arXiv subjects

R. A. Watson

Publications and source records attributed to R. A. Watson.

49 records · Page 3Linked to original sources

arXiv2001

A New Spin on Galactic Dust

We present a new puzzle involving Galactic microwave emission and attempt to resolve it. On one hand, a cross-correlation analysis of the WHAM H-alpha map with the Tenerife 10 and 15 GHz maps shows that the well-known DIRBE correlated microwave emission cannot be dominated by free-free emission. On the other hand, recent high resolution observations in the 8-10 GHz range with the Green Bank 140 ft telescope by Finkbeiner et al. failed to find the corresponding 8 sigma signal that would be expected in the simplest spinning dust models. So what physical mechanism is causing this ubiquitous dust-correlated emission? We argue for a model predicting that spinning dust is the culprit after all, but that the corresponding small grains are well correlated with the larger grains seen at 100 micron only on large angular scales. In support of this grain segregation model, we find the best spinning dust template to involve higher frequency maps in the range 12-60 micron, where emission from transiently heated small grains is important. Upcoming CMB experiments such as ground-based interferometers, MAP and Planck LFI with high resolution at low frequencies should allow a definitive test of this model.

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arXiv2001

COSMOSOMAS: A Circular Scanning Instrument to Map the Sky at Centimetric Wavelengths

We describe the first instrument of a Cosmic Microwave Background experiment for mapping cosmological structures on medium angular scales (the COSMOSOMAS experiment) and diffuse Galactic emission. The instrument is located at Teide Observatory (Tenerife) and is based on a circular scanning sky strategy. It consists of a 1 Hz spinning flat mirror directing the sky radiation into a 1.8 m off-axis paraboloidal antenna which focuses it on to a cryogenically cooled HEMT-based receiver operating in the frequency range 12--18 GHz. The signal is split by a set of three filters, allowing simultaneous observations at 13, 15 and 17 GHz, each with a 1 GHz bandpass. A 1-5 degree resolution sky map complete in RA and covering 20 degrees in declination is obtained each day at these frequencies. The observations presented here correspond to the first months of operation, which have provided a map of ~9000 square degrees on the sky centred at DEC = +31 degrees with sensitivities of 140, 150 and 250 microK per beam area in the channels at 13, 15 and 17 GHz respectively. We discuss the design and performance of the instrument, the atmospheric effects, the reliability of the data obtained and prospects of achieving a sensitivity of 30 microK per beam in two years of operation.

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arXiv2000

A measurement at the first acoustic peak of the CMB with the 33 GHz interferometer

This paper presents the results from the Jodrell Bank-IAC two-element 33 GHz interferometer operated with an element separation of 32.9 wavelengths and hence sensitive to 1 deg scale structure on the sky. The level of CMB fluctuations, assuming a flat CMB spatial power spectrum over the range of multipoles l = 208 +- 18, was found using a likelihood analysis to be ΔT_l = 63^{+7}_{-6} mu K at the 68% confidence limit, after the subtraction of the contribution of monitored point sources. Other possible foreground contributions have been assessed and are expected to have negligible impact on this result.

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arXiv2000

Populations of high-luminosity density-bounded H II regions in spiral galaxies? Evidence and implications

(Abridged) We present evidence that the HII regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escape from the regions. The key piece of evidence is the presence, in the Hαluminosity functions (LFs) of the populations of HII regions, of glitches, local sharp peaks at an apparently invariant luminosity, defined as the Stromgren luminosity (L_ Str), L_Hα= L_Str = 10^38.6 (\pm 10^0.1) erg/s (no other peaks are found in any of the LFs) accompanying a steepening of slope for L_Hα> L_Str. This behavior is readily explicable via a physical model whose basic premises are: (a) the transition at L_Hα= L_Str marks a change from essentially ionization bounding at low luminosities to density bounding at higher values, (b) for this to occur the law relating stellar mass in massive star-forming clouds to the mass of the placental cloud must be such that the ionizing photon flux produced within the cloud is a function which rises more steeply than the mass of the cloud. Supporting evidence for the hypothesis of this transition is presented. If confirmed, the density-bounding hypothesis would imply that the density-bounded regions were the main sources of the photons which ionize the diffuse gas in disk galaxies. We estimate that these regions emit sufficient Lyman continuum not only to ionize the diffuse medium, but to cause a typical spiral to emit significant ionizing flux into the intergalactic medium. The low scatter observed in L_Str, less than 0.1 mag rms in the still quite small sample measured to date, is an invitation to widen the data base, and to calibrate against primary standards, with the aim of obtaining a precise standard candle.

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arXiv2000

Cross-correlation of Tenerife data with Galactic templates - evidence for spinning dust?

The recent discovery of dust-correlated diffuse microwave emission has prompted two rival explanations: free-free emission and spinning dust grains. We present new detections of this component at 10 and 15 GHz by the switched-beam Tenerife experiment. The data show a turnover in the spectrum and thereby supports the spinning dust hypothesis. We also present a significant detection of synchrotron radiation at 10 GHz, useful for normalizing foreground contamination of CMB experiments at high-galactic latitudes.

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arXiv1999

CMB observations with the Jodrell Bank - IAC interferometer at 33 GHz

The paper presents the first results obtained with the Jodrell Bank - IAC two-element 33 GHz interferometer. The instrument was designed to measure the level of the Cosmic Microwave Background (CMB) fluctuations at angular scales of 1 - 2 degrees. The observations analyzed here were taken in a strip of the sky at Dec = +41 deg with an element separation of 16.7 lambda, which gives a maximum sensitivity to ~1.6 deg structures on the sky. The data processing and calibration of the instrument are described. The sensitivity achieved in each of the two channels is 7 micro K per resolution element. A reconstruction of the sky at Dec = +41 deg using a maximum entropy method shows the presence of structure at a high level of significance. A likelihood analysis, assuming a flat CMB spatial power spectrum, gives a best estimate of the level of CMB fluctuations of Delta Tl = 43 (+13,-12) micro K for the range l = 109 +/- 19; the main uncertainty in this result arises from sample variance. We consider that the contamination from the Galaxy is small. These results represent a new determination of the CMB power spectrum on angular scales where previous results show a large scatter; our new results are in agreement with the theoretical predictions of the standard inflationary cold dark matter models.

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arXiv1999

A 33 GHz interferometer for CMB observations on Tenerife

We describe a new high sensitivity experiment for observing cosmic microwave background (CMB) anisotropies. The instrument is a 2-element interferometer operating at 33 GHz with a ~3 GHz bandwidth. It is installed on the high and dry Teide Observatory site on Tenerife where successful beam-switching observations have been made at this frequency. Two realizations of the interferometer have been tested with element separations of 11.9 lambda and 16.7 lambda. The resulting angular resolution of ~2 deg was chosen to explore the amplitude of CMB structure on the large angular scale side of the Doppler (acoustic) peak. It is found that observations are unaffected by water vapour for more than 70 per cent of the time when the sensitivity is limited by the receiver noise alone. Observations over several months are expected to give an rms noise level of ~10 - 20 micro K covering ~100 resolution elements. Preliminary results show stable operation of the interferometer with the detection of discrete radio sources as well as the Galactic plane at Dec = +41 deg and -29 deg.

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arXiv1999

Cosmic Microwave background map making with data between 10 and 90GHz

We use data from the Tenerife 10, 15 and 33 GHz beamswitching experiments along with the COBE 53 and 90 GHz data to separate the cosmic microwave background (CMB) signal from the Galactic signal and create two maps at high Galactic latitude. The new multi-MEM technique is used to obtain the best reconstruction of the two channels. The two maps are presented and known features are identified within each. We find that the Galactic contribution to both the 15 and 33 GHz Tenerife data is small enough to be ignored when compared to the errors in the data and the magnitude of the CMB signal.

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arXiv1998

10 GHz Tenerife CMB observations at 8 degree resolution and their analysis using a new maximum entropy method

The complete set of data from the Tenerife 10 GHz (8 degree FWHM) twin-horn, drift scan experiment is described. These data are affected by both long-term atmospheric baseline drifts and short term noise. A new maximum entropy procedure, utilising the time invariance and spatial continuity of the astronomical signal, is used to achieve a clean separation of these effects from the astronomical signal, and to deconvolve the effects of the beam-switching. We use a fully positive/negative algorithm to produce two-dimensional maps of the intrinsic sky fluctuations. Known discrete sources and Galactic features are identified in the deconvolved map. The data from the 10 GHz experiment, after baseline subtraction with MEM, is then analysed using conventional techniques and new constraints on Galactic emission are made.

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arXiv1997

Studies of CMB structure at Dec=40. II: Analysis and cosmological interpretation

We present a detailed analysis of the cosmic microwave background structure in the Tenerife Dec=+40 degrees data. The effect of local atmospheric contributions on the derived fluctuation amplitude is considered, resulting in an improved separation of the intrinsic CMB signal from noise. Our analysis demonstrates the existence of common structure in independent data scans at 15 and 33 GHz. For the case of fluctuations described by a Gaussian auto-correlation function, a likelihood analysis of our combined results at 15 and 33 GHz implies an intrinsic rms fluctuation level of 48^{+21}_{-15} uK on a coherence scale of 4 degrees; the equivalent analysis for a Harrison-Zel'dovitch model gives a power spectrum normalisation of Q_{rms-ps} = 22^{+10}_{-6} uK. The fluctuation amplitude is seen to be consistent at the 68% confidence level with that reported for the COBE two-year data for primordial fluctuations described by a power law model with a spectral index in the range 1.0 \le n \le 1.6. This limit favours the large scale CMB anisotropy being dominated by scalar fluctuations rather than tensor modes from a gravitational wave background. The large scale Tenerife and COBE results are considered in conjunction with observational results from medium scale experiments in order to place improved limits on the fluctuation spectral index; we find n=1.10 +/- 0.10 assuming standard CDM with H_{0}=50 kms^{-1}Mpc^{-1}.

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arXiv1997

New Cosmological Structures on Medium Angular Scales Detected with the Tenerife Experiments

We present observations at 10 and 15 GHz taken with the Tenerife experiments in a band of the sky at Dec.=+35 degrees. These experiments are sensitive to multipoles in the range l=10-30. The sensitivity per beam is 56 and 20 microK for the 10 and the 15 GHz data, respectively. After subtraction of the prediction of known radio-sources, the analysis of the data at 15 GHz at high Galactic latitude shows the presence of a signal with amplitude Delta Trms ~ 32 microK. In the case of a Harrison-Zeldovich spectrum for the primordial fluctuations, a likelihood analysis shows that this signal corresponds to a quadrupole amplitude Q_rms-ps=20.1+7.1-5.4 microK, in agreement with our previous results at Dec.+=40 degrees and with the results of the COBE DMR. There is clear evidence for the presence of individual features in the RA range 190 degrees to 250 degrees with a peak to peak amplitude of ~110 microK. A preliminary comparison between our results and COBE DMR predictions for the Tenerife experiments clearly indicates the presence of individual features common to both. The constancy in amplitude over such a large range in frequency (10-90 GHz) is strongly indicative of an intrinsic cosmological origin for these structures.

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arXiv1996

Millimetric Ground-based Observations of Cosmic Microwave Background Anisotropy

First results of a Cosmic Microwave Background (CMB) anisotropy experiment conducted at the Observatorio del Teide (Tenerife, Spain) are presented. The instrument is a four channel (3.1, 2.1, 1.3 and 1.1 mm) $^3$He bolometer system coupled to a 45 cm diameter telescope. The resultant configuration is sensitive to structures on angular scales ~ 1-2 degrees. We use the channels at the two highest frequencies for monitoring the atmosphere, and apply a simple method to subtract this contribution in channels 1 (3.1 mm) and 2 (2.1 mm). The most intense structure at these two frequencies is the Galactic crossing with peak amplitudes of ~ 350 micro-K. These crossings have been clearly detected with the amplitude and shape predicted. This demonstrates that our multifrequency observations allow an effective assessment and subtraction of the atmospheric contribution. In the section of data at high Galactic latitude we obtain sensitivities ~ 40 micro-K per beam. The statistical analyses show the presence of common signals between channels 1 and 2. Assuming a simple Gaussian auto-correlation model with a scale of coherence $θ_c=1.32$ degrees for the signal, a likelihood analysis of this section of data reveals the presence of fluctuations with intrinsic amplitude $C_{0}^{1/2} = 76^{+43}_{-32}$ micro -K (68 % CL including a ~ 20% calibration uncertainty). Since residual atmospheric noise might still contaminate our results, we also give our result as an upper limit of 118 micro-K at 95% c.l.

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arXiv1994

Comparison of the COBE DMR and Tenerife Data

We have compared the Tenerife data with the {\it COBE} DMR two-year data in the declination +40°region of the sky observed by the Tenerife experiment. Using the Galactic plane signal at $\sim 30$ GHz, we show that the two data sets are cross-calibrated to within 5%. The high Galactic latitude data were investigated for the presence of common structures with the properties of cosmic microwave background (CMB) fluctuations. The most prominent feature in the Tenerife data ($ΔT \sim 80μ$K) is evident in both the 53 and 90 GHz DMR maps and has the Planckian spectrum expected for CMB anisotropy. The cross-correlation function of the Tenerife and DMR scans is indicative of common structure and at zero lag has the value $C(0)^{1/2}=34^{+13}_{-15}\: μ$K. The combination of the spatial and spectral information from the two data sets is consistent with the presence of cosmic microwave background anisotropies common to both. The probability that noise could produce the observed agreement is less than 5%.

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