1,396 research outputs found
Faint Gigahertz peaked spectrum sources and the evolution of young radio sources
GPS sources are the objects of choice to study the initial evolution of
extragalactic radio sources, since it is most likely that they are the young
counterparts of large scale radio sources. Correlations found between their
peak frequency, peak flux density and angular size provide strong evidence that
synchrotron self absorption is the cause of the spectral turnovers, and
indicate that young radio sources evolve in a self-similar way. The difference
in redshift distribution between young and old radio sources must be due to a
difference in slope of their luminosity functions, and we argue that this slope
is strongly affected by the luminosity evolution of the individual sources. A
luminosity evolution scenario is proposed in which GPS sources increase in
luminosity and large scale radio sources decrease in luminosity with time. It
is shown that such a scenario agrees with the local luminosity function of GPS
galaxies.Comment: Late, 6 pages, 2 figs. To appear in the proceedings of EVN/JIVE
Symposium No 4, New Astronomy Reviews (eds. Garrett et al.). For related
papers, see http://www.ast.cam.ac.uk/~snelle
Search for clusters at high redshift - I. Candidate Lya emitters near 1138-262 at z=2.2
Radio, optical and X-ray observations of the powerful radio galaxy PKS
1138-262 at z=2.156 have suggested that this galaxy is a massive galaxy in the
center of a forming cluster. We have imaged 1138-262 and the surrounding 38
square arcminute field with the Very Large Telescope in a broad band and a
narrow band encompassing the redshifted Lya emission. We detect 50 objects with
rest equivalent width larger than 20 A and a luminous, highly extended Lya halo
around 1138-262. If the radio galaxy is at the center of a forming cluster,
these objects are candidate Lya emitting cluster galaxies.Comment: Accepted for publication in A&A as Letter, 4 pages, 4 figure
WIMP searches with superheated droplet detectors: Status and Prospects
SIMPLE (Superheated Instrument for Massive ParticLE searches) employs
superheated droplet detectors (SDDs) to search for Weakly Interacting Massive
Particle (WIMP) dark matter. As a result of the intrinsic SDD insensitivity to
minimum ionizing particles and high fluorine content of target liquids,
competitive WIMP limits were already obtained at the early prototype stage. We
comment here on the expected immediate increase in sensitivity of the program
and on future plans to exploit this promising technnique.Comment: 6 pages LaTeX, incl. 2 figures. Presented at the 3rd Intl. Workshop
for the Identification of Dark Matter (IDM2000), York, Sept. 200
A new sample of faint Gigahertz Peaked Spectrum radio sources
The Westerbork Northern Sky Survey (WENSS) has been used to select a sample
of Gigahertz Peaked Spectrum (GPS) radio sources at flux densities one to two
orders of magnitude lower than bright GPS sources investigated in earlier
studies. Sources with inverted spectra at frequencies above 325 MHz have been
observed with the WSRT at 1.4 and 5 GHz and with the VLA at 8.6 and 15 GHz to
select genuine GPS sources. This has resulted in a sample of 47 GPS sources
with peak frequencies ranging from ~500 MHz to >15 GHz, and peak flux densities
ranging from ~40 to ~900 mJy. Counts of GPS sources in our sample as a function
of flux density have been compared with counts of large scale sources from
WENSS scaled to 2 GHz, the typical peak frequency of our GPS sources. The
counts can be made similar if the number of large scale sources at 2 GHz is
divided by 250, and their flux densities increase by a factor of 10. On the
scenario that all GPS sources evolve into large scale radio sources, these
results show that the lifetime of a typical GPS source is ~250 times shorter
than a typical large scale radio source, and that the source luminosity must
decrease by a factor of ~10 in evolving from GPS to large scale radio source.
However, we note that the redshift distributions of GPS and large scale radio
sources are different and that this hampers a direct and straightforward
interpretation of the source counts. Further modeling of radio source evolution
combined with cosmological evolution of the radio luminosity function for large
sources is required.Comment: LaTeX, 15 pages, 8 figs. To be published in A&AS. For more info see
http://www.ast.cam.ac.uk/~snelle
A CF3I-based SDD Prototype for Spin-independent Dark Matter Searches
The application of Superheated Droplet Detectors (SDDs) to dark matter
searches has so far been confined to the light nuclei refrigerants C2ClF5 and
C4F10 (SIMPLE and PICASSO, respectively), with a principle sensitivity to
spin-dependent interactions. Given the competitive results of these devices, as
a result of their intrinsic insensitivity to backgrounds, we have developed a
prototype trifluoroiodomethane (CF3I)-loaded SDD with increased sensitivity to
spin-independent interactions as well. A low (0.102 kgd) exposure test
operation of two high concentration, 1 liter devices is described, and the
results compared with leading experiments in both spin-dependent and
-independent sectors. Although competitive in both sectors when the difference
in exposures is accounted for, a problem with fracturing of the detector gel
must be addressed before significantly larger exposures can be envisioned.Comment: revised and updated; accepted Astrop. Phy
First Results of the Phase II SIMPLE Dark Matter Search
We report results of a 14.1 kgd measurement with 15 superheated droplet
detectors of total active mass 0.208 kg, comprising the first stage of a 30 kgd
Phase II experiment. In combination with the results of the neutron-spin
sensitive XENON10 experiment, these results yield a limit of |a_p| < 0.32 for
M_W = 50 GeV/c2 on the spin-dependent sector of weakly interacting massive
particle-nucleus interactions with a 50% reduction in the previously allowed
region of the phase space formerly defined by XENON, KIMS and PICASSO. In the
spin-independent sector, a limit of 2.3x10-5 pb at M_W = 45 GeV/c2 is obtained.Comment: 4 pages, 4 figures; PRL-accepted version with corrected SI contour
(Fig. 4
Final Analysis and Results of the Phase II SIMPLE Dark Matter Search
We report the final results of the Phase II SIMPLE measurements, comprising
two run stages of 15 superheated droplet detectors each, the second stage
including an improved neutron shielding. The analyses includes a refined signal
analysis, and revised nucleation efficiency based on reanalysis of
previously-reported monochromatic neutron irradiations. The combined results
yield a contour minimum of \sigma_{p} = 4.2 x 10^-3 pb at 35 GeV/c^2 on the
spin-dependent sector of WIMP-proton interactions, the most restrictive to date
from a direct search experiment and overlapping for the first time results
previously obtained only indirectly. In the spin-independent sector, a minimum
of 3.6 x 10^-6 pb at 35 GeV/c^2 is achieved, with the exclusion contour
challenging the recent CoGeNT region of current interest.Comment: revised, PRL-accepted version with slightly weakened limit contour
Galaxy protocluster candidates around z ~ 2.4 radio galaxies
We study the environments of 6 radio galaxies at 2.2 < z < 2.6 using
wide-field near-infrared images. We use colour cuts to identify galaxies in
this redshift range, and find that three of the radio galaxies are surrounded
by significant surface overdensities of such galaxies. The excess galaxies that
comprise these overdensities are strongly clustered, suggesting they are
physically associated. The colour distribution of the galaxies responsible for
the overdensity are consistent with those of galaxies that lie within a narrow
redshift range at z ~ 2.4. Thus the excess galaxies are consistent with being
companions of the radio galaxies. The overdensities have estimated masses in
excess of 10^14 solar masses, and are dense enough to collapse into virizalised
structures by the present day: these structures may evolve into groups or
clusters of galaxies. A flux-limited sample of protocluster galaxies with K <
20.6 mag is derived by statistically subtracting the fore- and background
galaxies. The colour distribution of the protocluster galaxies is bimodal,
consisting of a dominant blue sequence, comprising 77 +/- 10% of the galaxies,
and a poorly populated red sequence. The blue protocluster galaxies have
similar colours to local star-forming irregular galaxies (U -V ~ 0.6),
suggesting most protocluster galaxies are still forming stars at the observed
epoch. The blue colours and lack of a dominant protocluster red sequence
implies that these cluster galaxies form the bulk of their stars at z < 3.Comment: Accepted for publication in MNRA
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