Quasar Absorption Systems Research Articles

Convergence properties of fine structure constant measurements using quasar absorption systems

ABSTRACT Searches for space–time variations of fundamental constants have entered an era of unprecedented precision. New, high-quality quasar spectra require increasingly refined analytical methods. In this paper, a continuation in a series to establish robust and unbiased methodologies, we explore how convergence criteria in non-linear least-squares optimization impact on quasar absorption system measurements of the fine structure constant α. Given previous claims for high-precision constraints, we critically examine the veracity of a so-called blinding approach, in which α is fixed at the terrestrial value during the model building process, releasing it as a free parameter only after the ‘final’ absorption system kinematic structure has been obtained. We show that this approach results in such small consecutive parameter steps during minimization that convergence is unlikely to be reached, even after as many as 1000 iterations. The fix is straightforward: α must be treated as a free parameter from the earliest possible stages of absorption system model building. The implication of the results presented here is that all previous measurements that have used initially fixed α should be reworked.

Directly constraining the spatial coherence of the z ∼ 1 circumgalactic medium

One of the biggest puzzles regarding the circumgalactic medium (CGM) is the structure of its cool (T ∼ 104 K) gas phase. While the kinematics of quasar absorption systems suggests the CGM is composed of a population of different clouds, constraining their extent and spatial distribution has proven challenging, both from theoretical and observational points of view. In this work, we study the spatial structure of the z ∼ 1 CGM with unprecedented detail via resolved spectroscopy of giant gravitational arcs. We put together a sample of Mg IIλλ2796, 2803 detections obtained with VLT/MUSE in 91 spatially independent and contiguous sight lines toward 3 arcs, each probing an isolated star-forming galaxy believed to be detected in absorption. We constrain the coherence scale of this gas (Clength) – which represents the spatial scale over which the Mg II equivalent width (EW) remains constant – by comparing EW variations measured across all sight lines with empirical models. We find 1.4 < Clength/kpc < 7.8 (95% confidence). This measurement, of unprecedented accuracy, represents the scale over which the cool gas tends to cluster in separate structures. We argue that, if Clength is a universal property of the CGM, it needs to be reproduced by current and future theoretical models in order for us to understand the exact role of this medium in galaxy evolution.

Varying alpha, blinding, and bias in existing measurements

ABSTRACT The high resolution spectrograph ESPRESSO on the VLT allows measurements of fundamental constants at unprecedented precision and hence enables tests for space–time variations predicted by some theories. In a series of recent papers, we developed optimal analysis procedures that expose and eliminate the subjectivity and bias in previous quasar absorption system measurements. In this paper, we analyse the ESPRESSO spectrum of the absorption system at zabs = 1.15 towards the quasar HE 0515-4414. Our goal here is not to provide a new unbiased measurement of Δα/α in this system (that will be done separately). Rather, it is to carefully examine the impact of blinding procedures applied in many previous measurements of the fine structure constant in quasar absorption systems. To do this, we emulate previous procedures, using supercomputer Monte Carlo AI calculations to generate a large number of independently constructed models of the absorption complex. Each model is obtained using ai-vpfit, with Δα/α fixed until a ‘final’ model for the absorption system is obtained, at which point Δα/α is then released as a free parameter for one final optimization. The results show that the value of Δα/α obtained in this way is systematically biased towards the initially fixed value i.e. this process produces measurements that are unrelated to the true value of Δα/α. The implication is straightforward: to avoid bias, all future measurements must include Δα/α as a free parameter from the beginning of the modelling process.

Precision in high resolution absorption line modelling, analytic Voigt derivatives, and optimization methods

ABSTRACT This paper describes the optimization theory on which vpfit, a non-linear least-squares program for modelling absorption spectra, is based. Particular attention is paid to precision. Voigt function derivatives have previously been calculated using numerical finite difference approximations. We show how these can instead be computed analytically using Taylor series expansions and look-up tables. We introduce a new optimization method for an efficient descent path to the best fit, combining the principles used in both the Gauss–Newton and Levenberg–Marquardt algorithms. A simple practical fix for ill-conditioning is described, a common problem when modelling quasar absorption systems. We also summarize how unbiased modelling depends on using an appropriate information criterion to guard against overfitting or underfitting. The methods and the new implementations introduced in this paper are aimed at optimal usage of future data from facilities such as ESPRESSO/VLT and HIRES/ELT, particularly for the most demanding applications such as searches for space–time variations in fundamental constants and attempts to detect cosmological redshift drift.

Non-uniqueness in quasar absorption models and implications for measurements of the fine structure constant

ABSTRACT High resolution spectra of quasar absorption systems provide the best constraints on temporal or spatial changes of fundamental constants in the early Universe. An important systematic that has never before been quantified concerns model non-uniqueness. The absorption structure is generally complicated, comprising many blended lines. This characteristic means any given system can be fitted equally well by many slightly different models, each having a different value of α, the fine structure constant. We use AI Monte Carlo modelling to quantify non-uniqueness. Extensive supercomputer calculations are reported, revealing new systematic effects that guide future analyses: (i) Whilst higher signal to noise and improved spectral resolution produces a smaller statistical uncertainty for α, model non-uniqueness adds a significant additional uncertainty. (ii) Non-uniqueness depends on the line broadening mechanism used. We show that modelling the spectral data using turbulent line broadening results in far greater non-uniqueness, hence this should no longer be done. Instead, for varying α studies, it is important to use the more physically appropriate compound broadening. (iii) We have studied two absorption systems in detail. Generalising thus requires caution. Nevertheless, if non-uniqueness is present in all or most quasar absorption systems, it seems unavoidable that attempts to determine the existence (or non-existence) of spacetime variations of fundamental constants is best approached using a statistical sample.

Artificial intelligence and quasar absorption system modelling; application to fundamental constants at high redshift

ABSTRACT We have developed a new fully automated Artificial Intelligence (AI)-based method for deriving optimal models of complex absorption systems. The AI structure is built around VPFIT, a well-developed and extensively tested nonlinear least-squares code. The new method forms a sophisticated parallelized system, eliminating human decision-making and hence bias. Here, we describe the workings of such a system and apply it to synthetic spectra, in doing so establishing recommended methodologies for future analyses of Very Large Telescope (VLT) and Extremely Large Telescope (ELT) data. One important result is that modelling line broadening for high-redshift absorption components should include both thermal and turbulent components. Failing to do so means it is easy to derive the wrong model and hence incorrect parameter estimates. One topical application of our method concerns searches for spatial or temporal variations in fundamental constants. This subject is one of the key science drivers for the European Southern Observatory’s ESPRESSO spectrograph on the VLT and for the HIRES spectrograph on the ELT. The quality of new data demands completely objective and reproducible methods. The Monte Carlo aspects of the new method described here reveal that model non-uniqueness can be significant, indicating that it is unrealistic to expect to derive an unambiguous estimate of the fine structure constant α from one or a very small number of measurements. No matter how optimal the modelling method, it is a fundamental requirement to use a large sample of measurements to meaningfully constrain temporal or spatial α variation.

The impact of new d(p,γ)3 rates on Big Bang Nucleosynthesis

We consider the effect on Big Bang Nucleosynthesis (BBN) of new measurements of the d(p,γ)3 cross section by the LUNA Collaboration. These have an important effect on the primordial abundance of D/H which is also sensitive to the baryon density at the time of BBN. We have re-evaluated the thermal rate for this reaction, using a world average of cross section data, which we describe with model-independent polynomials; our results are in good agreement with a similar analysis by LUNA.We then perform a full likelihood analysis combining BBN and Planck cosmic microwave background (CMB) likelihood chains using the new rate combined with previous measurements and compare with the results using previous rates. Concordance between BBN and CMB measurements of the anisotropy spectrum using the old rates was excellent. The predicted deuterium abundance at the Planck value of the baryon density was (D/H)BBN+CMB old = (2.57 ± 0.13) × 10−5 which can be compared with the value determined from quasar absorption systems (D/H)obs = (2.55 ± 0.03) × 10−5. Using the new rates we find (D/H)BBN+CMB = (2.51 ± 0.11) × 10−5.We thus find consistency among BBN theory, deuterium and 4 observations, and the CMB, when using reaction rates fit in our data-driven approach. We also find that the new reaction data tightens the constraints on the number of relativistic degrees of freedom during BBN, giving the effective number of light neutrino species Nν = 2.880 ± 0.144 in good agreement with the Standard Model of particle physics.Finally, we note that the observed deuterium abundance continues to be more precise than the BBN+CMB prediction, whose error budget is now dominated by d(d,n)3 and d(d,p)3 H. More broadly, it is clear that the details of the treatment of nuclear reactions and their uncertainty have become critical for BBN.

Improved Ni II oscillator strengths from quasar absorption systems

Aims. We wish to improve the accuracy of oscillator strength values for several Ni II UV transitions and measure for the first time the f-value of a few other weak transitions for which no laboratory nor astronomical measurement is presently available. Methods. Four quasars displaying five damped Lyman α systems with relatively strong Ni II lines were selected. From the analysis of the excellent high resolution spectra available, we determined the relative f-value of Ni II transitions by comparing the strength of the corresponding absorption profiles. To quantify the latter, we used the apparent optical depth method for resolved features, equivalent width measurements for optically thin lines and line fitting with VPFIT. Absolute f-values were then derived by relating our determinations to the available laboratory measurements. Results. Thanks to the good signal-to-noise ratio of the spectra and to the suitable properties of the absorption systems investigated, we are able to significantly improve the determination of the f-value for 13 Ni II transitions falling in the 1317–1804 Å interval. Our results are found to be consistent with other earlier determinations for ten of these transitions; our median relative accuracy for these f-values is 6.5%. For three weak transitions near 1502, 1773, and 1804 Å, which have not been detected previously in astronomical spectra, we can get a first measurement of their f-value. Conclusions. Our work illustrates that, thanks to the redshift and the absence of variations of physical constants on cosmological scales, the analysis of absorption lines induced by remote gas in quasar spectra can nowadays provide valuable constraints on atomic data in the UV range.

Radio wave scattering by circumgalactic cool gas clumps

We consider the effects of radio-wave scattering by cool ionized clumps ($T\sim 10^4\,$K) in circumgalactic media (CGM). The existence of such clumps are inferred from intervening quasar absorption systems, but have long been something of a theoretical mystery. We consider the implications for compact radio sources of the `fog-like' two-phase model of the circumgalactic medium recently proposed by McCourt et al.(2018). In this model, the CGM consists of a diffuse coronal gas ($T\gtrsim 10^6\,$K) in pressure equilibrium with numerous $\lesssim 1\,$pc scale cool clumps or `cloudlets' formed by shattering in a cooling instability. The areal filling factor of the cloudlets is expected to exceed unity in $\gtrsim 10^{11.5} M_\odot$ haloes, and the ensuing radio-wave scattering is akin to that caused by turbulence in the Galactic warm ionized medium (WIM). If $30\,$per-cent of cosmic baryons are in the CGM, we show that for a cool-gas volume fraction of $f_{\rm v}\sim 10^{-3}$, sources at $z_{\rm s}\sim 1$ suffer angular broadening by $\sim 15\,\mu$as and temporal broadening by $\sim 1\,$ms at $\lambda = 30\,$cm, due to scattering by the clumps in intervening CGM. The former prediction will be difficult to test (the angular broadening will suppress Galactic scintillation only for $<10\,\mu$Jy compact synchrotron sources). However the latter prediction, of temporal broadening of localized fast radio bursts, can constrain the size and mass fraction of cool ionized gas clumps as function of halo mass and redshift, and thus provides a test of the model proposed by McCourt et al.(2018).

Spotting high-z molecular absorbers using neutral carbon

While molecular quasar absorption systems provide unique probes of the physical and chemical properties of the gas as well as original constraints on fundamental physics and cosmology, their detection remains challenging. Here we present the results from a complete survey for molecular gas in thirty-nine absorption systems selected solely upon the detection of neutral carbon lines in Sloan Digital Sky Survey (SDSS) spectra, without any prior knowledge of the atomic or molecular gas content. H2 is found in all twelve systems (including seven new detections) where the corresponding lines are covered by the instrument setups and measured to have logN(H2) ≳ 18, indicating a self-shielded regime. We also report seven CO detections (7/39) down to logN(CO) ~ 13.5, including a new one, and put stringent constraints on N(CO) for the remaining 32 systems. N(CO) and N(C I) are found to be strongly correlated with N(CO)/N(C I) ~ 1/10. This suggests that the C I-selected absorber population is probing gas deeper than the H I–H2 transition in which a substantial fraction of the total hydrogen in the cloud is in the form of H2. We conclude that targeting C I-bearing absorbers is a very efficient way to find high-metallicity molecular absorbers. However, probing the molecular content in lower-metallicity regimes as well as high-column-density neutral gas remains to be undertaken to unravel the processes of gas conversion in normal high-z galaxies.

Extent and structure of intervening absorbers from absorption lines redshifted on quasar emission lines

We wish to study the extent and subparsec scale spatial structure of intervening quasar absorbers, mainly those involving neutral and molecular gas. We have selected quasar absorption systems with high spectral resolution and good S/N data, with some of their lines falling on quasar emission features. By investigating the consistency of absorption profiles seen for lines formed either against the quasar continuum source or on the much more extended emission line region (ELR), we can probe the extent and structure of the foreground absorber over the extent of the ELR (0.3-1 pc). The spatial covering analysis provides constraints on the transverse size of the absorber and thus is complementary to variability or photoionisation modelling studies. The methods we used to identify spatial covering or structure effects involve line profile fitting and curve of growth analysis.We have detected three absorbers with unambiguous non uniformity effects in neutral gas. For one extreme case, the FeI absorber at z_abs=0.45206 towards HE 0001-2340, we derive a coverage factor of the ELR of at most 0.10 and possibly very close to zero; this implies an absorber overall size no larger than 0.06 pc. For the z_abs=2.41837 CI absorber towards QSO J1439+1117, absorption is significantly stronger towards the ELR than towards the continuum source in several CI and CI* velocity components pointing to factors of about two spatial variations of their column densities and the presence of structures at the 100 au - 0.1 pc scale. The other systems with firm or possible effects can be described in terms of partial covering of the ELR, with coverage factors in the range 0.7 - 1. The overall results for cold, neutral absorbers imply a transverse extent of about five times or less the ELR size, which is consistent with other known constraints.

Modelling long-range wavelength distortions in quasar absorption echelle spectra

Spectra observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the European Southern Observatory's VLT exhibit long-range wavelength distortions. These distortions impose a systematic error on high-precision measurements of the fine-structure constant, $\alpha$, derived from intervening quasar absorption systems. If the distortion is modelled using a model that is too simplistic, the resulting bias in $\Delta\alpha/\alpha$ away from the true value can be larger than the statistical uncertainty on the $\alpha$ measurement. If the effect is ignored altogether, the same is true. If the effect is modelled properly, accounting for the way in which final spectra are generally formed from the co-addition of exposures made at several different instrumental settings, the effect can be accurately removed and the correct $\Delta\alpha/\alpha$ recovered.

High-precision limit on variation in the fine-structure constant from a single quasar absorption system

The brightest southern quasar above redshift z = 1, HE 0515−4414, with its strong intervening metal absorption line system at zabs = 1.1508, provides a unique opportunity to precisely measure or limit relative variations in the fine-structure constant (Δα/α). A variation of just ∼3 parts per million (ppm) would produce detectable velocity shifts between its many strong metal transitions. Using new and archival observations from the Ultraviolet and Visual Echelle Spectrograph (UVES), we obtain an extremely high signal-to-noise ratio spectrum (peaking at S/N ≈ 250 pix−1). This provides the most precise measurement of Δα/α from a single absorption system to date, Δα/α = −1.42 ± 0.55stat ± 0.65sys ppm, comparable with the precision from previous, large samples of ∼150 absorbers. The largest systematic error in all (but one) previous similar measurements, including the large samples, was long-range distortions in the wavelength calibration. These would add an ∼2 ppm systematic error to our measurement and up to ∼10 ppm to other measurements using Mg and Fe transitions. However, we corrected the UVES spectra using well-calibrated spectra of the same quasar from the High Accuracy Radial velocity Planet Searcher, leaving a residual 0.59 ppm systematic uncertainty, the largest contribution to our total systematic error. A similar approach, using short observations on future well-calibrated spectrographs to correct existing high S/N spectra, would efficiently enable a large sample of reliable Δα/α measurements. The high-S/N UVES spectrum also provides insights into analysis difficulties, detector artefacts and systematic errors likely to arise from 25–40-m telescopes.

Constraints on a possible variation of the fine structure constant from galaxy cluster data

We propose a new method to probe a possible time evolution of the fine structure constant α from X-ray and Sunyaev-Zel'dovich measurements of the gas mass fraction (fgas) in galaxy clusters. Taking into account a direct relation between variations of α and violations of the distance-duality relation, we discuss constraints on α for a class of dilaton runaway models. Although not yet competitive with bounds from high-z quasar absorption systems, our constraints, considering a sample of 29 measurements of fgas, in the redshift interval 0.14 < z < 0.89, provide an independent estimate of α variation at low and intermediate redshifts. Furthermore, current and planned surveys will provide a larger amount of data and thus allow to improve the limits on α variation obtained in the present analysis.

A new analysis of fine-structure constant measurements and modelling errors from quasar absorption lines

We present an analysis of 23 absorption systems along the lines of sight towards 18 quasars in the redshift range of $0.4 \leq z_{abs} \leq 2.3$ observed on the Very Large Telescope (VLT) using the Ultraviolet and Visual Echelle Spectrograph (UVES). Considering both statistical and systematic error contributions we find a robust estimate of the weighted mean deviation of the fine-structure constant from its current, laboratory value of $\Delta\alpha/\alpha=\left(0.22\pm0.23\right)\times10^{-5}$, consistent with the dipole variation reported in Webb et al. and King et al. This paper also examines modelling methodologies and systematic effects. In particular we focus on the consequences of fitting quasar absorption systems with too few absorbing components and of selectively fitting only the stronger components in an absorption complex. We show that using insufficient continuum regions around an absorption complex causes a significant increase in the scatter of a sample of $\Delta\alpha/\alpha$ measurements, thus unnecessarily reducing the overall precision. We further show that fitting absorption systems with too few velocity components also results in a significant increase in the scatter of $\Delta\alpha/\alpha$ measurements, and in addition causes $\Delta\alpha/\alpha$ error estimates to be systematically underestimated. These results thus identify some of the potential pitfalls in analysis techniques and provide a guide for future analyses.

Connecting the Interstellar Gas and Dust Properties in Distant Galaxies Using Quasar Absorption Systems

AbstractGas and dust grains are fundamental components of the interstellar medium and significantly impact many of the physical processes driving galaxy evolution, such as star-formation, and the heating, cooling, and ionization of the interstellar material. Quasar absorption systems (QASs), which trace intervening galaxies along the sightlines to luminous quasars, provide a valuable tool to directly study the properties of the interstellar gas and dust in distant, normal galaxies. We have established the presence of silicate dust grains in at least some gas-rich QASs, and find that they exist at higher optical depths than expected for diffuse gas in the Milky Way. Differences in the absorption feature shapes additionally suggest variations in the silicate dust grain properties, such as in the level of grain crystallinity, from system-to-system. We present results from a study of the gas and dust properties of QASs with adequate archival IR data to probe the silicate dust grain properties. We discuss our measurements of the strengths of the 10 and 18 μm silicate dust absorption features in the QASs, and constraints on the grain properties (e.g., composition, shape, crystallinity) based on fitted silicate profile templates. We investigate correlations between silicate dust abundance, reddening, and gas metallicity, which will yield valuable insights into the history of star formation and chemical enrichment in galaxies.

CONSTRAINING THE VARIATION OF THE FINE-STRUCTURE CONSTANT WITH OBSERVATIONS OF NARROW QUASAR ABSORPTION LINES

Attempts to measure the variability of the fine structure constant alpha over cosmological time, using spectra of high redshift quasars have produced conflicting results. We use the Many Multiplet (MM) method with Mg II and Fe II lines on very high signal-to-noise, high resolution (R = 72,000) Keck HIRES spectra of eight narrow quasar absorption systems and consider both systematic uncertainties in spectrograph wavelength calibration and also velocity offsets introduced by internal velocity structure. We find no significant change in alpha, Delta(alpha)/alpha =(0.43 +/- 0.34)x 10^(-5), in the redshift range z = 0.7 - 1.5 (statistical and systematic). Scatter in measurements of Delta(alpha)/alpha arising from absorption line structure can be considerably larger than assigned statistical errors even for apparently simple and narrow absorption systems. We find a null result of Delta(alpha)/alpha = (-0.59 +/- 0.55) x 10^(-5) in a system at z = 1.7382 using Cr II, Zn II and Mn II, whereas using Cr II and Zn II in a system at z = 1.6614 we find a systematic velocity trend which, interpreted as a shift in alpha, would imply Delta(alpha)/alpha = (1.88 +/- 0.47) x 10^(-5) (statistical plus systematic). This latter result is almost certainly caused by varying ionic abundances in subcomponents of the line. We conclude that spectroscopic measurements of quasar absorption lines are not yet capable of unambiguously detecting variation in alpha using the MM method.

INTERSTELLAR SILICATE DUST IN THEz= 0.685 ABSORBER TOWARD TXS 0218+357

We report the detection of interstellar silicate dust in the z_abs=0.685 absorber along the sightline toward the gravitationally lensed blazar TXS 0218+357. Using Spitzer Space Telescope Infrared Spectrograph data we detect the 10 micron silicate absorption feature with a detection significance of 10.7-sigma. We fit laboratory-derived silicate dust profile templates obtained from literature to the observed 10 micron absorption feature, and find that the best single-mineral fit is obtained using an amorphous olivine template with a measured peak optical depth of tau_10=0.49+/-0.02, which rises to tau_10~0.67+/-0.04 if the covering factor is taken into account. We also detected the 18 micron silicate absorption feature in our data with a >3-sigma significance. Due to the proximity of the 18 micron absorption feature to the edge of our covered spectral range, and associated uncertainty about the shape of the quasar continuum normalization near 18 micron, we do not independently fit this feature. We find, however, that the shape and depth of the 18 micron silicate absorption are well-matched to the amorphous olivine template prediction, given the optical depth inferred for the 10 micron feature. The measured 10 micron peak optical depth in this absorber is significantly higher than those found in previously studied quasar absorption systems. The reddening, 21-cm absorption, and velocity spread of Mg II are not outliers relative to other studied absorption systems, however. This high optical depth may be evidence for variations in dust grain properties in the ISM between this and the previously studied high redshift galaxies.

Extremely metal-poor gas at a redshift of 7

In typical astrophysical environments, the abundance of heavy elements ranges from 0.001 to 2 times the solar value. Lower abundances have been seen in selected stars in the Milky Way's halo and in two quasar absorption systems at redshift z = 3 (ref. 4). These are widely interpreted as relics from the early Universe, when all gas possessed a primordial chemistry. Before now there have been no direct abundance measurements from the first billion years after the Big Bang, when the earliest stars began synthesizing elements. Here we report observations of hydrogen and heavy-element absorption in a spectrum of a quasar at z = 7.04, when the Universe was just 772 million years old (5.6 per cent of its present age). We detect a large column of neutral hydrogen but no corresponding metals (defined as elements heavier than helium), limiting the chemical abundance to less than 1/10,000 times the solar level if the gas is in a gravitationally bound proto-galaxy, or to less than 1/1,000 times the solar value if it is diffuse and unbound. If the absorption is truly intergalactic, it would imply that the Universe was neither ionized by starlight nor chemically enriched in this neighbourhood at z ≈ 7. If it is gravitationally bound, the inferred abundance is too low to promote efficient cooling, and the system would be a viable site to form the predicted but as yet unobserved massive population III stars.

Higher D or Li: probes of physics beyond the standard model

Standard Big Bang Nucleosynthesis at the baryon density determined by the microwave anisotropy spectrum predicts an excess of \li7 compared to observations by a factor of 4-5. In contrast, BBN predictions for D/H are somewhat below (but within ~2 \sigma) of the weighted mean of observationally determined values from quasar absorption systems. Solutions to the \li7 problem which alter the nuclear processes during or subsequent to BBN, often lead to a significant increase in the deuterium abundance consistent with the highest values of D/H seen in absorption systems. Furthermore, the observed D/H abundances show considerable dispersion. Here, we argue that those systems with D/H \simeq 4 \times 10^{-5} may be more representative of the primordial abundance and as a consequence, those systems with lower D/H would necessarily have been subject to local processes of deuterium destruction. This can be accounted for by models of cosmic chemical evolution able to destroy in situ Deuterium due to the fragility of this isotope.