HI Ly-alpha Research Articles

Rest-frame ultraviolet line emission from the intergalactic medium at

Rest-frame UV emission lines offer the possibility to directly image the gas around high-redshift galaxies with upcoming optical instruments. We use a suite of large, hydrodynamical simulations to predict the nature and detectability of emission lines from the intergalactic medium at 2<z<5. The brightest emission comes from HI Ly-alpha and the strongest metal line, CIII, is about an order of magnitude fainter, although HI Ly-alpha may be fainter if the gas is self-shielded to the UV background or if dust is important. The highest surface brightness regions for CIV, SiIII, SiIV and OVI are fainter than CIII by factors of a few. The NV and NeVIII lines, as well as HeII H-alpha, are substantially weaker but their maximum surface brightnesses still exceed 100 photon/cm^2/s/sr at z=2 (for 2" pixels). Lower ionisation lines arise in denser and colder gas that produces clumpier emission. The brightest HI Ly-alpha emission arises in highly overdense gas, but the highest surface brightness emission from high-ionisation metal lines traces a wider range of overdensities. Bright metal-line emission traces gas with temperatures close to the peak of the corresponding emissivity curve. While HI Ly-alpha, HeII H-alpha, CIII, SiIII, and SiIV are excellent probes of cold accretion flows and the colder parts of outflows, CIV, NV, OVI, and NeVIII are powerful tracers of the diffuse WHIM and galactic winds. A comparison of results from simulations with varying physical prescriptions demonstrates that the predictions for the brighter metal-line emission are robust to within factors of a few. Several emission lines from the high-redshift IGM will become detectable in the near future, possibly starting with the Cosmic Web Imager on Palomar. MUSE and the Keck Cosmic Web Imager have the potential to revolutionise studies of the interactions between high-redshift galaxies and their environment. (Abridged)

Calibrating galaxy redshifts using absorption by the surrounding intergalactic medium★

Rest-frame UV spectral lines of star-forming galaxies are systematically offset from the galaxies' systemic redshifts, probably because of large-scale outflows. We calibrate galaxy redshifts measured from rest-frame UV lines by utilizing the fact that the mean HI Ly-alpha absorption profiles around the galaxies, as seen in spectra of background objects, must be symmetric with respect to the true galaxy redshifts if the galaxies are oriented randomly with respect to the lines of sight to the background objects. We use 15 QSOs at z~2.5-3 and more than 600 foreground galaxies with spectroscopic redshifts at z~1.9-2.5. All galaxies are within 2 Mpc proper from the lines of sight to the background QSOs. We find that LyA emission and ISM absorption redshifts require systematic shifts of v_LyA=-295(+35)(-35) km/s and v_ISM=145(+70)(-35) km/s. Assuming a Gaussian distribution, we put 1-sigma upper limits on possible random redshift offsets of <220 km/s for LyA and <420 km/s for ISM redshifts. For the small subset (<10%) of galaxies for which near-IR spectra have been obtained, we can compare our results to direct measurements based on nebular emission lines which we confirm to mark the systemic redshifts. While our v_ISM agrees with the direct measurements, our v_LyA is significantly smaller. However, when we apply our method to the near-IR subsample which is characterized by slightly different selection effects, the best-fit velocity offset comes into agreement with the direct measurement. This confirms the validity of our approach, and implies that no single number appropriately describes the whole population of galaxies, in line with the observation that the line offset depends on galaxy spectral morphology. This method provides accurate redshift calibrations and will enable studies of circumgalactic matter around galaxies for which rest-frame optical observations are not available.

ANALYSIS OF METHODS FOR DETECTING THE PROXIMITY EFFECT IN QUASAR SPECTRA

Using numerical simulations of structure formation, we investigate multiple methods of determining the strength of the proximity effect in the HI Lyalpha forest. We analyze three high resolution (~10kpc) redshift snapshots (z=4,3,2.25) of a Hydro-Particle-Mesh simulation to obtain realistic absorption spectra of the HI Lyalpha forest. We begin our analysis investigating the intrinsic biases thought to arise in the widely adopted standard technique of combining multiple lines of sight when searching for the proximity effect. We confirm the existence of this biases. We then concentrate on the analysis of the proximity effect along individual lines of sight. We construct the proximity effect strength distribution (PESD) and confirm that the PESD inferred from a simple averaging technique accurately recovers the input strength of the proximity effect at all redshifts. Moreover, the PESD closely follows the behaviors found in observed samples of quasar spectra. However, the PESD obtained from our new simulated sight lines presents some differences to that of simple Monte Carlo simulations. After developing three new theoretical methods of recovering the strength of the proximity effect on individual lines of sight, we compare their accuracy to the PESD from the simple averaging technique. All our new approaches are based on the maximization of the likelihood function, albeit invoking some modifications. The new techniques presented here fail to recover the input proximity effect in an un-biased way. Thus, employing complex 3D simulations, we provide strong evidence in favor of the proximity effect strength distribution obtained from the simple averaging technique, as method of estimating the UV background intensity, free of any biases.

The He i 584 Å forest as a diagnostic of helium reionization

We discuss the potential of using the HeI 584 A forest to detect and study HeII reionization. Significant 584 A absorption is expected from intergalactic HeII regions, whereas there should be no detectable absorption from low density gas in HeIII regions. Unlike HeII Ly-alpha absorption (the subject of much recent study), the difficulty with using this transition to study HeII reionization is not saturation but rather that the absorption is weak. The Gunn-Peterson optical depth for this transition is tau ~ 0.1 x_{HeII} Delta^2 [(1+z)/5]^{9/2}, where x_{HeII} is the fraction of helium in HeII and Delta is the density in units of the cosmic mean. In addition, HeI 584 A absorption is contaminated by lower redshift HI Ly-alpha absorption with a comparable flux decrement. We estimate the requirements for a definitive detection of redshifted HeI absorption from low density gas (Delta ~ 1), which would indicate that HeII reionization was occurring. We find that this objective can be accomplished (using coeval HI Ly-alpha absorption to mask dense regions and in cross correlation) with a spectral resolution of 10^4 and a signal-to-noise ratio per resolution element of ~ 10. Such specifications may be achievable on a few known z ~ 3.5 quasar sightlines with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We also discuss how HeI absorption can be used to measure the hardness of the ionizing background above 13.6 eV.

An unbiased measurement of the UV background and its evolution via the proximity effect in quasar spectra

We used 40 high resolution, high S/N QSO spectra at 2.1<z<4.7 to search for the signature of the proximity effect in the HI Lyalpha forest. Comparing the effective optical depth near each QSO with the expected one, we clearly detect the proximity effect on the combined QSO sample and towards each individual QSO. The observed proximity effect strength distribution (PESD) is asymmetric towards a weak effect. We demonstrate that this is not simply an effect of gravitational clustering around QSOs. Comparing simulated PESDs with observations, we argue that the averaging method to determine the UVB intensity J is heavily biased towards high values because of the PESD asymmetry. Using instead the mode of the PESD provides an unbiased estimate of J. For our sample its modal value is log(J)=-21.51+/-0.15 (in units of ergcm^-2s^-1Hz^-1sr^-1) at z=2.73. We estimated the excess HI absorption attributed to gravitational clustering. On scales of ~3 Mpc, only a minority of QSOs shows overdensities of up to a factor of a few in tau_eff; these are exactly the objects with the weakest proximity effects. After removing them, we redetermined the UVB intensity arriving at log(J)=-21.46+0.14-0.21. This is the most accurate measurement of J to date. We present a new diagnostic based on the shape of the PESD which strongly supports our conclusion that there is no systematic overdensity bias for the proximity effect. This additional diagnostic breaks the otherwise unavoidable degeneracy of the proximity effect between UVB and overdensity. We estimated the redshift evolution of J and found tentative evidence for a mild decrease with increasing redshift. Our results are in excellent agreement with predictions for the evolving UVB intensity, supporting the notion of a substantial contribution of star-forming galaxies.

EUV spectral energy distribution of quasars restored from associated absorbers

(abridged) Aims. To reconstruct the spectral shape of the quasar ionizing radiation in the extreme-UV range (1Ryd <= E < 10Ryd) from the analysis of narrow absorption lines (NAL) of the associated systems. Methods. Computational technique for inverse spectroscopic problems - Monte Carlo Inversion augmented by procedure of the spectral shape recovering and modified to account for the incomplete coverage of the light source. Results. The ionizing spectra responsible for the ionization structure of the NAL systems require an intensity depression at E > 4Ryd which is attributed to the HeII Lyman continuum opacity (tau^HeII_c ~ 1). A most likely source of this opacity is a quasar accretion disk wind. The corresponding column density of HI in the wind is estimated as a few times 10^16 cm^-2. This amount of neutral hydrogen should cause a weak continuum depression at lamb <= 912A (rest-frame), and a broad and shallow absorption in HI Ly-alpha. If metallicity of the wind is high enough, other resonance lines of OVI, NeVI-NeVIII, etc. are expected. In the analyzed QSO spectra we do observe broad (stretching over 1000s km/s) and shallow (tau << 1) absorption troughs of HI Ly-alpha and OVI 1031,1037A...

ITAL]Chandra[/ITAL] Detection of O [CSC]viii[/CSC] L[CLC]y[/CLC]α Absorption from an Overdense Region in the Intergalactic Medium

We report the first detection of an OVIII Ly-alpha absorption line associated with an overdense region in the intergalactic medium (IGM) along the sightline towards PKS 2155-304 with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). The absorption line is detected at 4.5 sigma level with cz~16,600 km/s. At the same velocity Shull et al.(1998) detected a small group of spiral galaxies (with an overdensity of delta_gal ~ 100) and low metallicity HI Ly-alpha clouds. We constrain the intragroup gas that gives rise to the OVIII Ly-alpha line to a baryon density in the range 1e-5 < n_b < 7.5e-5 cm^-3 (50 < delta_b < 350) and a temperature of (4-5) x 10^6 K, assuming 0.1 solar abundance. These estimates are in accordance with those of the warm/hot intergalactic medium (WHIM) that are predicted from hydrodynamic simulations. Extrapolating from this single detection implies a large fraction of the ``missing baryons'' (~ 10%, or ~ 30-40% of the WHIM) are probed by the OVIII absorber.