161 research outputs found
Artificial intelligence as writing: knowledge-based hypertext systems as a medium for communication
This thesis is an exploration of a new metaphor for artificial intelligence (AI). Traditionally, the computer within AI has been viewed as an agent, one with which the user engages in a conversation. More recently certain researchers have proposed the notion that artificial intelligence (and indeed computing in general) can be more appropriately seen as a form of writing. Initially this thesis reviews the literature in this area, and aspects of AI which support the approach. Features of writing are then described which show parallels with AI. This then allows us to take lessons from the history and development of both traditional writing and the new computer-based writing systems to inform the design of a new type of artificial intelligence system. A design based on these features, called Running Texts is presented through a number of small examples. Issues that arise from these and possible future developments, based on the implementation are then discussed. A rationale for users choosing to learn a system such as Running Texts is proposed, as benefits from the psychological and social implications of writing can be applied to AI systems, when they are seen as writing. The same parallels point out potential problems, and suggest new ways to see the relation between AI and thought
ACS photometry of extended, luminous globular clusters in the outskirts of M31
A new population of extended, luminous globular clusters has recently been
discovered in the outskirts of M31. These objects have luminosities typical of
classical globular clusters, but much larger half-light radii. We report the
first results from deep ACS imaging of four such clusters, one of which is a
newly-discovered example lying at a projected distance of ~60 kpc from M31. Our
F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the
horizontal branch level, and clearly demonstrate, for the first time, that all
four clusters are composed of >10 Gyr old, metal-poor stellar populations. No
evidence for multiple populations is observed. From a comparison with Galactic
globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H]
< -1.8. The observed horizontal branch morphologies show a clear second
parameter effect between the clusters. Preliminary radial luminosity profiles
suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median
value of the globular cluster luminosity function. Our results confirm that
these four objects are bona fide old, metal-poor globular clusters, albeit with
combined structures and luminosities unlike those observed for any other
globular clusters in the Local Group or beyond.Comment: 12 pages, 3 figures, 1 table; accepted for publication in ApJ Letter
N-body Models of Extended Clusters
We use direct N-body simulations to investigate the evolution of star
clusters with large size-scales with the particular goal of understanding the
so-called extended clusters observed in various Local Group galaxies, including
M31 and NGC6822. The N-body models incorporate a stellar mass function, stellar
evolution and the tidal field of a host galaxy. We find that extended clusters
can arise naturally within a weak tidal field provided that the tidal radius is
filled at the start of the evolution. Differences in the initial tidal
filling-factor can produce marked differences in the subsequent evolution of
clusters and the size-scales that would be observed. These differences are more
marked than any produced by internal evolution processes linked to the
properties of cluster binary stars or the action of an intermediate-mass black
hole, based on models performed in this work and previous work to date. Models
evolved in a stronger tidal field show that extended clusters cannot form and
evolve within the inner regions of a galaxy such as M31. Instead our results
support the suggestion many extended clusters found in large galaxies were
accreted as members of dwarf galaxies that were subsequently disrupted. Our
results also enhance the recent suggestion that star clusters evolve to a
common sequence in terms of their size and mass.Comment: 12 pages, 8 figures, accepted by MNRA
An HST/ACS View of the Inhomogeneous Outer Halo of M31
We present a high precision photometric view of the stellar populations in
the outer halo of M31, using data taken with the Hubble Space Telescope
Advanced Camera for Surveys (HST/ACS). We analyse the field populations
adjacent to 11 luminous globular clusters which sample the galactocentric
radial range 18 < R < 100 kpc and reach a photometric depth of ~2.5 magnitudes
below the horizontal branch (m_F814W ~27 mag). The colour-magnitude diagrams
(CMDs) are well populated out to ~60 kpc and exhibit relatively metal-rich red
giant branches, with the densest fields also showing evidence for prominent red
clumps. We use the Dartmouth isochrones to construct metallicity distribution
functions (MDFs) which confirm the presence of dominant populations with
= -0.6 to -1.0 dex and considerable metallicity dispersions of 0.2 to
0.3 dex (assuming a 10 Gyr population and scaled-Solar abundances). The average
metallicity over the range 30 - 60 kpc is [Fe/H] = -0.8 +/- 0.14 dex, with no
evidence for a significant radial gradient. Metal-poor stars ([Fe/H] <= -1.3)
typically account for < 10-20 % of the population in each field, irrespective
of radius. Assuming our fields are unbiased probes of the dominant stellar
populations in these parts, we find that the M31 outer halo remains
considerably more metal-rich than that of the Milky Way out to at least 60 kpc.Comment: Accepted for publication in MNRAS. 10 pages, 6 figure
Young accreted globular clusters in the outer halo of M31
We report on Gemini/GMOS observations of two newly discovered globular
clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at
a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc,
onto a field halo substructure known as the South-West Cloud. We measure radial
velocities for the two clusters which confirm that they are almost certainly
physically associated with this feature. Colour-magnitude diagrams reveal
strikingly short, exclusively red horizontal branches in both PA-7 and PA-8;
both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the
morphology of the horizontal branch is maximally sensitive to age, and we use
the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both
objects are very likely to be at least 2 Gyr younger than the oldest Milky Way
globular clusters. Our observations provide strong evidence for young globular
clusters being accreted into the remote outer regions of M31 in a manner
entirely consistent with the established picture for the Milky Way, and add
credence to the idea that similar processes play a central role in determining
the composition of globular cluster systems in large spiral galaxies in
general.Comment: 14 pages, 8 figures, accepted for publication in MNRA
Newly-Discovered Globular Clusters in NGC 147 and NGC 185 from PAndAS
Using data from the Pan-Andromeda Archaeological Survey (PAndAS), we have
discovered four new globular clusters (GCs) associated with the M31 dwarf
elliptical (dE) satellites NGC 147 and NGC 185. Three of these are associated
with NGC 147 and one with NGC 185. All lie beyond the main optical boundaries
of the galaxies and are the most remote clusters yet known in these systems.
Radial velocities derived from low resolution spectra are used to argue that
the GCs are bound to the dwarfs and are not part of the M31 halo population.
Combining PAndAS with UKIRT/WFCAM data, we present the first homogeneous
optical and near-IR photometry for the entire GC systems of these dEs.
Colour-colour plots and published colour-metallicity relations are employed to
constrain GC ages and metallicities. It is demonstrated that the clusters are
in general metal poor ([Fe/H] < -1.25 dex), while the ages are more difficult
to constrain. The mean (V-I) colours of the two GC systems are very similar
to those of the GC systems of dEs in the Virgo and Fornax clusters, as well as
the extended halo GC population in M31. The new clusters bring the GC specific
frequency (S_N) to ~9 in NGC 147 and ~5 in NGC 185, consistent with values
found for dEs of similar luminosity residing in a range of environments.Comment: 14 pages, 6 figures, 6 tables, accepted for publication in MNRA
Dynamics in the satellite system of Triangulum: Is AndXXII a dwarf satellite of M33?
We present results from a spectroscopic survey of the dwarf spheroidal And
XXII and the two extended clusters EC1 and EC2. These three objects are
candidate satellites of the Triangulum galaxy, M33, which itself is likely a
satellite of M31. We use the DEep Imaging Multi-Object Spectrograph mounted on
the Keck-II telescope to derive radial velocities for candidate member stars of
these objects and thereby identify the stars that are most likely actual
members. Eleven most probable stellar members (of 13 candidates) are found for
AndXXII. We obtain an upper limit of sigma_v < 6.0 km s-1 for the velocity
dispersion of AndXXII, [Fe/H] ~ -1.6 for its metallicity, and 255pc for the
Plummer radius of its projected density profile. We construct a colour
magnitude diagram for AndXXII and identify both the red giant branch and the
horizontal branch. The position of the latter is used to derive a heliocentric
distance to And XXII of 853 pm 26 kpc. The combination of the radial velocity,
distance, and angular position of AndXXII indicates that it is a strong
candidate for being the first known satellite of M33 and one of the very few
examples of a galactic satellite of a satellite. N-body simulations imply that
this conclusion is unchanged even if M31 and M33 had a strong encounter in the
past few Gyr. We test the hypothesis that the extended clusters highlight
tidally stripped galaxies by searching for an excess cloud of halo-like stars
in their vicinity. We find such a cloud for the case of EC1 but not EC2. The
three objects imply a dynamical mass for M33 that is consistent with previous
estimates.Comment: 14 pages, 14 figures, revised for MNRAS publicatio
Major Substructure in the M31 Outer Halo: the South-West Cloud
We undertake the first detailed analysis of the stellar population and
spatial properties of a diffuse substructure in the outer halo of M31. The
South-West Cloud lies at a projected distance of ~100 kpc from the centre of
M31, and extends for at least ~50 kpc in projection. We use Pan-Andromeda
Archaeological Survey photometry of red giant branch stars to determine a
distance to the South-West Cloud of 793 +/- 45 kpc. The metallicity of the
cloud is found to be [Fe/H] = -1.3 +/- 0.1. This is consistent with the
coincident globular clusters PAndAS-7 and PAndAS-8, which have metallicities
determined using an independent technique of [Fe/H] = -1.35 +/- 0.15. We
measure a brightness for the Cloud of M_V = -12.1 mag; this is ~75 per cent of
the luminosity implied by the luminosity-metallicity relation. Under the
assumption that the South-West Cloud is the visible remnant of an accreted
dwarf satellite, this suggests that the progenitor object was amongst M31's
brightest dwarf galaxies prior to disruption.Comment: 13 pages, 9 figures, accepted for publication in MNRA
The kinematic footprints of five stellar streams in Andromeda's halo
(abridged) We present a spectroscopic analysis of five stellar streams (`A',
`B', `Cr', `Cp' and `D') as well as the extended star cluster, EC4, which lies
within streamC, all discovered in the halo of M31 from our CFHT/MegaCam survey.
These spectroscopic results were initially serendipitous, making use of our
existing observations from the DEep Imaging Multi-Object Spectrograph mounted
on the Keck II telescope, and thereby emphasizing the ubiquity of tidal streams
that account for ~70% of the M31 halo stars in the targeted fields. Subsequent
spectroscopy was then procured in streamCr/p and streamD to trace the velocity
gradient along the streams. For the cluster EC4, candidate member stars with
average [Fe/H]~-1.4 (Fe/H_spec=-1.6), are found at v_{hel}=-285 km/s suggesting
it could be related to streamCp. No similarly obvious cold kinematic candidate
is found for streamD, although candidates are proposed in both of two
spectroscopic pointings along the stream (both at -400 km/s). Spectroscopy near
the edge of streamB suggests a likely kinematic detection, while a candidate
kinematic detection of streamA is found (plausibly associated to M33 rather
than M31). The low dispersion of the streams in kinematics, physical thickness,
and metallicity makes it hard to reconcile with a scenario whereby these stream
structures as an ensemble are related to the giant southern stream. We conclude
that the M31 stellar halo is largely made up of multiple kinematically cold
streams.Comment: 19 pages, 12 figures, accepted in MNRAS. High resolution version,
with fig10 here: http://www.ast.cam.ac.uk/~schapman/streams.pd
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