12,503 research outputs found
MegaMorph: classifying galaxy morphology using multi-wavelength S\'ersic profile fits
Aims. This work investigates the potential of using the wavelength-dependence
of galaxy structural parameters (S\'ersic index, n, and effective radius, Re)
to separate galaxies into distinct types. Methods. A sample of nearby galaxies
with reliable visual morphologies is considered, for which we measure
structural parameters by fitting multi-wavelength single-S\'ersic models.
Additionally, we use a set of artificially redshifted galaxies to test how
these classifiers behave when the signal-to-noise decreases. Results. We show
that the wavelength-dependence of n may be employed to separate
visually-classified early- and late-type galaxies, in a manner similar to the
use of colour and n. Furthermore, we find that the wavelength variation of n
can recover galaxies that are misclassified by these other morphological
proxies. Roughly half of the spiral galaxies that contaminate an early-type
sample selected using (u-r) versus n can be correctly identified as late-types
by N, the ratio of n measured in two different bands. Using a set of
artificially-redshifted images, we show that this technique remains effective
up to z ~ 0.1. N can therefore be used to achieve purer samples of early-types
and more complete samples of late-types than using a colour-n cut alone. We
also study the suitability of R, the ratio of Re in two different bands, as a
morphological classifier, but find that the average sizes of both early- and
late-type galaxies do not change substantially over optical wavelengths.Comment: 6 pages, 2 figures, 2 tables, Accepted for publication in A&
On the monotonicity of the correction term in Ramanujan's factorial approximation
We present two new proofs of the monotonicity of the correction term
in Ramanujan's refinement of Stirling's formula.Comment: Latex, 5 page
The Open String Regge Trajectory and Its Field Theory Limit
We study the properties of the leading Regge trajectory in open string theory
including the open string planar one-loop corrections. With SU(N) Chan-Paton
factors, the sum over planar open string multi-loop diagrams describes the 't
Hooft limit N\to\infty. Our motivation is to improve the understanding of open
string theory at finite \alpha' as a model of gauge theories. SU(N) gauge
theories in D space-time dimensions are described by requiring open strings to
end on a stack of N Dp-branes of space-time dimension D=p+1. The large N
leading trajectory \alpha(t)=1+\alpha' t+\Sigma(t) can be extracted, through
order g^2, from the s\to-\infty limit, at fixed t, of the four open string tree
and planar loop diagrams. We analyze the t\to0 behavior with the result that
\Sigma(t)\sim-Cg^2(-\alpha' t)^{(D-4)/2}/(D-4). This result precisely tracks
the 1-loop Reggeized gluon of gauge theory in D>4 space-time dimensions. In
particular, for D\to4 it reproduces the known infrared divergences of gauge
theory in 4 dimensions with a Regge trajectory behaving as -\ln(-\alpha^\prime
t). We also study \Sigma(t) in the limit t\to-\infty and show that, when D<8,
it behaves as \alpha^\prime t/(\ln(-\alpha^\prime t))^{\gamma}, where \gamma>0
depends on D and the number of massless scalars. Thus, as long as 4<D<8, the
1-loop correction stays small relative to the tree trajectory for the whole
range -\infty<t<0. Finally we present the results of numerical calculations of
\Sigma(t) for all negative t.Comment: 19 pages, 5 figure
Spectroscopic characterization of X-ray emitting young stars associated with the Sh 2-296 nebula
We studied a sample of stars associated with the Sh 2-296 nebula, part of the
reflection nebulae complex in the region of Canis Major (CMa R1). Our sample
corresponds to optical counterparts of X-ray sources detected from observations
with the XMM-Newton satellite, which revealed dozens of possible low-mass young
stars not yet known in this region.
A sample of 58 young star candidates were selected based on optical spectral
features, mainly H{\alpha} and lithium lines, observed with multi-objects
spectroscopy performed by the Gemini South telescope. Among the candidates, we
find 41 confirmed T Tauri and 15 very likely young stars. Based on the
H{\alpha} emission, the T Tauri stars were distinguished between classical
(17%) and weak-lined (83%), but no significant difference was found in the age
and mass distribution of these two classes.
The characterization of the sample was complemented by near- and mid-infrared
data, providing an estimate of ages and masses from the comparison with
pre-main-sequence evolutionary models. While half of the young stars have an
age of 1-2 Myrs or less, only a small fraction (~25%) shows evidence of IR
excess revealing the presence of circumstellar discs. This low fraction is
quite rare compared to most young star-forming regions, suggesting that some
external factor has accelerated the disc dissipation
Insights into the quark-gluon vertex from lattice QCD and meson spectroscopy
By comparing successful quark-gluon vertex interaction models with the
corresponding interaction extracted from lattice-QCD data on the quark's
propagator, we identify common qualitative features which could be important to
tune future interaction models beyond the rainbow ladder approximation.
Clearly, a quantitative comparison is conceptually not simple, but
qualitatively the results suggest that a realistic interaction should be
relatively broad with a strong support at about ~GeV and
infrared-finite
Towards flavored bound states beyond rainbows and ladders
We give a snapshot of recent progress in solving the Dyson-Schwinger equation
with a beyond rainbow-ladder ansatz for the dressed quark-gluon vertex which
includes ghost contributions. We discuss the motivations for this approach with
regard to heavy-flavored bound states and form factors and briefly describe
future steps to be taken.Comment: Contribution to the proceedings of the XXXVI Reuni\~ao de Trabalho
sobre F\'isica Nuclear no Brasil which took place in Maresias, S\~ao Paulo,
Brazil. 8 pages, 3 figures. AIP proceeding styl
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