Measuring gravitational lens time delays using low-resolution radio monitoring observations

Obtaining lensing time delay measurements requires long-term monitoring campaigns with a high enough resolution (< 1 arcsec) to separate the multiple images. In the radio, a limited number of high-resolution interferometer arrays make these observations difficult to schedule. To overcome this problem, we propose a technique for measuring gravitational time delays which relies on monitoring the total flux density with low-resolution but high-sensitivity radio telescopes to follow the variation of the brighter image. This is then used to trigger high-resolution observations in optimal numbers which then reveal the variation in the fainter image. We present simulations to assess the efficiency of this method together with a pilot project observing radio lens systems with the Westerbork Synthesis Radio Telescope (WSRT) to trigger Very Large Array (VLA) observations. This new method is promising for measuring time delays because it uses relatively small amounts of time on high-resolution telescopes. This will be important because instruments that have high sensitivity but limited resolution, together with an optimum usage of followup high-resolution observations from appropriate radio telescopes may in the future be useful for gravitational lensing time delay measurements by means of this new method.Comment: 10 pages, 7 figures, accepted by MNRA

A new perspective on the submillimetre galaxy MM 18423+5938 at redshift 3.9296 from radio continuum imaging

The bright submillimetre (sub-mm) galaxy MM 18423+5938 at redshift 3.9296 has been predicted from mid-infrared and millimetre photometry to have an exceptionally large total infrared (IR) luminosity. We present new radio imaging at 1.4 GHz with the Westerbork Synthesis Radio Telescope that is used to determine a radio-derived total IR luminosity for MM 18423+5938 via the well established radio-far-infrared correlation. The flux density is found to be S_1.4 GHz = 217 +/- 37 \mu Jy, which corresponds to a rest-frame luminosity density of L_1.4 GHz = 2.32 +/- 0.40 x 10^25 / u W / Hz, where u is the magnification from a probable gravitational lens. The radio-derived total IR luminosity and star-formation rate are L_8-1000 \mu m = 5.6^+4.1_-2.4 x 10^13 / u L_sol and SFR = 9.4^+7.4_-4.9 x 10^3 / u M_sol / yr, respectively, which are ~9 times smaller than those previously reported. These differences are attributed to the IR spectral energy distribution of MM 18423+5938 being poorly constrained by the limited number of reliable photometric data that are currently available, and from a previous misidentification of the object at 70 \mu m. Using the radio derived total IR luminosity as a constraint, the temperature of the cold dust component is found to be T ~ 24^+7_-5 K for a dust emissivity of \beta = 1.5 +/- 0.5. The radio-derived properties of this galaxy are still large given the low excitation temperature implied by the CO emission lines and the temperature of the cold dust. Therefore, we conclude that MM 18423+5938 is probably gravitationally lensed.Comment: 5 pages, 2 figures, 1 table, accepted for publication in MNRAS Letter

Comparison of an X-ray selected sample of massive lensing clusters with the MareNostrum Universe LCDM simulation

A long-standing problem of strong lensing by galaxy clusters regards the observed high rate of giant gravitational arcs as compared to the predictions in the framework of the "standard" cosmological model. Recently, few other inconsistencies between theoretical expectations and observations have been claimed which regard the large size of the Einstein rings and the high concentrations of few clusters with strong lensing features. All of these problems consistently indicate that observed galaxy clusters may be gravitational lenses stronger than expected. We use clusters extracted from the MareNostrum Universe to build up mock catalogs of galaxy clusters selected through their X-ray flux. We use these objects to estimate the probability distributions of lensing cross sections, Einstein rings, and concentrations for the sample of 12 MACS clusters at $z>0.5$ presented in Ebeling et al. (2007) and discussed in Zitrin et al. (2010). We find that simulated clusters produce $\sim 50$ less arcs than observed clusters do. The medians of the distributions of the Einstein ring sizes differ by $\sim 25$ between simulations and observations. We estimate that, due to cluster triaxiality and orientation biases affecting the lenses with the largest cross sections, the concentrations of the individual MACS clusters inferred from the lensing analysis should be up to a factor of $\sim 2$ larger than expected from the $\Lambda$CDM model. The arc statistics, the Einstein ring, and the concentration problems in strong lensing clusters are mitigated but not solved on the basis of our analysis. Nevertheless, due to the lack of redshifts for most of the multiple image systems used for modeling the MACS clusters, the results of this work will need to be verified with additional data. The upcoming CLASH program will provide an ideal sample for extending our comparison (abridged).Comment: 11 pages, 9 figures, accepted for publication on A&

Highly-magnified, Multiply-imaged radio counterparts of the Sub-mm Starburst Emission in the Cluster-Lens MS0451.6-0305

Previous authors have reported the detection of intrinsically faint sub-mm emission lensed by the cluster MS0451.6-0305. They suggest that this emission arises from a merging system composed of a Ly-break galaxy and a pair of extremely red objects which are multiply-imaged in the optical/NIR observations. Since the submm emission presents an unusually large angular extent (~1 arcmin), the possible radio emission asociatted with that system can help to identify optical/NIR counterparts due to the higher spatial resolution and astrometric accuracy of the radio observations. Archive VLA data (BnA configuration at 1.4 GHz) was reduced and analysed. A simple lens model was constructed to aid the interpretation of the radio and pre-existing sub-mm and optical/NIR data. We present a 1.4 GHz map of the central region of MS0451.6-0305 and report the detection of gravitationally lensed radio emission, coincident with the previously discovered sub-mm lensed emission. The overall morphology and scale of the radio and sub-mm emission are strikingly similar, extending ~1 arcmin across the sky. This observation strongly suggests that the radio and sub-mm emission arise from the same sources. Preliminary estimates of the total S_850 microns/S_1.4 GHz flux density ratio appear to be consistent with that expected from distant star forming galaxies. The radio emission is resolved into 7 distinct components, and the overall structure can be explained, using a simple lens model, with three multiply-imaged radio sources at z~2.9. One of these sources is predicted to lie in the middle of the previously mentioned system in the source plane, suggesting that it is related to the intense star formation generated during the merging process.Comment: 11 pages, 3 figures. Acepted for publication in A&A on 05/10/200

Gravitational lensing reveals extreme dust-obscured star formation in quasar host galaxies

We have observed 104 gravitationally-lensed quasars at $z\sim1-4$ with Herschel/SPIRE, the largest such sample ever studied. By targeting gravitational lenses, we probe intrinsic far-infrared (FIR) luminosities and star formation rates (SFRs) more typical of the population than the extremely luminous sources that are otherwise accessible. We detect 72 objects with Herschel/SPIRE and find 66 percent (69 sources) of the sample have spectral energy distributions (SEDs) characteristic of dust emission. For 53 objects with sufficiently constrained SEDs, we find a median effective dust temperature of $38^{+12}_{-5}$ K. By applying the radio-infrared correlation, we find no evidence for an FIR excess which is consistent with star-formation-heated dust. We derive a median magnification-corrected FIR luminosity of $3.6^{+4.8}_{-2.4}~\times 10^{11}~{\rm L_{\odot}}$ and median SFR of $120^{+160}_{-80}~{\rm M_{\odot}~yr^{-1}}$ for 94 quasars with redshifts. We find $\sim10$ percent of our sample have FIR properties similar to typical dusty star-forming galaxies at $z\sim2-3$ and a range of SFRs $<20-10000~{\rm M_{\odot}~yr^{-1}}$ for our sample as a whole. These results are in line with current models of quasar evolution and suggests a coexistence of dust-obscured star formation and AGN activity is typical of most quasars. We do not find a statistically-significant difference in the FIR luminosities of quasars in our sample with a radio excess relative to the radio-infrared correlation. Synchrotron emission is found to dominate at FIR wavelengths for $<15$ percent of those sources classified as powerful radio galaxies.Comment: 47 pages, 89 figures, accepted for publication in MNRA

On the abundance of gravitational arcs produced by submillimeter galaxies at radio and submm wavelengths

We predict the abundance of giant gravitational arcs produced by submillimeter galaxies (SMGs) lensed by foreground galaxy clusters, both at radio and submm wavelengths. The galaxy cluster population is modeled in a realistic way with the use of semi-analytic merger trees, while the density profiles of individual deflectors take into account ellipticity and substructures. The adopted typical size of the radio and submm emitting regions of SMGs is based on current radio/CO observations and the FIR-radio correlation. The source redshift distribution has been modeled using three different functions (based on spectroscopic/photometric redshift measurements and a simple evolutionary model) to quantify the effect of a high redshift tail on the number of arcs. The source number counts are compatible with currently available observations, and were suitably distorted to take into account the lensing magnification bias. We present tables and plots for the numbers of radio and submm arcs produced by SMGs as a function of surface brightness, useful for the planning of future surveys aimed at arc statistics studies. They show that e. g., the detection of several hundred submm arcs on the whole sky with a signal-to-noise ratio of at least 5 requires a sensitivity of 1 mJy arcsec(-2) at 850 mu m. Approximately the same number of radio arcs should be detected with the same signal-to-noise ratio with a surface brightness threshold of 20 mu y arcsec(-2) at 1.4 GHz. Comparisons of these results with previous work found in the literature are also discussed