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Register a new userDetection of x-rays from galaxy groups associated with the gravitationally lensed systems PG 1115+080 and B1422+231
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Gravitational lenses that produce multiple images of background quasars can be an invaluable cosmological tool. Deriving cosmological parameters, however, requires modeling the potential of the lens itself. It has been estimated that up to a quarter of lensing galaxies are associated with a group or cluster which perturbs the gravitational potential. Detection of X-ray emission from the group or cluster can be used to better model the lens. We report on the first detection in X-rays of the group associated with the lensing system PG 1115+080 and the first X-ray image of the group associated with the system B1422+231. We find a temperature and rest-frame luminosity of 0.8 +/- 0.1 keV and 7 +/- 2 x 10^{42} ergs/s for PG 1115+080 and 1.0 +infty/-0.3 keV and 8 +/- 3 x 10^{42} ergs/s for B1422+231. We compare the spatial and spectral characteristics of the X-ray emission to the properties of the group galaxies, to lens models, and to the general properties of groups at lower redshift.
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We analyzed the microlensing of the X-ray and optical emission of the lensed quasar PG 1115+080. We find that the effective radius of the X-ray emission is 1.3(+1.1 -0.5) dex smaller than that of the optical emission. Viewed as a thin disk observed at inclination angle i, the optical accretion disk has a scale length, defined by the point where the disk temperature matches the rest frame energy of the monitoring band (kT=hc/lambda_rest with lambda_rest=0.3 micron), of log[(r_{s,opt}/cm)(cos(i) / 0.5)^{1/2}] = 16.6 pm 0.4. The X-ray emission region (1.4-21.8 keV in the rest frame) has an effective half-light radius of log[r_{1/2,X}/cm] = 15.6 (+0.6-0.9}. Given an estimated black hole mass of 1.2 * 10^9 M_sun, corresponding to a gravitational radius of log[r_g/cm] = 14.3, the X-ray emission is generated near the inner edge of the disk while the optical emission comes from scales slightly larger than those expected for an Eddington-limited thin disk. We find a weak trend supporting models with low stellar mass fractions near the lensed images, in mild contradiction to inferences from the stellar velocity dispersion and the time delays.
Hubble Space Telescope observations of the gravitational lens PG 1115+080 in the infrared show the known z =0.310 lens galaxy and reveal the z = 1.722 quasar host galaxy. The main lens galaxy G is a nearly circular (ellipticity < 0.07) elliptical galaxy with a de Vaucouleurs profile and an effective radius of R_e = 0.59 +/- 0.06 arcsec (1.7 +/- 0.2 h^{-1} kpc for Omega = 1 and h = H_0/100 km/s/Mpc). G is part of a group of galaxies that is a required component of all successful lens models. The new quasar and lens positions (3 milliarcsecond errors) yield constraints for these models that are statistically degenerate, but several conclusions are firmly established. (1) The principal lens galaxy is an elliptical galaxy with normal structural properties, lying close to the fundamental plane for its redshift. (2) The potential of the main lens galaxy is nearly round, even when not constrained by the small ellipticity of the light of this galaxy. (3) All models involving two mass distributions place the group component near the luminosity-weighted centroid of the brightest nearby group members. (4) All models predict a time delay ratio r_{ABC} = 1.3. (5) Our lens models predict H_0 = 44 +/- 4 km/s/Mpc if the lens galaxy contains dark matter and has a flat rotation curve, and H_0 = 65 +/- 5 km/s/Mpc if it has a constant mass-to-light ratio. (6) Any dark halo of the main lens galaxy must be truncated near 1.5 arcsec (4 h^{-1} kpc) before the inferred Ho rises above 60 km/s/Mpc. (7) The quasar host galaxy is lensed into an Einstein ring connecting the four quasar images, whose shape is reproduced by the models. Improved NICMOS imaging of the ring could be used to break the degeneracy of the lens models.
Time delay measurements have recently been reported for the lensed quasar PG 1115+080. These measurements can be used to derive Ho, but only if we can constrain the lensing potential. We have applied a recently developed deconvolution technique to analyse sub-arcsecond I band images of PG 1115+080, obtained at the Nordic Optical Telescope (NOT) and the Canada France Hawaii Telescope (CFHT). The high performance of the deconvolution code allows us to derive precise positions and magnitudes for the four lensed images of the quasar, as well as for the lensing galaxy. The new measurement of the galaxy position improves its precision by a factor of 3 and thus strengthens the constraints on the lensing potential. With the new data, a range of models incorporating some of the plausible systematic uncertainties yields Ho = 53 (+10/-7) km/s/mpc.
We present time-delay estimates for the quadruply imaged quasar PG 1115+080. Our resuls are based on almost daily observations for seven months at the ESO MPIA 2.2m telescope at La Silla Observatory, reaching a signal-to-noise ratio of about 1000 per quasar image. In addition, we re-analyse existing light curves from the literature that we complete with an additional three seasons of monitoring with the Mercator telescope at La Palma Observatory. When exploring the possible source of bias we consider the so-called microlensing time delay, a potential source of systematic error so far never directly accounted for in previous time-delay publications. In fifteen years of data on PG 1115+080, we find no strong evidence of microlensing time delay. Therefore not accounting for this effect, our time-delay estimates on the individual data sets are in good agreement with each other and with the literature. Combining the data sets, we obtain the most precise time-delay estimates to date on PG 1115+080, with Dt(AB) = 8.3+1.5-1.6 days (18.7% precision), Dt(AC) = 9.9+1.1-1.1 days (11.1%) and Dt(BC) = 18.8+1.6-1.6 days (8.5%). Turning these time delays into cosmological constraints is done in a companion paper that makes use of ground-based Adaptive Optics (AO) with the Keck telescope.
We present mid-infrared imaging at 11.7 mu m for the quadruple lens systems, PG1115+080 and B1422+231, using the cooled mid-infrared camera and spectrometer (COMICS) attached on the Subaru telescope. These lensed QSOs are characterized by their anomalous optical and radio flux ratios as obtained for (A1, A2) images of PG1115+080 and (A, B, C) images of B1422+231, respectively, i.e., such flux ratios are hardly reproduced by lens models with smooth mass distribution. Our mid-infrared observations for these images have revealed that the mid-infrared flux ratio A2/A1 of PG1115+080 is virtually consistent with smooth lens models (but inconsistent with the optical flux ratio), whereas for B1422+231, the mid-infrared flux ratios among (A, B, C) are in good agreement with the radio flux ratios. We also identify a clear infrared bump in the spectral energy distributions of these QSOs, thereby indicating that the observed mid-infrared fluxes originate from a hot dust torus around a QSO nucleus. Based on the size estimate of the dust torus, we place limits on the mass of a substructure in these lens systems, causing the anomalous optical or radio flux ratios. For PG1115+080, the mass of a substructure inside an Einstein radius, M_E, is < 16 Msun, corresponding to either a star or a low-mass CDM subhalo having the mass of M_{100}^{SIS} < 2.2 * 10^4 Msun inside radius of 100 pc if modeled as a singular isothermal sphere (SIS). For B1422+231, we obtain M_E > 209 Msun, indicating that a CDM subhalo is more likely, having the mass of M_{100}^{SIS} > 7.4 * 10^4 Msun
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