Juan M. Uson

Correcting direction-dependent gains in the deconvolution of radio interferometric images

Astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of instrumental and atmospheric effects. In general, such effects are time-variable and vary across the field of view as well, resulting in direction-dependent (DD), time-varying gains. Most existing imaging and calibration algorithms assume that the corruptions are direction independent, preventing even moderate dynamic range full-beam, full-Stokes imaging. We present a general framework for imaging algorithms which incorporate DD errors. We describe as well an iterative deconvolution algorithm that corrects known DD errors due to the antenna power patterns (including errors due to the antenna polarization response) as well as pointing errors for high dynamic range full-beam polarimetric imaging. Using simulations we demonstrate that errors due to realistic primary beams as well as antenna pointing errors will limit the dynamic range of upcoming higher sensitivity instruments like the EVLA and ALMA and that our new algorithm can be used to correct for such errors. We show that the technique described here corrects for effects that can be described as approximate unitary operators in the interferometric measurement equation, such as those due to antenna pointing errors and non-azimuthally symmetric antenna power patterns. We have applied this algorithm to VLA 1.4 GHz observations of a field that contains two “4C” sources and have obtained Stokes I and V images with systematic errors that are one order of magnitude lower than those obtained with conventional imaging tools. Residual systematic errors that are seen at a level slightly above that of the thermal noise are likely due to selfcalibration instabilities that are triggered by a combination of unknown pointing errors and errors in our assumption of the shape of the primary beam of each antenna. We hope to present a more refined algorithm to deal with the fully general case in due course. Our simulations show that on data with no other calibration errors, the algorithm corrects pointing errors as well as errors due to known asymmetries in the antenna pattern.

Detections of CO in Late-Type, Low Surface Brightness Spiral Galaxies

Using the IRAM 30 m telescope, we have obtained (CO)-C-12 J = 1-0 and 2-1 spectral line observations toward the nuclear regions of 15 edge-on, low surface brightness (LSB) spiral galaxies. Our sample comprises extreme late-type LSB spirals with disk-dominated morphologies and rotational velocities V-rot less than or similar to 120 km s(-1). We report detections of four galaxies in at least one transition (greater than or similar to 5 sigma); for the remainder of the sample we provide upper limits on the nuclear CO content. Adopting a standard Galactic I-CO-to-H-2 conversion factor implies molecular gas masses of (3.3-9.8) x 10(6) M-circle dot in the nuclear regions (inner 1.1-1.8 kpc) of the detected galaxies. Combining our new data with samples of late-type spirals from the literature, we find that CO-detected LSB spirals adhere to the same M-H2-far-infrared correlation as more luminous and higher surface brightness galaxies. The amount of CO in the central regions of late-type spirals appears to depend more strongly on mass than on central optical surface brightness, and CO detectability declines significantly for moderate to low surface brightness spirals with V-rot less than or similar to 90 km s(-1); no LSB spirals have so far been detected in CO below this threshold. Metallicity effects alone are unlikely to account for this trend, and we speculate that we are seeing the effects of a decrease in the mean fraction of a galaxy disk able to support giant molecular cloud formation with decreasing galaxy mass.

H I IMAGING OBSERVATIONS OF SUPERTHIN GALAXIES. II. IC 2233 AND THE BLUE COMPACT DWARF NGC 2537

We have used the Very Large Array to image the H I 21 cm line emission in the edge-on Sd galaxy IC 2233 and the blue compact dwarf NGC 2537. We also present new optical B, R, and Hα imaging of IC 2233 obtained with the WIYN telescope. Despite evidence of localized massive star formation in the form of prominent H II regions and shells, supergiant stars, and a blue integrated color, IC 2233 is a low surface brightness system with a very low global star formation rate (0.05 M☉ yr−1), and we detect no significant 21 cm radio continuum emission from the galaxy. The H I and ionized gas disks of IC 2233 are clumpy and vertically distended, with scale heights comparable to that of the young stellar disk. Both the stellar and H I disks of IC 2233 appear flared, and we also find a vertically extended, rotationally anomalous component of H I extending to ~ 2.4d10 kpc from the midplane. The H I disk exhibits a mild lopsidedness as well as a global corrugation pattern with a period of ~7d10 kpc and an amplitude of ~150d10 pc. To our knowledge, this is the first time corrugations of the gas disk have been reported in an external galaxy; these undulations may be linked to bending instabilities or to underlying spiral structure and suggest that the disk is largely self-gravitating. Lying at a projected distance of from IC 2233, NGC 2537 has an H I disk with a bright, tilted inner ring and a flocculent, dynamically cold outer region that extends to ~3.5 times the extent of the stellar light (D25). Although NGC 2537 is rotationally-dominated, we measure H I velocity dispersions as high as km s−1 near its center, indicative of significant turbulent motions. The inner rotation curve rises steeply, implying a strong central mass concentration. Our data indicate that IC 2233 and NGC 2537 do not constitute a bound pair and most likely lie at different distances. We also find no compelling evidence of a recent minor merger in either IC 2233 or NGC 2537, suggesting that both are examples of small disk galaxies evolving in relative isolation.

The central galaxy in abell 2029 : an old supergiant

A mosaic of images shows the extended structure of the cD galaxy that resides at the center of the rich cluster of galaxies Abell 2029. After correcting for the scattered light of nearby stars and galaxies, the faint halo of this giant can be traced out to a distance of more than 1 megaparsec, making it one of the largest and most luminous galaxies known. The smoothness of this halo suggests that it was formed early in the history of the cluster.

Pixelization and dynamic range in radio interferometry

This study investigates some of the consequences of representing the sky by a rectangular grid of pixels on the dynamic range of images derived from radio interferometric measurements. In particular, the effects of image pixelization coupled to the CLEAN deconvolution representation of the sky as a set of discrete delta functions can limit the dynamic range obtained when representing bright emission not confined to pixels on the grid. Sky curvature effects on non-coplanar arrays will limit the dynamic range even if strong sources are centered on a pixel in a “fly’s eye” representation when such pixel is not located at the corresponding facet’s tangent point. Uncertainties in the response function of the individual antennas as well as in the calibration of actual data due to ionospheric, atmospheric or other effects will limit the dynamic range even when using grid-less subtraction (i.e. in the visibility domain) of strong sources located within the field of view of the observation. A technique to reduce these effects is described and examples from an implementation in the Obit package are given. Application of this technique leads to significantly superior results without a significant increase in the computing time.

Soft X-Ray Absorption Due to a Foreground Edge-on Spiral Galaxy toward the Core of A2029

We have detected an X-ray absorption feature against the core of the galaxy cluster Abell 2029 (z=0.0767) which we identify with the foreground galaxy UZC J151054.6+054313 (z=0.0221). Optical observations (B, V, R, and I) indicate that it is an Scd galaxy seen nearly edge-on at an inclination of 87 \pm 3 degrees. HI observations give a rotation velocity of 108 kms and an atomic hydrogen mass of M_{HI} = 3.1 X 10^9 d_{90}^2 \msun, where d_{90} is the distance to the galaxy in units of 90 Mpc. X-ray spectral fits to the Chandra absorption feature yield a hydrogen column density of (2.0 \pm 0.4) X 10^{21} cm^{-2} assuming solar abundances. If the absorber is uniformly distributed over the disk of the galaxy, the implied hydrogen mass is M_H = (6.2 \pm 1.2) X 10^8 d_{90}^2 \msun. Since the absorbing gas in the galaxy is probably concentrated to the center of the galaxy and the middle of the disk, this is a lower limit to the total hydrogen mass. On the other hand, the absorption measurements imply that the dark matter in UZC J151054.6+054313 is not distributed in a relatively uniform diffuse gas.We have detected an X-ray absorption feature against the core of the galaxy cluster A2029 (z = 0.0767) that we identify with the foreground galaxy UZC J151054.6+054313 (z = 0.0221). Optical observations (B, V, R, and I) indicate that it is an Scd galaxy seen nearly edge-on at an inclination of 87° ± 3°. H I observations give a rotation velocity of 108 km s-1 and an atomic hydrogen mass of M = 3.1 × 109d M☉, where d90 is the distance to the galaxy in units of 90 Mpc. X-ray spectral fits to the Chandra absorption feature yield a hydrogen column density of (2.0 ± 0.4) × 1021 cm-2, assuming solar abundances. If the absorber is uniformly distributed over the disk of the galaxy, the implied hydrogen mass is MH = (6.2 ± 1.2) × 108d M☉. Since the absorbing gas in the galaxy is probably concentrated toward the center of the galaxy and the middle of the disk, this is a lower limit to the total hydrogen mass. On the other hand, the absorption measurements imply that the dark matter in UZC J151054.6+054313 is not distributed in a relatively uniform diffuse gas.

Signatures of Interstellar-Intracluster Medium Interactions: Spiral Galaxy Rotation Curves in Abell 2029

We investigate the rich cluster Abell 2029 (z ~ 0.08) using optical imaging and long-slit spectral observations of 52 disk galaxies distributed throughout the cluster field. No strong emission-line galaxies are present within ~400 kpc of the cluster center, a region largely dominated by the similarly shaped X-ray and low surface brightness optical envelopes centered on the giant cD galaxy. However, two-thirds of the galaxies observed outside the cluster core exhibit line emission. Hα rotation curves of 14 cluster members are used in conjunction with a deep I-band image to study the environmental dependence of the Tully-Fisher relation. The Tully-Fisher zero point of Abell 2029 matches that of clusters at lower redshifts, although we do observe a relatively larger scatter around the Tully-Fisher relation. We do not observe any systematic variation in the data with projected distance to the cluster center. We see no environmental dependence of Tully-Fisher residuals, R-I color, Hα equivalent width, or the shape and extent of the rotation curves.

Large-aperture BVRJK photometry of rich Abell clusters - Constraints on dark matter

This paper presents results of large (arcmin), single-aperture JK photometry and CCD BVR photometry on and near the centers of distant (Z between values of 0.14 and 0.20), rich Abell clusters. The colors of the integrated light of the cores of these clusters are consistent with those of nearby E and S0 galaxies. The absence of anomalous infrared emission provides the strongest constraint to date on the possible stellar make-up of the dark matter in clusters of galaxies, requiring K band (2.2 microns) mass-to-light ratios of the dark matter of (M/L)K not less than 400h. Indeed, no more than 5/h percent of the dark mass can be made of objects with mass greater than 0.1 solar mass. Although these results do not require a contribution of nonbaryonic matter, they provide some constraints on the mass function for baryonic objects which might compose the dark matter. These conclusions do not depend on the detailed distribution of the dark matter in the clusters. 35 refs.

Signatures of Galaxy-Cluster Interactions: Tully-Fisher Observations atz~0.1

We have obtained new optical imaging and spectroscopic observations of 78 galaxies in the fields of the rich clusters Abell 1413 (z = 0.14), Abell 2218 (z = 0.18), and Abell 2670 (z = 0.08). We have detected line emission from 25 cluster galaxies plus an additional six galaxies in the foreground and background, a much lower success rate than what was found (65%) for a sample of 52 lower richness Abell clusters in the range 0.02 z 0.08. We have combined these data with our previous observations of Abell 2029 and Abell 2295 (both at z = 0.08), which yields a sample of 156 galaxies. We evaluate several parameters as a function of cluster environment: Tully-Fisher residuals, Hα equivalent width, and rotation curve asymmetry, shape, and extent. Although Hα is more easily detectable in galaxies that are located further from the cluster cores, we fail to detect a correlation between Hα extent and galaxy location in those where it is detected, again in contrast with what is found in the clusters of lesser richness. We fail to detect any statistically significant trends for the other parameters in this study. The zero point in the z ~ 0.1 Tully-Fisher relation is marginally fainter (by 1.5 σ) than that found in nearby clusters, but the scatter is essentially unchanged.

The Microwave Background Radiation

The microwave background radiation was first detected in 1965 by A. Penzias and R. Wilson, who received the 1978 Nobel Prize for Physics for this measurement [see Wilson (1983b) for his delightful account of this discovery].