347 research outputs found
Nowhere to Hide: Radio-faint AGN in the GOODS-N field. I. Initial catalogue and radio properties
(Abridged) Conventional radio surveys of deep fields ordinarily have
arc-second scale resolutions often insufficient to reliably separate radio
emission in distant galaxies originating from star-formation and AGN-related
activity. Very long baseline interferometry (VLBI) can offer a solution by
identifying only the most compact radio emitting regions in galaxies at
cosmological distances where the high brightness temperatures (in excess of
K) can only be reliably attributed to AGN activity. We present the first
in a series of papers exploring the faint compact radio population using a new
wide-field VLBI survey of the GOODS-N field. The unparalleled sensitivity of
the European VLBI Network (EVN) will probe a luminosity range rarely seen in
deep wide-field VLBI observations, thus providing insights into the role of AGN
to radio luminosities of the order across cosmic
time. The newest VLBI techniques are used to completely cover an entire 7'.5
radius area to milliarcsecond resolutions, while bright radio sources ( mJy) are targeted up to 25 arcmin from the pointing centre. Multi-source
self-calibration, and a primary beam model for the EVN array are used to
correct for residual phase errors and primary beam attenuation respectively.
This paper presents the largest catalogue of VLBI detected sources in GOODS-N
comprising of 31 compact radio sources across a redshift range of 0.11-3.44,
almost three times more than previous VLBI surveys in this field. We provide a
machine-readable catalogue and introduce the radio properties of the detected
sources using complementary data from the e-MERLIN Galaxy Evolution survey
(eMERGE).Comment: 15 pages, 8 figures, accepted in A&A. Machine-readable table
available upon reques
High-z radio starbursts host obscured X-ray AGN
We use Virtual Observatory methods to investigate the association between
radio and X-ray emission at high redshifts. Fifty-five of the 92 HDF(N) sources
resolved by combining
MERLIN+VLA data were detected by Chandra, of which 18 are hard enough and
bright enough to be obscured AGN. The high-z population of microJy radio
sources is dominated by starbursts an order of magnitude more active and more
extended than any found at z<1 and at least a quarter of these simultaneously
host highly X-ray-luminous obscured AGN.Comment: 4 pages, 2 figures, To appear in the proceedings of 'At the Edge of
the Universe' (9-13 October 2006, Sintra, Portugal
23 GHz VLBI Observations of SN 2008ax
We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI)
observations of the type IIb supernova SN 2008ax, made with the Very Long
Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These
observations resulted in a marginal detection of the supernova. The total flux
density recovered from our VLBI image is 0.80.3 mJy (one standard
deviation). As it appears, the structure may be interpreted as either a
core-jet or a double source. However, the supernova structure could be somewhat
confused with a possible close by noise peak. In such a case, the recovered
flux density would decrease to 0.480.12 mJy, compatible with the flux
densities measured with the VLA at epochs close in time to our VLBI
observations. The lowest average expansion velocities derived from our
observations are km s (case of a double
source) and km s (taking the weaker source
component as a spurious, close by, noise peak, which is the more likely
interpretation). These velocities are 7.3 and 2 times higher, respectively,
than the maximum ejecta velocity inferred from optical-line observations.Comment: 4 pages, 3 figures. Accepted in A&A on 24/03/200
Proton Zemach radius from measurements of the hyperfine splitting of hydrogen and muonic hydrogen
While measurements of the hyperfine structure of hydrogen-like atoms are
traditionally regarded as test of bound-state QED, we assume that theoretical
QED predictions are accurate and discuss the information about the
electromagnetic structure of protons that could be extracted from the
experimental values of the ground state hyperfine splitting in hydrogen and
muonic hydrogen. Using recent theoretical results on the proton polarizability
effects and the experimental hydrogen hyperfine splitting we obtain for the
Zemach radius of the proton the value 1.040(16) fm. We compare it to the
various theoretical estimates the uncertainty of which is shown to be larger
that 0.016 fm. This point of view gives quite convincing arguments in support
of projects to measure the hyperfine splitting of muonic hydrogen.Comment: Submitted to Phys. Rev.
Double radio peak and non-thermal collimated ejecta in RS Ophiuchi following the 2006 outburst
We report Multi-Element Radio-Linked Interferometer Network, Very Large Array, One-Centimetre Radio Array, Very Long Baseline Array (VLBA), Effelsberg and Giant Metrewave Radio Telescope observations beginning 4.5 days after the discovery of RS Ophiuchi undergoing its 2006 recurrent nova outburst. Observations over the first 9 weeks are included, enabling us to follow spectral development throughout the three phases of the remnant development. We see dramatic brightening on days 4 to 7 at 6 GHz and an accompanying increase in other bands, particularly 1.46 GHz, consistent with transition from the initial âfree expansionâ phase to the adiabatic expansion phase. This is complete by day 13 when the flux density at 5 GHz is apparently declining from an unexpectedly early maximum (compared with expectations from observations of the 1985 outburst). The flux density recovered to a second peak by approximately day 40, consistent with behaviour observed in 1985. At all times the spectral index is consistent with mixed non-thermal and thermal emission. The spectral indices are consistent with a non-thermal component at lower frequencies on all dates, and the spectral index changes show that the two components are clearly variable. The estimated extent of the emission at 22 GHz on day 59 is consistent with the extended east and west features seen at 1.7 GHz with the VLBA on day 63 being entirely non-thermal. We suggest a two-component model, consisting of a decelerating shell seen in mixed thermal and non-thermal emission plus faster bipolar ejecta generating the non-thermal emission, as seen in contemporaneous VLBA observations. Our estimated ejecta mass of 4 ± 2 Ă 10â7 Mâ is consistent with a white dwarf (WD) mass of 1.4 Mâ. It may be that this ejecta mass estimate is a lower limit, in which case a lower WD mass would be consistent with the data
Observations of the neutral gas and dust in the radio galaxy 3C305
We present MERLIN and Hubble Space Telescope (HST) observations of the
central region of the nearby radio galaxy 3C305 and use them to study the gas
and dust in this object. The MERLIN observations are of neutral hydrogen (HI)
absorption against the strong non-thermal 20cm continuum seen towards the
central 4kpc of 3C305. Our 0.2-arcsec (160 pc) resolution observations show
that the HI absorption is highly localised against the south-western
radio-emission with column densities 1.9x10^21/cm2. The absorption is broad and
red-shifted by 130 km/s relative to the systemic velocity. The HST images in
430nm, 702nm, [OIII] 500.7nm, [FeII] 1.64um and K-band polarization are
presented. Evidence is seen for coincidence of the [FeII] emission with the
knot at the end of the radio jet, suggesting the presence of shocks. We compare
the optical and radio images in order to investigate the relationship between
the dust and neutral gas. An unresolved (0.07-arcsec) nucleus is detected in H
and K and its properties are consistent with a quasar reddened by A(V)>4. We
propose that the absorption arises in a region of neutral gas and dust whose
structure is complex but is broadly consistent with an inclined disc
encircling, but not covering, the active galactic nucleus. A comparison of the
neutral gas observations and previous emission-line observations suggests that
both the neutral and ionised gas are undergoing galactic rotation towards the
observer in the north-east and away from the observer in the south-west. We
propose that the outflow giving rise to the radio emission has a component
towards the observer in the north-east and away from the observer in the
south-west.Comment: 13 pages, 11 figures, accepted by MNRA
White-etching matter in bearing steel. Part II: Distinguishing cause and effect in bearing steel failure
The premature failure of large bearings of the type used in wind turbines, possibly through a mechanism called âwhite-structure flakingâ, has triggered many studies of microstructural damage associated with âwhite-etching areasâ created during rolling contact fatigue, although whether they are symptoms or causes of failure is less clear. Therefore, some special experiments have been conducted to prove that white-etching areas are the consequence, and not the cause, of damage. By artificially introducing a fine dispersion of microcracks in the steel through heat treatment and then subjecting the sample to rolling contact fatigue, manifestations of hard white-etching matter have been created to a much greater extent than samples similarly tested without initial cracks. A wide variety of characterization tools has been used to corroborate that the white areas thus created have the same properties as reported observations on real bearings. Evidence suggests that the formation mechanism of the white-etching regions involves the rubbing and beating of the free surfaces of cracks, debonded inclusions, and voids under repeated rolling contact. It follows that the focus in avoiding early failure should be in enhancing the toughness of the bearing steel in order to avoid the initial microscopic feature event.Funding by CONACyT, the
Cambridge Overseas Trust, and the Roberto Rocca Education Programme
is highly appreciated and acknowledged.This is the accepted manuscript version. The final published version is available from Springer at http://link.springer.com/article/10.1007%2Fs11661-014-2431-x
Probing the accelerating Universe with radio weak lensing in the JVLA Sky Survey
We outline the prospects for performing pioneering radio weak gravitational
lensing analyses using observations from a potential forthcoming JVLA Sky
Survey program. A large-scale survey with the JVLA can offer interesting and
unique opportunities for performing weak lensing studies in the radio band, a
field which has until now been the preserve of optical telescopes. In
particular, the JVLA has the capacity for large, deep radio surveys with
relatively high angular resolution, which are the key characteristics required
for a successful weak lensing study. We highlight the potential advantages and
unique aspects of performing weak lensing in the radio band. In particular, the
inclusion of continuum polarisation information can greatly reduce noise in
weak lensing reconstructions and can also remove the effects of intrinsic
galaxy alignments, the key astrophysical systematic effect that limits weak
lensing at all wavelengths. We identify a VLASS "deep fields" program (total
area ~10-20 square degs), to be conducted at L-band and with high-resolution
(A-array configuration), as the optimal survey strategy from the point of view
of weak lensing science. Such a survey will build on the unique strengths of
the JVLA and will remain unsurpassed in terms of its combination of resolution
and sensitivity until the advent of the Square Kilometre Array. We identify the
best fields on the JVLA-accessible sky from the point of view of overlapping
with existing deep optical and near infra-red data which will provide crucial
redshift information and facilitate a host of additional compelling
multi-wavelength science.Comment: Submitted in response to NRAO's recent call for community white
papers on the VLA Sky Survey (VLASS
- âŠ