223 research outputs found
On High-Rate Cryptographic Compression Functions
The security of iterated hash functions relies on the properties of underlying compression functions. We study highly efficient compression functions based on block ciphers. We propose a model for high-rate compression functions, and give an upper bound for the rate of any collision resistant compression function in our model. In addition, we show that natural generalizations of constructions by Preneel, Govaerts, and Vandewalle to the case of rate-2 compression functions are not collision resistant
Identification and dynamics of a beneficial mutation in a long-term evolution experiment with Escherichia coli
<p>Abstract</p> <p>Background</p> <p>Twelve populations of <it>E. coli </it>were serially propagated for 20,000 generations in a glucose-supplemented minimal medium in order to study the dynamics of evolution. We sought to find and characterize one of the beneficial mutations responsible for the adaptation and other phenotypic changes, including increased cell size, in one of these populations.</p> <p>Results</p> <p>We used transposon-tagging followed by P1-transduction into the ancestor, screening for increased cell size and fitness, co-transduction analysis, and DNA sequencing. We identified a 1-bp insertion in the BoxG1 region located upstream of <it>glmUS</it>, an operon involved in cell-wall biosynthesis. When transduced into the ancestor, this mutation increased competitive fitness by about 5%. This mutation spread through its population of origin between 500 and 1500 generations. Mutations in this region were not found in the other 11 evolving populations, even after 20,000 generations.</p> <p>Conclusion</p> <p>The 1-bp insertion in the BoxG1 region near <it>glmUS </it>was demonstrably beneficial in the environment in which it arose. The absence of similar mutations in the other evolved populations suggests that they substituted other mutations that rendered this particular mutation unimportant. These results show the unpredictability of adaptive evolution, whereas parallel substitutions at other loci in these same populations reveal the predictability.</p
Outliers from the Mass--Metallicity Relation II: A Sample of Massive Metal-Poor Galaxies from SDSS
We present a sample of 42 high-mass low-metallicity outliers from the
mass--metallicity relation of star-forming galaxies. These galaxies have
stellar masses that span log(M_*/M_sun) ~9.4 to 11.1 and are offset from the
mass--metallicity relation by -0.3 to -0.85 dex in 12+log(O/H). In general,
they are extremely blue, have high star formation rates for their masses, and
are morphologically disturbed. Tidal interactions are expected to induce
large-scale gas inflow to the galaxies' central regions, and we find that these
galaxies' gas-phase oxygen abundances are consistent with large quantities of
low-metallicity gas from large galactocentric radii diluting the central
metal-rich gas. We conclude with implications for deducing gas-phase
metallicities of individual galaxies based solely on their luminosities,
specifically in the case of long gamma-ray burst host galaxies.Comment: Accepted for publication in ApJ; 11 pages, 11 figure
Revealing the High-Redshift Star Formation Rate with Gamma-Ray Bursts
While the high-z frontier of star formation rate (SFR) studies has advanced
rapidly, direct measurements beyond z ~ 4 remain difficult, as shown by
significant disagreements among different results. Gamma-ray bursts, owing to
their brightness and association with massive stars, offer hope of clarifying
this situation, provided that the GRB rate can be properly related to the SFR.
The Swift GRB data reveal an increasing evolution in the GRB rate relative to
the SFR at intermediate z; taking this into account, we use the highest-z GRB
data to make a new determination of the SFR at z = 4-7. Our results exceed the
lowest direct SFR measurements, and imply that no steep drop exists in the SFR
up to at least z ~ 6.5. We discuss the implications of our result for cosmic
reionization, the efficiency of the universe in producing stellar-mass black
holes, and ``GRB feedback'' in star-forming hosts.Comment: 4 pages, 2 figures; ApJ Letters, in pres
SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy
While performing a survey to detect recoiling supermassive black holes, we
have identified an unusual source having a projected offset of 800 pc from a
nearby dwarf galaxy. The object, SDSS J113323.97+550415.8, exhibits broad
emission lines and strong variability. While originally classified as a
supernova (SN) because of its nondetection in 2005, we detect it in recent and
past observations over 63 yr and find over a magnitude of rebrightening in the
last 2 years. Using high-resolution adaptive optics observations, we constrain
the source emission region to be <12 pc and find a disturbed host-galaxy
morphology indicative of recent merger activity. Observations taken over more
than a decade show narrow [O III] lines, constant ultraviolet emission, broad
Balmer lines, a constant putative black hole mass over a decade of observations
despite changes in the continuum, and optical emission-line diagnostics
consistent with an active galactic nucleus (AGN). However, the optical spectra
exhibit blueshifted absorption, and eventually narrow Fe II and [Ca II]
emission, each of which is rarely found in AGN spectra. While this peculiar
source displays many of the observational properties expected of a potential
black hole recoil candidate, some of the properties could also be explained by
a luminous blue variable star (LBV) erupting for decades since 1950, followed
by a Type IIn SN in 2001. Interpreted as an LBV followed by a SN analogous to
SN 2009ip, the multi-decade LBV eruptions would be the longest ever observed,
and the broad Halpha emission would be the most luminous ever observed at late
times (>10 yr), larger than that of unusually luminous supernovae such as SN
1988Z, suggesting one of the most extreme episodes of pre-SN mass loss ever
discovered.Comment: Accepted for publication in MNRA
Impact of baryons on the cluster mass function and cosmological parameter determination
Recent results by the Planck collaboration have shown that cosmological
parameters derived from the cosmic microwave background anisotropies and
cluster number counts are in tension, with the latter preferring lower values
of the matter density parameter, , and power spectrum
amplitude, . Motivated by this, we investigate the extent to which
the tension may be ameliorated once the effect of baryonic depletion on the
cluster mass function is taken into account. We use the large-volume Millennium
Gas simulations in our study, including one where the gas is pre-heated at high
redshift and one where the gas is heated by stars and active galactic nuclei
(in the latter, the self-gravity of the baryons and radiative cooling are
omitted). In both cases, the cluster baryon fractions are in reasonably good
agreement with the data at low redshift, showing significant depletion of
baryons with respect to the cosmic mean. As a result, it is found that the
cluster abundance in these simulations is around 15 per cent lower than the
commonly-adopted fit to dark matter simulations by Tinker et al (2008) for the
mass range . Ignoring this effect
produces a significant artificial shift in cosmological parameters which can be
expressed as at
(the median redshift of the cluster sample) for the
feedback model. While this shift is not sufficient to fully explain the
discrepancy, it is clear that such an effect cannot be
ignored in future precision measurements of cosmological parameters with
clusters. Finally, we outline a simple, model-independent procedure that
attempts to correct for the effect of baryonic depletion and show that it works
if the baryon-dark matter back-reaction is negligible.Comment: 10 pages, 5 figures, Accepted by MNRA
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