Result Of Gravitational Lensing Research Articles

Active galactic nuclei cores in infrared-faint radio sources

Infrared-faint radio sources (IFRS) form a new class of galaxies characterised by radio flux densities between tenths and tens of mJy and faint or absent infrared counterparts. It has been suggested that these objects are radio-loud active galactic nuclei (AGNs) at significant redshifts (z >~ 2). Whereas the high redshifts of IFRS have been recently confirmed based on spectroscopic data, the evidence for the presence of AGNs in IFRS is mainly indirect. So far, only two AGNs have been unquestionably confirmed in IFRS based on very long baseline interferometry (VLBI) observations. In this work, we test the hypothesis that IFRS contain AGNs in a large sample of sources using VLBI. We observed 57 IFRS with the Very Long Baseline Array (VLBA) down to a detection sensitivity in the sub-mJy regime and detected compact cores in 35 sources. Our VLBA detections increase the number of VLBI-detected IFRS from 2 to 37 and provide strong evidence that most - if not all - IFRS contain AGNs. We find that IFRS have a marginally higher VLBI detection fraction than randomly selected sources with mJy flux densities at arcsec-scales. Moreover, our data provide a positive correlation between compactness - defined as the ratio of milliarcsec- to arcsec-scale flux density - and redshift for IFRS, but suggest a decreasing mean compactness with increasing arcsec-scale radio flux density. Based on these findings, we suggest that IFRS tend to contain young AGNs whose jets have not formed yet or have not expanded, equivalent to very compact objects. We found two IFRS that are resolved into two components. The two components are spatially separated by a few hundred milliarcseconds in both cases. They might be components of one AGN, a binary black hole, or the result of gravitational lensing.

Close separation triple system QSO 1009-0252 with discordant redshifts: Is the spectrum of one component blueshifted?

1009-0252 is a Quasi Stellar Object (QSO) with three components A, B, C. A, B are thought to be the result of gravitational lensing of one object, and A, C constitute a close pair with redshifts 2.74 and 1.62 respectively. Close separation pairs of QSOs with discordant redshifts have received special attention in recent years, probably because of the possibility that they may be physically associated, implying non-cosmological redshifts. Attempts have been made to explain their occurrences due to the effect of gravitational lensing. However, gravitational lensing has not offered a completely satisfactory explanation for this triplet. Furthermore, examination revealed some inadequacies and inconsistencies in the redshift identification of the observed lines in the component A. Observational results of 1009-0252 therefore remain puzzling. We propose an alternative explanation by suggesting that A, B actually constitute a close pair and C is an unrelated object in the field. We show that the observed spectrum of A can be interpreted as blueshifted. This implies that A, B are two separate objects, one (A) approaching us and the other (B) receding from us, and are not the result of gravitational lensing of a single object. The oppositely directed pair A, B may have been ejected due to the merger of two galaxies.

Binary quasars

Quasar pairs are either physically distinct binary quasars or the result of gravitational lensing. The majority of known pairs are in fact lenses, with a few confirmed as binaries, leaving a population of objects that have not yet been successfully classified. Building on the arguments of Kochanek, Falco & Munoz (1999), it is shown that there are no objective reasons to reject the binary interpretation for most of these. In particular, the similarity of the spectra of the quasar pairs appears to be an artifact of the generic nature of quasar spectra. The two ambiguous pairs discovered as part of the Large Bright Quasar Survey (Q 1429-053 and Q 2153-0256) are analysed using principle components analysis, which shows that their spectral similarities are not greater than expected for a randomly chosen pair of quasars from the survey. The assumption of the binary hypothesis allows the dynamics, time-scales and separation distribution of binary quasars to be investigated and constrained. The most plausible model is that the quasars' activity is triggered by tidal interactions in a galatic merger, but that the (re-)activation of the galactic nuclei occurs quite late in the interaction, when the nuclei are within 80+/-30 kpc of each other. A simple dynamical friction model for the decaying orbits reproduces the observed distribution of projected separations, but the decay time inferred is comparable to a Hubble time. Hence it is predicted that binary quasars are only observable as such in the early stages of galactic collisions, after which the quiescent super-massive black holes orbit in the merger remnant for some time.

Two Close Separation Quasar-Quasar Pairs in the Large Bright Quasar Survey

We present photometric and spectroscopic observations of two close quasar-quasar pairs found in the Large Bright Quasar Survey (LBQS) (Hewett et al. 1995, AJ, 109, 1498). The two components of the 2153-2056 pair ($z=1.845$, $\Delta\theta=7\farcs8$, $B=17.9$ and 21.3) have the same redshifts within the relatively large uncertainty, $\Delta v_{A-B} =-1100\pm1500 km s^{-1}$, of the observations. The quasars are most likely spatially coincident although the possibility that the pair is the result of gravitational lensing cannot be ruled out. The two components of 1148+0055 ($z=1.879$, $B=18.5$ and $z=1.409$, $B=21.1$, $\Delta\theta=3\farcs9$) have disjoint redshifts and the pair has attracted some attention in the context of gravitational lensing (e.g. Wampler 1997, ApJ, 476, L55) following the independent discovery of the pair by Surdej and collaborators. Four close, $\Delta\theta \le10\farcs0$, quasar-quasar pairs have now been discovered in the LBQS and we discuss the probability of identifying pairs with disjoint redshifts and of locating spatially coincident pairs from the systematic investigation of the well-defined quasar catalogue.

Environmental Effect on the Associations of Background Quasars with Foreground Objects. I. Analytic Investigation

The associations of the angular positions of background quasars with foreground galaxies, clusters of galaxies, and quasars are often attributed to statistical lensing by gravitational potentials of the matter along the lines of sight, although it is known that the presence of none of the individual objects (galaxies, clusters, or quasars) is sufficient to explain fully the reported amplitudes of the quasar number enhancements. This probably arises from the fact that the gravitational lensing effect of the environmental matter surrounding these objects has been ignored. In this paper we conduct an extensive study of the influence of the environmental matter on the prediction of the quasar enhancement factor by employing a spatial two-point correlation function. Assuming a singular isothermal sphere for the mass density profile in galaxies and cluster of galaxies, we estimate the average surface mass density bar-Σ around galaxies, clusters, and quasars from the galaxy-galaxy, cluster-cluster, cluster-galaxy, and quasar-galaxy correlations. Our results show that the bar-Σ induced quasar number enhancement in the scenario of gravitational magnification depends critically on the mass density parameters of galaxies (Ωg) and clusters of galaxies (Ωc) in the universe. For a flat cosmological model with Ω0 = 1 the environmental matter can indeed play an important role in the lensing origin of the quasar-quasar and quasar-galaxy associations if Ωg ~ Ωc ~ Ω0, while it is unlikely that bar-Σ is sufficient to account for the reported quasar overdensity behind quasars/galaxies if galaxies and clusters of galaxies contribute no more than 25% to the matter of the universe. Nonetheless, the recently observed quasar-cluster associations on a scale of ~ 10' cannot be the result of gravitational lensing by the cluster environmental matter even if Ωg = Ωc = Ω0.

The distribution of spectral index in the structure of quasar 3C 286

High dynamic range images on mas scale and the distribution of spectral index in the structure of the quasar 3C 286 were obtained from observations of global VLBI arrays at 5 and 1.66 GHz. There are two components located the center of the quasar, but we cannot decide which one is the real core from the spectral index. It is impossible that the structure of 3C 286 is the result of gravitational lensing. There may be two active black holes at the center, i.e., two central engines, each with its own jet extending in different directions.

Radio continuum polarimetric imaging of high redshift radio galaxies

Multifrequency images of total and polarized radio continuum emission from the two high redshift radio galaxies 0902+343 (z = 3.40) and 0647+415 (4C 41.17, z = 3.80) are presented. These images represent the most sensitive polarimetric study of high redshift ratio galaxies to date. The emission from both galaxies is substantially polarized, up to 30% in some regions, and both sources sit behind deep 'Faraday screens,' producing large rotation measures, over 10(exp 3) rad/sq. m in magnitude, and large rotation measure gradients across the sources. Such large rotation measures provide further evidence that high redshift radio galaxies are situated in very dense environments. Drawing the analogy to a class of low redshift powerful radio galaxies with similarly large rotation measures, we suggest that 0902+343 and 0647+415 are situated at the centers of dense, x-ray 'colling flow' clusters, and that the cluster gas is substantially magnetized. The remarkable similarity between the optical and radio morphologies of 0647+415 on scales as small as 0.1 sec is presented. We consider, and reject, both synchrotron and inverse Compton radiation as possible sources of the optical emission. We also consider both scattering of light out of a 'cone' of radiation from an obscured nucleus, and jet-induced star formation, and find that both models encounter difficulties in explaining this remarkably close radio-optical alignment. High resolution spectral index images reveal compact, flat spectrum components in both sources. We suggest that these components are the active nuclei of the galaxies. Lastly, high resolution images of 0902+343 show that the southernmost component forms a 'ring' of 0.2 sec radius. We discuss the possibility that this ring is the result of gravitational lensing, along the lines proposed by Kochanek & Lawrence (1990).

Observations of a High Surface Brightness Arclet Associated with the Cluster of Galaxies GHO 2154+0508

We present the results of photometric and spectroscopic observations of a high surface brightness arclet associated with the cluster of galaxies GHO 2154+0508. High-resolution imaging (FWHM ≃0″.45) of this arclet, presumably the result of gravitational lensing of a background galaxy, show it to be resolved, having FWHM ≃1″.1, with lengths of 3″.3 and 4″.5 in the V band and R band, respectively. We find the arclet to have V=22.07±0.08, (V−R) KC =0.7, and a mean central surface brightness in the V band of 23.7 mag arcsec −2

H0414 + 009 - an X-ray bright BL Lac with a radio tail in a distant cluster of galaxies

VLA observations of the X-ray selected BL Lac H0414 + 009 at 6, 20 and 90 cm show that it has a steep-spectrum (⁠|$\alpha \simeq 1; f_\nu \propto\nu^{-\alpha}$|⁠) radio tail. The tail extends ∼ 40 arcsec from the core, in a gentle curve. There is no evidence of extended emission on the opposite side of the core. This is by far the clearest example of a single-sided radio tail associated with a BL Lac source. The low radio luminosity (⁠|$1.6\times 10^{25} \text {W Hz}^{-1}$| at 20 cm for H0 = 50 km s−1 Mpc−1 and q0 = 1/2 at z = 0.287) and morphology of the extended emission strengthens the proposed association of BL Lacs with Fanaroff–Riley type I (FR I) radio sources. Deep V, R and I CCD images show that the BL Lac is located in a galaxy whose surface brightness and colours are best fitted by an elliptical model with absolute total magnitude Mv = − 23.53. The elliptical host galaxy is the brightest of a poor cluster. We confirm the recently published redshift (z = 0.287) for the BL Lac and find similar redshifts for five of the surrounding, faint galaxies, supporting the conclusion that the BL Lac and the cluster are physically associated and are not merely a chance super-position. We conclude, contrary to earlier suggestions, that BL Lacs do not avoid the cluster environment but are found in groups and clusters just as often as are normal radio galaxies, consistent with the hypothesis that BL Lacs are simply normal radio galaxies seen ‘end-on’. The appearance of the cluster is striking, containing a number of ‘twin’ galaxy pairs. It has been suggested that the pairs could be the result of gravitational lensing by a foreground cosmic string, but our observations do not support this hypothesis. We have analysed the cluster properties and present VRI CCD photometry for 108 galaxies fainter than the BL Lac host galaxy in the CCD images (1.1 × 1.2 Mpc2). In the Bautz–Morgan morphological classification scheme we classify this cluster as BM I. The cluster could be as rich as Abell class 0, however, the richness may be lower due to considerable uncertainties in the corrections for the field galaxy background and the small field of view of the CCD. We have searched deep EXOSAT images for evidence of extended X-ray emission from a hot intracluster medium, but find none. The upper limit to the luminosity of any cluster-type emission from 0414 + 009, however, is still rather high, |$L_\text x(2-10\text {keV})\le 4 \times 10^{37}\text W$|⁠, comparable with that of moderately X-ray luminous Abell clusters. If hot gas emitting at that luminosity does exist in the cluster, it could help explain the unusual structure of the radio source.

The snapshot survey - A search for gravitationally lensed quasars with the Hubble Space Telescope

In a systematic attempt to find new examples of multiply imaged quasars that are the result of gravitational lensing, 89 short exposures, through two filters, of high-luminosity quasars from a well-defined sample have been obtained with the Hubble Space Telecope's Planetary Camera. Useful high-resolution images of approximately 30 quasars have resulted. None of the quasars show evidence of multiple images due to gravitational lensing. Simulations show that multiple images with brightness ratios of up to several magnitudes would have been detected, down to image separations of ~0.1". These results are compared with lower resolution ground-based surveys and current theoretical predictions. The Snapshot Survey has uncovered several engineering problems in the observatory's performance, some of which have already been corrected. In particular, we find that the large telescope pointing errors and drift rates are primarily the result of the lack of correction for stellar aberration when pointing and tracking are performed solely with gyroscopes. The implications of the possibly low intrinsic gyro drift rate on future observations are briefly discussed.

Pavo XD-10, an X-ray QSO with extended optical structure

Deep charge-coupled device (CCD) images of the quasi-stellar object (QSO) identified with the Einstein X-ray source Pavo XD-10 show an optical structure that is dramatically extended. Here we discuss the morphology, the photometry from the CCD data, and the limited spectral information which indicates a redshift (z) of 0.72 for the system. We show that if the extended structure represents a galaxy hosting the z = 0.72 QSO, the galaxy luminosity is 2 mag brighter than the host galaxies detected for QSOs at lower redshifts. Instead, the system may be the result of a galaxy-galaxy interaction. A third and more speculative possibility is that it is the result of gravitational lensing, with much of the ‘underlying’ structure representing the lens galaxy in the line-of-sight.