218 research outputs found
Discovery of a Methane Dwarf from the IfA-Deep Survey
We present the discovery of a distant methane dwarf, the first from the
Institute for Astronomy (IfA) Deep Survey. The object ("IfA 0230-Z1") was
identified from deep optical I and z'-band imaging, being conducted as an
IfA-wide collaboration using the prime-focus imager Suprime-Cam on the Subaru
8.2-m Telescope. IfA 0230-Z1 is extremely red in the Iz'J (0.8--1.2 micron)
bands but relatively blue in J-H; such colors are uniquely characteristic of T
dwarfs. A near-IR spectrum taken with the Keck Telescope shows strong H2O
absorption and a continuum break indicative of CH4, confirming the object has a
very cool atmosphere. Comparison with nearby T dwarfs gives a spectral type of
T3-T4 and a distance of ~45 pc. Simple estimates based on previous T dwarf
discoveries suggest that the IfA survey will find a comparable number of T
dwarfs as the 2MASS survey, albeit at a much larger average distance. We also
discuss the survey's ability to probe the galactic scale height of ultracool (L
and T) dwarfs.Comment: Astrophysical Journal Letters, in pres
Detection efficiency and photometry in supernova surveys - the Stockholm VIMOS Supernova Survey I
The aim of the work presented in this paper is to test and optimise supernova
detection methods based on the optimal image subtraction technique. The main
focus is on applying the detection methods to wide field supernova imaging
surveys and in particular to the Stockholm VIMOS Supernova Survey (SVISS). We
have constructed a supernova detection pipeline for imaging surveys. The core
of the pipeline is image subtraction using the ISIS 2.2 package. Using real
data from the SVISS we simulate supernovae in the images, both inside and
outside galaxies. The detection pipeline is then run on the simulated frames
and the effects of image quality and subtraction parameters on the detection
efficiency and photometric accuracy are studied. The pipeline allows efficient
detection of faint supernovae in the deep imaging data. It also allows
controlling and correcting for possible systematic effects in the SN detection
and photometry. We find such a systematic effect in the form of a small
systematic flux offset remaining at the positions of galaxies in the subtracted
frames. This offset will not only affect the photometric accuracy of the
survey, but also the detection efficiencies. Our study has shown that ISIS 2.2
works well for the SVISS data. We have found that the detection efficiency and
photometric accuracy of the survey are affected by the stamp selection for the
image subtraction and by host galaxy brightness. With our tools the subtraction
results can be further optimised, any systematic effects can be controlled and
photometric errors estimated, which is very important for the SVISS, as well as
for future SN searches based on large imaging surveys such as Pan-STARRS and
LSST.Comment: 17 pages, 10 figure, accepted for publication in A&
Naked Singularity in a Modified Gravity Theory
The cosmological constant induced by quantum fluctuation of the graviton on a
given background is considered as a tool for building a spectrum of different
geometries. In particular, we apply the method to the Schwarzschild background
with positive and negative mass parameter. In this way, we put on the same
level of comparison the related naked singularity (-M) and the positive mass
wormhole. We discuss how to extract information in the context of a f(R)
theory. We use the Wheeler-De Witt equation as a basic equation to perform such
an analysis regarded as a Sturm-Liouville problem . The application of the same
procedure used for the ordinary theory, namely f(R)=R, reveals that to this
approximation level, it is not possible to classify the Schwarzschild and its
naked partner into a geometry spectrum.Comment: 8 Pages. Contribution given to DICE 2008. To appear in the
proceeding
Scale-Free model for governing universe dynamics
We investigate the effects of scale-free model on cosmology, providing, in
this way, a statistical background in the framework of general relativity. In
order to discuss properties and time evolution of some relevant universe
dynamical parameters (cosmographic parameters), such as (Hubble
parameter), (deceleration parameter), (jerk parameter) and
(snap parameter), which are well re-defined in the framework of scale-free
model, we analyze a comparison between WMAP data. Hence the basic purpose of
the work is to consider this statistical interpretation of mass distribution of
universe, in order to have a mass density dynamics, not inferred from
Friedmann equations, via scale factor . This model, indeed, has been used
also to explain a possible origin and a viable explanation of cosmological
constant, which assumes a statistical interpretation without the presence of
extended theories of gravity; hence the problem of dark energy could be
revisited in the context of a classical probability distribution of mass, which
is, in particular, for the scale-free model, , with
. The CDM model becomes, with these considerations, a
consequence of the particular statistics together with the use of general
relativity.Comment: 7 pages, 4 figure
23 High Redshift Supernovae from the IfA Deep Survey: Doubling the SN Sample at z>0.7
We present photometric and spectroscopic observations of 23 high redshift
supernovae spanning a range of z=0.34-1.03, 9 of which are unambiguously
classified as Type Ia. These supernovae were discovered during the IfA Deep
Survey, which began in September 2001 and observed a total of 2.5 square
degrees to a depth of approximately m=25-26 in RIZ over 9-17 visits, typically
every 1-3 weeks for nearly 5 months, with additional observations continuing
until April 2002. We give a brief description of the survey motivations,
observational strategy, and reduction process. This sample of 23 high-redshift
supernovae includes 15 at z>0.7, doubling the published number of objects at
these redshifts, and indicates that the evidence for acceleration of the
universe is not due to a systematic effect proportional to redshift. In
combination with the recent compilation of Tonry et al. (2003), we calculate
cosmological parameter density contours which are consistent with the flat
universe indicated by the CMB (Spergel et al. 2003). Adopting the constraint
that Omega_total = 1.0, we obtain best-fit values of (Omega_m,
Omega_Lambda)=(0.33, 0.67) using 22 SNe from this survey augmented by the
literature compilation. We show that using the empty-beam model for
gravitational lensing does not eliminate the need for Omega_Lambda > 0.
Experience from this survey indicates great potential for similar large-scale
surveys while also revealing the limitations of performing surveys for z>1 SNe
from the ground.Comment: 67 pages, 12 figures, 12 tables, accepted for publication in the
Astrophysical Journa
Hubble Space Telescope Observations of Nine High-Redshift ESSENCE Supernovae
We present broad-band light curves of nine supernovae ranging in redshift
from 0.5 to 0.8. The supernovae were discovered as part of the ESSENCE project,
and the light curves are a combination of Cerro Tololo 4-m and Hubble Space
Telescope (HST) photometry. On the basis of spectra and/or light-curve fitting,
eight of these objects are definitely Type Ia supernovae, while the
classification of one is problematic. The ESSENCE project is a five-year
endeavor to discover about 200 high-redshift Type Ia supernovae, with the goal
of tightly constraining the time average of the equation-of-state parameter [w
= p/(rho c^2)] of the "dark energy." To help minimize our systematic errors,
all of our ground-based photometry is obtained with the same telescope and
instrument. In 2003 the highest-redshift subset of ESSENCE supernovae was
selected for detailed study with HST. Here we present the first photometric
results of the survey. We find that all but one of the ESSENCE SNe have slowly
declining light curves, and the sample is not representative of the
low-redshift set of ESSENCE Type Ia supernovae. This is unlikely to be a sign
of evolution in the population. We attribute the decline-rate distribution of
HST events to a selection bias at the high-redshift edge of our sample and find
that such a bias will infect other magnitude-limited SN Ia searches unless
appropriate precautions are taken.Comment: 62 pages, 18 numbered figures, accepted for publication in the
Astronomical Journa
Averaging inhomogeneities in scalar-tensor cosmology
The backreaction of inhomogeneities on the cosmic dynamics is studied in the
context of scalar-tensor gravity. Due to terms of indefinite sign in the
non-canonical effective energy tensor of the Brans-Dicke-like scalar field,
extra contributions to the cosmic acceleration can arise. Brans-Dicke and
metric f(R) gravity are presented as specific examples. Certain representation
problems of the formalism peculiar to these theories are pointed out.Comment: Comments and references added. 14 page
Testing homogeneity with galaxy number counts : light-cone metric and general low-redshift expansion for a central observer in a matter dominated isotropic universe without cosmological constant
As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test this hypothesis we calculate the general analytical formula to fifth order
for the redshift spherical shell mass. Using the same analytical method we
write the metric in the light-cone by introducing a gauge invariant quantity
which together with the luminosity distance completely
determine the light-cone geometry of a LTB model.Comment: 13 page
Corrections to the apparent value of the cosmological constant due to local inhomogeneities
Supernovae observations strongly support the presence of a cosmological
constant, but its value, which we will call apparent, is normally determined
assuming that the Universe can be accurately described by a homogeneous model.
Even in the presence of a cosmological constant we cannot exclude nevertheless
the presence of a small local inhomogeneity which could affect the apparent
value of the cosmological constant. Neglecting the presence of the
inhomogeneity can in fact introduce a systematic misinterpretation of
cosmological data, leading to the distinction between an apparent and true
value of the cosmological constant. We establish the theoretical framework to
calculate the corrections to the apparent value of the cosmological constant by
modeling the local inhomogeneity with a solution. Our assumption
to be at the center of a spherically symmetric inhomogeneous matter
distribution correspond to effectively calculate the monopole contribution of
the large scale inhomogeneities surrounding us, which we expect to be the
dominant one, because of other observations supporting a high level of isotropy
of the Universe around us.
By performing a local Taylor expansion we analyze the number of independent
degrees of freedom which determine the local shape of the inhomogeneity, and
consider the issue of central smoothness, showing how the same correction can
correspond to different inhomogeneity profiles. Contrary to previous attempts
to fit data using large void models our approach is quite general. The
correction to the apparent value of the cosmological constant is in fact
present for local inhomogeneities of any size, and should always be taken
appropriately into account both theoretically and observationally.Comment: 16 pages,new sections added analyzing central smoothness and accuracy
of the Taylor expansion approach, Accepted for publication by JCAP. An essay
based on this paper received honorable mention in the 2011 Essay Context of
the Gravity Research Foundatio
Can the cosmological constant be mimicked by smooth large-scale inhomogeneities for more than one observable?
As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test such an hypothesis we calculate the low redshift expansion of the
luminosity distance and the redshift spherical shell mass density
for a central observer in a LTB space without cosmological constant and
show how they cannot fit the observations implied by a model if
the conditions to avoid a weak central singularity are imposed, i.e. if the
matter distribution is smooth everywhere. Our conclusions are valid for any
value of the cosmological constant, not only for as
implied by previous proofs that has to be positive in a smooth LTB
space, based on considering only the luminosity distance.
The observational signatures of smooth LTB matter dominated models are
fundamentally different from the ones of models not only because
it is not possible to reproduce a negative apparent central deceleration
, but because of deeper differences in their space-time geometry
which make impossible the inversion problem when more than one observable is
considered, and emerge at any redshift, not only for .Comment: 18 pages, corrected a typo in the definition of the energy density
which doesn't change the conclusion, references adde
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