arXiv subjects

J. K. Webb

Publications and source records attributed to J. K. Webb.

At least 55 records · Page 3Linked to original sources

arXiv2006

Laboratory spectroscopy and the search for space-time variation of the fine structure constant using QSO spectra

Theories unifying gravity with other interactions suggest spatial and temporal variation of fundamental ``constants'' in the Universe. A change in the fine structure constant, alpha, could be detected via shifts in the frequencies of atomic transitions in quasar absorption systems. Previous studies of three independent samples of data, containing 143 absorption systems spread from 2 to 10 billion years after big bang, hint that alpha was smaller 7 - 11 billion years ago. To continue this study we urgently need accurate laboratory measurements of atomic transition frequencies. In this paper, we present a current list of transitions of importance to the search for alpha variation. They are E1 transitions to the ground state in several different atoms and ions, with wavelengths ranging from around 900 - 6000 Angstroms, and require an accuracy of better than 10^(-4) Angstroms. We also discuss isotope shift measurements that are needed in order to resolve systematic effects in the study. Researchers who are interested in performing these measurements should contact the authors directly.

physics.atom-ph↗

arXiv2005

The UNSW Extrasolar Planet Search: Methods and First Results from a Field Centred on NGC 6633

We report on the current status of the University of New South Wales Extrasolar Planet Search project, giving details of the methods we use to obtain millimagnitude precision photometry using the 0.5m Automated Patrol Telescope. We use a novel observing technique to optimally broaden the PSF and thus largely eliminate photometric noise due to intra-pixel sensitivity variations on the CCD. We have observed 8 crowded Galactic fields using this technique during 2003 and 2004. Our analysis of the first of these fields (centred on the open cluster NGC 6633) has yielded 49 variable stars and 4 shallow transit candidates. Follow-up observations of these candidates have identified them as eclipsing binary systems. We use a detailed simulation of our observations to estimate our sensitivity to short-period planets, and to select a new observing strategy to maximise the number of planets detected.

astro-ph↗

arXiv2005

Limits on variations in fundamental constants from 21-cm and ultraviolet quasar absorption lines

Quasar absorption spectra at 21-cm and UV rest-wavelengths are used to estimate the time variation of x=alpha^2 g_p mu, where alpha is the fine structure constant, g_p the proton g factor, and m_e/m_p=mu the electron/proton mass ratio. Over a redshift range 0.24 ~< z_abs ~< 2.04, _total_weighted=(1.17+-1.01) x 10^-5. A linear fit gives d/dt{x}/x=(-1.43+-1.27) x 10^-15 yr^-1. Two previous results on varying alpha yield the strong limits Delta mu/mu=(2.31+-1.03) x 10^-5 and Delta mu/mu=(1.29+-1.01) x 10^-5. Our sample, 8x larger than any previous, provides the first direct estimate of the intrinsic 21-cm and UV velocity differences ~6 km s^-1.

astro-ph↗

arXiv2004

Spin temperatures and covering factors for HI 21-cm absorption in damped Lyman-alpha systems

We investigate the practice of assigning high spin temperatures to damped Lyman-alpha absorption systems (DLAs) not detected in HI 21-cm absorption. In particular, Kanekar & Chengaulr (2003) have attributed the mix of 21-cm detections and non-detections in low redshift (z<2.04) DLAs to a mix of spin temperatures, while the non-detections at high redshift were attributed to high spin temperatures. Below z=0.9, where some of the DLA host galaxy morphologies are known, we find that 21-cm absorption is normally detected towards large radio sources when the absorber is known to be associated with a large intermediate (spiral) galaxy. Furthermore, at these redshifts, only one of the six 21-cm non-detections has an optical identification and these DLAs tend to lie along the sight-lines to the largest background radio continuum sources. For these and many of the high redshift DLAs occulting large radio continua, we therefore expect covering factors of less than the assumed/estimated value of unity. This would have the effect of introducing a range of spin temperatures considerably narrower than the current range of >9000 K, while still supporting the hypothesis that the high redshift DLA sample comprises a larger proportion of compact galaxies than the low redshift sample.

astro-ph↗

arXiv2004

D/H in a new Lyman limit quasar absorption system at z=3.256 towards PKS1937-1009

We have identified a new Lyman limit absorption system towards PKS1937-1009, with log N(HI)=18.25 +/- 0.02 at z=3.256. It is suitable for measuring D/H, and we find a 68.3% confidence range for D/H of 1.6^{+0.25}_{-0.30} \times 10^{-5}, and a 95.4% range of 1.6^{+0.5}_{-0.4} \times 10^{-5}. The metallicity of the cloud where D/H was measured is low, [Si/H] = -2.0 +/- 0.5. At these metallicities we expect that D/H will be close to the primordial value. Our D/H disagrees at a level of 99.4% with the predicted D/H using the baryon density calculated from the cosmic background radiation measured by WMAP, 2.60^{+0.19}_{-0.17} \times 10^{-5}. Our result also exacerbates the scatter in D/H values around the mean primordial D/H.

astro-ph↗

arXiv2004

Atomic and Molecular Absorption at High Redshift

Strong constraints on possible variations in fundamental constants can be derived from HI 21-cm and molecular rotational absorption lines observed towards quasars. With the aim of forming a statistical sample of constraints we have begun a program of systematic searches for such absorption systems. Here we describe molecular rotational searches in 25 damped Lyman-alpha systems where, in many cases, we set optical depth limits an order of magnitude better than that required to detect the 4 known redshifted millimeter-wave absorbers. We also discuss the contributory factors in the detectability of HI 21-cm absorption, focusing on possible biases (e.g.low covering factors) in the currently known sample of absorbers and non-detections.

astro-ph↗

arXiv2004

Molecular fraction limits in damped Lyman-alpha absorption systems

We have used the Green Bank Telescope (GBT) and Berkeley-Illinois-Maryland Association (BIMA) array to search for redshifted millimetre absorption in a sample of damped Lyman-alpha absorption systems (DLAs). This brings the number of published systems searched from 18 to 30. In 17 cases we reach $3σ$ limits of $τ\leq0.1$, which is a significant improvement over the previous searches and more than sufficient to detect the 4 known redshifted millimetre absorbers ($τ\gapp1$). While the CO rotational (millimetre) column density limits obtained are weaker than the electronic (optical) limits, they may provide useful limits below the atmospheric cut-off for the Lyman and Werner \MOLH-bands in the UV ($z_{\rm abs}\lapp1.8$). Using a model for the DLA metallicity evolution combined with assumed HCO$^+$/\MOLH ~and CO/\MOLH conversion ratios, we use the molecular column density limits to calculate plausible \MOLH ~molecular fraction limits. Finally, we use these results to discuss the feasibility of detecting rotational CO transitions in DLAs with the next generation of large radio telescopes.

astro-ph↗

arXiv2003

H_2-bearing damped Lyman-alpha systems as tracers of cosmological chemical evolution

The chemical abundances in damped Lyman-alpha systems (DLAs) show more than 2 orders of magnitude variation at a given epoch, possibly because DLAs arise in a wide variety of host galaxies. This could significantly bias estimates of chemical evolution. We explore the possibility that DLAs in which H_2 absorption is detected may trace cosmological chemical evolution more reliably since they may comprise a narrower set of physical conditions. The 9 known H_2 absorption systems support this hypothesis: metallicity exhibits a faster, more well-defined evolution with redshift than in the general DLA population. The dust-depletion factor and, particularly, H_2 molecular fraction also show rapid increases with decreasing redshift. We comment on possible observational selection effects which may bias this evolution. Larger samples of H_2-bearing DLAs are clearly required and may constrain evolution of the UV background and DLA galaxy host type with redshift.

astro-ph↗

arXiv2003

Constraining Variations in the Fine-structure Constant, Quark Masses and the Strong Interaction

We present evidence for variations in the fine-structure constant from Keck/HIRES spectra of 143 quasar absorption systems over the redshift range 0.2 < z_abs < 4.2. This includes 15 new systems, mostly at high-z (z_abs > 1.8). Our most robust estimate is a weighted mean da/a=(-0.57 +/- 0.11)x10^{-5}. We respond to recent criticisms of the many-multiplet method used to extract these constraints. The most important potential systematic error at low-z is the possibility of very different Mg heavy isotope abundances in the absorption clouds and laboratory: {\it higher} abundances of {25,26}Mg in the absorbers may explain the low-z results. Approximately equal mixes of {24}Mg and {25,26}Mg are required. Observations of Galactic stars generally show {\it lower} {25,26}Mg isotope fractions at the low metallicities typifying the absorbers. Higher values can be achieved with an enhanced population of intermediate mass stars at high redshift, a possibility at odds with observed absorption system element abundances. At present, all observational evidence is consistent with the varying-alpha results. Another promising method to search for variation of fundamental constants involves comparing different atomic clocks. Here we calculate the dependence of nuclear magnetic moments on quark masses and obtain limits on the variation of alpha and m_q/Lambda_QCD from recent atomic clock experiments with hyperfine transitions in H, Rb, Cs, Hg+ and an optical transition in Hg+.

astro-ph↗

arXiv2003

Cosmological variation of deuteron binding energy, strong interaction and quark masses from big bang nucleosynthesis

We use Big Bang Nucleosynthesis calculations and light element abundance data to constrain the relative variation of the deuteron binding energy since the universe was a few minutes old, $δQ = Q(BBN)-Q(present)$. Two approaches are used, first treating the baryon to photon ratio, $η$, as a free parameter, but with the additional freedom of varying $δQ$, and second using the WMAP value of $η$ and solving only for $δQ$. Including varying $Q$ yields a better fit to the observational data than imposing the present day value, rectifying the discrepancy between the $^4He$ abundance and the deuterium and $^7Li$ abundances, {\it and} yields good agreement with the independently determined $η_{WMAP}$. The minimal deviation consistent with the data is significant at about the 4-$σ$ level; $δQ/Q= -0.019 \pm 0.005$. If the primordial $^4$He abundance lies towards the low end of values in the literature, this deviation is even larger and more statistically significant. Taking the light element abundance data at face-value, our result may be interpreted as variation of the dimensionless ratio $X=m_s/Λ_{QCD}$ of the strange quark mass and strong scale: $δX/X=(1.1 \pm 0.3) \times 10^{-3}$. These results provide a strong motivation for a more thorough exploration of the potential systematic errors in the light element abundance data.

astro-ph↗

arXiv2003

Deep Searches for High Redshift Molecular Absorption

Millimetre-band scans of the frequency space towards optically dim quasars is potentially a highly efficient method for detecting new high redshift molecular absorption systems. Here we describe scans towards 7 quasars over wide bandwidths (up to 23 GHz) with sensitivity limits sufficient to detect the 4 redshifted absorbers already known. With wider frequency bands, highly efficient searches of large numbers of possibly obscured objects will yield many new molecular absorbers.

astro-ph↗

arXiv2003

Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra

[Abridged] We previously presented evidence for a varying fine-structure constant, alpha, in two independent samples of Keck/HIRES QSO spectra. Here we present a detailed many-multiplet analysis of a third Keck/HIRES sample containing 78 absorption systems. We also re-analyse the previous samples, providing a total of 128 absorption systems over the redshift range 0.2<z_abs<3.7. All three samples separately yield consistent, significant values of da/a. The analyses of low- and high-z systems rely on different ions/transitions with very different dependencies on alpha, yet they also give consistent results. We identify additional random errors in 22 high-z systems characterized by transitions with a large dynamic range in apparent optical depth. Increasing the statistical errors on da/a for these systems gives our fiducial result, a weighted mean da/a=(-0.543+/-0.116)x10^-5, representing 4.7-sigma evidence for a smaller weighted mean alpha in the absorption clouds. Assuming that da/a=0 at z_abs=0, the data marginally prefer a linear increase in alpha with time: dota/a=(6.40+/-1.35)x10^-16 yr^-1. The two-point correlation function for alpha is consistent with zero over 0.2-13 Gpc comoving scales and the angular distribution of da/a shows no significant dipolar anisotropy. We therefore have no evidence for spatial variations in da/a. We extend our previous searches for possible systematic errors, identifying atmospheric dispersion and isotopic structure effects as potentially the most significant. However, overall, known systematic errors do not explain the results. Future many-multiplet analyses of QSO spectra from different telescopes and spectrographs will provide a now crucial check on our Keck/HIRES results.

astro-ph↗

arXiv2003

A search for high redshift molecular absorption lines toward millimetre-loud, optically faint quasars

We describe initial results of a search for redshifted molecular absorption toward four millimetre-loud, optically faint quasars. A wide frequency bandwidth of up to 23 GHz per quasar was scanned using the Swedish-ESO Sub-millimetre Telescope at La Silla. Using a search list of commonly detected molecules, we obtained nearly complete redshift coverage up to z_abs=5. The sensitivity of our data is adequate to have revealed absorption systems with characteristics similar to those seen in the four known redshifted millimetre-band absorption systems, but none were found. Our frequency-scan technique nevertheless demonstrates the value of wide-band correlator instruments for searches such as these. We suggest that a somewhat larger sample of similar observations should lead to the discovery of new millimetre-band absorption systems.

astro-ph↗

arXiv2002

A 22GHz search for molecular absorption at z~3 with the upgraded ATCA

We report a lambda > 1cm search for rotational molecular absorption towards quasars, now possible with the upgraded Australia Telescope Compact Array (ATCA). The targets were PKS 0201+113, PKS 0336-017 and Q 0537-286, where known damped Lyman-alpha absorption systems (DLAs) could cause redshifted molecular absorption in the 12mm band of the telescope. We place 3-sigma upper limits on any HCO+ 0-1 absorption features of < 30mJy per 3.4km/s channel. The non-detections could be attributed to the inherent low metallicities in DLAs leading to generally low H_2, and thus HCO+, column densities. In general, the detection of molecular rotational transitions in DLAs could be further hindered by a lower than expected CO-to-H_2 conversion ratio, whether due either to photoionization of carbon or its relative under-abundance at high redshift.

astro-ph↗

arXiv2002

Does the fine structure constant vary? A third quasar absorption sample consistent with varying alpha

We report preliminary results from a third sample of quasar absorption line spectra from the Keck telescope which has been studied to search for any possible variation of the fine structure constant, alpha. This third sample, which is larger than the sum of the two previously published samples, shows the same effect, and also gives, as do the previous two samples, a significant result. The combined sample yields a highly significant effect, da/a = (alpha_z - alpha_0)/alpha_0 = -0.57 +/- 0.10 x 10^{-5}, averaged over the redshift range 0.2 < z < 3.7. We include a brief discussion of small-scale kinematic structure in quasar absorbing clouds. However, kinematics are unlikely to impact significantly on the averaged non-zero da/a above, and we have so far been unable to identify any systematic effect which can explain it. New measurements of quasar spectra obtained using independent instrumentation and telescopes are required to properly check the Keck results.

astro-ph↗

arXiv2002

Does the fine structure constant vary? A detailed investigation into systematic effects

We have previously presented 5.7-sigma evidence for a smaller alpha at redshifts 0.2 < z_abs < 3.7 from a sample of 128 Keck/HIRES quasar absorption systems: da/a = (-0.57 +/- 0.10) x 10^{-5}. A non-zero da/a manifests itself as a distinct pattern of shifts in the measured absorption line wavelengths. The statistical error is now small: we do detect small line shifts in the HIRES data. Whether these shifts are due to systematic errors or due to real variation in alpha is now the central question. Here we summarize the two potentially most important systematic effects: atmospheric dispersion and isotopic abundance evolution. Previously, these have been difficult to quantify/model but here we find that neither of them can explain our results. Furthermore, the HIRES spectra themselves contain no evidence for these effects. Independent measurements of da/a with a different telescope and spectrograph are now crucial if we are to rule out or confirm the present evidence for a variable alpha.

astro-ph↗

arXiv2002

Time evolution of the fine structure constant

We present a short review of the current quasar (QSO) absorption line constraints on possible variation of the fine structure constant, alpha = e^2/(hbar*c). Particular attention is paid to recent optical Keck/HIRES spectra of 49 absorption systems which indicate a smaller alpha in the past (Murphy et al. 2001, Webb et al. 2001). Here we present new preliminary results from 128 absorption systems: da/a = (-0.57 +/- 0.10) x 10^{-5} over the redshift range 0.2 < z < 3.7, in agreement with the previous results. Known potential systematic errors cannot explain these results. We compare them with strong `local' constraints and discuss other (radio and millimetre-wave) QSO absorption line constraints on variations in alpha^2 * g_p and alpha^2 * g_p * m_e/m_p (g_p is the proton g-factor and m_e/m_p is the electron/proton mass ratio). Finally, we discuss future efforts to rule out or confirm the current 5.7 sigma optical detection.

astro-ph↗

arXiv2002

An Automated Search for Extrasolar Planet Transits

We are setting up a new search for transiting extrasolar planets using the 0.5m Automated Patrol Telescope at Siding Spring Observatory, Australia. We will begin regular observations in September 2002. We expect to find ~7 new planets per year.

astro-ph↗