29 research outputs found
New Hubble Space Telescope Discoveries of Type Ia Supernovae at z > 1: Narrowing Constraints on the Early Behavior of Dark Energy
We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble Space
Telescope (HST) and have used them to trace the history of cosmic expansion
over the last 10 billion years. These objects, which include 13
spectroscopically confirmed SNe Ia at z > 1, were discovered during 14 epochs
of reimaging of the GOODS fields North and South over two years with the
Advanced Camera for Surveys on HST. Together with a recalibration of our
previous HST-discovered SNe Ia, the full sample of 23 SNe Ia at z > 1 provides
the highest-redshift sample known. Combined with previous SN Ia datasets, we
measured H(z) at discrete, uncorrelated epochs, reducing the uncertainty of
H(z>1) from 50% to under 20%, strengthening the evidence for a cosmic jerk--the
transition from deceleration in the past to acceleration in the present. The
unique leverage of the HST high-redshift SNe Ia provides the first meaningful
constraint on the dark energy equation-of-state parameter at z >1.
The result remains consistent with a cosmological constant (w(z)=-1), and
rules out rapidly evolving dark energy (dw/dz >>1). The defining property of
dark energy, its negative pressure, appears to be present at z>1, in the epoch
preceding acceleration, with ~98% confidence in our primary fit. Moreover, the
z>1 sample-averaged spectral energy distribution is consistent with that of the
typical SN Ia over the last 10 Gyr, indicating that any spectral evolution of
the properties of SNe Ia with redshift is still below our detection threshold.Comment: typos, references corrected, minor additions to exposition 82 pages,
17 figures, 6 tables. Data also available at:
http://braeburn.pha.jhu.edu/~ariess/R06. Accepted, Astrophysical Journal vol.
656 for March 10, 200
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
Type Ia Supernova Discoveries at z>1 From the Hubble Space Telescope: Evidence for Past Deceleration and Constraints on Dark Energy Evolution
We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space
Telescope (HST) and have used them to provide the first conclusive evidence for
cosmic deceleration that preceded the current epoch of cosmic acceleration.
These objects, discovered during the course of the GOODS ACS Treasury program,
include 6 of the 7 highest-redshift SNe Ia known, all at z>1.25, and populate
the Hubble diagram in unexplored territory. The luminosity distances to these
and 170 previous SNe Ia are provided. A purely kinematic interpretation of the
SN Ia sample provides evidence at the > 99% confidence level for a transition
from deceleration to acceleration or similarly, strong evidence for a cosmic
jerk. Using a simple model of the expansion history, the transition between the
two epochs is constrained to be at z=0.46 +/- 0.13. The data are consistent
with the cosmic concordance model of Omega_M ~ 0.3, Omega_Lambda~0.7
(chi^2_dof=1.06), and are inconsistent with a simple model of evolution or dust
as an alternative to dark energy. For a flat Universe with a cosmological
constant. When combined with external flat-Universe constraints we find w=-1.02
+ 0.13 - 0.19 (and w_0$, and its time evolution, dw/dz,
are a factor of ~8 more precise than its first estimate and twice as precise as
those without the SNe Ia discovered with HST. Our constraints are consistent
with the static nature of and value of w expected for a cosmological constant
(i.e., w_0 = -1.0, dw/dz = 0), and are inconsistent with very rapid evolution
of dark energy. We address consequences of evolving dark energy for the fate of
the Universe.Comment: Accepted Astrophysical Journal, to Appear June 2004 72 pages, 17
Figure
Twenty-Three High-Redshift Supernovae from the Institute for Astronomy Deep Survey: Doubling the Supernova Sample at z > 0.7
We present photometric and spectroscopic observations of 23 high-redshift supernovae (SNe) spanning a range of z = 0.34-1.03, nine of which are unambiguously classified as Type la. These SNe were discovered during the IfA Deep Survey, which began in 2001Partial support for this work was provided by NASA grants
GO-08641 and GO-09118 from the Space Telescope Science
Institute, which is operated by AURA, Inc., under NASA
contract NAS5-26555. Funding was also provided by NSF
grant AST 02-06329. S. T. H. acknowledges support from the
NASA LTSA grant NAG5-9364
The Hubble Higher-Z Supernova Search: Supernovae to z=1.6 and Constraints on Type Ia Progenitor Models
We present results from the Hubble Higher-z Supernova Search, the first
space-based open field survey for supernovae (SNe). In cooperation with the
Great Observatories Origins Deep Survey, we have used the Hubble Space
Telescope with the Advanced Camera for Surveys to cover 300 square arcmin in
the area of the Chandra Deep Field South and the Hubble Deep Field North on
five separate search epochs (separated by 45 day intervals) to a limiting
magnitude of z'=26. These deep observations have allowed us to discover 42 SNe
in the redshift range 0.2 < z < 1.6. As these data span a large range in
redshift, they are ideal for testing the validity of Type Ia supernova
progenitor models with the distribution of expected ``delay times,'' from
progenitor star formation to SN Ia explosion, and the SN rates these models
predict. Through a Bayesian maximum likelihood test, we determine which
delay-time models best reproduce the redshift distribution of SNe Ia discovered
in this survey. We find that models that require a large fraction of ``prompt''
(less than 2 Gyr) SNe Ia poorly reproduce the observed redshift distribution
and are rejected at 95% confidence. We find that Gaussian models best fit the
observed data for mean delay times in the range of 3 to 4 Gyr.Comment: 32 pages, 15 figures, accepted for publication in the Astrophysical
Journa
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
Postural Stability Margins as a Function of Support Surface Slopes.
This investigation examined the effects of slope of the surface of support (35°, 30°, 20°, 10° Facing(Toe) Down, 0° Flat and 10°, 20°, 25° Facing (Toe) Up) and postural orientation on the margins of postural stability in quiet standing of young adults. The findings showed that the center of pressure-CoP (displacement, area and length) had least motion at the baseline (0° Flat) platform condition that progressively increased as a function of platform angle in both facing up and down directions. The virtual time to collision (VTC) dynamics revealed that the spatio-temporal margins to the functional stability boundary were progressively smaller and the VTC time series also more regular (SampEn-Sample Entropy) as slope angle increased. Surface slope induces a restricted stability region with lower dimension VTC dynamics that is more constrained when postural orientation is facing down the slope. These findings provide further evidence that VTC acts as a control variable in standing posture that is influenced by the emergent dynamics of the individual-environment-task interaction