arXiv subjects

A. G. de Bruyn

Publications and source records attributed to A. G. de Bruyn.

At least 127 records · Page 7Linked to original sources

arXiv1999

CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey

The third phase of the Cosmic Lens All-Sky Survey (CLASS) has recently been completed, bringing the total number of sources imaged to over 15000 in the CLASS and JVAS combined survey. In the VLA observations carried out in March and April of 1998, two new candidate lensed systems were discovered: CLASS B1152+199 and B1359+154. B1152+199 is a 1.6 arcsecond double, with a background quasar at z=1.019 lensed by a foreground galaxy at z=0.439. The relatively flat radio spectra of the lensed images, combined with a previous ROSAT detection of the source, make B1152+199 a strong candidate for time delay studies at both radio and X-ray wavelengths. B1359+154 is a quadruply lensed quasar at z=3.235, with a maximum image separation of 1.7 arcseconds. As yet, the redshift of the lensing object in this system is undetermined. The steep spectral index of the source suggests that B1359+154 will not exhibit strong variability, and is therefore unlikely to be useful for determining the Hubble constant from measured time delays.

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arXiv1999

HI and OH absorption at z=0.89

We report on WSRT observations of HI and OH absorption at z=0.885 towards the radio lens PKS 1830-21, mm wave transitions of several molecular species have already been observed at this redshift. At mm wavelengths the source structure is dominated by two extremely compact components, the northeast (NE) and southwest (SW) components. At lower frequencies the continuum emission is much more extended and there is also a broad Einstein ring connecting the NE and SW components. This means that the HI and OH spectra sample a much larger region of the absorber than the mm wave spectra. The HI spectrum that we obtain is asymmetric, with a peak at -147 km/s with respect to the main molecular line redshift of z=0.88582. Weak mm wave molecular absorption has also been detected towards the NE component at this same velocity. The HI absorption, however, covers a total velocity width of 300 km/sec, i.e. including velocities well to the red of molecular features suggesting that it is spatially widespread. In OH we detect both the 1667 and the 1665 MHz transitions. The OH spectrum has a velocity width comparable to that of the HI spectrum, suggesting that it too is widespread in the absorber. The lack of a prominent HI peak in the spectrum at the velocity corresponding to the SW component, suggests that the galaxy responsible for the absorption at z=0.885 has a central molecular disk many kpc in size, and that HI is deficient in this central region. Our observations are sensitive to the large scale kinematics of the absorber, and to first order the implied dynamical mass is consistent with the lens models of Nair et. al. (1993).

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arXiv1999

Measuring Cosmological Parameters with the JVAS and CLASS Gravitational Lens Surveys

The JVAS (Jodrell Bank-VLA Astrometric Survey) and CLASS (Cosmic Lens All-Sky Survey) are well-defined surveys containing about ten thousand flat-spectrum radio sources. For many reasons, flat-spectrum radio sources are particularly well-suited as a population from which one can obtain unbiased samples of gravitational lenses. These are by far the largest gravitational (macro)lens surveys, and particular attention was paid to constructing a cleanly-defined sample for the survey itself and for the underlying luminosity function. Here we present the constraints on cosmological parameters, particularly the cosmological constant, derived from JVAS and combine them with constraints from optical gravitational lens surveys, `direct' measurements of $Ω_{0}$, $H_{0}$ and the age of the universe, and constraints derived from CMB anisotropies, before putting this final result into the context of the latest results from other, independent cosmological tests.

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arXiv1999

Optical spectroscopy of faint gigahertz peaked spectrum sources

We present spectroscopic observations of a sample of faint Gigahertz Peaked Spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS). Redshifts have been determined for 19 (40%) of the objects. The optical spectra of the GPS sources identified with low redshift galaxies show deep stellar absorption features. This confirms previous suggestions that their optical light is not significantly contaminated by AGN-related emission, but is dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (probably) young radio sources as probes of galaxy evolution. The optical spectra of GPS sources identified with quasars are indistinguishable from those of flat spectrum quasars, and clearly different from the spectra of Compact Steep Spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples presented in the literature, peaking at z ~ 2-3. It is unlikely that a significant population of low redshift GPS quasars is missed due to selection effects in our sample. We therefore claim that there is a genuine difference between the redshift distributions of GPS galaxies and quasars, which, because it is present in both the radio-faint and bright samples, can not be due to a redshift-luminosity degeneracy. It is therefore unlikely that the GPS quasars and galaxies are unified by orientation, unless the quasar opening angle is a strong function of redshift. We suggest that the GPS quasars and galaxies are unrelated populations and just happen to have identical observed radio-spectral properties, and hypothesise that GPS quasars are a sub-class of flat spectrum quasars.

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arXiv1999

Spectral Energy Distributions and Light Curves of GRB 990123 and its Afterglow

Gamma-ray bursts (GRBs) are thought to result from the interaction of an extremely relativistic outflow interacting with a small amount of material surrounding the site of the explosion. Multi-wavelength observations covering the gamma-ray to radio wavebands allow investigations of this `fireball' model. On 23 January 1999 optical emission was detected while the gamma-ray burst was still underway. Here we report the results of gamma-ray, optical/infra-red, sub-mm, mm and radio observations of this burst and its afterglow, which indicate that the prompt and afterglow emissions from GRB 990123 are associated with three distinct regions in the fireball. The afterglow one day after the burst has a much lower peak frequency than those of previous bursts; this explains the short-lived nature of the radio emission, which is not expected to reappear. We suggest that such differences reflect variations in the magnetic-field strengths in the afterglow emitting regions.

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arXiv1999

Can the reionization epoch be detected as a global signature in the cosmic background?

The reionization of the Universe is expected to leave a signal in the form of a sharp step in the spectrum of the sky. If reionization occurs at 5 < z < 20, a feature should appear in the radio sky at 70 - 240 MHz due to redshifted HI 21-cm line emission, accompanied by another feature in the optical/near-IR at 0.7 - 2.6 micron due to hydrogen recombination radiation. The expected amplitude is well above fundamental detection limits, and the sharpness of the feature may make it distinguishable from variations due to terrestrial, galactic and extragalactic foregrounds. Because this is essentially a continuum measurement of a signal which occurs over the whole sky, relatively small telescopes may suffice for detection in the radio. In the optical/near-IR, a space telescope is needed with the lowest possible background conditions, since the experiment will be severely background-limited.

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arXiv1998

A new radio double lens from CLASS: B1127+385

We present the discovery of a new gravitational lens system with two compact radio images separated by 0.701+-0.001 arcsec. The lens system was discovered in the Cosmic Lens All Sky Survey (CLASS) as a flat spectrum radio source. Both radio components show structure in a VLBA 8.4 GHz radio image. No further extended structure is seen in either the VLA, MERLIN or VLBA images. Hubble Space Telescope (HST) WFPC2 images in F555W and F814W show two extended objects close to the radio components, which we identify as two lens galaxies. Their colours and mass-to-light ratios seem to favour two late-type spiral galaxies at relatively high redshifts (z_d>~0.5). Faint emission is also detected at positions corresponding to the radio images. A two-lens mass model can explain the observed VLBA structure. The best fit model has a reduced chi^2 of 1.1. The relative positions of the VLBA subcomponents are reproduced within 0.08 mas, the flux density ratios within 0.19. We also reproduce the position angle and separation of the two VLBA subcomponents in A and B within the observational errors, which we consider strong evidence for the validity of the lens model. Moreover, we find a surface density axis ratio of 0.74^{+0.10}_{-0.12} for the primary lens (G1), consistent with the surface brightness axis ratio of 0.69+-0.15. Also, the surface density position angle of 4.9^{+28.2}_{-22.4} degree of G1 compares well with the -6+-13 degree position angle of the surface brightness distribution. The errors indicate the 99 per cent confidence interval.

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arXiv1998

B2045+265: A New Four-Image Gravitational Lens from CLASS

We have discovered a new gravitational lens in the Cosmic Lens All-Sky Survey (CLASS). B2045+265 is a four-image system with a maximum separation of 1.9 arcsec. A fifth radio component is detected, but its radio spectrum and its positional coincidence with infrared emission from the lensing galaxy strongly suggests that it is the radio core of the lensing galaxy. This implies that the B2045+265 system consists of a flat-spectrum radio source being lensed by another flat-spectrum radio source. Infrared images detect the lensed images of the background source and the lensing galaxy. The lensed images have relative positions and flux densities that are consistent with those seen at radio wavelengths. Spectra of the system reveal a lens redshift of z_l=0.8673 and a source redshift of z_s=1.28. The image splitting and system redshifts imply that the projected mass inside the Einstein radius of the lensing galaxy is M_E=4.7x10^{11} h^{-1} M_sun. An estimate of the light emitted inside the Einstein radius gives a mass-to-light ratio in the rest frame B band of (M/L_B)_E = 20 h (M/L_B)_sun. Both the mass and mass-to-light ratio are higher than what is seen in nearby Sa galaxies. In fact, the implied rotation velocity for the lensing galaxy is two to three times higher than what is seen in nearby spirals. The large projected mass inside the Einstein ring radius may be the result of a significant amount of dark matter in the system, perhaps from a compact group of galaxies; however, it may also arise from a misidentification of the source redshift. A simple model of the gravitational potential of the lens reproduces the image positions well, but further modeling is required to satisfy the constraints from the image flux density ratios. With further observations and modeling, this lens may yield an estimate of H_0.

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arXiv1998

The Mpc-scale radio source associated with the GPS galaxy B1144+352

We present the results of new observations of the enigmatic radio source B1144+352 with the WSRT at 1.4 GHz. The WENSS and NVSS surveys have revealed faint extended radio structures on Mpc-scales. We have investigated these large-scale radio components. One of the radio structures has a leading hotspot and we conclude that this structure is a large radio lobe associated with the GPS source. The other contains two separate radio sources which are superposed on the sky: A low-power radio source at a similar redshift (z=0.065) to the GPS host galaxy and an extended radio lobe, which we believe is associated with the GPS host galaxy. Further, the 1.4-GHz flux density of the core has decreased by 70 mJy between 1994 and 1997. We have also retrieved and analysed unpublished archival ROSAT HRI data of B1144+352. The source has been detected and appears to be slightly extended in X-rays. The ROSAT data have not detected a large extended X-ray halo surrounding the host galaxy, limiting the luminosity of the intra-cluster gas within a radius of 250 kpc to < 2.3 x 10^42 erg/s. B1144+352 is the second GPS galaxy known to be associated with a Mpc-sized radio source, the other being B1245+676. We argue that the observed structures must be the result of an interrupted central jet-activity, and that as such these sources may well be the progenitors of double-double radio galaxies.

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arXiv1998

Optical and near-infrared imaging of faint Gigahertz Peaked Spectrum sources

A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected from the Westerbork Northern Sky Survey (WENSS, Rengelink et al. 1997), has been imaged in the optical and near infrared, resulting in an identification fraction of 87%. The R-I and R-K colours of the faint optical counterparts are as expected for passively evolving elliptical galaxies, assuming that they follow the R band Hubble diagram as determined for radio-bright GPS galaxies. We have found evidence that the radio spectral properties of the GPS quasars are different from those of GPS galaxies: The observed distribution of radio spectral peak frequencies for GPS sources optically identified with bright stellar objects (presumably quasars) is shifted compared with GPS sources identified with faint or extended optical objects (presumably galaxies), in the sense that a GPS quasar is likely to have a higher peak frequency than a GPS galaxy. This means that the true peak frequency distribution is different for the GPS galaxies and quasars, because the sample selection effects are independent of optical identification. The correlation between peak frequency and redshift as has been suggested for bright sources has not been found in this sample; no correlation exists between R magnitude (and therefore redshift) and peak frequency for the GPS galaxies. We therefore believe that the claimed correlation is actually caused by the dependence of the peak frequency on optical host, because the GPS galaxies are in general at lower redshifts than the quasars. The difference in the peak frequency distributions of the GPS galaxies and quasars is further evidence against the hypothesis that they form a single class of object.

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arXiv1998

WNB 0313+683: Analysis of a Newly Discovered Giant Radio Galaxy

We present the results of a detailed analysis of the newly discovered 15 arcmin large FRII-type radio galaxy WNB 0313+683. It has a redshift of 0.0901 +/- 0.0002 and a projected linear size of 2.0 Mpc. Using multi-frequency radio data from the WENSS and NVSS surveys, the VLA, the WSRT and the 100m Effelsberg telescope, we have derived the age of the source (1.4 +/- 0.1 x 10^8 yrs) and the particle densities around the two radio lobes (>1.6 x 10^(-6) per cubic cm for the southern lobe and >5.8 x 10^(-7) per cubic cm for the northern). From optical spectroscopy we find that the H$α$ emission line has a broad component, and that the extinction is large (with colour index E(B-V) = 0.98 +/- 0.10 mag), but is probably mostly galactic in origin (b= +9.8 degrees). Further, the radio core has an inverted spectrum with a spectral index of +0.42 +/- 0.03, suggesting a very compact structure. At 10.5 GHz, it contributes 25% of the total flux density of the source, which is extremely high for radio galaxies. WNB 0313+683 has a very large optical emission line flux with respect to its estimated jet power, when compared with the correlation between these two properties found by Rawlings & Saunders (1991). We argue that this, together with the relatively high radio power and the inverted radio spectrum of the radio core, is suggestive of a new phase of radio activity in this source.

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arXiv1998

The 1.4 GHz light curve of GRB 970508

We report on Westerbork 1.4 GHz radio observations of the radio counterpart to $γ$-ray burst GRB~970508, between 0.80 and 138 days after this event. The 1.4 GHz light curve shows a transition from optically thick to thin emission between 39 and 54 days after the event. We derive the slope $p$ of the spectrum of injected electrons ($dN/dγ_{e}\proptoγ_{e}^{-p}$) in two independent ways which yield values very close to $p=2.2$. This is in agreement with a relativistic dynamically near-adiabatic blast wave model whose emission is dominated by synchrotron radiation and in which a significant fraction of the electrons cool fast.

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arXiv1998

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.

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arXiv1998

Deceleration of Relativistic Radio Components and the morphologies of Gigahertz Peaked Spectrum Sources

A relativistic radio component, which moves in a direction close to the sky plane, will increase in flux density when it decelerates. This effect is the basis for the qualitative model for GPS galaxies we present in this paper, which can explain their low-variability convex spectrum, their compact double or compact symmetric morphology, and the lack of GPS quasars at similar redshifts. Components are expelled from the nucleus at relativistic speeds at a large angle to the line of sight, and are decelerated (eg. by ram-pressure or entrainment of the external gas) before contributing to a mini-lobe. The young components are Doppler boosted in the direction of motion but appear fainter for the observer. The non-relativistic mini-lobes dominate the structure and are responsible for the low variability in flux density and the convex radio spectrum as well as the compact double angular morphology. Had the same source been orientated at a small angle to the line of sight, the young components would be boosted in the observer's direction resulting in a flat and variable radio spectrum at high frequencies. Hence the characteristic convex spectrum of a GPS source would not be seen. These sources at small angles to the line of sight are probably identified with quasars, and are not recognized as GPS sources, but are embedded in the large population of flat spectrum variable quasars and BL Lac objects. This leads to a deficiency in GPS/CSOs identified with quasars.

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arXiv1998

The edge-on spiral gravitational lens B1600+434

We present new observations of the gravitational lens (GL) system B1600+434, strongly suggesting that the lens is an edge-on spiral galaxy. These observations are used to constrain the mass model of the system, in particular the oblateness and velocity dispersion of the dark matter halo around the lensing galaxy. From an analytical model we find a lower limit on the halo oblateness $q_\rmn{halo}=(c/a)_ρ\ga0.4$; more detailed numerical models give a lower limit $q_\rmn{h}\ga0.5$. We determine an average halo velocity dispersion of $σ_{halo}=190\pm 15$ km/s over all non-singular isothermal elliptical (NIE) halo models. Constraining the models to larger and more massive disks, decreases this average by only 10 km/s. A lower limit of $σ_{halo}\ga 150$ km/s is found, even for disk masses larger then the mass inside the Einstein radius. This lower limit indicates the need for a massive dark matter halo, contributing at least half of the mass inside the Einstein radius. Time delay calculations give $(54\pm3)/h_{50}$ days for the NIE halo model and $(70\pm4)/h_{50}$ days for the modified Hubble profile (MHP) halo model. Although the time delay for both NIE and MHP halo models is well constrained on our parameter grid, it strongly depends on the halo surface density profile. We furthermore find that the presence of a flat luminous mass distribution can severely alter the statistical properties of the lens.

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arXiv1997

The complex gravitational lens system B1933+503

We report the discovery of the most complex arcsec-scale radio gravitational lens system yet known. B1933+503 was found during the course of the CLASS survey and MERLIN and VLA radio maps reveal up to 10 components. Four of these are compact and have flat spectra; the rest are more extended and have steep spectra. The background lensed object appears to consist of a flat spectrum core (quadruply imaged) and two compact "lobes" symmetrically disposed relative to the core. One of the lobes is quadruply imaged while the other is doubly imaged. An HST observation of the system with the WFPC2 shows a galaxy with an axial ratio of 0.5, but none of the images of the background object are detected. A redshift of 0.755 has been measured for the lens galaxy.

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arXiv1997

A complete infrared Einstein ring in the gravitational lens system B1938+666

We report the discovery, using NICMOS on the Hubble Space Telescope, of an arcsecond-diameter Einstein ring in the gravitational lens system B1938+666. The lensing galaxy is also detected, and is most likely an early-type. Modelling of the ring is presented and compared with the radio structure from MERLIN maps. We show that the Einstein ring is consistent with the gravitational lensing of an extended infrared component, centred between the two radio components.

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arXiv1997

Radio and Optical Follow-up Observations and Improved IPN Position of GRB 970111

We report on Westerbork 840 MHz, 1.4 and 5 GHz radio observations of the improved IPN-WFC error box of the gamma-ray burst GRB 970111, between 26.4 hours and 120 days after the event onset. In the ~ 13 sq arcmin area defined by the IPN (BATSE and Ulysses) annulus and the published refined BeppoSAX Wide Field Camera error box we detected no steady sources brighter than 0.56 mJy (4 sigma), and no varying radio emission, down to 1.0 mJy (4 sigma). We also report on B, V, R and I band observations of the error box with the 4.2 m William Herschel Telescope at La Palma.

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