Uffe Hellsten

The X-Ray Forest: A New Prediction of Hierarchical Structure Formation Models

We use numerical simulations of structure formation in a cold dark matter model to predict the absorption lines in the soft X-rays produced by heavy elements in the shock-heated intergalactic medium at low redshift. The simulation incorporates a model for heavy-element production in galaxies and the subsequent dispersion of the metals to the intergalactic medium. We analyze in particular absorption lines produced by oxygen and calculate the ionization stage, taking into account the observed X-ray background at the present time. We find that oxygen is fully ionized by the X-ray background in low-density voids and is mostly in the form of O VII and O VIII in the sheets and filamentary regions. Strong absorption lines of O VII and O VIII with equivalent widths W ~ 100 km s-1 are produced in filamentary regions of overdensities of ~100 and temperatures of ~106 K, located in the outskirts of groups and clusters of galaxies. The O VII line at E = 574 eV is generally the strongest one in these systems. Our model predicts that any X-ray source (such as a quasar) should typically show about one O VII absorption line with W > 100 km s-1 in the interval from z = 0 to z = 0.3. These lines could be detected with the upcoming generation of X-ray telescopes, and their origin in intervening systems could be confirmed by the association with groups of galaxies and X-ray-emitting halos near the line of sight at the same redshift. The hot intergalactic medium may be one of the main reservoirs of baryons in the present universe, and the heavy-element X-ray absorption lines offer a promising possibility of detecting this new component in the near future.

Constraining the metallicity of the low-density Lyα forest using O VI absorption

We present a systematic search for O VI (1032, 1037 A) absorption in a Keck High-Resolution Echelle Spectrometer spectrum of the z = 3.62 quasar Q1422+231, with the goal of constraining the metallicity and ionization state of the low-density intergalactic medium (IGM). Comparison of C IV absorption measurements to models of the Lyα forest based on cosmological simulations shows that absorbers with NH I 1014.5 cm-2 have a mean carbon abundance [C/H] ≈ -2.5, assuming a metagalactic photoionizing background with the spectral shape predicted by Haardt and Madau in 1996. In these models, lower column density absorption arises in lower density gas where most C IV is photoionized to C V. Therefore, O VI should be the most sensitive tracer of metallicity in Lyα absorbers with NH I 1014.5 cm-2. O VI lines lie at wavelengths heavily contaminated by Lyman series absorption, so we interpret the search results by comparing them to carefully constructed, mock Q1422 spectra drawn from a hydrodynamic simulation of a Λ-dominated cold dark matter model. A search for deep, narrow absorption features yields only a few candidate O VI lines in the spectrum of Q1422. H I absorption blankets the position of the doublet companion line in each case, and the total number of narrow lines is statistically consistent with that in zero-metallicity artificial spectra. Artificial spectra generated with the Haardt and Madau background and [O/H] -2.5 predict too many narrow lines and are statistically inconsistent with the data. We also search for O VI associated with C IV systems, using the optical depth ratio technique of Songaila. With this method we do find significant O VI absorption; matching the data requires [O/C] ≈ 0.5 and corresponding [O/H] ≈ -2.0. Taken together, the narrow-line and optical depth ratio results imply that (1) the metallicity in the low-density regions of the IGM is at least a factor of 3 below that in the overdense regions where C IV absorption is detectable, and (2) oxygen is overabundant in the C IV regions, consistent with the predictions of Type II supernova-enrichment models and the observed abundance pattern in old halo stars. The photoionizing background spectrum would be truncated above 4 Ryd in regions that have not undergone helium reionization (He II → He III), and in this case matching the Q1422 data requires lower [C/H] but higher [O/H]. Taking [O/C] ≈ 1 as the maximum plausible overabundance of oxygen, we conclude that helium must have been reionized through at least 50% of the volume from z ~ 3-3.6.

Metal Lines Associated with Lyα Absorbers: A Comparison of Theory and Observations

We study the metal-line absorption of C IV, C II, Si IV, and N V at redshifts z = 3.5 to z = 2 within the framework of a cosmological model for the Lyα forest, comparing the results of numerical simulations to recent observations by Songaila & Cowie (SC). We find that the observed mean value of the C IV/H I ratio at z 3 is reproduced if a uniform metallicity of [C/H] ~ -2.5 is assumed in our model, but that the observed scatter in this ratio is larger than predicted, implying a scatter of roughly an order of magnitude in the metallicity of the absorbing systems. The enrichment pattern that is required for our model to match SCs C IV observations is very similar to that predicted by other simulations of reionization and early metal enrichment in this type of cosmological scenario. Our model predicts no significant evolution in the mean values of metal-line column densities between z = 3.5 and z = 2. Comparison of the predicted and observed numbers of Si IV and N V systems suggests that the photoionizing background radiation field at z ~ 3 is somewhat softer than that proposed earlier. Our model does not account for the increase in the Si IV/C IV ratio at z 3.2 found by SC. While the latter study suggested that the increase could be explained by a softening of the radiation field caused by He II absorption at z 3, such a modification does not raise the mean value of Si IV/C IV significantly in our simulation, because it shifts numerous weak Si IV systems to just above the detection limit, thus keeping the mean column density of observable Si IV systems low.

The Observability of Metal Lines Associated with the Lyα Forest

We develop a prescription for characterizing the strengths of metal lines associated with Lyα forest absorbers (LYFAs) of a given neutral hydrogen column density NH I and metallicity [O/H]. This line observability index (LOX) is line specific and translates, for weak lines, into a measure of the equivalent width. It can be evaluated quickly for thousands of transitions within the framework of a given model of the Lyα forest, providing a ranking of the absorption lines in terms of their strengths and enabling model builders to select the lines that deserve more detailed consideration, i.e., those that should be detectable in observed spectra of a given resolution and signal-to-noise ratio. We compute the LOX for a large number of elements and transitions in two cosmological models of the Lyα forest at z ~ 3 derived from hydrodynamic simulations of structure formation. We present results for a cold dark matter universe with a cosmological constant; an Ω = 1 cold dark matter model yields nearly identical results, and we argue more generally that the LOX predictions are insensitive to the specific choice of cosmology. We also discuss how the LOX depends on redshift and on model parameters such as the mean baryonic density and radiation field. We find that the O VI (λλ1032, 1038) doublet is the best probe of the metallicity in low column density LYFAs (NH I ≈ 1014.5 cm-2). Metallicities down to [O/H] ~ -3 yield O VI absorption features that should be detectable in current high-quality spectra, provided that the expected position of the O VI feature is not contaminated by H I absorption. The strongest transitions in lower ionization states of oxygen are O V (λ630), O IV (λ788), and O III (λ833). These absorption lines are all predicted to be stronger than the O VI feature, but even at redshifts 3-4 they will have to be observed in the ultraviolet, and they are extremely difficult to detect with present UV instruments, such as the Space Telescope Imaging Spectrograph (STIS). At lower redshifts, detection of these lines may be possible in STIS spectra of the very brightest QSOs, while one may have to wait for next-generation instruments such as the Cosmic Origins Spectrograph (COS) to detect such lines in a number of high-redshift QSOs. The strongest metal lines with rest frame wavelength larger than 912 A associated with higher column density LYFAs at z ~ 3 are C III (λ977) and Si III (λ1206.5), which peak at NH I ~ 1017 cm-2. Of the lines with rest wavelengths λr > 1216 A, which can potentially be observed redward of the Lyα forest, the C IV (λλ1548, 1551) doublet is expected to dominate in all LYFAs, regardless of the value of NH I. We argue that C IV and C II absorption may peak in different spatial regions, and that comparison of single-phase models of the C IV/C II ionization ratios with observed C IV/C II column density ratios can lead to an overestimate of the ionization parameter in the central parts of the absorbers.

The Structure of Isothermal, Self-Gravitating, Stationary Gas Spheres for Softened Gravity

A theory for the structure of isothermal, self-gravitating gas spheres in pressure equilibrium is developed for softened gravity, assuming an ideal gas equation of state. The one-parameter spline softening proposed by Hernquist & Katz is used. We show that the addition of this extra scale parameter implies that the set of equilibrium solutions constitute a one-parameter family, rather than the one and only one isothermal sphere solution for Newtonian gravity, and we develop a number of approximate, analytical or semianalytical solutions. For softened gravity, the structure of isothermal spheres is, in general, very different from the Newtonian isothermal sphere. For example, for any finite choice of softening length and temperature T, it is possible to deposit an arbitrarily large mass of gas in pressure equilibrium and with a nonsingular density distribution inside of r0 for any r0 > 0. Furthermore, it is sometimes claimed that the size of the small-scale, self-gravitating gas structures formed in dissipative Tree-SPH simulations is simply of the order the gravitational softening length. We demonstrate, that this claim, in general, is not correct. The main purpose of the paper is to compare the theoretical predictions of our models with the properties of the small, massive, quasi-isothermal gas clumps (r ~ 1 kpc, M ~ 1010 M?, and T 104 K) which form in numerical Tree-SPH simulations of the formation of Milky Way-sized galaxies when effects of stellar feedback processes are not included. We find reasonable agreement, despite the neglect of effects of rotational support in the models presented in this paper. We comment on whether the hydrodynamical resolution is sufficient in our numerical simulations of galaxy formation involving highly supersonic, radiative shocks, and we give a necessary condition, in the form of a simple test, that the hydrodynamical resolution in any such simulations is sufficient. Finally, we conclude that one should be cautious, when comparing results of numerical simulations, involving gratitational softening and hydrodynamical smoothing, with reality.

On the global structure of self-gravitating discs for softened gravity

Effects of gravitational softening on the global structure of self-gravitating disks in centrifugal equillibrium are examined in relation to hydrodynamical/gravitational simulations. The one-parameter spline softening proposed by Hernquist&Katz is used. It is found that if the characteristic size of a disk, r, is comparable or less than the gravitational softening length, epsilon, then the cross section of the simulated disk is significantly larger than that of a no-softening (Newtonian) disk with the same mass and angular momentum. We furthermore demonstrate that if r is less than or about epsilon/2 then the scaling relation r proportional to epsilon^3/4 holds for a given mass and specific angular momentum distribution with mass. Finally we compare some of the theoretical results obtained in this and a previous paper with the results of numerical Tree-SPH simulations and find qualitative agreement.