Showing papers by "Robert F. Carswell published in 2011"

Indications of a Spatial Variation of the Fine Structure Constant

Abstract: We previously reported Keck telescope observations suggesting a smaller value of the fine structure constant α at high redshift. New Very Large Telescope (VLT) data, probing a different direction in the Universe, shows an inverse evolution; α increases at high redshift. Although the pattern could be due to as yet undetected systematic effects, with the systematics as presently understood the combined data set fits a spatial dipole, significant at the 4.2 σ level, in the direction right ascension 17.5 ± 0.9 h, declination -58 ± 9 deg. The independent VLT and Keck samples give consistent dipole directions and amplitudes, as do high and low redshift samples. A search for systematics, using observations duplicated at both telescopes, reveals none so far which emulate this result.

Iron and alpha-element Production in the First One Billion Years after the Big Bang

Abstract: We present measurements of carbon, oxygen, silicon, and iron in quasar absorption systems existing when the universe was roughly one billion years old. We measure column densities in nine low-ionization systems at 4.7 4.7 systems, as in the lower-redshift DLAs, and that the column density ratios are close to the intrinsic relative element abundances. The abundances in our z > 4.7 systems are therefore likely to represent the typical integrated yields from stellar populations within the first gigayear of cosmic history. Due to the time limit imposed by the age of the universe at these redshifts, our measurements thus place direct constraints on the metal production of massive stars, including iron yields of prompt supernovae. The lack of redshift evolution further suggests that the metal inventories of most metal-poor absorption systems at z > 2 are also dominated by massive stars, with minimal contributions from delayed Type Ia supernovae or AGB winds. The relative abundances in our systems broadly agree with those in very metal-poor, non-carbon-enhanced Galactic halo stars. This is consistent with the picture in which present-day metal-poor stars were potentially formed as early as one billion years after the Big Bang.

A cold component and the complex velocity structure of DLA1331 + 170

Abstract: We examine the velocity structure in the gas associated with H I in the damped Lyα absorption system at redshift z = 1.7764 towards the QSO 1331 + 170 using Arecibo H i 21-cm data, optical spectra from the Keck High Resolution Echelle Spectrograph (HIRES) and European Southern Observatory (ESO) Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES), and a previously published Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) ultraviolet spectrum. From the optical data we find at least two, and possibly three, components showing C I lines. One of these has very narrow lines with Doppler parameter b = 0.55 km s ―1 , corresponding to a kinetic temperature of 220 K if the broadening is thermal and with a 2σ upper limit of 480 K. We re-examine the H 2 analysis undertaken by Cui et al. using the neutral carbon velocity structure, and find a model which is, unlike theirs, consistent with a mixture of collisional and background radiation excitation of the observed H 2 rotational levels. Using Voigt profile fits to absorption lines from a range of singly ionized heavy elements we find eight components covering a velocity range of ∼110 km s ―1 , with a further outlier over 120 km s ―1 away from the nearest in the main group. The H I structure is expected to follow some combination of the singly ionized and neutral gas, but the 21-cm absorption profile is considerably different. We suggest, as have others, that this may be because the different extent and brightness distributions of the radio and optical background sources mean that the sightlines are not the same, and so the spin temperature derived by comparing the Lyα and 21-cm line strengths has little physical meaning. The neutral and singly ionized heavy element line profiles also show significant differences, and so the dominant components in each appear to be physically distinct. Attempts to use the range of atomic masses to separate thermal and turbulent components of their Doppler widths were not generally successful, since there are several blended components and the useful mass range (about a factor of 2) is not very large. The velocity structure in all ionization stages up to +3, apart from the neutral heavy elements, is sufficiently complex that it is difficult to separate out the corresponding velocity components for different ionization levels and determine their column densities.