arXiv subjects

E. E. Falco

Publications and source records attributed to E. E. Falco.

61 records · Page 4Linked to original sources

arXiv1997

Limits on Cosmological Models From Radio-Selected Gravitational Lenses

We are conducting a redshift survey of 177 flat-spectrum radio sources in 3 samples covering the 5GHz flux ranges 50-100, 100-200 and 200-250 mJy. So far, we have measured 124 redshifts with completenesses of 80%, 68% and 58% for the bright, intermediate, and faint flux ranges. Using the newly determined redshift distribution we can derive cosmological limits from the statistics of the 6 gravitational lenses in the JVAS sample of 2500 flat-spectrum radio sources brighter than 200 mJy at 5GHz. For flat cosmological models with a cosmological constant, the limit using only radio data is Omega_0 > 0.27 at 2-sigma (0.47 < Omega_0 < 1.38 at 1-sigma). The limits are statistically consistent with those for lensed quasars, and the combined radio + optical sample requires Omega_0 > 0.38 at 2-sigma (0.64 < Omega_0 < 1.66 at 1-sigma) for our most conservative redshift completeness model and assuming that there are no quasar lenses produced by spiral galaxies. Our best fit model improves by approximately 1-sigma if extinction in the early-type galaxies makes the lensed quasars fainter by Delta m = 0.58 +- 0.45 mag, but we still find a limit of Omega_0 > 0.26 at 2-sigma in flat cosmologies. The increasing fraction of radio galaxies as compared to quasars at fainter radio fluxes (rising from ~10% at 1 Jy to ~50% at 0.1 Jy) explains why lensed optical emission is common for radio lenses and partly explains the red color of radio-selected lenses.

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arXiv1997

The Gravitationally-Lensed Radio Source MG0751+2716

We report the discovery of a new gravitationally lensed radio source. Radio maps of MG0751+2716 show four lensed images, which, at higher resolution, are resolved into long arcs of emission. A group of galaxies is present in optical images, including the principal lensing galaxy, with a much brighter galaxy just a few arcseconds away. We have measured the redshift of this brighter galaxy. No optical counterpart to the background source has been detected. Lens models that can readily reproduce the lensed image positions all require a substantial shear component. However, neither the very elongated lens nor the bright nearby galaxy are correctly positioned to explain the shear. Lens models which associate the mass with the light of galaxies in the group can produce an acceptable fit, but only with an extreme mass-to-light ratio in one of the minor group members.

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arXiv1996

SBS 0909+532: A New Double Gravitational Lens or Binary Quasar?

The z=1.377, B=17.0 mag quasar SBS 0909+532 A, B is a double with two images separated by 1.107 +/- 0.006 arcsec. Because the faint image has an emission line at the same wavelength as the MgII 2798 A emission line of the quasar, and lacks the broad MgIb absoption feature expected for a star with the same colors (a K star), we conclude that image B is a quasar with similar redshift to image A. The relative probabilities that the double is the smallest separation (4.8/h kpc for Ω_0=1) correlated quasar pair or a gravitational lens are \sim 1:10^6. If the object is a lens, the mean lens redshift is $ =0.5$ with 90% confidence bounds of 0.18 < z_l < 0.83 for Ω_0=1. If the lens is an elliptical galaxy, we expect it to be brighter than I < 19.5 mag. The broad band flux ratio varies with wavelength, with ΔI=0.31, ΔR=0.58, and ΔB=1.29 magnitudes, which is difficult to reconcile with the lensing hypothesis.

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arXiv1996

The Quadruple Gravitational Lens PG1115+080: Time Delays and Models

Optical photometry is presented for the quadruple gravitational lens PG1115+080. A preliminary reduction of data taken from November 1995 to June 1996 gives component ``C'' leading component ``B'' by 23.7+/-3.4 days and components ``A1'' and ``A2'' by 9.4 days. A range of models has been fit to the image positions, none of which gives an adequate fit. The best fitting and most physically plausible of these, taking the lensing galaxy and the associated group of galaxies to be singular isothermal spheres, gives a Hubble constant of 42 km/s/Mpc for Omega=1, with an observational uncertainty of 14%, as computed from the B-C time delay measurement. Taking the lensing galaxy to have an approximately E5 isothermal mass distribution yields H0=64 km/sec/Mpc while taking the galaxy to be a point mass gives H0=84 km/sec/Mpc. The former gives a particularly bad fit to the position of the lensing galaxy, while the latter is inconsistent with measurements of nearby galaxy rotation curves. Constraints on these and other possible models are expected to improve with planned HST observations.

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arXiv1996

VLA Observations of the Gravitational Lens System Q2237+0305

We report observations of the four-image gravitational lens system Q2237+0305 with the VLA at 20 cm and 3.6 cm. The quasar was detected at both frequencies (\approx 0.7 mJy) with a flat spectrum. All four lensed images are clearly resolved at 3.6 cm, and the agreement of the radio and optical image positions is excellent. No radio emission is detected from the lensing galaxy, and any fifth lensed quasar image must be fainter than \sim 20% of the A image flux density. Since the optical quasar images are variable and susceptible to extinction, radio flux ratios provide the best measurement of the macrolensing magnification ratios. The radio B/A and C/A image flux ratios are consistent with the observed range of optical variations, but the D/A ratio is consistently higher in the radio than in the optical. The radio ratios are consistent with magnification ratios predicted by lens models, and weaken alternative interpretations for Q2237+0305. More accurate radio ratios can distinguish between the models, as well as improve our understanding of both microlensing and extinction in this system.

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arXiv1995

VLBI Observations of the Gravitational Lens System 0957+561

We present hybrid maps of the A and B images of 0957+561 from each of four sessions of 6 cm VLBI observations that span the six-year interval 1987-1993. The inner- and outer-jets are clearly detected, and confirm the structures reported previously. There is no evidence of change in the separation between the core and inner-jet components, so the prospect of measuring the time delay using differential proper motions is not promising. The flux density in the core of each image peaked between 1989 and 1992. From the variation in these flux densities, we obtain a time-delay estimate of $\sim$1 yr.

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arXiv1994

The detectability of planetary companions of compact galactic objects from their effects on microlensed lightcurves of distant stars

We discuss a possible method for detection of dark companions of galactic objects of stellar mass. Such binary systems are likely to occur in the galactic disk and possibly also in the halo. The high incidence of binary and higher-multiplicity systems in the solar neighborhood, if indicative of the galactic disk at large, implies that current searches for the gravitational microlensing signature of massive compact objects in our galaxy would yield a significant fraction of binary systems. Our calculations suggest that 40% of the lightcurves that will be obtained in such searches may be sufficiently perturbed to reveal, if sufficiently well-sampled, the presence of a compact dark companion of Jovian mass orbiting a primary. The likelihood of occurrence of perturbed lightcurves must also be taken into account by systematic search programs, to improve their event detection efficiency. The statistics of perturbed lensing events, if carefully interpreted, could yield estimates of the incidence of binary systems with low mass ratios, including that of systems with planets.

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