69 research outputs found
Stars quenching stars: how photoionization by local sources regulates gas cooling and galaxy formation
Current models of galaxy formation lack an efficient and physically
constrained mechanism to regulate star formation (SF) in low and intermediate
mass galaxies. We argue that the missing ingredient could be the effect of
photoionization by local sources on the gas cooling. We show that the soft
X-ray and EUV flux generated by SF is able to efficiently remove the main
coolants (e.g., HeII, OV and FeIX) from the halo gas via direct
photoionization. As a consequence, the cooling and accretion time of the gas
surrounding star-forming galaxies may increase by one or two orders of
magnitude. For a given halo mass and redshift, the effect is directly related
to the value of the star formation rate (SFR). Our results suggest the
existence of a critical SFR above which "cold" mode accretion is stopped, even
for haloes with virial masses well below the critical shock-heating mass
suggested by previous studies.The evolution of the critical SFR with redshift,
for a given halo mass, resembles the respective steep evolution of the observed
SFR for z<1. This suggests that photoionization by local sources would be able
to regulate gas accretion and star formation, without the need for additional,
strong feedback processes.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
A time-domain control signal detection technique for OFDM
Transmission of system-critical control information plays a key role in efficient management of limited wireless network resources and successful reception of payload data information. This paper uses an orthogonal frequency division multiplexing (OFDM) architecture to investigate the detection performance of a time-domain approach used to detect deterministic control signalling information. It considers a type of control information chosen from a finite set of information, which is known at both transmitting and receiving wireless terminals. Unlike the maximum likelihood (ML) estimation method, which is often used, the time-domain detection technique requires no channel estimation and no pilots as it uses a form of time-domain correlation as the means of detection. Results show that when compared with the ML method, the time-domain approach improves detection performance even in the presence of synchronisation error caused by carrier frequency offset
End-to-End Joint Antenna Selection Strategy and Distributed Compress and Forward Strategy for Relay Channels
Multi-hop relay channels use multiple relay stages, each with multiple relay
nodes, to facilitate communication between a source and destination.
Previously, distributed space-time codes were proposed to maximize the
achievable diversity-multiplexing tradeoff, however, they fail to achieve all
the points of the optimal diversity-multiplexing tradeoff. In the presence of a
low-rate feedback link from the destination to each relay stage and the source,
this paper proposes an end-to-end antenna selection (EEAS) strategy as an
alternative to distributed space-time codes. The EEAS strategy uses a subset of
antennas of each relay stage for transmission of the source signal to the
destination with amplify and forwarding at each relay stage. The subsets are
chosen such that they maximize the end-to-end mutual information at the
destination. The EEAS strategy achieves the corner points of the optimal
diversity-multiplexing tradeoff (corresponding to maximum diversity gain and
maximum multiplexing gain) and achieves better diversity gain at intermediate
values of multiplexing gain, versus the best known distributed space-time
coding strategies. A distributed compress and forward (CF) strategy is also
proposed to achieve all points of the optimal diversity-multiplexing tradeoff
for a two-hop relay channel with multiple relay nodes.Comment: Accepted for publication in the special issue on cooperative
communication in the Eurasip Journal on Wireless Communication and Networkin
A High Throughput Configurable SDR Detector for Multi-user MIMO Wireless Systems
Spatial division multiplexing (SDM) in
MIMO technology significantly increases the spectral
efficiency, and hence capacity, of a wireless communication
system: it is a core component of the next generation
wireless systems, e.g. WiMAX, 3GPP LTE and
other OFDM-based communication schemes. Moreover,
spatial division multiple access (SDMA) is one
of the widely used techniques for sharing the wireless
medium between different mobile devices. Sphere detection
is a prominent method of simplifying the detection
complexity in both SDM and SDMA systems
while maintaining BER performance comparable with
the optimum maximum-likelihood (ML) detection. On
the other hand, with different standards supporting
different system parameters, it is crucial for both base
station and handset devices to be configurable and
seamlessly switch between different modes without the
need for separate dedicated hardware units. This challenge
emphasizes the need for SDR designs that target
the handset devices. In this paper, we propose the
architecture and FPGA realization of a configurable
sort-free sphere detector, Flex-Sphere, that supports
4, 16, 64-QAM modulations as well as a combination of 2, 3 and 4 antenna/user configuration for handsets.
The detector provides a data rate of up to 857.1 Mbps
that fits well within the requirements of any of the next
generation wireless standards. The algorithmic optimizations
employed to produce an FPGA friendly realization
are discussed.Xilinx Inc.National Science Foundatio
Specific star-formation and the relation to stellar mass from 0<z<2 as seen in the far-infrared at 70 and 160mu
We use the Spitzer Wide-area InfraRed Extragalactic Legacy Survey (SWIRE) to
explore the specific star-formation activity of galaxies and their evolution
near the peak of the cosmic far-infrared (FIR) background at 70 and 160um. We
use a stacking analysis to determine the mean FIR properties of well defined
subsets of galaxies at flux levels well below the FIR catalogue detection
limits of SWIRE and other Spitzer surveys. We tabulate the contribution of
different subsets of galaxies to the FIR background at 70um and 160um. These
long wavelengths provide a good constraint on the bolometric, obscured
emission. The large area provides good constraints at low z and in finer
redshift bins than previous work. At all redshifts we find that the specific
FIR Luminosity (sLFIR) decreases with increasing mass, following a trend
L_FIR/M* propto M_* ^beta with beta =-0.38\pm0.14. This is a more continuous
change than expected from the {Delucia2007} semi-analytic model suggesting
modifications to the feedback prescriptions. We see an increase in the sLFIR by
about a factor of ~100 from 0<z<2 and find that the sLFIR evolves as
(1+z)^alpha with alpha=4.4\pm0.3 for galaxies with 10.5 < log M*/Msun < 12.
This is considerably steeper than the {Delucia2007} semi-analytic model (alpha
\sim 2.5). When separating galaxies into early and late types on the basis of
the optical/IR spectral energy distributions we find that the decrease in sLFIR
with stellar mass is stronger in early type galaxies (beta ~ -0.46), while late
type galaxies exhibit a flatter trend (beta \sim -0.15). The evolution is
strong for both classes but stronger for the early type galaxies. The early
types show a trend of decreasing strength of evolution as we move from lower to
higher masses while the evolution of the late type galaxies has little
dependence on stellar mass. We suggest that in late-type galaxies we are seeing
a consistently declining sSFR..Comment: v2 Update doesn't change the content of the paper, but now includes
data files for the plots Fig 5-13 (all.plotdat, spi.plotdat and ell.plotdat
on arXiv package
Enriched haloes at redshift with no star-formation: Implications for accretion and wind scenarios
[Abridged] In order to understand which process (e.g. galactic winds, cold
accretion) is responsible for the cool (T~10^4 K) halo gas around galaxies, we
embarked on a program to study the star-formation properties of galaxies
selected by their MgII absorption signature in quasar spectra. Specifically, we
searched for the H-alpha line emission from galaxies near very strong z=2 MgII
absorbers (with rest-frame equivalent width EW>2 \AA) because these could be
the sign-posts of outflows or inflows. Surprisingly, we detect H-alpha from
only 4 hosts out of 20 sight-lines (and 2 out of the 19 HI-selected
sight-lines), despite reaching a star-formation rate (SFR) sensitivity limit of
2.9 M/yr (5-sigma) for a Chabrier initial mass function. This low success rate
is in contrast with our z=1 survey where we detected 66%\ (14/21) of the MgII
hosts. Taking into account the difference in sensitivity between the two
surveys, we should have been able to detect >11.4 of the 20 z=2 hosts whereas
we found only 4 galaxies. Interestingly, all the z=2 detected hosts have
observed SFR greater than 9 M/yr, well above our sensitivity limit, while at
z=1 they all have SFR less than 9 M/yr, an evolution that is in good agreement
with the evolution of the SFR main sequence. Moreover, we show that the z=2
undetected hosts are not hidden under the quasar continuum after stacking our
data and that they also cannot be outside our surveyed area. Hence, strong MgII
absorbers could trace star-formation driven winds in low-mass halos (Mhalo <
10^{10.6} Msun). Alternatively, our results imply that z=2 galaxies traced by
strong MgII absorbers do not form stars at a rate expected (3--10 M/yr) for
their (halo or stellar) masses, supporting the existence of a transition in
accretion efficiency at Mhalo ~ 10^{11} Msun. This scenario can explain both
the detections and the non-detections.Comment: 14 pages, 4 fig.; MNRAS in press, minor corrections to match proof
Correction of anaemia through the use of darbepoetin alfa improves chemotherapeutic outcome in a murine model of Lewis lung carcinoma
Darbepoetin alfa (Aranesp®, Amgen) is a novel erythropoiesis-stimulating protein with a serum half-life longer than recombinant human erythropoietin (Epo), used in the treatment of cancer-associated anaemia. Anaemia is known to adversely affect prognosis and response to treatment in cancer patients. Solid tumours contain regions of hypoxia due to poor vascular supply and cellular compaction. Although hypoxic stress usually results in cell death, hypoxia-resistant tumour cells are genetically unstable and often acquire a drug-resistant phenotype. Increasing tumour oxygenation and perfusion during treatment could have the doubly beneficial outcome of reducing the fraction of treatment-resistant cells, while increasing drug delivery to previously hypoxic tissue. In this study, we examined the effect of darbepoetin alfa on chemotherapy sensitivity and delivery in an in vivo model of Lewis lung carcinoma, shown here to express the Epo receptor (EpoR). We identified that weekly darbepoetin alfa treatment, commencing 10 days before chemotherapy, resulted in a significant reduction in tumour volume compared to chemotherapy alone. This was mediated by the prevention of anaemia, a reduction in tumour hypoxia and a concomitant increase in drug delivery. Darbepoetin alfa treatment alone did not modulate the growth of the EpoR-expressing tumour cells. This study identifies an important role for darbepoetin alfa in increasing the therapeutic index of chemotherapy
Pre-Bilaterian Origins of the Hox Cluster and the Hox Code: Evidence from the Sea Anemone, Nematostella vectensis
BACKGROUND: Hox genes were critical to many morphological innovations of bilaterian animals. However, early Hox evolution remains obscure. Phylogenetic, developmental, and genomic analyses on the cnidarian sea anemone Nematostella vectensis challenge recent claims that the Hox code is a bilaterian invention and that no “true” Hox genes exist in the phylum Cnidaria. METHODOLOGY/PRINCIPAL FINDINGS: Phylogenetic analyses of 18 Hox-related genes from Nematostella identify putative Hox1, Hox2, and Hox9+ genes. Statistical comparisons among competing hypotheses bolster these findings, including an explicit consideration of the gene losses implied by alternate topologies. In situ hybridization studies of 20 Hox-related genes reveal that multiple Hox genes are expressed in distinct regions along the primary body axis, supporting the existence of a pre-bilaterian Hox code. Additionally, several Hox genes are expressed in nested domains along the secondary body axis, suggesting a role in “dorsoventral” patterning. CONCLUSIONS/SIGNIFICANCE: A cluster of anterior and posterior Hox genes, as well as ParaHox cluster of genes evolved prior to the cnidarian-bilaterian split. There is evidence to suggest that these clusters were formed from a series of tandem gene duplication events and played a role in patterning both the primary and secondary body axes in a bilaterally symmetrical common ancestor. Cnidarians and bilaterians shared a common ancestor some 570 to 700 million years ago, and as such, are derived from a common body plan. Our work reveals several conserved genetic components that are found in both of these diverse lineages. This finding is consistent with the hypothesis that a set of developmental rules established in the common ancestor of cnidarians and bilaterians is still at work today
Travelling-wave ion mobility and negative ion fragmentation of high-mannoseN-glycans
The isomeric structure of high-mannose N-glycans can significantly impact biological recognition events. Here, the utility of travelling-wave ion mobility mass spectrometry for isomer separation of high-mannose N-glycans is investigated. Negative ion fragmentation using collision-induced dissociation gave more informative spectra than positive ion spectra with mass-different fragment ions characterizing many of the isomers. Isomer separation by ion mobility in both ionization modes was generally limited, with the arrival time distributions (ATD) often showing little sign of isomers. However, isomers could be partially resolved by plotting extracted fragment ATDs of the diagnostic fragment ions from the negative ion spectra, and the fragmentation spectra of the isomers could be extracted by using ions from limited areas of the ATD peak. In some cases, asymmetric ATDs were observed, but no isomers could be detected by fragmentation. In these cases, it was assumed that conformers or anomers were being separated. Collision cross sections of the isomers in positive and negative fragmentation mode were estimated from travelling-wave ion mobility mass spectrometry data using dextran glycans as calibrant. More complete collision cross section data were achieved in negative ion mode by utilizing the diagnostic fragment ions. Examples of isomer separations are shown for N-glycans released from the well-characterized glycoproteins chicken ovalbumin, porcine thyroglobulin and gp120 from the human immunodeficiency virus. In addition to the cross-sectional data, details of the negative ion collision-induced dissociation spectra of all resolved isomers are discussed
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