Wesley N. Colley

A Robust Determination of the Time Delay in 0957+561A, B and a Measurement of the Global Value of Hubble's Constant

Continued photometric monitoring of the gravitational lens system 0957+561A, B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g-band event in the trailing (B) image light curve at the precise time predicted in an earlier paper. The prediction was based on the observation of the event during 1995 in the leading (A) image and on a differential time delay of 415 days. This success confirms the so-called short delay, and the absence of any such feature at a delay near 540 days rejects the long delay for this system, thus resolving a long-standing controversy. A series of statistical analyses of our light-curve data yield a best-fit delay of 417 ? 3 days (95% confidence interval) and demonstrate that this result is quite robust against variations in the analysis technique, data subsamples, and assumed parametric relationship of the two light curves. Recent improvements in the modeling of the lens system (consisting of a galaxy plus a galaxy cluster) allow us to derive a value of the global value (at z = 0.36) of Hubbles constant H0 using Refsdals method, a simple and direct (single-step) distance determination based on experimentally verified and securely understood physics and geometry. The result is H0 = 64 ? 13 km s-1 Mpc-1 (for ? = 1), where this 95% confidence interval is dominantly due to remaining lens model uncertainties. However, it is reassuring that available observations of the lensing mass distribution overconstrain the model and thus provide an internal consistency check on its validity. We argue that this determination of the extragalactic distance scale (10% accurate at 1 ?) is now of comparable quality, in terms of both statistical and systematic uncertainties, to those based on more conventional techniques. Finally, we briefly discuss the prospects for improved H0 determinations using gravitational lenses, and some other possible implications and uses of the 0957+561A, B light curves.

Unlensing Multiple Arcs in 0024+1654: Reconstruction of the Source Image

A unique reconstruction of the image of a high-redshift source galaxy responsible for multiple long arcs in the z = 0.4 cluster 0024+1654 is obtained by inverse lensing. Deep B- and I-band imaging with the Hubble Space Telescope enables high resolution of the arcs due to strong gravitational lensing of the background source. The gravitational lens in the foreground cluster is thus used to obtain a magnified view of the distant source. Four strongly lensed images of the source lead to a unique reconstruction. Each of the long arcs, when unlensed, leads to the same reconstructed source image, exhibiting a beaded, ringlike morphology. The U luminosity of the ring alone is equivalent to that of a normal galaxy. This is likely a galaxy in formation.

An Event in the Light Curve of 0957+561A and Prediction of the 1996 Image B Light Curve

CCD photometry of the gravitational lens system 0957+561A, B in the g and r bands was obtained on alternate nights, weather permitting, from 1994 December through 1995 May using the Double Imaging Spectrograph (DIS) on the Apache Point Observatory (APO) 3.5 m telescope. The remote observing and fast instrument change capabilities of this facility allowed accumulation of light curves sampled frequently and consistently. The Honeycutt ensemble photometry algorithm was applied to the data set and yielded typical relative photometric errors of approximately 0.01 mag. Image A exhibited a sharp drop of about 0.1 mag in late 1994 December; no other strong features were recorded in either image. This event displays none of the expected generic features of a microlensing-induced flux variation and is likely to be intrinsic to the quasar; if so, it should also be seen in the B image with the lensing differential time delay. We give the expected 1996 image B light curves based on two values of the time delay and brightness ratio which have been proposed and debated in the literature. Continued monitoring of the system in the first half of 1996 should easily detect the image B event and thus resolve the time-delay controversy.

Two-dimensional Topology of Large-Scale Structure in the Las Campanas Redshift Survey

We have measured the topology (genus) of the density distribution of large-scale structure observed in the Las Campanas Redshift Survey (LCRS). The LCRS is complete to magnitude 17.5 and contains nearly 24,000 galaxies with median redshift of 30,000 km s-1. The large volume and large number of galaxies permit sampling of nearly 100 independent structures with which to compute the genus topology, a vast improvement over previous studies. We find that the genus is consistent with a random-phase Gaussian distribution of initial density fluctuations, as would be produced naturally in inflationary models. When we combine these results with the genus measurements of the COBE microwave background fluctuations, we find that two orthogonal projections of the three-dimensional distribution of initial density fluctuations are consistent with Gaussian random-phase behavior, in agreement with standard inflationary models. Particular attention is given to statistical significance of the genus test.

Topology of COBE microwave background fluctuations

We have measured the topology (genus) of the fluctuations in the cosmic microwave background that have been seen in the recently completed (4-yr) data set produced by the COBE satellite. We find that the genus is consistent with that expected from a random phase Gaussian distribution, as might be produced naturally in inflationary models.

Observational prospects for extragalactic microlensing events

We consider the feasibility of directly observing gravitational microlensing in extra-galactic sources, whose stars are not generally resolved. This precludes use of the simple optical depth to microlensing formulation, which is applicable only to resolved stars. We, instead, extend this method to consider observational constraints, such as seeing, sky background and minimum detectable change in magnitude. Our analysis provides quantitative relations between these constraints and the expected observational results, event duration and number of detections per year. We find that an ambitious ground-based observer should detect several events per year in M31. We also consider detection of microlensing in visual binary galaxies. We find that, although these objects may present hundreds of events per year, extremely short durations would yield poor prospects for observation.