1,894 research outputs found
Evaluating the New Automatic Method for the Analysis of Absorption Spectra Using Synthetic Spectra
We recently presented a new "artificial intelligence" method for the analysis
of high-resolution absorption spectra (Bainbridge and Webb, Mon. Not. R.
Astron. Soc. 2017, 468,1639-1670). This new method unifies three established
numerical methods: a genetic algorithm (GVPFIT); non-linear least-squares
optimisation with parameter constraints (VPFIT); and Bayesian Model Averaging
(BMA). In this work, we investigate the performance of GVPFIT and BMA over a
broad range of velocity structures using synthetic spectra. We found that this
new method recovers the velocity structures of the absorption systems and
accurately estimates variation in the fine structure constant. Studies such as
this one are required to evaluate this new method before it can be applied to
the analysis of large sets of absorption spectra. This is the first time that a
sample of synthetic spectra has been utilised to investigate the analysis of
absorption spectra. Probing the variation of nature's fundamental constants
(such as the fine structure constant), through the analysis of absorption
spectra, is one of the most direct ways of testing the universality of physical
laws. This "artificial intelligence" method provides a way to avoid the main
limiting factor, i.e., human interaction, in the analysis of absorption
spectra.Comment: 9 pages, 5 figures, published on 5 April 2017 in Univers
Keck constraints on a varying fine-structure constant: wavelength calibration errors
The Keck telescope's HIRES spectrograph has previously provided evidence for
a smaller fine-structure constant, alpha, compared to the current laboratory
value, in a sample of 143 quasar absorption systems:
da/a=(-0.57+/-0.11)x10^{-5}. This was based on a variety of metal-ion
transitions which, if alpha varies, experience different relative velocity
shifts. This result is yet to be robustly contradicted, or confirmed, by
measurements on other telescopes and spectrographs; it remains crucial to do
so. It is also important to consider new possible instrumental systematic
effects which may explain the Keck/HIRES results. Griest et al. (2009,
arXiv:0904.4725v1) recently identified distortions in the echelle order
wavelength scales of HIRES with typical amplitudes +/-250m/s. Here we
investigate the effect such distortions may have had on the Keck/HIRES varying
alpha results. We demonstrate that they cause a random effect on da/a from
absorber to absorber because the systems are at different redshifts, placing
the relevant absorption lines at different positions in different echelle
orders. The typical magnitude of the effect on da/a is ~0.4x10^{-5} per
absorber which, compared to the median error on da/a in the sample,
~1.9x10^{-5}, is relatively small. Consequently, the weighted mean value
changes by less than 0.05x10^{-5} if the corrections we calculate are applied.
Nevertheless, we urge caution, particularly for analyses aiming to achieve high
precision da/a measurements on individual systems or small samples, that a much
more detailed understanding of such intra-order distortions and their
dependence on observational parameters is important if they are to be avoided
or modelled reliably. [Abridged]Comment: 9 pages, 2 figures, 2 tables. Invited contribution to Proc. IAU
XXVIIth General Assembly, Joint Discussion 9, "Are the fundamental constants
varying with time?". To appear in P. Molaro, E. Vangioni-Flam, eds, Memorie
della Societa Astronomica Italiana (MmSAIt), Vol. 80. Complete version of
Table 1 available at http://astronomy.swin.edu.au/~mmurphy/pub.htm
Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects
We use the dynamics of a galaxy, set up initially at a constant proper
distance from an observer, to derive and illustrate two counter-intuitive
general relativistic results. Although the galaxy does gradually join the
expansion of the universe (Hubble flow), it does not necessarily recede from
us. In particular, in the currently favored cosmological model, which includes
a cosmological constant, the galaxy recedes from the observer as it joins the
Hubble flow, but in the previously favored cold dark matter model, the galaxy
approaches, passes through the observer, and joins the Hubble flow on the
opposite side of the sky. We show that this behavior is consistent with the
general relativistic idea that space is expanding and is determined by the
acceleration of the expansion of the universe -- not a force or drag associated
with the expansion itself. We also show that objects at a constant proper
distance will have a nonzero redshift; receding galaxies can be blueshifted and
approaching galaxies can be redshifted.Comment: 8 pages including 6 figures, to appear in Am. J. Phys., 2003.
Reference added in postscrip
Development of group accounting in the United Kingdom to 1933
The publication of consolidated accounts is an early example of innovative financial reporting procedures being introduced by U.S. companies before they were adopted in the U.K., where Nobel Industries (1922) is generally cited as the first holding company to prepare economic entity based financial reports. This paper produces evidence which shows that the publication of consolidated accounts, by British companies, began at least as early as 1910. Our research nevertheless confirms the generally held view that U.S. developments occurred earlier, and we explore a range of possible explanations for this phenomenon
Convergence properties of fine structure constant measurements using quasar absorption systems
Searches for spacetime variations of fundamental constants have entered an
era of unprecedented precision. New, high quality quasar spectra require
increasingly refined analytic methods. In this article, a continuation in a
series to establish robust and unbiased methodologies, we explore how
convergence criteria in non-linear least squares optimisation impact on quasar
absorption system measurements of the fine structure constant alpha. Given
previous claims for high-precision constraints, we critically examine the
veracity of a so-called ``blinding'' approach, in which alpha 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 an extended flat canyon in
chi squared-alpha space, such that convergence is unlikely to be reached, even
after as many as 1000 iterations. The fix is straightforward: alpha 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 alpha should be
reworked.Comment: 10 pages, 13 figures. Submitted for publicatio
The Origin of C IV Absorption Systems at Redshifts z<1---Discovery of Extended C IV Envelopes Around Galaxies
(Abridged) We report the discovery of extended CIV gaseous envelopes around
galaxies of a wide range of luminosity and morphological type. First, we show
that CIV absorption systems are strongly clustered around galaxies on velocity
scales of v < 250 km/s and impact parameter scales of rho < 100 h^{-1} kpc but
not on larger velocity or impact parameter scales. Next, adopting measurements
of galaxy properties presented in previous papers, we examine how properties of
the CIV absorption systems depend on properties of the galaxies. On the basis
of 14 galaxy and absorber pairs and 36 galaxies that do not produce
corresponding CIV absorption lines to within sensitive upper limits, we find
that: (1) Galaxies of a range of morphological type and luminosity appear to
possess extended CIV gaseous envelopes of radius R ~ 100 h^{-1} kpc, with
abrupt boundaries between the CIV absorbing and non-absorbing regions. (2) The
extent of CIV-absorbing gas around galaxies scales with galaxy B-band
luminosity as R \propto L_B^{0.5 +/- 0.1} but does not depend strongly on
galaxy surface brightness, redshift, or morphological type. And (3) the
covering factor of CIV clouds within ~ 100 h^{-1} kpc of galaxies is nearly
unity, but there is a large scatter in the mean number of clouds encountered
along the line of sight. The most significant implication of the study is that
galaxies of a wide range of luminosity and morphological type are surrounded by
chemically enriched gas that extends for at least ~ 100 h^{-1} kpc. We consider
various scenarios that may have produced metals at large galactic distance and
conclude that accreting satellites are most likely to be responsible for
chemically enriched gas at large galactic distances to regular looking
galaxies.Comment: 19 pages, 3 figures, to appear in ApJ, July 20 200
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