19 research outputs found
Stellar Disk Truncations: Where do we stand ?
In the light of several recent developments we revisit the phenomenon of
galactic stellar disk truncations. Even 25 years since the first paper on outer
breaks in the radial light profiles of spiral galaxies, their origin is still
unclear. The two most promising explanations are that these 'outer edges'
either trace the maximum angular momentum during the galaxy formation epoch, or
are associated with global star formation thresholds. Depending on their true
physical nature, these outer edges may represent an improved size
characteristic (e.g., as compared to D_25) and might contain fossil evidence
imprinted by the galaxy formation and evolutionary history. We will address
several observational aspects of disk truncations: their existence, not only in
normal HSB galaxies, but also in LSB and even dwarf galaxies; their detailed
shape, not sharp cut-offs as thought before, but in fact demarcating the start
of a region with a steeper exponential distribution of starlight; their
possible association with bars; as well as problems related to the
line-of-sight integration for edge-on galaxies (the main targets for truncation
searches so far). Taken together, these observations currently favour the
star-formation threshold model, but more work is necessary to implement the
truncations as adequate parameters characterising galactic disks.Comment: LaTeX, 10 pages, 6 figures, presented at the "Penetrating Bars
through Masks of Cosmic Dust" conference in South Africa, proceedings
published by Kluwer, and edited by Block, D.L., Freeman, K.C., Puerari, I., &
Groess, R; v3 to match published versio
Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii
Petrosian magnitudes were designed to help with the difficult task of
determining a galaxy's total light. Although these magnitudes (taken here as
the flux within 2R_P, with the inverted Petrosian index 1/eta(R_P)=0.2) can
represent most of an object's flux, they do of course miss the light outside of
the Petrosian aperture (2R_P). The size of this flux deficit varies
monotonically with the shape of a galaxy's light-profile, i.e., its
concentration. In the case of a de Vaucouleurs R^{1/4} profile, the deficit is
0.20 mag; for an R^{1/8} profile this figure rises to 0.50 mag. Here we provide
a simple method for recovering total (Sersic) magnitudes from Petrosian
magnitudes using only the galaxy concentration (R_90/R_50 or R_80/R_20) within
the Petrosian aperture. The corrections hold to the extent that Sersic's model
provides a good description of a galaxy's luminosity profile. We show how the
concentration can also be used to convert Petrosian radii into effective
half-light radii, enabling a robust measure of the mean effective surface
brightness. Our technique is applied to the SDSS DR2 Petrosian parameters,
yielding good agreement with the total magnitudes, effective radii, and mean
effective surface brightnesses obtained from the NYU--VAGC Sersic R^{1/n} fits
by Blanton et al. (2005). Although the corrective procedure described here is
specifically applicable to the SDSS DR2 and DR3, it is generally applicable to
all imaging data where any Petrosian index and concentration can be
constructed.Comment: AJ, in press after minor additions to the text (7 pages of text, plus
6 of figures and tables
On the Efficiency of the Tidal Stirring Mechanism for the Origin of Dwarf Spheroidals: Dependence on the Orbital and Structural Parameters of the Progenitor Disky Dwarfs
(Abridged) The tidal stirring model posits the formation of dSph galaxies via
the tidal interactions between rotationally-supported dwarfs and MW-sized host
galaxies. Using a set of collisionless N-body simulations, we investigate the
efficiency of the tidal stirring mechanism. We explore a wide variety of dwarf
orbital configurations and initial structures and demonstrate that in most
cases the disky dwarfs experience significant mass loss and their stellar
components undergo a dramatic morphological and dynamical transformation: from
disks to bars and finally to pressure-supported spheroidal systems with
kinematic and structural properties akin to those of the classic dSphs in the
Local Group (LG). Our results suggest that such tidal transformations should be
common occurrences within the currently favored cosmological paradigm and
highlight the key factor responsible for an effective metamorphosis to be the
strength of the tidal shocks at the pericenters of the orbit. We demonstrate
that the combination of short orbital times and small pericenters,
characteristic of dwarfs being accreted at high redshift, induces the strongest
transformations. Our models also indicate that the transformation efficiency is
affected significantly by the structure of the progenitor disky dwarfs. Lastly,
we find that the dwarf remnants satisfy the relation Vmax = \sqrt{3} * sigma,
where sigma is the 1D, central stellar velocity dispersion and Vmax is the
maximum halo circular velocity, with intriguing implications for the missing
satellites problem. Overall, we conclude that the action of tidal forces from
the hosts constitutes a crucial evolutionary mechanism for shaping the nature
of dwarf galaxies in environments such as that of the LG. Environmental
processes of this type should thus be included as ingredients in models of
dwarf galaxy formation and evolution.Comment: submitted to ApJ, 34 pages, 15 figures, LaTeX (uses emulateapj.cls
The kinematic identification of a thick stellar disc in M31
We present the first characterization of a thick disc component in the
Andromeda galaxy (M31) using kinematic data from the DEIMOS multi-object
spectrograph instrument on Keck II. Using 21 fields in the South West of the
galaxy, we measure the lag of this component with respect to the thin disc, as
well as the dispersion, metallicity and scale length of the component. We find
an average lag between the two components of =46.0+/-3.9km/s. The velocity
dispersion of the thick disc is sigma_{thick}=50.8+/-1.9km/s, greater than the
value of dispersion we determine for the thin disc,
sigma_{thin}=35.7+/-1.0km/s. The thick disc is more metal poor than the thin
disc, with [Fe/H]_{spec}=-1.0+/-0.1 compared to [Fe/H]_{spec}=-0.7+/-0.05 for
the thin disc. We measure a radial scale length of the thin and thick discs of
h_r=7.3+/-1.0 kpc and h_r=8.0+/-1.2 kpc. From this, we infer scale heights for
both discs of 1.1+/-0.2 kpc and 2.8+/-0.6 kpc, both of which are ~2--3 times
larger than those observed in the Milky Way. We estimate a mass range for the
thick disc component of 2.4x10^{10}Msun< M_{*,thick} <4.1x10^{10}Msun. This
value provides a useful constraint on possible formation mechanisms, as any
proposed method for forming a thick disc must be able to heat (or deposit) at
least this amount of material.Comment: 22 pages, 17 figures. Minor revisions made to text following referee
report. Accepted for publication in MNRA
Maintainable Concurrent Software
Abstract Constructing software that is easy to maintain and reuse is not a trivial task. When the software being constructed exploits concurrency, the task is complicated further. This thesis attempts to determine what shape an object-oriented programming language's concurrency support should take in order to aid the construction of concurrent software that is easy to maintain and reuse. This is achieved by analysing the component parts of concurrency, comparing and contrasting the way various object-oriented programming languages support concurrency, and analysing problems that arise when using these languages. Using this, the concurrency constructs that an object-oriented programming language should provide, and their inter-relationships, are described. Finally solutions to common problems requiring concurrency, constructed using these constructs, are provided. The particular contributions this thesis makes are: a taxonomy for classifying concurrency support in object-oriented programming languages, analysis of the issues arising when introducing concurrency support into object-oriented programming languages, and a working implementation of the concurrency constructs found to be necessary for an object-oriented programming language to support concurrency in a fashion that promotes the construction of software that is easy to maintain and reuse. Acknowledgements My supervisor, Ewan Tempero, not only got me interested in object-oriented programming and concurrency, but taught me how to research, how to write, was patient when I went AWOL, and was generally an excellent all round supervisor. Other students have always been amazed at the regularity with which Ewan would meet and chat with me. For all this I am truly grateful. When I first embarked upon my masters, three years ago, many people had trouble understanding why I was leaving a well paid job to return to academia. Luckily I had the support of several friends. To Lisa, Michele, Karen, Miki, Regan, and Simon I am grateful for encouraging me to leave my job and be poor
The Herschel-SPIRE Legacy Survey (HSLS): the scientific goals of a shallow and wide submillimeter imaging survey with SPIRE
A large sub-mm survey with Herschel will enable many exciting science opportunities, especially in an era of wide-field optical and radio surveys and high resolution cosmic microwave background experiments. The Herschel-SPIRE Legacy Survey (HSLS), will lead to imaging data over 4000 sq. degrees at 250, 350, and 500 micron. Major Goals of HSLS are: (a) produce a catalog of 2.5 to 3 million galaxies down to 26, 27 and 33 mJy (50% completeness; 5 sigma confusion noise) at 250, 350 and 500 micron, respectively, in the southern hemisphere (3000 sq. degrees) and in an equatorial strip (1000 sq. degrees), areas which have extensive multi-wavelength coverage and are easily accessible from ALMA. Two thirds of the of the sources are expected to be at z > 1, one third at z > 2 and about a 1000 at z > 5. (b) Remove point source confusion in secondary anisotropy studies with Planck and ground-based CMB data. (c) Find at least 1200 strongly lensed bright sub-mm sources leading to a 2% test of general relativity. (d) Identify 200 proto-cluster regions at z of 2 and perform an unbiased study of the environmental dependence of star formation. (e) Perform an unbiased survey for star formation and dust at high Galactic latitude and make a census of debris disks and dust around AGB stars and white dwarfs
An examination of the intersection environment associated with perceived crash risk among school-aged children: using street-level imagery and computer vision
While computer vision techniques and big data of street-level imagery are getting increasing attention, a "black-box" model of deep learning hinders the active application of these techniques to the field of traffic safety research. To address this issue, we presented a semantic scene labeling approach that leverages wide-coverage street-level imagery for the purpose of exploring the association between built environment characteristics and perceived crash risk at 533 intersections. The environmental attributes were measured at eye-level using scene segmentation and object detection algorithms, and they were classified as one of four intersection typologies using the k-means clustering method. Data on perceived crash risk were collected from a questionnaire conducted on 799 children 10 to 12 years old. Our results showed that environmental features derived from deep learning algorithms were significantly associated with perceived crash risk among school-aged children. The results have revealed that some of the intersection characteristics including the proportional area of sky and roadway were significantly associated with the perceived crash risk among school-aged children. In particular, road width had dominant influence on risk perception. The findings provide information useful to providing appropriate and proactive interventions that may reduce the risk of crashes at intersections