Low-redshift Intergalactic Medium Research Articles

Jet feedback and the photon underproduction crisis in simba

ABSTRACT We examine the impact of black hole jet feedback on the properties of the low-redshift intergalactic medium (IGM) in the simba simulation, with a focus on the Lyα forest mean flux decrement DA. Without jet feedback, we confirm the photon underproduction crisis (PUC) in which ΓH i at $z$ = 0 must be increased by 6 times over the Haardt & Madau value in order to match the observed DA. Turning on jet feedback lowers this discrepancy to ∼2.5 times, and additionally using the recent Faucher–Giguère background mostly resolves the PUC, along with producing a flux probability distribution function in accord with observations. The PUC becomes apparent at late epochs ($z \lesssim 1$) where the jet and no-jet simulations diverge; at higher redshifts simba reproduces the observed DA with no adjustment, with or without jets. The main impact of jet feedback is to lower the cosmic baryon fraction in the diffuse IGM from 39 per cent to 16 per cent at $z$ = 0, while increasing the warm-hot intergalactic medium (WHIM) baryon fraction from 30 per cent to 70 per cent; the lowering of the diffuse IGM content directly translates into a lowering of DA by a similar factor. Comparing to the older mufasa simulation that employs different quenching feedback but is otherwise similar to simba, mufasa matches DA less well than simba, suggesting that low-redshift measurements of DA and ΓH i could provide constraints on feedback mechanisms. Our results suggest that widespread IGM heating at late times is a plausible solution to the PUC, and that simba’s jet active galactic nucleus feedback model, included to quench massive galaxies, approximately yields this required heating.

The Spread of Metals into the Low-redshift Intergalactic Medium

Abstract We investigate the association between galaxies and metal-enriched and metal-deficient absorbers in the local universe (z < 0.16) using a large compilation of far-ultraviolet spectra of bright active galactic nuclei targets observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. In this homogeneous sample of 18 O vi detections at and 18 nondetections at using absorbers with , the maximum distance O vi extends from galaxies of various luminosities is ∼0.6 Mpc, or ∼5 virial radii, confirming and refining earlier results. This is an important value that must be matched by numerical simulations, which input the strength of galactic winds at the sub-grid level. We present evidence that the primary contributors to the spread of metals into the circum- and intergalactic media are sub-L* galaxies ( ). The maximum distances that metals are transported from these galaxies is comparable to, or less than, the size of a group of galaxies. These results suggest that, where groups are present, the metals produced by the group galaxies do not leave the group. Since many O vi nondetections in our sample occur at comparably close impact parameters as those of the metal-bearing absorbers, some more pristine intergalactic material appears to be accreting onto groups where it can mix with metal-bearing clouds.

AN HST/COS SURVEY OF THE LOW-REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS*

ABSTRACT We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright active galactic nuclei (AGNs) at redshifts z AGN < 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H i and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs < 0.75 covering total redshift pathlengths Δz H i = 21.7 and Δz O vi = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of H i systems can be parametrized d  ( > N ) / dz ?> = C 14 ( N / 10 14 cm − 2 ) − ( β − 1 ) ?> , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 ∝ ( 1 + z ) 2.3 ± 0.1 ?> , and slope β(z) = 1.75–0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N H I ?> . The distribution of O vi absorbers appears to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H i absorbers on scales of Δv = 50–300 km s−1 with no significant clustering at Δv ≳ 1000 km s−1. Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N H i ≳ 1013.5 cm−2, metal-bearing IGM systems. The full catalog of absorption lines and fully reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at http://archive.stsci.edu/prepds/igm/.

THE METAGALACTIC IONIZING BACKGROUND: A CRISIS IN UV PHOTON PRODUCTION OR INCORRECT GALAXY ESCAPE FRACTIONS?

Recent suggestions of a "photon underproduction crisis" (Kollmeier \etal\ 2014) have generated concern over the intensity and spectrum of ionizing photons in the metagalactic ultraviolet background (UVB). The balance of hydrogen photoionization and recombination determines the opacity of the low-redshift intergalactic medium (IGM). We calibrate the hydrogen photoionization rate ($\Gamma_{\rm H}$) by comparing {\it Hubble Space Telescope} spectroscopic surveys of the low-redshift column density distribution of \HI\ absorbers and the observed ($z < 0.4$) mean \Lya\ flux decrement, $D_A = (0.014)(1+z)^{2.2}$, to new cosmological simulations. The distribution, $f(N_{\rm HI}, z) \equiv d^2 {\cal N} / d(\log N_{\rm HI}) dz$, is consistent with an increased UVB that includes contributions from both quasars and galaxies. Our recommended fit, $\Gamma_{\rm H}(z) = (4.6 \times 10^{-14}$ s$^{-1})(1+z)^{4.4}$ for $0 < z < 0.47$, corresponds to unidirectional LyC photon flux $\Phi_0 \approx 5700$~cm$^{-2}$~s$^{-1}$ at $z = 0$. This flux agrees with observed IGM metal ionization ratios (\CIII/\CIV\ and \SiIII/\SiIV) and suggests a 25-30\% contribution of \Lya\ absorbers to the cosmic baryon inventory. The primary uncertainties in the low-redshift UVB are the contribution from massive stars in galaxies and the LyC escape fraction ($f_{\rm esc}$), a highly directional quantity that is difficult to constrain statistically. We suggest that both quasars and low-mass starburst galaxies are important contributors to the ionizing UVB at $z < 2$. Their additional ionizing flux would resolve any crisis in photon underproduction.

Photon underproduction crisis: Are QSOs sufficient to resolve it?

Abstract We investigate the recent claim of ‘photon underproduction crisis’ by Kollmeier et al. which suggests that the known sources of ultraviolet (UV) radiation may not be sufficient to generate the inferred H i photoionization rate ($\Gamma _{\rm H\,\small {i}}$) in the low-redshift intergalactic medium. Using the updated QSO emissivities from the recent studies and our cosmological radiative transfer code developed to estimate the UV background, we show that the QSO contributions to $\Gamma _{\rm H\,\small {i}}$ is higher by a factor ∼2 as compared to the previous estimates. Using self-consistently computed combinations of star formation rate density and dust attenuation, we show that a typical UV escape fraction of 4 per cent from star-forming galaxies should be sufficient to explain the inferred $\Gamma _{\rm H\,\small {i}}$ by Kollmeier et al. Interestingly, we find that the contribution from QSOs alone can explain the recently inferred $\Gamma _{\rm H\,\small {i}}$ by Shull et al. which used the same observational data but different simulation. Therefore, we conclude that the crisis is not as severe as it was perceived before and there seems no need to look for alternate explanations such as low luminosity hidden QSOs or decaying dark matter particles.

Thinking outside the halo: tracing the large-scale distribution of diffuse cosmic metals with semi-analytic models

With the installation of the Cosmic Origins Spectrograph on the Hubble Space Telescope, measurements of the metal content of the low redshift intergalactic medium (IGM) are now available. Using a new grid-based model for diffuse gas coupled to the SAGE semi-analytic model of galaxy formation, we examine the impact of supernova feedback on the pollution of the IGM. We consider different assumptions for the reheating and ejection of gas by supernovae and their dependence on galaxy circular velocity and gas surface density. Where metals are present, we find the most likely metallicity to be $-1.5 < $log$_{10}$(Z/Z$_{\odot}$)$< -1.0$ at $z = 0$, consistent with both observations and more sophisticated hydrodynamic simulations. Our model predicts that the regions of the IGM with the highest metallicities will be near galaxies with M$_{\star} \sim 10^{10.5}h^{-1}$M$_{\odot}$ and in environments of densities $\sim 10 \times$ the mean. We also find that 90% of IGM metals at $z = 0$ are ejected by galaxies with stellar masses less than $10^{10.33}h^{-1}$M$_{\odot}$.

TRACING THE COSMIC METAL EVOLUTION IN THE LOW-REDSHIFT INTERGALACTIC MEDIUM

Using the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, we measured the abundances of six ions (C III, C IV, Si III, Si IV, N V, O VI) in the low-redshift (z < 0.4) intergalactic medium and explored C and Si ionization corrections from adjacent ion stages. Both C IV and Si IV have increased in abundance by a factor of ~10 from z = 5.5 to the present. We derive ion mass densities, (rho_ion) = (Omega_ion)(rho_cr) with Omega_ion expressed relative to closure density. Our models of the mass-abundance ratios, (Si III / Si IV) = 0.67(+0.35,-0.19), (C III / C IV) = 0.70(+0.43,-0.20), and (Omega_CIII + Omega_CIV) / (Omega_SiIII + Omega_SiIV) = 4.9(+2.2,-1.1), are consistent with a hydrogen photoionization rate Gamma_H = (8 +/- 2) x 10^{-14} s^{-1} at z < 0.4 and specific intensity I_0 = (3 +/- 1) x 10^{-23} erg/(cm^2 s Hz sr) at the Lyman limit. We find mean photoionization parameter log U = -1.5 +/- 0.4, baryon overdensity Delta_b = 200 +/- 50, and Si/C enhanced to three times its solar ratio (enhancement of alpha-process elements). We compare these metal abundances to the expected IGM enrichment and abundances in higher photoionized states of carbon (C V) and silicon (Si V, Si VI, Si VII). Our ionization modeling infers IGM metal densities of (5.4 +/- 0.5) x 10^5 M_sun / Mpc^3 in the photoionized Lya forest traced by the C and Si ions and (9.1 +/- 0.6) x 10^5 M_sun / Mpc^3 in hotter gas traced by O VI. Combining both phases, the heavy elements in the IGM have mass density rho_Z = (1.5 +/- 0.8) x 10^6 M_sun / Mpc^3 or Omega_Z = 10^{-5}. This represents 10 +/- 5 percent of the metals produced by (6 +/- 2) x 10^8 M_sun / Mpc^3 of integrated star formation with yield y_m = 0.025 +/- 0.010. The missing metals at low redshift may reside within galaxies and in undetected ionized gas in galaxy halos and circumgalactic medium.

The impact of feedback on the low-redshift intergalactic medium

We analyse the evolution of the properties of the low-redshift Intergalactic Medium (IGM) using high-resolution hydrodynamic simulations that include a detailed chemical evolution model. We focus on the effects that two different forms of energy feedback, strong galactic winds driven by supernova explosion and Active Galactic Nuclei (AGN) powered by gas accretion onto super-massive black holes (BHs), have on the thermo- and chemo-dynamical properties of of the low redshift IGM. We find that feedback associated to winds (W) and BHs leave distinct signatures in both the chemical and thermal history of the baryons, especially at redshift z<3 [..] We present results for the enrichment in terms of mass and metallicity distributions for the WHIM phase, both as a function of density and temperature. Finally, we compute the evolution of the relative abundances between different heavy elements, namely Oxygen, Carbon and Iron. While both C/O and O/Fe evolve differently at high redshifts for different feedback models, their values are similar at z=0 [..]. The sensitivity of WHIM properties on the implemented feedback scheme could be important both for discriminating between different feedback physics and for detecting the WHIM with future far-UV and X-ray telescopes.

A LARGE RESERVOIR OF IONIZED GAS IN THE GALACTIC HALO: IONIZED SILICON IN HIGH-VELOCITY AND INTERMEDIATE-VELOCITY CLOUDS

The low Galactic halo is enveloped by a sheath of ionized, low-metallicity gas, which can provide a substantial (1 M☉ yr−1) cooling inflow to replenish star formation in the disk. Using absorption spectra from the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer toward 37 active galactic nuclei at high latitude, we detect widespread interstellar Si iii λ1206.50 absorption: 61 high-velocity clouds (HVCs) along 30 sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We find a segregation of redshifted and blueshifted absorbers across the Galactic rotation axis at ℓ = 180°, consistent with a lag in the rotation velocity above the Galactic plane. The HVC sky coverage is large (81% ± 5% for 30 out of 37 directions) with Si iii optical depth typically 4–5 times that of O vi λ1031.926. The mean HVC column density per sight line, 〈log NSi III〉 = 13.42 ± 0.21, corresponds to total column density NHII ≈ (6 × 1018 cm−2)(ZSi/0.2Z☉)−1 of ionized low-metallicity gas, similar to that inferred in O vi. This reservoir could total 108M☉ and produce a mass infall rate ∼1 M☉ yr−1. By modeling Si ii, Si iii, Si iv, and H i in a subset of absorbers, we constrain the mean photoionization parameter in the low halo, 〈log U〉 = −3.0+0.3−0.4, approximately 10 times lower than observed in the low-redshift intergalactic medium. The metallicities in some HVCs, derived from [Si ii/H i], are 10%–30% solar, whereas values found from all three silicon ions are lower in the pure-photoionization models. These formally lower matallicities are highly uncertain, since some of the higher ions may be collisionally ionized. The HVC and IVC metallicities may be compared with the median photometric metallicity, [Fe/H] = −1.46 ± 0.30, for ∼200,000 halo F/G stars in the Sloan Digital Sky Survey.

The nature and origin of low-redshift O vi absorbers

The O VI ion observed in quasar absorption-line spectra is the most accessible tracer of the cosmic metal distribution in the low-redshift ( z< 0.5) intergalactic medium (IGM). We explore the nature and origin of O VI absorbers using cosmological hydrodynamic simulations including galactic outflows with a range of strengths. We consider the effects of ionization background variations, non-equilibrium ionization and cooling, uniform metallicity and small-scale (subresolution) turbulence. Our main results are as follows. (1) IGM O VI is predominantly photoionized with T ≈ 10 4.2±0.2 K. A key reason for this is that O VI absorbers preferentially trace overenriched (by ∼× 5) regions of the IGM at a given density, which enhances metal-line cooling such that absorbers can cool to photo-ionized temperatures within a Hubble time. As such, O VI is not a good tracer of the warm-hot intergalactic medium. (2) The predicted O VI properties fit observables if and only if sub-resolution turbulence is added, regardless of any other model variations. The required turbulence increases with O VI absorber strength. Stronger absorbers arise from more recent outflows, so qualitatively this can be understood if IGM turbulence dissipates on the order of a Hubble time. The amount of turbulence is consistent with other examples of turbulence observed in the IGM and galactic haloes. (3) Metals traced by O VI and H I do not trace exactly the same baryons, but reside in the same large-scale structure. Our simulations reproduce observed alignment statistics between O VI and H I, yet aligned absorbers typically have O VI arising from cooler gas, and for stronger absorbers lower densities, than H I. Owing to peculiar velocities dominating the line structure, coincident absorption often arises from spatially distinct gas. (4) Photo-ionized O VI traces gas in a variety of environments, and is not directly associated with the nearest galaxy, though is typically nearest to ∼0.1L∗ galaxies. Weaker O VI components trace some of the oldest cosmic metals. (5) Very strong absorbers (EW 100 mA) are more likely to be collisionally ionized, tracing more recent enrichment (2 Gyr) within or near galactic haloes.

AGN Feedback Causes Downsizing

We study the impact of outflows driven by active galactic nuclei (AGNs) on galaxy formation. Outflows move into the surrounding intergalactic medium (IGM) and heat it sufficiently to prevent it from condensing onto galaxies. In the dense, high-redshift IGM, such feedback requires a highly energetic outflow, driven by a large AGN. However, in the more tenuous low-redshift IGM, equivalently strong feedback can be achieved by less energetic winds (and thus smaller galaxies). Using a simple analytic model, we show that this leads to the antihierarchical quenching of star formation in large galaxies, consistent with current observations. At redshifts prior to the formation of large AGNs, galaxy formation is hierarchical and follows the growth of dark matter halos. The transition between the two regimes lies at the z ≈ 2 peak of AGN activity..

Lyα flux power spectrum and its covariance

We analyse the flux power spectrum and its covariance using simulated Lyα forests. We find that pseudo-hydro techniques are good approximations of hydrodynamical simulations at high redshift. However, the pseudo-hydro techniques fail at low redshift because they are insufficient for characterizing some components of the low-redshift intergalactic medium, notably the warm-hot intergalactic medium. Hence, to use the low-redshift Lyα flux power spectrum to constrain cosmology, one would need realistic hydrodynamical simulations. By comparing (one-dimensional) mass statistics with flux statistics, we show that the non-linear transform between density and flux quenches the fluctuations so that the flux power spectrum is much less sensitive to cosmological parameters than the one-dimensional mass power spectrum. The covariance of the flux power spectrum is nearly Gaussian. As such, the uncertainties of the underlying mass power spectrum could still be large, even though the flux power spectrum can be precisely determined from a small number of lines of sight.

Hot Baryons and the Distribution of Metals in the Intergalactic Medium

We use the observed number and column density distributions of intergalactic O VI absorbers to constrain the distribution of metals in the low-redshift intergalactic medium (IGM). In this simple model, the metals in the O VI absorbers are assumed to be produced in and propagated from low-redshift galaxies drawn from a real sample, in this case the Sloan Digital Sky Survey. This model can explain the observed d/dz of metals borne by O VI absorbers if these metals are dispersed out to ~200 kpc by galaxies down to L ~ L. Massive galaxies (L ~ L) by themselves cannot provide the metals unless they can enrich volumes out to 0.5-1 Mpc. This model suggests that the turnover in d/dN dz below log N(O ) 13.5 (Wλ = 50 mA) is likely a real effect resulting from the apparently limited volumes over which galaxies can disperse metals, rather than a consequence of survey incompleteness. If so, it would indicate that metals are not as widespread throughout the IGM, as they are assumed to be in cosmological simulations of the warm-hot IGM. Alternatively, our model estimates the fraction of O VI absorbers directly caused by galaxies, such as the known populations of highly ionized high-velocity clouds surrounding the Milky Way, rather than the hot IGM.

Mapping the Cosmic Web with Ly Emission

We use a high-resolution cosmological simulation to predict the distribution of H I Lyα emission from the low-redshift (z 0.5) intergalactic medium (IGM). Our simulation can be used to reliably compute the emission from optically thin regions of the IGM but not that of self-shielded gas. We therefore consider several models that bracket the expected emission from self-shielded regions. Most galaxies are surrounded by extended (102 kpc) of optically thin gas with Lyα surface brightness close to the expected background. Most of these regions contain smaller cores of dense, cool gas. Unless self-shielded gas is able to cool to T < 104.1 K, these cores are much brighter than the background. The Lyα coronae represent cooling flows of IGM gas accreting onto galaxies. We also estimate the number of Lyα photons produced through the reprocessing of stellar ionizing radiation in the interstellar medium of galaxies; while this mechanism is responsible for the brightest Lyα emission, it occurs on small physical scales and can be separated using high-resolution observations. In all cases, we find that Lyα emitters are numerous (with a space density of ~0.1 h3 Mpc-3) and closely trace the filamentary structure of the IGM, providing a new way to map gas inside the cosmic web.

Intergalactic shock acceleration and the cosmic gamma-ray background

We investigate numerically the contribution to the cosmic gamma-ray background from cosmic-ray ions and electrons accelerated at intergalactic shocks associated with cosmological structure formation. We show that the kinetic energy of accretion flows in the low-redshift intergalactic medium is thermalized primarily through moderately strong shocks, which allow for an efficient conversion of shock ram pressure into cosmic-ray pressure. Cosmic rays accelerated at these shocks produce a diffuse gamma-ray flux which is dominated by inverse Compton emission from electrons scattering off cosmic microwave background photons. Decay of neutral π mesons generated in p—p inelastic collisions of the ionic cosmic-ray component with the thermal gas contribute about 30 per cent of the computed emission. Based on experimental upper limits on the photon flux above 100 MeV from nearby clusters we constrain the efficiency of conversion of shock ram pressure into relativistic CR electrons to ≲1 per cent. Thus, we find that cosmic rays of cosmological origin can generate an overall significant fraction of order 20 per cent and no more than 30 per cent of the measured gamma-ray background.

ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Observations of the Low-Redshift L[CLC]y[/CLC]β Forest

We describe a moderate-resolution (20-25 km s-1) Far Ultraviolet Spectroscopic Explorer (FUSE) study of the low-redshift intergalactic medium. We report on studies of seven extragalactic sight lines and 12 Lyβ absorbers that correspond to Lyα lines detected by Hubble Space Telescope (HST)/Goddard High Resolution Spectrograph and Space Telescope Imaging Spectrograph. These absorbers appear to contain a significant fraction of the low-z baryons and were a major discovery of the HST spectrographs. Using FUSE data, with 40 mA (4 σ) Lyβ detection limits, we have employed the equivalent width ratio of Lyβ/Lyα, and occasionally higher Lyman lines, to determine the Doppler parameter b and accurate column densities N for moderately saturated lines. We detect Lyβ absorption corresponding to all Lyα lines with Wλ ≥ 200 mA. The Lyβ/Lyα ratios yield a preliminary distribution function of Doppler parameters, with mean b = 31.4 ± 7.4 km s-1 and median 28 km s-1, comparable to values at redshifts z = 2.0-2.5. If thermal, these b-values correspond to T ≈ 50,000 K, although the inferred Doppler parameters are considerably less than the widths derived from Lyα profile fitting, b/bwidth = 0.52. The typical increase in column density over that derived from profile fitting is Δ log N = 0.3, but ranges up to 1.0 dex. Our data suggest that the low-z Lyα absorbers contain sizable nonthermal motions or velocity components in the line profile, perhaps arising from cosmological expansion and infall.

Intervening O vi Quasar Absorption Systems at Low Redshift: A Significant Baryon Reservoir.

Far-UV echelle spectroscopy of the radio-quiet QSO H1821+643 (zem=0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) at approximately 7 km s-1 resolution, reveals four definite O vi absorption-line systems and one probable O vi absorber at 0.15<zabs<0.27. The four definite O vi absorbers are located near galaxies and are highly displaced from the quasar in redshift; these are likely intervening systems unrelated to the background QSO. In the case of the strong O vi system at zabs=0.22497, multiple components are detected in Si iii and O vi as well as H i Lyman series lines, and the differing component velocity centroids and b-values firmly establish that this is a multiphase absorption system. A weak O vi absorber is detected at zabs=0.22637, i.e., offset by approximately 340 km s-1 from the zabs=0.22497 system. Lyalpha absorption is detected at zabs=0.22613, but no Lyalpha absorption is significantly detected at 0.22637. Other weak O vi absorbers at zabs=0.24531 and 0.26659 and the probable O vi system at 0.21326 have widely diverse O vi/H i column density ratios with N(O vi)/N(H i) ranging from </=0.14+/-0.03 to 5.2+/-1.2. The number density of O vi absorbers with rest equivalent width greater than 30 mÅ in the H1821+643 spectrum is remarkably high, dN&solm0;dz approximately 48, which implies with a high (90%) confidence that it is greater than 17 in the low-redshift intergalactic medium. We conservatively estimate that the cosmological mass density of the O vi systems is Omegab(Ovi&parr0; greater, similar0.0008 h-175. With an assumed metallicity of 1/10 solar and a conservative assumption that the fraction of oxygen in the O vi ionization stage is 0.2, we obtain Omegab(Ovi&parr0; greater, similar0.004 h-175. This is comparable to the combined cosmological mass density of stars and cool gas in galaxies and X-ray-emitting gas in galaxy clusters at low redshift.