Quasar Absorption Spectra Research Articles

Fine structure constant variation or spacetime anisotropy?

Recent observations on the quasar absorption spectra supply evidence for the variation of the fine structure constant α. In this paper, we propose another interpretation of the observational data on the quasar absorption spectra: a scenario with spacetime inhomogeneity and anisotropy. Maybe the spacetime is characterized by the Finsler geometry instead of the Riemann one. The Finsler geometry admits fewer symmetries than the Riemann geometry does. We investigate the Finslerian geodesic equations in the Randers spacetime (a special Finsler spacetime). It is found that the cosmological redshift in this spacetime deviates from the one in general relativity. The modification term to the redshift could be generally revealed as a monopole plus dipole function of spacetime locations and directions. We suggest that this modification corresponds to the spatial monopole and dipole of α variation in the quasar absorption spectra.

Isotope shifts and relativistic shifts of Cr ii for the study ofαvariation in quasar absorption spectra

We use the combination of configuration interaction and many-body perturbation theory method (CI+MBPT) to perform ab initio calculations the low-energy spectra of Cr II with high accuracy. It is found that second-order MBPT diagrams should be included in a consistent and complete way for the MBPT to improve the accuracy of calculations in this five-valence-electron system. This contrasts with previous ions with fewer valence electrons where it was found that single-valence-electron diagrams dominate the corrections. Isotope shifts and relativistic shifts (q-values) are calculated for use in astronomical determination of the fine-structure constant in quasar absorption spectra.

Multiscale inference of matter fields and baryon acoustic oscillations from the Lyα forest

We present a novel Bayesian method for the joint reconstruction of cosmological matter density fields, peculiar velocities and power-spectra in the quasi-nonlinear regime. We study its applicability to the Ly-alpha forest based on multiple quasar absorption spectra. Our approach to this problem includes a multiscale, nonlinear, two-step scheme since the statistics describing the matter distribution depends on scale, being strongly non-Gaussian on small scales (< 0.1 h^{-1} Mpc) and closely lognormal on scales >~10 h^{-1} Mpc. The first step consists on performing 1D highly resolved matter density reconstructions along the line-of-sight towards z~2-3 quasars based on an arbitrary non-Gaussian univariate model for matter statistics. The second step consists on Gibbs-sampling based on conditional PDFs. The matter density field is sampled in real space with Hamiltonian-sampling using the Poisson/Gamma-lognormal model, while redshift distortions are corrected with linear Lagrangian perturbation theory. The power-spectrum of the lognormal transformed variable which is Gaussian distributed (and thus close to the linear regime) can consistently be sampled with the inverse Gamma distribution function. We test our method through numerical N-body simulations with a computational volume large enough (> 1 h^{-3} Gpc^3) to show that the linear power-spectra are nicely recovered over scales larger than >~20 h^{-1} Mpc, i.e. the relevant range where features imprinted by the baryon-acoustics oscillations (BAOs) appear.

A ground-based imaging study of galaxies causing damped Lyman α (DLA), sub-DLA and Lyman limit system absorption in quasar spectra★

We present results from a search for galaxies that give rise to damped Lyman alpha (DLA), subDLA, and Lyman limit system (LLS) absorption at redshifts 0.1 ~< z ~< 1 in the spectra of background quasars. The sample was formed from a larger sample of strong MgII absorbers (W_0^(2796) >= 0.3 A) whose HI column densities were determined by measuring the Ly-alpha line in HST UV spectra. Photometric redshifts, galaxy colors, and proximity to the quasar sightline, in decreasing order of importance, were used to identify galaxies responsible for the absorption. Our sample includes 80 absorption systems for which the absorbing galaxies have been identified, of which 54 are presented here for the first time. The main results of this study are: (i) the surface density of galaxies falls off exponentially with increasing impact parameter, b, from the quasar sightline relative to a constant background of galaxies, with an e-folding length of ~46 kpc. Galaxies with b >~ 100 kpc calculated at the absorption redshift are statistically consistent with being unrelated to the absorption system. (ii) log N(HI) is inversely correlated with b at the 3.0 sigma level of significance. DLA galaxies are found systematically closer to the quasar sightline, by a factor of two, than are galaxies which give rise to subDLAs or LLSs. The median impact parameter is 17.4 kpc for the DLA galaxy sample, 33.3 kpc for the subDLA sample, and 36.4 kpc for the LLS sample. (iii) Absorber galaxy luminosity relative to L*, L/L*, is not significantly correlated with W_0^(2796), log N(HI), or b. (iv) DLA, subDLA, and LLS galaxies comprise a mix of spectral types, but are inferred to be predominantly late type galaxies based on their spectral energy distributions. The implications of these results are discussed. (Abridged)

Is there further evidence for spatial variation of fundamental constants?

The detection of a spatial variation of the fine-structure constant, alpha, based on study of quasar absorption systems has recently been reported. The physics that causes this alpha-variation should have other observable manifestations, and this motivates us to look for complementary astrophysical effects. In this paper we propose a method to test whether spatial variation of fundamental constants existed during the epoch of big bang nucleosynthesis. Using existing measurements of primordial deuterium abundance we find very weak indications that such a signature might exist, but the paucity of measurements precludes any firm conclusion. We also examine existing quasar absorption spectra data that are sensitive to variation of the electron-to-proton mass ratio, mu, and x = (alpha^2 mu g_p) for spatial variation.

Cosmic density field reconstruction from Lyα forest data

Abstract We present a novel, fast method to recover the density field through the statistics of the transmitted flux in high-redshift quasar absorption spectra. The proposed technique requires the computation of the probability distribution function of the transmitted flux (PF) in the Lyαforest region and, as a sole assumption, the knowledge of the probability distribution function of the matter density field (PΔ). We show that the probability density conservation of the flux and matter density unveils a flux–density (F–Δ) relation which can be used to invert the Lyαforest without any assumption on the physical properties of the intergalactic medium. We test our inversion method atz= 3through the following steps: (i) simulation of a sample of synthetic spectra for whichPΔis known; (ii) computation ofPF; and (iii) inversion of the Lyαforest through theF–Δrelation. Our technique, when applied to only 10 observed spectra characterized by a signal-to-noise ratio≥100, provides an exquisite (relative errorεΔ≲ 12per cent in≳50 per cent of the pixels) reconstruction of the density field in≳90 per cent of the line of sight. We finally discuss strengths and limitations of the method.

Varying Constants, Gravitation and Cosmology.

Fundamental constants are a cornerstone of our physical laws. Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. Thus, it is of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy. We detail the relations between the constants, the tests of the local position invariance and of the universality of free fall. We then review the main experimental and observational constraints that have been obtained from atomic clocks, the Oklo phenomenon, solar system observations, meteorite dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic microwave background and big bang nucleosynthesis. At each step we describe the basics of each system, its dependence with respect to the constants, the known systematic effects and the most recent constraints that have been obtained. We then describe the main theoretical frameworks in which the low-energy constants may actually be varying and we focus on the unification mechanisms and the relations between the variation of different constants. To finish, we discuss the more speculative possibility of understanding their numerical values and the apparent fine-tuning that they confront us with.

The impact of spatial fluctuations in the ultraviolet background on intergalactic carbon and silicon

Spatial inhomogeneities in the spectral shape of the ultra-violet background (UVB) at the tail-end of HeII reionisation are thought to be the primary cause of the large fluctuations observed in the HeII to HI Ly-a forest optical depth ratio, tau_HeII/tau_HI, at z~2-3. These spectral hardness fluctuations will also influence the ionisation balance of intergalactic metals; we extract realistic quasar absorption spectra from a large hydrodynamical simulation to examine their impact on intergalactic SiIV and CIV absorbers. Using a variety of toy UVB models, we find that while the predicted spatial inhomogeneities in spectral hardness have a significant impact on tau_HeII/tau_HI, the longer mean free path for photons with frequencies above and below the HeII ionisation edge means these fluctuations have less effect on the SiIV and CIV ionisation balance. Furthermore, UVB models which produce the largest fluctuations in specific intensity at the HeII ionisation edge also have the softest ionising spectra, and thus result in photo-ionisation rates which are too low to produce significant fluctuations in the observed tau_SiIV/tau_CIV. Instead, we find spatial variations in the IGM metallicity will dominate any scatter in tau_SiIV/tau_CIV. Our results suggest that observational evidence for homogeneity in the observed tau_SiIV/tau_CIV distribution does not rule out the possibility of significant fluctuations in the UVB spectral shape at z~2-3. On the other hand, the scatter in metallicity inferred from observations of intergalactic CIV and SiIV absorption at z~2-3 using spatially uniform ionisation corrections is likely intrinsic, and therefore provides a valuable constraint on intergalactic metal enrichment scenarios at these redshifts.

What can we learn from quasar absorption spectra?

We analyze optical-near infrared spectra of a large sample of quasars at high redshift with the aim of investigating both the cosmic reionization history at z ~ 6 and the properties of dust extinction at z > 4. In order to constrain cosmic reionization, we study the transmitted flux in the region blueward the Lya emission line in a sample of 17 quasars spectra at 5.7 ≤ zem ≤ 6.4. By comparing the properties of the observed spectra with the results of a semi-analytical model of the Lyα forest we find that actual data favor a model in which the Universe is highly ionized at z ∼ 6, thus being consistent with an epoch of reionization at higher redshifts. For what concerns the study of the high-z dust, we focus our attention on the region redward the Lya emission line of 33 quasars at 4 ≤ zem ≤ 6.4. We compute simulated dust-absorbed quasar spectra by taking into account a large grid of extinction curves. We find that the SMC extinction curve, which has been shown to reproduce the dust reddening of most quasars at z < 4, is not a good prescription for describing dust extinction also at higher redshifts.

Astronomical and laboratory searches for space-time variation of fundamental constants

We review recent activity searching for variations in the fundamental constants of nature in quasar absorption spectra and in the laboratory. While research in this direction has been ongoing for many decades, the topic has recently been stimulated by astronomical evidence for spatial variation in the fine-structure constant, α. This result could be confirmed using different quasar data and atomic clock measurements, but there are significant challenges to obtain the required accuracy. We review existing measurements and discuss some of the most promising systems where any variations would be strongly enhanced.

GMRT observation towards detecting the post-reionization 21-cm signal

The redshifted 21-cm signal from neutral hydrogen (H i) is an important future probe of the high-redshift Universe. We have analysed 610 MHz Giant Metrewave Radio Telescope (GMRT) observations towards detecting this signal from z= 1.32. The multi-frequency angular power spectrum Cℓ(Δν) is used to characterize the statistical properties of the background radiation across angular scales ∼20 arcsec to 10 arcmin, and a frequency bandwidth of 7.5 MHz with resolution 125 kHz. The measured Cℓ(Δν) which ranges from 7 to 18 mK2 is dominated by foregrounds, the expected H i signal CHIℓ (Δν) ∼ 10−6 to 10−7 mK2 is several orders of magnitude smaller and detecting this is a big challenge. The foregrounds, believed to originate from continuum sources, is expected to vary smoothly with Δν whereas the H i signal decorrelates within ∼0.5 MHz, and this holds the promise of separating the two. For each ℓ, we use the interval 0.5 ≤Δν≤ 7.5 MHz to fit a fourth-order polynomial which is subtracted from the measured Cℓ(Δν) to remove any smoothly varying component across the entire bandwidth Δν≤ 7.5 MHz. The residual Cℓ(Δν), we find, has an oscillatory pattern with amplitude and period, respectively, ∼0.1 mK2 and Δν= 3 MHz at the smallest ℓ value of 1476, and the amplitude and period decreasing with increasing ℓ. Applying a suitably chosen high pass filter, we are able to remove the residual oscillatory pattern for ℓ= 1476 where the residual Cℓ(Δν) is now consistent with zero at the 3σ noise level. Based on this we conclude that we have successfully removed the foregrounds at ℓ= 1476 and the residuals are consistent with noise. We use this to place an upper limit on the H i signal whose amplitude is determined by ⁠, where and b are the H i neutral fraction and the H i bias, respectively. A value of greater than 7.95 would have been detected in our observation, and is therefore ruled out at the 3σ level. For comparison, studies of quasar absorption spectra indicate which is ∼330 times smaller than our upper limit. We have not succeeded in completely removing the residual oscillatory pattern, whose cause is presently unknown to us, for the larger ℓ values.

Log-Poisson non-Gaussianity of Lyα transmitted flux fluctuations at high redshift

We investigate the non-Gaussian features of the IGM at redshift $z\sim 5 - 6$ using Ly$\alpha$ transmitted flux of quasar absorption spectra and cosmological hydrodynamic simulation of the concordance $\Lambda$CDM universe. We show that the neutral hydrogen mass density field and Ly$\alpha$ transmitted flux fluctuations possess all the non-Gaussian features predicted by the log-Poisson hierarchy, which depends only on two dimensionless parameters $\beta$ and $\gamma$, describing, respectively, the intermittence and singularity of the random fields. We find that the non-Gaussianity of the Ly$\alpha$ transmitted flux of quasars from $z=4.9$ to $z=6.3$ can be well reconstructed by the hydrodynamical simulation samples. Although the Gunn-Peterson optical depth and its variance underwent a significant evolution in the redshift range of $5 - 6$, the intermittency measured by $\beta$ is almost redshift-independent in this range. More interesting, the intermittency of quasar's absorption spectra on physical scales $0.1-1$ h$^{-1}$Mpc in redshift $5 - 6$ are found to be about the same as that on physical scales $1-10$ h$^{-1}$Mpc at redshifts $2 - 4$. Considering the Jeans length is less than 0.1 h$^{-1}$Mpc at $z\sim 5$, and $1$ h$^{-1}$Mpc at $z\sim 2$, these results imply that the nonlinear evolution in high and low redshifts will lead the cosmic baryon fluid to a state similar to fully developed turbulence. The log-Poisson high order behavior of current high redshift data of quasar's spectrum can be explained by uniform UV background in the redshift range considered. We also studied the log-Poisson non-Gaussianity by considering inhomogeneous background. With several simplified models of inhomogeneous background, we found the effect of the inhomogeneous background on the log-Poisson non-Gaussianity is not larger than 1-sigma.

The effect of neutrinos on the matter distribution as probed by the intergalactic medium

We present a suite of full hydrodynamical cosmological simulations that quantitatively address the impact of neutrinos on the (mildly non-linear) spatial distribution of matter and in particular on the neutral hydrogen distribution in the Intergalactic Medium (IGM), which is responsible for the intervening Lyman-α absorption in quasar spectra. The free-streaming of neutrinos results in a (non-linear) scale-dependent suppression of power spectrum of the total matter distribution at scales probed by Lyman-α forest data which is larger than the linear theory prediction by about 25 % and strongly redshift dependent. By extracting a set of realistic mock quasar spectra, we quantify the effect of neutrinos on the flux probability distribution function and flux power spectrum. The differences in the matter power spectra translate into a ∼ 2.5% (5%) difference in the flux power spectrum for neutrino masses with Σmν = 0.3 eV (0.6 eV). This rather small effect is difficult to detect from present Lyman-α forest data and nearly perfectly degenerate with the overall amplitude of the matter power spectrum as characterised by σ8. If the results of the numerical simulations are normalized to have the same σ8 in the initial conditions, then neutrinos produce a smaller suppression in the flux power of about 3% (5%) for Σmν = 0.6 eV (1.2 eV) when compared to a simulation without neutrinos. We present constraints on neutrino masses using the Sloan Digital Sky Survey flux power spectrum alone and find an upper limit of Σmν < 0.9 eV (2σ C.L.), comparable to constraints obtained from the cosmic microwave background data or other large scale structure probes.

Sensitivity of the energy levels of singly ionized cobalt to the variation of the fine-structure constant

We use relativistic Hartree-Fock and configuration interaction methods to calculate the dependence of transition frequencies for singly ionized cobalt on the fine structure constant. The results are to be used in the search for variation of the fine structure constant in quasar absorption spectra.

Searching for space-time variation of the fine structure constant using QSO spectra: overview and future prospects

Current theories that seek to unify gravity with the other fundamental interactions suggest that spatial and temporal variation of fundamental constants is a possibility, or even a necessity, in an expanding Universe. Several studies have tried to probe the values of constants at earlier stages in the evolution of the Universe, using tools such as big-bang nucleosynthesis, the Oklo natural nuclear reactor, quasar absorption spectra, and atomic clocks (see, e.g. Flambaum &amp; Berengut (2009)).

Varying alpha driven by the Dirac–Born–Infeld scalar field

Since about ten years ago, varying α theories attracted many attentions, mainly due to the first observational evidence from the quasar absorption spectra that the fine structure “constant” might change with cosmological time. In this Letter, we investigate the cosmic evolution of α driven by the Dirac–Born–Infeld (DBI) scalar field. To be general, we consider various couplings between the DBI scalar field and the electromagnetic field. We also confront the resulting Δα/α with the observational constraints, and find that various cosmological evolution histories of Δα/α are allowed. Comparing with the case of varying α driven by quintessence, the corresponding constraints on the parameters of coupling have been relaxed, thanks to the relativistic correction of the DBI scalar field.

Precision spectroscopy of the3s−3pfine-structure doublet inMg+

We apply a recently demonstrated method for precision spectroscopy on strong transitions in trapped ions to measure both fine structure components of the 3s-3p transition in 24-Mg+ and 26-Mg+. We deduce absolute frequency reference data for transition frequencies, isotope shifts and fine structure splittings that are in particular useful for comparison with quasar absorption spectra, which test possible space-time variations of the fine structure constant. The measurement accuracy improves previous literature values, when existing, by more than two orders of magnitude.

SPACE-TIME VARIATION OF COUPLING CONSTANTS AND FUNDAMENTAL MASSES

We review recent works discussing the effects of variation of fundamental "constants" on a variety of physical systems. These are motivated by theories unifying gravity with other interactions that suggest the possibility of temporal and spatial variation of the fundamental constants in an expanding Universe. The effects of any potential variation of the fine-structure constant and fundamental masses could be seen in phenomena covering the lifespan of the Universe, from Big Bang nucleosynthesis to quasar absorption spectra to modern atomic clocks. We review recent attempts to find such variations and discuss some of the most promising new systems where huge enhancements of the effects may occur.

Interpreting the transmission windows of distant quasars

We propose the apparent shrinking criterion (ASC) to interpret the spatial extent, R W , of transmitted flux windows in the absorption spectra of high-z quasars. The ASC can discriminate between the two regimes in which R w corresponds either to the physical size, R HII , of the quasar H II region or to the distance, R max W , at which the transmitted flux drops to =0.1 and a Gunn-Peterson (GP) trough appears. In the first case [H II region (HR) regime], one can determine the intergalactic medium mean H I fraction, x HI ; it the second [proximity region (PR) regime], the value of R W allows one to measure the local photoionization rate and the local enhancement of the photoionization rate, Γ G , due to nearby/intervening galaxies. The ASC has been tested against radiative transfer+smoothed particle hydrodynamics numerical simulations, and applied to a sample of 15 high-z (z > 5.8) quasar spectra. All sample quasars are found to be in the PR regime; hence, their observed spectral properties (inner flux profile, extent of transmission window) cannot reliably constrain the value of x HI . Four sample quasars show evidence for a local enhancement (up to 50 per cent) in the local photoionization rate possibly produced by a galaxy overdensity. We discuss the possible interpretations and uncertainties of this result.

Ionization state of cosmic hydrogen by early stars and quasars

Cosmic hydrogen is reionized and maintained in its highly ionized state by the ultraviolet emission attributed to an early generation of stars and quasars. The Lyα opacity observed in absorption spectra of high-redshift quasars permits more stringent constraints on the ionization state of cosmic hydrogen. Based on density perturbation and structure formation theory, we develop an analytic model to trace the evolution of the ionization state in the post-overlap epoch of reionization, in which the bias factor is taken into account. For quasars, we represent an improved luminosity function by utilizing a hybrid approach for the halo formation rate that is in reasonable agreement with the published measurements at 2 ≲ z ≲ 6. Comparison with the classic Press–Schechter mass function of dark matter halos, we demonstrate that the biased mass distribution indeed enhances star formation efficiency in the overdense environment by more than 25 per cent following the overlap of ionized bubbles. In addition, an alternative way is introduced to derive robust estimates of the mean free path for ionizing photons. In our model, star-forming galaxies are likely to dominate the ionizing background radiation beyond z = 3, and quasars contribute equally above a redshift of z ∼ 2.5. From 5 ≤ z ≤ 6, the lack of evolution in photoionization rate can thus be explained by the relatively flat evolution in star formation efficiency, although the mean free path of ionizing photons increases rapidly. Moreover, in the redshift interval z ∼ 2–6, the expected mean free path and Gunn–Peterson optical depth obviously evolve by a factor of ∼ 500 and ∼ 50 respectively. We find that the relative values of critical overdensities for hydrogen ionization and collapse could be 430% at z ≈ 2 and 2% at z ≈ 6, suggesting a rapid overlap process in the overdense regions around instant quasars following reionization. We further illustrate that the absolute estimates of the fraction of neutral hydrogen computed from theoretical models may not be important because of comparable uncertainties in the computation.