James E. Rhoads

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.

Radio transients from gamma-ray bursters

The rapid time variability of gamma-ray bursts implies the sources are very compact, and the peak luminosities are so high that some matter must be ejected at ultra-relativistic speeds. The very large Lorentz factors of the bulk flow are also indicated by the very broad and hard spectra. It is natural to expect that when the relativistic ejecta interact with the interstellar matter a strong synchrotron radio emission is generated, as is the case with supernova remnants and radio galaxies. We estimate that the strongest gamma-ray bursts may be followed by radio transients with peak fluxes as high as 0.1 Jy. The time of peak radio emission depends on the distance scale; it is less than a minute if the bursts are in the galactic halo, and about a week if the bursts are at cosmological distances.

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.

Magnetic merging in colliding flux tubes

We develop an analytical theory of reconnection between colliding, twisted magnetic flux tubes. Our analysis is restricted to direct collisions between parallel tubes and is based on the collision dynamics worked out by Bogdan (1984). We show that there is a range of collision velocities for which neutral point reconnection of the Parker-Sweet type can occur, and a smaller range for which reconnection leads to coalescence. Mean velocities within the solar convection zone are probably significantly greater than the upper limit for coalescence. This suggests that the majority of flux tube collisions do not result in merging, unless the frictional coupling of the tubes to the background flow is extremely strong.

OH and H I absorption at the nucleus of NGC 660

High-resolution interferometric data of the H I and OH absorption in the nuclear region of NGC 660 reveal three distinct absorbing structures. The central disk of the galaxy with a large velocity gradient dominates the absorption signature. The gas in the warped outer disk appears in absorption close to the systemic velocity; the outer rings of the warp located at large radii are moving in front of the nuclear radio source. Third, an outflowing feature can be seen at the center of the radio source at 100 km s −1 below the systemic velocity. This mostly molecular feature could be due to a perturbed spiral structure in the inner regions of the disk

Determining the Contribution of Young Stars to Near Infrared Light

As part of an effort to determine the stellar origins of near infrared (NIR) light, we have measured the NIR photometric CO absorption index in different regions of the galaxy NGC 1309. This index measures the strength of the gravity-sensitive 2.3µm stellar absorption band and is a good indicator of young, cool supergiant stars. Our data suggest that such young supergiants dominate the 2µm light from active star forming regions in NGC 1309. The galaxy’s quiescent regions, in contrast, do not show evidence of young supergiants. It follows that the 2/µm light comes from different stellar populations in different places, and large changes in the 2µm surface brightness need not imply correspondingly large features in the galaxy’s mass distribution.

ATLAS probe for the study of galaxy evolution with 300,000,000 galaxy spectra

ATLAS (Astrophysics Telescope for Large Area Spectroscopy) Probe is a mission concept for a NASA probe-class space mission with primary science goal the definitive study of galaxy evolution through the capture of 300,000,000 galaxy spectra up to z=7. It is made of a 1.5-m Ritchey-Chretien telescope with a field of view of solid angle 0.4 deg2. The wavelength range is at least 1 μm to 4 μm with a goal of 0.9 μm to 5 μm. Average resolution is 600 but with a possible trade-off to get 1000 at the longer wavelengths. The ATLAS Probe instrument is made of 4 identical spectrographs each using a Digital Micro-mirror Device (DMD) as a multi-object mask. It builds on the work done for the ESA SPACE and Phase-A EUCLID projects. Three-mirror fore-optics re-image each sub-field on its DMD which has 2048 x 1080 mirrors 13.6 μm wide with 2 possible tilts, one sending light to the spectrograph, the other to a light dump. The ATLAS Probe spectrographs use prisms as dispersive elements because of their higher and more uniform transmission, their larger bandwidth, and the ability to control the resolution slope with the choice of glasses. Each spectrograph has 2 cameras. While the collimator is made of 4 mirrors, each camera is made of only one mirror which reduces the total number of optics. All mirrors are aspheric but with a relatively small P-V with respect to their best fit sphere making them easily manufacturable. For imaging, a simple mirror to replace the prism is not an option because the aberrations are globally corrected by the collimator and camera together which gives large aberrations when the mirror is inserted. An achromatic grism is used instead. There are many variations of the design that permit very different packaging of the optics. ATLAS Probe will enable ground-breaking science in all areas of astrophysics. It will (1) revolutionize galaxy evolution studies by tracing the relation between galaxies and dark matter from the local group to cosmic voids and filaments, from the epoch of reionization through the peak era of galaxy assembly; (2) open a new window into the dark universe by mapping the dark matter filaments to unveil the nature of the dark Universe using 3D weak lensing with spectroscopic redshifts, and obtaining definitive measurements of dark energy and modification of gravity using cosmic large-scale structure; (3) probe the Milky Ways dust-shrouded regions, reaching the far side of our Galaxy; and (4) characterize asteroids and other objects in the outer solar systems.

LAE Galaxies at High Redshift: Formation Sites for Low-Metal Globular Clusters

Lyman-α emitting (LAE) galaxies observed at intermediate to high redshift have the correct size, mass, star formation rate, metallicity, and space density to have been the formation sites of metal-poor globular clusters. LAEs are typically small galaxies with transient starbursts. They should accrete onto spiral and elliptical galaxies over time, delivering metal-poor clusters into the larger galaxies’ halos as they themselves get dispersed by tidal forces. The galaxy WLM is a good example of a dwarf remnant from a very early starburst that contains a metal-poor globular cluster but failed to get incorporated into the Milky Way or M31 because of its remote location in the local group.

Measuring Spiral Arm Torques: M100 in K

Spiral arms exert gravitational torques on stars that lead to redistribution of angular momentum within galactic disks: in the inner part stars lose angular momentum and fall towards the center of galaxy and in the outer part stars gain angular momentum and move farther away from the center. These torques depend only on the non-axisymmetric mass distribution and can be measured directly from photometry, if light traces mass. The method to calculate the gravitational torques is described in Gnedin, Goodman, and Frei 1995 (hereafter GGF).