47 research outputs found
The Hierarchical Formation of the Galactic Disk
I review the results of recent cosmological simulations of galaxy formation
that highlight the importance of satellite accretion in the formation of
galactic disks. Tidal debris of disrupted satellites may contribute to the disk
component if they are compact enough to survive the decay and circularization
of the orbit as dynamical friction brings the satellite into the disk plane.
This process may add a small but non-negligible fraction of stars to the thin
and thick disks, and reconcile the presence of very old stars with the
protracted merging history expected in a hierarchically clustering universe. I
discuss various lines of evidence which suggest that this process may have been
important during the formation of the Galactic disk.Comment: paper to be read at the "Penetrating Bars through Masks of Cosmic
Dust" conference in South Afric
Age-related changes in global motion coherence: conflicting haemodynamic and perceptual responses
Our aim was to use both behavioural and neuroimaging data to identify indicators of perceptual decline in motion processing. We employed a global motion coherence task and functional Near Infrared Spectroscopy (fNIRS). Healthy adults (n = 72, 18-85) were recruited into the following groups: young (n = 28, mean age = 28), middle-aged (n = 22, mean age = 50), and older adults (n = 23, mean age = 70). Participants were assessed on their motion coherence thresholds at 3 different speeds using a psychophysical design. As expected, we report age group differences in motion processing as demonstrated by higher motion coherence thresholds in older adults. Crucially, we add correlational data showing that global motion perception declines linearly as a function of age. The associated fNIRS recordings provide a clear physiological correlate of global motion perception. The crux of this study lies in the robust linear correlation between age and haemodynamic response for both measures of oxygenation. We hypothesise that there is an increase in neural recruitment, necessitating an increase in metabolic need and blood flow, which presents as a higher oxygenated haemoglobin response. We report age-related changes in motion perception with poorer behavioural performance (high motion coherence thresholds) associated with an increased haemodynamic response
Structure and Evolution of the Milky Way
This review discusses the structure and evolution of the Milky Way, in the
context of opportunities provided by asteroseismology of red giants. The review
is structured according to the main Galactic components: the thin disk, thick
disk, stellar halo, and the Galactic bar/bulge. The review concludes with an
overview of Galactic archaeology and chemical tagging, and a brief account of
the upcoming HERMES survey with the AAT.Comment: Proc. of the workshop "Red Giants as Probes of the Structure and
Evolution of the Milky Way" (Roma, 15-17 Nov 2010), Astrophysics and Space
Science Proceedings, ISBN 978-3-642-18417-8 (eds. A. Miglio, J. Montalban, A.
Noels). Part of RedGiantsMilkyWay/2011/ proceedings available at
http://arxiv.org/html/1108.4406v
The stellar halo of the Galaxy
Stellar halos may hold some of the best preserved fossils of the formation
history of galaxies. They are a natural product of the merging processes that
probably take place during the assembly of a galaxy, and hence may well be the
most ubiquitous component of galaxies, independently of their Hubble type. This
review focuses on our current understanding of the spatial structure, the
kinematics and chemistry of halo stars in the Milky Way. In recent years, we
have experienced a change in paradigm thanks to the discovery of large amounts
of substructure, especially in the outer halo. I discuss the implications of
the currently available observational constraints and fold them into several
possible formation scenarios. Unraveling the formation of the Galactic halo
will be possible in the near future through a combination of large wide field
photometric and spectroscopic surveys, and especially in the era of Gaia.Comment: 46 pages, 16 figures. References updated and some minor changes.
Full-resolution version available at
http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pd
Metal-Poor Stars and the Chemical Enrichment of the Universe
Metal-poor stars hold the key to our understanding of the origin of the
elements and the chemical evolution of the Universe. This chapter describes the
process of discovery of these rare stars, the manner in which their surface
abundances (produced in supernovae and other evolved stars) are determined from
the analysis of their spectra, and the interpretation of their abundance
patterns to elucidate questions of origin and evolution. More generally,
studies of these stars contribute to other fundamental areas that include
nuclear astrophysics, conditions at the earliest times, the nature of the first
stars, and the formation and evolution of galaxies -- including our own Milky
Way. We illustrate this with results from studies of lithium formed during the
Big Bang; of stars dated to within ~1 Gyr of that event; of the most metal-poor
stars, with abundance signatures very different from all other stars; and of
the build-up of the elements over the first several Gyr. The combination of
abundance and kinematic signatures constrains how the Milky Way formed, while
recent discoveries of extremely metal-poor stars in the Milky Way's dwarf
galaxy satellites constrain the hierarchical build-up of its stellar halo from
small dark-matter dominated systems. [abridged]Comment: Book chapter, emulated version, 34 pages; number of references are
limited by publisher; to appear in Vol. 5 of textbook "Planets, Stars and
Stellar Systems", by Springer, in 201
Iron Behaving Badly: Inappropriate Iron Chelation as a Major Contributor to the Aetiology of Vascular and Other Progressive Inflammatory and Degenerative Diseases
The production of peroxide and superoxide is an inevitable consequence of
aerobic metabolism, and while these particular "reactive oxygen species" (ROSs)
can exhibit a number of biological effects, they are not of themselves
excessively reactive and thus they are not especially damaging at physiological
concentrations. However, their reactions with poorly liganded iron species can
lead to the catalytic production of the very reactive and dangerous hydroxyl
radical, which is exceptionally damaging, and a major cause of chronic
inflammation. We review the considerable and wide-ranging evidence for the
involvement of this combination of (su)peroxide and poorly liganded iron in a
large number of physiological and indeed pathological processes and
inflammatory disorders, especially those involving the progressive degradation
of cellular and organismal performance. These diseases share a great many
similarities and thus might be considered to have a common cause (i.e.
iron-catalysed free radical and especially hydroxyl radical generation). The
studies reviewed include those focused on a series of cardiovascular, metabolic
and neurological diseases, where iron can be found at the sites of plaques and
lesions, as well as studies showing the significance of iron to aging and
longevity. The effective chelation of iron by natural or synthetic ligands is
thus of major physiological (and potentially therapeutic) importance. As
systems properties, we need to recognise that physiological observables have
multiple molecular causes, and studying them in isolation leads to inconsistent
patterns of apparent causality when it is the simultaneous combination of
multiple factors that is responsible. This explains, for instance, the
decidedly mixed effects of antioxidants that have been observed, etc...Comment: 159 pages, including 9 Figs and 2184 reference
Tamoxifen in breast conservation therapy.
Breast conservation surgery for early breast cancer requires post-operative radiotherapy to give local control rates equivalent to mastectomy. Three hundred and thirty-five women presenting with symptomatic breast lumps and receiving radiotherapy and adjuvant systemic therapy at St. Bartholomew's, have actuarial 8-year local relapse-free rates of 90% for T1 and 83% for T2,3 presenting cases. Amongst 49 elderly and/or medically frail patients treated with a similar surgical policy but post-operative tamoxifen only (for standard risk features), the local relapse-free rates were 96% for T1 and 43% for T2,3. Most relapses occurred in the first 2 years in both groups. We conclude that, in the absence of high risk features (defined), breast conservation surgery and tamoxifen only is a safe option for T1 disease in the elderly, but that the risk of local relapse is considerably higher when this policy is employed for patients presenting with larger tumors