock-heated to $T \sim 10^{5}-10^{7}$K yielding a warm-hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation. We present a novel experiment to detect the WHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable QSO sightline that passes within $Δd = 3$ Mpc from the 7 inter-cluster ax…
The methods by which one characterizes the distribution of matter in cosmological simulations is intrinsically different from how one performs the same task observationally. In this paper, we make substantial steps towards comparing simulations and observations of the intergalactic medium (IGM) in a more sensible way. We present a pipeline that generates and fits synthetic QSO absorption spectra using sight lines cast through a cosmological simulation, and simultaneously identifies structure by directly analyzing the variations in HI and OVI number density. We compare synthetic absorption spec…
Pinpointing the cosmic web between massive galaxy clusters I. The incidence of HI and OVI as WHIM tracers
Pontificia Universidad Católica de Valparaíso（智利天主教瓦帕莱索大学）；Universidad Técnica Federico Santa María（费德里科·圣塔玛丽亚理工大学）；Leibniz-Institut for Astrophysik Potsdam (AIP)（波茨坦阿基米德天体物理研究所）；The University of Tokyo（东京大学）；University of California, Santa Cruz（加州大学圣克鲁兹分校）；New Mexico State University（新墨西哥州立大学）；University of Edinburgh（爱丁堡大学）；University of Washington（华盛顿大学）；CONICET-Universidad Nacional de Córdoba（阿根廷国家科学研究委员会-科尔多瓦国立大学）
We assess the possibility to detect the warm-hot intergalactic medium (WHIM) in emission and to characterize its physical conditions and spatial distribution through spatially resolved X-ray spectroscopy, in the framework of the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are equipped with microcalorimeter-based detectors. For this purpose we analyze a large set of mock emission spectra, extracted from a cosmological hydrodynamical simulation. These mock X-ray spectra are searched for emission features showing both the OVII K alpha triplet and OVIII Ly alpha line, wh…
For low-redshift cosmology and galaxy formation rates, it is important to account for all the baryons synthesized in the Big Bang. Although galaxies and clusters contain 10% of the baryons, many more reside in the photoionized Lyman-alpha forest and shocked-heated warm-hot intergalactic medium (WHIM) at T = 10^5 to 10^7 K. Current tracers of WHIM at 10^5 to 10^6 K include the O VI 1032, 1038 absorption lines, together with broad Lyman-alpha absorbers (BLAs) and EUV/X-ray absorption lines from Ne VIII, O VII, and O VIII. We improve the O VI baryon surveys with corrections for oxygen metallicity…
At present, 30-40 per cent of the baryons in the local Universe is still undetected. According to theoretical predictions, this gas should reside in filaments filling the large-scale structure (LSS) in the form of a Warm-Hot Intergalactic Medium (WHIM), at temperatures of 10^5 - 10^7 K, thus emitting in the soft X-ray energies via free-free interaction and line emission from heavy elements. In this work we characterize the properties of the X-ray emission of the WHIM, and the LSS in general, focusing on the influence of different physical mechanisms, namely galactic winds (GWs), black-hole fee…
We have used the angular Autocorrelation Function (AcF) in a pointing with Chandra's ACIS-S instrument. However, we determined that the contribution of Chandra's background below 1 keV is too high to achieve reasonable sensitivity.
We analyze the association of galaxies to Lya and OVI absorption, the most commonly detected transitions in the low-z intergalactic medium (IGM), in the fields of 14 quasars with z_em = 0.06-0.57. Confirming previous studies, we observe a high covering fraction for Lya absorption to impact parameter rho = 300kpc: 33/37 of our L>0.01L* galaxies show Lya equivalent width W_Lya>50mA. Galaxies of all luminosity L>0.01L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ~10^(10.5 +/- 0.3) Msun for a constant N_H. The virialize…
We present a re-analysis, with newly acquired atomic data, of the two detections of two highly ionized intervening OVII absorbers reported by Nicastro and collaborators (2018). We confirm both intervening Warm-Hot Intergalactic Medium OVII detections, and revise statistical significance and physical parameters of the absorber at $z=0.4339$ in light of its partial contamination by Galactic interstellar medium NII K$α$ absorption.
Both active and sterile sub-eV neutrinos can serve for hot dark matter (DM). On the other hand, keV sterile neutrinos could be a good candidate for warm DM. The beta-decaying (e.g., H-3 and Ru-106) and EC-decaying (e.g., Ho-163) nuclei are considered as the most promising targets to capture those extremely low energy neutrinos and antineutrinos, respectively. We calculate the capture rates of relic electron neutrinos and antineutrinos against the corresponding beta-decay or EC-decay backgrounds in different flavor mixing schemes. We stress that such direct laboratory measurements of hot or war…
For the first time, we present the simultaneous detection and characterization of three distinct phases at $>10^5$ K in $z=0$ absorption, using deep $\it{Chandra}$ observations toward Mrk 421. The extraordinarily high signal-to-noise ratio ($\geqslant60$) of the spectra has allowed us to detect a $\it{hot}$ phase of the Milky Way circumgalactic medium (CGM) at 3.2$^{+1.5}_{-0.5}\times$ 10$^7$ K, coexisting with a $\textit{warm-hot}$ phase at 1.5$\pm$0.1$\times$10$^6$ K and a $\textit{warm}$ phase at 3.0$\pm$0.4$\times$10$^5$ K. The $\textit{warm-hot}$ phase is at the virial temperature of the …
(Abridged) We have analyzed Chandra LETG and XMM-Newton RGS spectra towards the z=0.177 quasar PG 1116+215, a sightline that is rendered particularly interesting by the HST detection of several OVI and HI broad Lyman-alpha absorption lines that may be associated with the warm-hot intergalactic medium. We performed a search for resonance K-alpha absorption lines from OVII and OVIII at the redshifts of the detected far-ultraviolet lines. We detected an absorption line in the Chandra spectra at 5.2 sigma confidence level at wavelengths corresponding to OVIII K-alpha at z=0.0911+-0.0004+-0.0005 (s…
It has been known for decades that the observed number of baryons in the local universe falls about 30-40% 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-alpha Forest. 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 consti…
We have analyzed all available archival XMM-Newton observations of X Comae, a bright X-ray quasar behind the Coma cluster, to study the properties of the warm-hot intergalactic medium in the vicinity of the nearest massive galaxy cluster. The RGS observations confirm the possible presence of a Ne IX K-alpha absorption line at the redshift of Coma, although with a limited statistical significance. This analysis is therefore in line with the earlier analysis by Takei et al. (2007) based on a sub-set of these data. Its large column density and optical depth, however, point to implausible conditio…
Capturing low-energy electron antineutrinos on radioactive Ho-163 nuclei, which decay into Dy-163 via electron capture (EC), is a noteworthy opportunity to detect relic sterile antineutrinos. Such hypothetical particles are more or less implied by current experimental and cosmological data, and they might be a part of hot dark matter or a candidate for warm dark matter in the Universe. Using the isotope Ho-163 as a target and assuming reasonable active-sterile antineutrino mixing angles, we calculate the capture rate of relic electron antineutrinos against the corresponding EC-decay background…
We investigate the thermal relics as hot, warm and cold dark matter in $\mathscr{L}=\varepsilon^{2-2β}R^β+{16π}m_{\text{Pl}}^{-2}\mathscr{L}_m$ gravity, where $\varepsilon$ is a constant balancing the dimension of the field equation, and $1<β<(4+\sqrt{6})/5$ for the positivity of energy density and temperature. If light neutrinos serve as hot/warm relics, the entropic number of statistical degrees of freedom $g_{*s}$ at freeze-out and thus the predicted fractional energy density $Ω_ψh^2$ are $β-$dependent, which relaxes the standard mass bound $Σm_ν$. For cold relics, by exactly solve the simp…
The backbone of the large-scale structure of the Universe is determined by processes on a cosmological scale and by the gravitational interaction of the dominant dark matter. However, the mobile baryon population shapes the appearance of these structures. Theory predicts that most of the baryons reside in vast unvirialized filamentary structures that connect galaxy groups and clusters, but the observational evidence is currently lacking. Because the majority of the baryons are supposed to exist in a large-scale, hot and dilute gaseous phase, X-rays provide the ideal tool to progress our unders…
Using Suzaku data, we had found a $3.4σ$ evidence for the X-ray emitting warm-hot circumgalactic medium (CGM) in the L$^\star$ galaxy NGC 3221. Here we present XMM-Newton data and outline an efficient, rigorous and well-defined method to extract the faint CGM signal. We confirm the CGM detection at $4σ$ significance within 30-200 kpc of the galaxy. We claim with $99.62\%$ confidence that the CGM is extended beyond $150$ kpc. The average temperature of the CGM is 2.0$^{+0.2}_{-0.3} \times 10^6$ K, but it is not isothermal. We find suggestive evidence for a declining temperature gradient out to …
