VI Absorption Research Articles

Fabrication of CuO/MoO3 p-n heterojunction for enhanced dyes degradation and hydrogen production from water splitting

The development of excellent photocatalytic material is highly required for energy and environmental applications. In this study, visible light responsive p-n heterojunction photocatalysts based on CuO/MoO3 with varying ratios of CuO were prepared by the facile hydrothermal method. The crystalline structure, surface morphology, chemical compositions and optical properties of the synthesized photocatalysts were studied using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), photoluminescence (PL) techniques and UV–Vi's absorption spectroscopy. The results showed that the 5%CuO/MoO3 nanocomposite displayed enhanced photocatalytic performance for the production of hydrogen (98.5 μmol h−1g−1) and degradation of dyes rhodamine B (RhB) and alizarine yellow (AY) than all other samples. Furthermore, 5% CuO/MoO3 composite exhibited excellent stability after five consecutive cycles for both RhB and AY dyes. Overall, the improved photocatalytic performance of 5%CuO/MoO3 composite was due to increased adsorption of visible light, good surface morphology, enhanced charge separation/transfer which inhibited recombination of electrons and holes. This study could encourage the synthesis of novel and effective p-n heterojunction photocatalysts for practical applications.

Discovery of a multiphase O VI and O VII absorber in the circumgalactic/intergalactic transition region

Aims. The observational constraints on the baryon content of the warm-hot intergalactic medium (WHIM) rely almost entirely on far ultraviolet (FUV) measurements. However, cosmological, hydrodynamical simulations predict strong correlations between the spatial distributions of FUV and X-ray absorbing WHIM. We investigate this prediction by analyzing XMM-Newton X-ray counterparts of FUV-detected intergalactic O VI absorbers known from FUSE and HST/STIS data, thereby aiming to gain understanding on the properties of the hot component of FUV absorbers and to compare this information to the predictions of simulations. Methods. We study the X-ray absorption at the redshift of the only significantly detected O VI absorber in the Ton S 180 sightline’s FUV spectrum, found at zOVI = 0.04579 ± 0.00001. We characterize the spectral properties of the O VI-O VIII absorbers and explore the ionization processes behind the measured absorption. The observational results are compared to the predicted warm-hot gas properties in the EAGLE simulation to infer the physical conditions of the absorber. Results. We detect both O VI and O VII absorption at a 5σ confidence level, whereas O VIII absorption is not significantly detected. Collisional ionization equilibrium (CIE) modeling constrains the X-ray absorbing gas temperature to log TCIE (K) = 6.22 ± 0.05 with a total hydrogen column density NH = 5.8−2.2+3.0 × Z⊙/Zabs × 1019 cm−2. This model predicts an O VI column density consistent with that measured in the FUV, but our limits on the O VI line width indicate > 90% likelihood that the FUV-detected O VI arises from a different, cooler phase. We find that the observed absorber lies about a factor of two further away from the detected galaxies than is the case for similar systems in EAGLE Conclusions. The analysis suggests that the detected O VI and O VII trace two different – warm and hot – gas phases of the absorbing structure at z ≈ 0.046, of which the hot component is likely in collisional ionization equilibrium. As the baryon content information of the studied absorber is primarily imprinted in the X-ray band, understanding the abundance of similar systems helps to define the landscape for WHIM searches with future X-ray telescopes. Our results highlight the crucial role of line widths for the interpretation and detectability of WHIM absorbers.

Hot WHIM counterparts of FUV O VI absorbers: Evidence in the line-of-sight towards quasar 3C 273

Aims. We explore the high spectral resolution X-ray data towards the quasar 3C 273 to search for signals of hot (∼106−7 K) X-ray-absorbing gas co-located with two established intergalactic far-ultraviolet (FUV) O VI absorbers. Methods. 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 detailed comparison of the measurement results to the WHIM properties in the EAGLE cosmological, hydrodynamical simulation. Results. At one of the examined FUV redshifts, z = 0.09017 ± 0.00003, we measured signals of two hot ion species, O VIII and Ne IX, with a 3.9σ combined significance level. While the absorption signal is only marginally detected in individual co-added spectra, considering the line features in all instruments collectively and assuming collisional equilibrium for absorbing gas, we were able to constrain the temperature (kT = 0.26 ± 0.03 keV) and the column density (NH × Z⊙/Z = 1.3−0.5+0.6 × 1019 cm−2) of the absorber. Thermal analysis indicates that FUV and X-ray absorption relate to different phases, with estimated temperatures, TFUV ≈ 3 × 105, and, TX − ray ≈ 3 × 106 K. These temperatures match the EAGLE predictions for WHIM at the FUV/X-ray measured Nion-ranges. We detected a large scale galactic filament crossing the sight-line at the redshift of the absorption, linking the absorption to this structure. Conclusions. This study provides observational insights into co-existing warm and hot gas within a WHIM filament and estimates the ratio of the hot and warm phases. Because the hot phase is thermally distinct from the O VI gas, the estimated baryon content of the absorber is increased, conveying the promise of X-ray follow-up studies of FUV detected WHIM in refining the picture of the missing baryons.

To be or not to be: the case of the hot WHIM absorption in the blazar PKS 2155–304 sight line

The cosmological missing baryons at z < 1 most likely hide in the hot (T ≳ 105.5 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 ≲ 105.5 K) phase of the WHIM has been very robustly detected in the far-ultraviolet (FUV) band. We adopted the assumption that the hot and warm WHIM phases are co-located and therefore used the FUV-detected warm WHIM as a tracer for the cosmologically interesting hot WHIM. We performed an X-ray follow-up in the sight line of the blazar PKS 2155–304 at the redshifts where previous FUV measurements of O VI, Si IV, and broad Lyman-alpha (BLA) absorption have indicated the existence of the warm WHIM. We looked for the O VII Heα and O VIII Lyα absorption lines, the most likely hot WHIM tracers. Despite the very large exposure time (≈1 Ms), the Reflection Grating Spectrometer unit 1 (RGS1) on-board XMM-Newton data yielded no significant detection which corresponds to upper limits of log N(O VII(cm−2)) ≤ 14.5−15.2 and log N(O VIII(cm−2)) ≤ 14.5−15.2. An analysis of the data obtained with the combination of the Low Energy Transmission Grating (LETG) and the High Resolution Camera (HRC) on-board Chandra yielded consistent results. However, the data obtained with the LETG, combined with the Advanced CCD Imaging Spectrometer (ACIS) lead to the detection of an feature resembling an absorption line at λ ≈ 20 Å at simple one-parameter confidence level of 3.7σ, consistent with several earlier LETG/ACIS reports. Given the high statistical quality of the RGS1 data, the possibility of RGS1 accidentally missing the true line at λ ∼ 20 Å is very low: 0.006%. Neglecting this, the LETG/ACIS detection can be interpreted as Lyα transition of O VIII at one of the redshifts (z ≈ 0.054) of FUV-detected warm WHIM. Given the very convincing X-ray spectral evidence for and against the existence of the λ ∼ 20 Å feature, we cannot conclude whether or not it is a true astrophysical absorption line. Considering cosmological simulations, the probability of the LETG/ACIS λ ∼ 20 Å feature being due to the astrophysical O VIII absorber co-located with the FUV-detected O VI absorber is at the very low level of ≲0.1%. We cannot completely rule out the very unlikely possibility that the LETG/ACIS 20 Å feature is due to a transient event located close to the blazar.

Warm-hot gas in X-ray bright galaxy clusters and the H i-deficient circumgalactic medium in dense environments

We analyze the intracluster medium (ICM) and circumgalactic medium (CGM) in 7 X-ray detected galaxy clusters using spectra of background QSOs (HST-COS/STIS), optical spectroscopy of the cluster galaxies (MMT/Hectospec and SDSS), and X-ray imaging/spectroscopy (XMM-Newton and Chandra). First, we report a very low covering fraction of H I absorption in the CGM of these cluster galaxies, f_c = 25% +25%/-15%, to stringent detection limits (log N(H I) < 13 cm^-2). As field galaxies have an H I covering fraction of ~100% at similar radii, the dearth of CGM H I in our data indicates that the cluster environment has effectively stripped or overionized the gaseous halos of these cluster galaxies. Second, we assess the contribution of warm-hot (10^5 - 10^6 K) gas to the ICM as traced by O VI and broad Ly-alpha (BLA) absorption. Despite the high signal-to-noise of our data, we do not detect O VI in any cluster, and we only detect BLA features in the QSO spectrum probing one cluster. We estimate that the total column density of warm-hot gas along this line of sight totals to ~3% of that contained in the hot T > 10^7 K X-ray emitting phase. Residing at high relative velocities, these features may trace pre-shocked material outside the cluster. Comparing gaseous galaxy halos from the low-density 'field' to galaxy groups and high-density clusters, we find that the CGM is progressively depleted of H I with increasing environmental density, and the CGM is most severely transformed in galaxy clusters. This CGM transformation may play a key role in environmental galaxy quenching.

The Extent of Chemically Enriched Gas around Star-forming Dwarf Galaxies

Abstract Supernova driven winds are often invoked to remove chemically enriched gas from dwarf galaxies to match their low observed metallicities. In such shallow potential wells, outflows may produce massive amounts of enriched halo gas (circumgalactic medium, CGM) and pollute the intergalactic medium (IGM). Here, we present a survey of the CGM and IGM around 18 star-forming field dwarfs with stellar masses of at . Eight of these have CGM probed by quasar absorption spectra at projected distances, d, less than that of the host virial radius, . Ten are probed in the surrounding IGM at . The absorption measurements include neutral hydrogen, the dominant silicon ions for diffuse cool gas (T ∼ 104 K; Si ii, Si iii, and Si iv), moderately ionized carbon (C iv), and highly ionized oxygen (O vi). Metal absorption from the CGM of the dwarfs is less common and weaker compared to massive star-forming galaxies, though O vi absorption is still common. None of the dwarfs probed at have definitive metal-line detections. Combining the available silicon ions, we estimate that the cool CGM of the dwarfs accounts for only 2%–6% of the expected silicon budget from the yields of supernovae associated with past star formation. The highly ionized O vi accounts for ≈8% of the oxygen budget. As O vi traces an ion with expected equilibrium ion fractions of ≲0.2, the highly ionized CGM may represent a significant metal reservoir even for dwarfs not expected to maintain gravitationally shock heated hot halos.

Feeding the fire: Tracing the mass-loading of 107 K galactic outflows with O vi absorption.

Galactic outflows regulate the amount of gas galaxies convert into stars. However, it is difficult to measure the mass outflows remove because they span a large range of temperatures and phases. Here, we study the rest-frame ultraviolet spectrum of a lensed galaxy at z ~ 2.9 with prominent interstellar absorption lines from O i, tracing neutral gas, up to O vi, tracing transitional phase gas. The O vi profile mimics weak low-ionization profiles at low velocities, and strong saturated profiles at high velocities. These trends indicate that O vi gas is co-spatial with the low-ionization gas. Further, at velocities blueward of -200 km s-1 the column density of the low-ionization outflow rapidly drops while the O vi column density rises, suggesting that O vi is created as the low-ionization gas is destroyed. Photoionization models do not reproduce the observed O vi, but adequately match the low-ionization gas, indicating that the phases have different formation mechanisms. Photoionized outflows are more massive than O vi outflows for most of the observed velocities, although the O vi mass outflow rate exceeds the photoionized outflow at velocities above the galaxy's escape velocity. Therefore, most gas capable of escaping the galaxy is in a hot outflow phase. We suggest that the O vi absorption is a temporary by-product of conduction transferring mass from the photoionized phase to an unobserved hot wind, and discuss how this mass-loading impacts the observed circum-galactic medium.

Bimodality of low-redshift circumgalactic O vi in non-equilibrium eagle zoom simulations

We introduce a series of 20 cosmological hydrodynamical simulations of Lstar (M_200 =10^11.7 - 10^12.3 Msol) and group-sized (M_200 = 10^12.7 - 10^13.3 Msol) haloes run with the model used for the EAGLE project, which additionally includes a non-equilibrium ionization and cooling module that follows 136 ions. The simulations reproduce the observed correlation, revealed by COS-Halos at z~0.2, between O VI column density at impact parameters b < 150 kpc and the specific star formation rate (sSFR=SFR/Mstar) of the central galaxy at z~0.2. We find that the column density of circumgalactic O VI is maximal in the haloes associated with Lstar galaxies, because their virial temperatures are close to the temperature at which the ionization fraction of O VI peaks (T~10^5.5 K). The higher virial temperature of group haloes (> 10^6 K) promotes oxygen to higher ionization states, suppressing the O VI column density. The observed NO VI-sSFR correlation therefore does not imply a causal link, but reflects the changing characteristic ionization state of oxygen as halo mass is increased. In spite of the mass-dependence of the oxygen ionization state, the most abundant circumgalactic oxygen ion in both Lstar and group haloes is O VII; O VI accounts for only 0.1% of the oxygen in group haloes and 0.9-1.3% with Lstar haloes. Nonetheless, the metals traced by O VI absorbers represent a fossil record of the feedback history of galaxies over a Hubble time; their characteristic epoch of ejection corresponds to z > 1 and much of the ejected metal mass resides beyond the virial radius of galaxies. For both Lstar and group galaxies, more of the oxygen produced and released by stars resides in the circumgalactic medium (within twice the virial radius) than in the stars and ISM of the galaxy.

A pair of O vi and broad Ly α absorbers probing warm gas in a galaxy group environment atz∼ 0.4

We report on the detection of two O VI absorbers separated in velocity by 710 km/s at z ~ 0.4 towards the background quasar SBS0957+599. Both absorbers are multiphase systems tracing substantial reservoirs of warm baryons. The low and intermediate ionization metals in the first absorber is consistent with an origin in photoionized gas. The O VI has a velocity structure different from other metal species. The Ly-alpha shows the presence of a broad feature. The line widths for O VI and the broad Ly-alpha suggest T = 7.1 x 10^5 K. This warm medium is probing a baryonic column which is an order of magnitude more than the total hydrogen in the cooler photoionized gas. The second absorber is detected only in H I and O VI. Here the temperature of 4.6 x 10^4 K supports O VI originating in a low-density photoionized gas. A broad component is seen in the Ly-alpha, offset from the O VI. The temperature in the broad Ly-alpha is T < 2.1 x 10^5 K. The absorbers reside in a galaxy overdensity region with 7 spectroscopically identified galaxies within ~ 10 Mpc and delta_v ~ 1000 km/s of the first absorber, and 2 galaxies inside a similar separation from the second absorber. The distribution of galaxies relative to the absorbers suggest that the line of sight could be intercepting a large-scale filament connecting galaxy groups, or the extended halo of a sub-L* galaxy. Though kinematically proximate, the two absorbers reaffirm the diversity in the physical conditions of low redshift O VI systems and the galactic environments they inhabit.

HALO MASS DEPENDENCE OF H I AND O VI ABSORPTION: EVIDENCE FOR DIFFERENTIAL KINEMATICS

We studied a sample of 14 galaxies (0.1 < z < 0.7) using HST/WFPC2 imaging and high-resolution HST/COS or HST/STIS quasar spectroscopy of Lya, Lyb, OVI1031, and OVI1037 absorption. The galaxies, having 10.8 < log(M/M_solar) < 12.2, lie within D = 300 kpc of quasar sightlines, probing out to D/R_vir = 3. When the full range of galaxy virial masses and D/R_vir of the sample are examined, 40% of the HI absorbing clouds can be inferred to be escaping their host halo. The fraction of bound clouds decreases as D/R_vir increases such that the escaping fraction is around 15% for D/R_vir < 1, around 45% for 1 < D/R_vir < 2, and around 90% for 2 < D/R_vir < 3. Adopting the median mass log(M/M_solar) = 11.5 to divide the sample into "higher" and "lower" mass galaxies, we find mass dependency for the hot CGM kinematics. To our survey limits, OVI absorption is found in only 40% of the HI clouds in and around lower mass halos as compared to 85% around higher mass halos. For D/R < 1, lower mass halos have an escape fraction of 65%, whereas higher mass halos have an escape fraction of 5%. For 1 < D/R_vir < 2, the escape fractions are 55% and 35% for lower mass and higher mass halos, respectively. For 2 < D/R_vir < 3, the escape fraction for lower mass halos is around 90%. We show that it is highly likely that the absorbing clouds reside within 4 virial radii of their host galaxies and that the kinematics are dominated by outflows. Our finding of "differential kinematics" is consistent with the scenario of "differential wind recycling" proposed by Oppenheimer et al. We discuss the implications for galaxy evolution, the stellar to halo mass function, and the mass metallicity relationship of galaxies.

GALACTIC AND CIRCUMGALACTIC O VI AND ITS IMPACT ON THE COSMOLOGICAL METAL AND BARYON BUDGETS AT 2 &lt;z≲ 3.5

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 high redshift galaxies and their circumgalactic medium (CGM) at 2<z<4. We select absorbers optically thick at the Lyman limit ({\tau}LL > 1, log N(HI) > 17.3) as probes of these galaxies and their CGM where both transitions of the O VI doublet have little contamination from the Ly {\alpha}, {\beta} forests. We found 20 absorbers that satisfy these rules: 7 Lyman limit systems (LLSs), 8 super-LLSs (SLLSs) and 5 damped Ly{\alpha} (DLAs). The O VI detection rate is 100% for the DLAs, 71% for the LLSs, and 63% for the SLLSs. When O VI is detected, log N(OVI)=14.9+/-0.3, an average O VI column density substantially larger and with a smaller dispersion than found in blind O VI surveys at similar redshifts. Strong O VI absorption is therefore nearly ubiquitous in the CGM of z~2-3 galaxies. The total velocity widths of the O VI profiles are also large (200<Dv(OVI)<400 km/s). These properties are quite similar to those seen for O VI in low z star-forming galaxies, and therefore we hypothesize that these strong CGM O VI absorbers (with {\tau}LL > 1) at 2<z<3.5 also probe outflows of star-forming galaxies. 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}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.

THE PROPERTIES OF LOW REDSHIFT INTERGALACTIC O VI ABSORBERS DETERMINED FROM HIGH S/N OBSERVATIONS OF 14 QSOs WITH THE COSMIC ORIGINS SPECTROGRAPH

We report on the observed properties of the plasma revealed through high signal-to-noise (S/N) observations of 54 intervening O VI absorption systems containing 85 O VI and 133 H I components in a blind survey of 14 QSOs observed at ~18 km s-1 resolution with the Cosmic Origins Spectrograph (COS) over a redshift path of 3.52 at z < 0.5. Simple systems with one or two H I components and one O VI component comprise 50% of the systems. For a sample of 45 well-aligned absorption components where the temperature can be estimated, we find evidence for cool photoionized gas in 31 (69%) and warm gas (6 > log T > 5) in 14 (31%) of the components. The total hydrogen content of the 14 warm components can be estimated from the temperature and the measured value of log N(H I). The very large implied values of log N(H) range from 18.38 to 20.38 with a median of 19.35. The metallicity, [O/H], in the 6 warm components with log T > 5.45 ranges from -1.93 to 0.03 with a median value of -1.0 dex. Ground-based galaxy redshift studies reveal that most of the absorbers we detect sample gas in the IGM extending 200 to 600 kpc beyond the closest associated galaxy. We estimate the warm aligned O VI absorbers contain (4.1+/-1.1)% of the baryons at low z. The warm plasma traced by the aligned O VI and H I absorption contains nearly as many baryons as are found in galaxies.

WHERE DO GALAXIES END?

Our current view of galaxies considers them as systems of stars and gas embedded in extended halos of dark matter, much of it formed by the infall of smaller systems at earlier times. The true extent of a galaxy remains poorly determined, with the virial radius (R_vir) providing a characteristic separation between collapsed structures in dynamical equilibrium and external infalling matter. Other physical estimates of the extent of gravitational influence include the gravitational radius, gas accretion radius, and "galactopause" arising from outflows that stall at 100-200 kpc over a range of outflow parameters and confining gas pressures. Physical criteria are proposed to define bound structures, including a more realistic definition of R_vir (M*, M_h, z_a) for stellar mass M* and halo mass M_h, half of which formed at "assembly redshifts" z_a = 0.7-1.3. We estimate the extent of bound gas and dark matter around L* galaxies to be ~200 kpc. The new virial radii, with mean R_vir ~ 200 kpc, are 40-50% smaller than values estimated in recent HST/COS detections of H I and O VI absorbers around galaxies. In the new formalism, the Milky Way stellar mass, log M* = 10.7 +/- 0.1, would correspond to R_vir = 153 (+25,-16) kpc for half-mass halo assembly at z_a = 1.06 +/- 0.03. The frequency per unit redshift of low-redshift O VI absorption lines in QSO spectra suggests absorber sizes ~150 kpc when related to intervening 0.1L* galaxies. This formalism is intended to clarify semantic differences arising from observations of extended gas in galactic halos, circumgalactic medium (CGM), and filaments of the intergalactic medium (IGM). Astronomers should refer to bound gas in the galactic halo or CGM, and unbound gas at the CGM-IGM interface, on its way into the IGM.

PROBING THE LARGE AND MASSIVE CIRCUMGALACTIC MEDIUM OF A GALAXY ATz∼ 0.2 USING A PAIR OF QUASARS

We present analysis of two O VI absorbers at redshift z_{abs} = 0.227, detected in the spectra of two closely spaced QSO sightlines (Q 0107-025 A and B), observed with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). At the same redshift, presence of a single bright (~ 1.2 L*) galaxy at an impact parameter of ~ 200 kpc (proper) from both the sightlines was reported by Crighton et al. (2010). Using detailed photoionization models we show that the high ionization phases of both the O VI absorbers have similar ionization conditions (e.g. log U ~ -1.1 to -0.9), chemical enrichment (e.g. log Z ~ -1.4 to -1.0), total hydrogen column density (e.g. log N_{H} (cm^{-2}) ~ 19.6-19.7) and line of sight thickness (e.g. l_{los} ~ 600-800 kpc). Therefore we speculate that the O VI absorbers are tracing different parts of same large scale structure, presumably the circumgalactic medium (CGM) of the identified galaxy. Using sizes along and transverse to the line of sight, we estimate the size of the CGM to be R ~ 330 kpc. The baryonic mass associated with this large CGM as traced by O VI absorption is ~ 1.2x10^{11} M_{sun}. A low ionization phase is detected in one of the O VI systems with near solar metallicity (log Z = 0.20\pm0.20) and parsec scale size (l_{los} ~ 6 pc), possibly tracing the neutral phase of a high velocity cloud (HVC) embedded within the CGM.

Absence of hot gas within the Wolf-Rayet bubble around WR 16

We present the analysis of XMM-Newton archival observations towards the Wolf-Rayet (WR) bubble around WR16. Despite the closed bubble morphology of this WR nebula, the XMM-Newton observations show no evidence of diffuse emission in its interior as in the similar WR bubbles NGC6888 and S308. We use the present observations to estimate a 3-\sigma upper limit to the X-ray luminosity in the 0.3-1.5 keV energy band equal to 7.4x10^{32} erg s^{-1} for the diffuse emission from the WR nebula, assuming a distance of 2.37 kpc. The WR nebula around WR16 is the fourth observed by the current generation of X-ray satellites and the second not detected. We also examine FUSE spectra to search for nebular O VI absorption lines in the stellar continuum of WR16. The present far-UV data and the lack of measurements of the dynamics of the optical WR bubble do not allow us to confirm the existence of a conductive layer of gas at T~3x10^5 K between the cold nebular gas and the hot gas in its interior. The present observations result in an upper limit of n_e < 0.6 cm^-3 on the electron density of the X-ray emitting material within the nebula.

DETECTION OF DIFFUSE X-RAY EMISSION FROM PLANETARY NEBULAE WITH NEBULAR O VI

The presence of O VI ions can be indicative of plasma temperatures of a few times 10^5 K that is expected in heat conduction layers between the hot shocked stellar wind gas at several 10^6 K and the cooler (~10,000 K) nebular gas of planetary nebulae (PNe). We have used FUSE observations of PNe to search for nebular O VI emission or absorption as a diagnostic of conduction layer to ensure the presence of hot interior gas. Three PNe showing nebular O VI, namely IC 418, NGC 2392, and NGC 6826, have been selected for Chandra observations and diffuse X-ray emission is indeed detected in each of these PNe. Among the three, NGC 2392 has peculiarly high diffuse X-ray luminosity and plasma temperature compared with those expected from its stellar wind's mechanical luminosity and terminal velocity. The limited effects of heat conduction on the plasma temperature of a hot bubble at the low terminal velocity of the stellar wind of NGC 2392 may partially account for its high plasma temperature, but the high X-ray luminosity needs to be powered by processes other than the observed stellar wind, probably caused by the presence of an unseen binary companion of the CSPN of NGC 2392. We have compiled relevant information on the X-ray, stellar, and nebular properties of PNe with a bubble morphology and found that the expectations of bubble models including heat conduction compare favorably with the present X-ray observations of hot bubbles around H-rich CSPNe, but have notable discrepancies for those around H-poor [WR] CSPNe. We note that PNe with more massive central stars can produce hotter plasma and higher X-ray surface brightness inside central hot bubbles.

Observations of O vi Absorption from the Superbubbles of the Large Magellanic Cloud

AbstractWe have presented the observations of O VI absorption at 1032 Å towards 22 sightlines in 10 superbubbles (SBs) of the Large Magellanic Cloud (LMC) using the data obtained from the Far Ultraviolet Spectroscopic Explorer (FUSE). The estimated abundance of O VI in the SBs varies from a minimum of (1.09 ±0.22)×1014 atoms/cm2 in SB N206 to a maximum of (3.71±0.23)×1014 atoms/cm2 in SB N70. We find about a 46% excess in the abundance of O VI in the SBs compared to the non-SB lines of sight. Even inside a SB, O VI column density (N(O VI)) varies by about a factor of 2 to 2.5. These data are useful in understanding the nature of the hot gas in SBs.

THE BARYON CENSUS IN A MULTIPHASE INTERGALACTIC MEDIUM: 30% OF THE BARYONS MAY STILL BE MISSING

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 (Z/Zsun) and O VI ionization fraction (f_OVI) using cosmological simulations of heating, cooling, and metal transport in a density-temperature structured medium. Statistically, their product correlates with column density, (Z/Zsun)(f_OVI) = (0.015)(N_OVI/10^{14} cm^-2)^0.70. The N_OVI-weighted mean is 0.01, which doubles previous estimates of WHIM baryon content. We also reanalyze H I data from the Hubble Space Telescope, applying redshift corrections for absorber density, photoionizing background, and proper length, dl/dz. We find substantial baryon fractions in the photoionized Lya forest (28 +/- 11%), O VI/BLA-traced WHIM (25 +/- 8%), and collapsed phase (18 +/- 4%) in galaxies, groups, clusters, and circumgalactic gas. The baryon shortfall is 29 +/- 13%, which may be detected in X-ray absorbers from hotter WHIM or in weaker Lya and O VI absorbers. Further progress will require higher-precision baryon surveys of weak absorbers at column densities N_HI > 10^{12.0} cm^-2, N_OVI > 10^{12.5} cm^-2, and N_OVII > 10^{14.5} cm^-2, with moderate-resolution UV and X-ray spectrographs.

Magicc haloes: confronting simulations with observations of the circumgalactic medium at z=0

We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminosities L ~ 0.1 and 1 L* generated using the smooth particle hydrodynamic code GASOLINE. These simulations are part of the Making Galaxies In a Cosmological Context (MAGICC) program in which the stellar feedback is tuned to match the stellar mass-halo mass relationship. For comparison, each galaxy was also simulated using a 'lower feedback' (LF) model which has strength comparable to other implementations in the literature. The 'MAGICC feedback' (MF) model has a higher incidence of massive stars and an approximately two times higher energy input per supernova. Apart from the low-mass halo using LF, each galaxy exhibits a metal-enriched CGM that extends to approximately the virial radius. A significant fraction of this gas has been heated in supernova explosions in the disc and subsequently ejected into the CGM where it is predicted to give rise to substantial O VI absorption. The simulations do not yet address the question of what happens to the O VI when the galaxies stop forming stars. Our models also predict a reservoir of cool H I clouds that show strong Ly\alpha absorption to several hundred kpc. Comparing these models to recent surveys with the Hubble Space Telescope, we find that only the MF models have sufficient O VI and H I gas in the CGM to reproduce the observed distributions. In separate analyses, these same MF models also show better agreement with other galaxy observables (e.g. rotation curves, surface brightness profiles and H I gas distribution). We infer that the CGM is the dominant reservoir of baryons for galaxy haloes.

THE PROPERTIES OF TWO LOW-REDSHIFT O VI ABSORBERS AND THEIR ASSOCIATED GALAXIES TOWARD 3C 263,

Ultraviolet observations of the QSO 3C 263 (zem = 0.652) with COS and FUSE reveal O VI absorption systems at z = 0.06342 and 0.14072 . WIYN multi-object spectrograph observations provide information about the galaxies associated with the absorbers. The multi-phase system at z = 0.06342 traces cool photoionized gas and warm collisionally ionized gas associated with a L ~ 0.31L* compact spiral emission line galaxy with an impact parameter of 63 kpc. The cool photoionized gas in the absorber is well modeled with log U ~ -2.6, log N(H) ~17.8, log n(H) ~ -3.3 and [Si/H] = -0.14\pm0.23. The collisionally ionized gas containing C IV and O VI probably arises in cooling shock heated transition temperature gas with log T ~ 5.5. The absorber is likely tracing circumgalactic gas enriched by gas ejected from the spiral emission line galaxy. The simple system at z = 0.14072 only contains O VI and broad and narrow H I. The O VI with b = 33.4\pm11.9 km s-1 is likely associated with the broad H I {\lambda}1215 absorption with b = 86.7\pm15.4 km s-1. The difference in Doppler parameters implies the detection of a very large column of warm gas with log T = 5.61(+0.16, -0.25), log N(H) = 19.54(+0.26, -0.44) and [O/H] = -1.48 (+0.46, -0.26). This absorber is possibly associated with a 1.6L* absorption line galaxy with an impact parameter of 617 kpc although an origin in warm filament gas or in the halo of a fainter galaxy is more likely.