Browsing by Author "Lombard, Fred"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Is there evolution in the infrared Tully–Fisher relation? Comparing two linear regressions
    (Oxford University Press, 2009) Koen, Chris; Lombard, Fred
    In a recent paper, Puech and co-workers compared K-band Tully–Fisher relations derived for nearby and distant galaxies, respectively. They concluded that the two relations differ, and deduced that there is evolution in the Tully–Fisher relations. The statistical comparison between the two regression lines is re-examined, and it is shown that the statistical test used gives non-significant results. It is argued that better results can be obtained by comparing the ‘inverse’ Tully–Fisher relations, and it is demonstrated by two different methods that the nearby- and distant-sample relations do indeed differ at a very high significance level. One of the statistical methods described is non-parametric, and can be applied very generally to compare linear regressions from two different samples.
  • Thumbnail Image
    Item
    A re-assessment of the nearest neighbour alignment of the X-ray isophotes of galaxy clusters
    (Oxford University Press, 2006) Koen, Chris; Lombard, Fred
    Alignment is defined as the tendency of the distribution of pointing angles between the major axes of clusters and their nearest neighbours to be more concentrated towards small values for small nearest neighbour distances, whereas the distribution is expected to be uniform over all angles at larger distances. Conflicting pronouncements on the reality of this effect have been published in the astronomy literature. A re-assessment of the evidence for alignment is presented, based on three recently published X-ray data sets. We find that whereas there is evidence for alignment, it is not as convincing as previously claimed. In particular, the scale to which the effect has been claimed to extend seems to have been severely overstated.
  • Thumbnail Image
    Item
    What can be deduced from open cluster metallicity measurements?
    (Oxford University Press, 2007) Koen, Chris; Lombard, Fred
    The dependence of [Fe/H] on galactocentric distance, distance from the galactic mid-plane and age is studied. Both ordinary least-squares and non-parametric regression in the form of a ‘generalized additive model’ are used. The radial metallicity slope is found to be shallower than previously claimed in the literature, and there is a significant abundance gradient perpendicular to the galactic plane. There may be a tendency for metallicity to increase with cluster age.