Figure 3. Integrated intensity distribution of HI from W51C for VLSR=+85 to +183 kms-1. A smooth background was subtracted. The contours are -10,10,20,30,60,90,120, and 150 kms-1.
NAIC/AO Newsletter, July 1997
Kiriaki Xilouris has been responsible for the data acquisition for the searches on behalf of the five teams involved. She finds that presently the telescope pointing deviates quite significantly from the indicated encoder positions. However, using continuum records taken in parallel with the pulsar searches, positions to a fraction of a beamwidth can be estimated retrospectively. In respect of the search observations post-October 1996, analysis of the data from the PSPM has meant fairly significant modifications to search software, plus the handling of much greater volumes of data per unit observing time. However, all five groups have either modified, or are close to completing modification of, their software to accommodate the PSPM data. Results reported so far include both new candidate detections and the re-detection of some known pulsars.
In addition to our return to observations, the recent months have seen the publication of a number of exciting results from data collected by Arecibo users during upgrade-phase observations, or from experiments conducted in the late pre-upgrade era. A brief review of these is of especial interest as the seeds of the future should be visible in the successes of the past.
Anderson, Wolszczan, Kulkarni and Prince timed the two millisecond pulsars in globular cluster, NGC 5904, at 430 MHz between April 1989 and July 1994. This represents 103 separate observing sessions of average duration 72 min. The data from 73 of these sessions were also incoherently combined to search for fainter pulsars in the cluster. This gave a detection limit of 40 microJy for isolated pulsars. No new objects were found. For the binary pulsar, PSR B1516+02B, offset from the center of the cluster by 1.5 core radii, they find the orbit to be precessing. If due to General Relativity, the total mass of this binary lies between 0.45 and 5.5 Mo. Continued observations over the next decade should resolve the issue between a low-mass and high-mass binary pulsar. This pulsar is also found to have a negative period derivative, a property it shares with 5 other globular-cluster pulsars. This is attributed to the pulsar lying behind the cluster and being accelerated towards the cluster center of mass. In addition, the large orbital eccentricity (e = 0.138) of PSR B1516+02B is consistent with theoretical calculations by Rasio and Heggie of the collective perturbations of passing nearby stars on an initially circular orbit.
Figure 3. Integrated intensity distribution of HI from W51C
for VLSR=+85 to +183 kms-1. A smooth background was
subtracted. The contours are -10,10,20,30,60,90,120, and 150
kms-1.
Koo and Moon have used Arecibo and VLA data to investigate high-velocity HI found towards the SNR W51C with the Hat Creek 26-m telescope. This HI has velocities well in excess of those permitted by Galactic rotation. The frequency-switched Arecibo spectra were used to make channel maps of the area, and the same data were also combined with VLA D-configuration observations to fill in the low spatial frequencies missing from the synthesis. The Arecibo images show HI up to +180 kms-1 near the field center, with the map of integrated HI emission revealing an extended ridge-like feature (Fig 3). For an assumed distance of 6 kpc, an HI mass of 1200 Mo is derived. The higher-resolution VLA image reveals the feature to have a loop-like morphology, although HI is only detected up to +140 kms-1 due to lower brightness sensitivity. The authors surmise that local dynamical phenomena are causing the high-velocity HI, and investigate its relation to the SNR. The high-velocity HI is found to delineate the western boundary of the bright central region of the X-ray image of the remnant for over 20 arcmin (~35 pc), suggesting that it is in contact with the hot (3.4 x 106 K) X-ray emitting gas in the central region. They also find positional correlation between the high-velocity HI and the eastern boundary of a high-velocity molecular stream, defined by its 13CO emission. They conclude that W51C is interacting with an ambient molecular cloud, and that the high-velocity HI lies at the interface between the remnant and molecular cloud, it being accelerated by the SNR shock which has a velocity of ~ 100 kms-1. In addition to this work, Koo has used Arecibo and VLA data to carry out an HI-absorption study of the W51 complex, using the data to determine the relative positions of many of the components in the region.
Tamanaha has employed HI spectra taken with the Hat-Creek 26-m telescope and the Arecibo 305-m dish to map the neutral hydrogen towards the Galactic anticenter, and particularly over the anticenter HI shell. Instead of appearing as an almost-complete shell, this supershell' decomposes into two separate filaments elongated parallel to the plane. These features are explained by a global model involving the continued impact of anticenter chain high-velocity clouds with the disk of the Galaxy.
Troland, et al. have used the Arecibo telescope to perform a Zeeman-effect study of the OH mainlines from the dark cloud, TMC-1C. TMC1 is 140 pc distant, and the Arecibo resolution represents a linear scale there of 0.1 pc, the smallest to date for any single-dish Zeeman study of a dark cloud. In addition, the Arecibo beam is well matched to the dimensions of the cloud's dense core. The observations yield a magnetic-field estimate of B|| = +1.4+/-2.4 microG. For the column density estimated for this region, a value of |Bvirial| ~ 100 microG results, > 10 times the 3 sigma upper limit on |B|| | of 8.4 microG. If indeed |B| << |Bvirial|, then the core cannot be supported by the magnetic field and could be in a state of supercritical collapse. This seems unlikely given the very narrow OH and NH3 line widths of 0.44 and 0.24 kms-1 respectively. If |B|~|Bvirial|, then either |B||| << |B| because of geometrical factors, including field reversals, or high OH optical depths result in poor sampling of the dense core gas. The authors note that such a field would be considerably greater than that of 25 microG expected from the Alfven velocity derived from the OH line width and the estimated density from NH3 data. Such a cloud is magnetically supported more by static field pressure than by motions associated with Alfven waves, and should flatten into a disk perpendicular to the field. In fact, the NH3 core of TMC-1C is elongated in the ratio 3:1, and if this is from flattening along the field lines, then the high ratio implies that the symmetry axis of the cloud (and hence B) lies at a large angle to the line-of-sight, so |B|| | << |B|.
Lewis has published his catalog of main-line OH maser emission from the complete sample of 393 color-selected IRAS sources with 25 micron flux densities > 2 Jy which he had previously detected in the 1612-MHz OH line. Main-line masers were detected in 238 objects, with 18 only showing 1665-MHz emission and 119 both main lines. Main lines were not detected in 159 objects. He concludes that, a) main-line OH masers are in general weaker than their companion 1612-MHz masers, with the 1665-MHz line often marginally weaker than the 1667-MHz line, b) while some objects exhibit both main lines, many show the 1667-MHz line alone, while ~ 10% are detected in only the 1665-MHz line, and c) there seems to be a decided tendency for the main-line peaks to fall at a slightly different velocity to the 1612-MHz peak, such that |Vmain - V1612| has a maximum frequency at 0.3 +/-0.2 kms-1.
Alef, Benz, and Güdel have taken VLBI observations of YY Gem (Gl 278C), a well-known source of gyrosynchrotron radio emission, and used their results to test different coronal models against the sizes they derive. YY Gem, which lies 13.9 pc from the Sun, consists of two near-identical dM1e stars in a quasi edge-on orbit. It is believed that the stars are young (~ 108 yr) and in their late pre-main-sequence contraction phase. Evidence for eclipses are seen in high-frequency radio, optical and X-ray observations. These 1.6-GHz Mk-III VLBI observations were made with an array which included Arecibo, along with other of the world's largest telescopes. The observations took place at an epoch when the YY-Gem system showed a steady flux density of 1.5 mJy, a remarkably low value for VLBI observations. Very clear fringes were obtained on most baselines, and an image was synthesized of beam size 5.5 mas x 2.7 mas. A simple, slightly-resolved, symmetric source is revealed, with the image having a dynamic range of 70:1. The source is, in fact, resolved at the 4 sigma significance level, the first time that this has been achieved for a dMe star. The best size estimate is 0.94 +/-0.24 mas = 2 +/-0.5 x 1011cm = 2.1 +/-0.6 photospheric diameters. The derived brightness temperature, Tb (1.6 GHz) ~ 109 K, is acceptable for the gyrosynchrotron mechanism to be operative, as is the measure circular polarization limit of < 8 %. The lack of any eclipse in the correlated visibilities places a lower limit of 0.43 mas on the size for each star.
The relatively large neutral hydrogen contents of, and enhanced density of companion galaxies around, Seyfert-2 galaxies suggest that tidal interaction could play a major role in the evolution of Seyfert-2 galaxies. Freudling & Prieto had previously found that the type-S0 Seyfert-2 galaxy, NGC 5252 contains 5x109 Mo of neutral hydrogen. This amount was enough to map the galaxy with the VLA D-array. The VLA maps showed that several galaxies within the primary beam are at the same redshift as the Seyfert galaxy. This, together with the distribution of the hydrogen in the Seyfert galaxy, led to the scenario of neutral hydrogen accumulating on to NGC 5252 through interaction with other members of the group. Deep HI spectra were taken with the Arecibo lambda 22-cm feed at several positions within this group to search for signs of tidal interaction between the galaxies. No HI outside of the galaxies was detected suggesting that no recent tidal interaction has taken place within this group. The lack of evidence for interaction among the galaxies could be interpreted in two different ways. Either interaction occurred in the distant past and triggered activity in NGC 5252 over a long period, or factors other than the gas supply might be responsible for the observation that Seyfert-2 galaxies tend to be surrounded by a region of enhanced galaxy density.
Pildis, Schombert & Eder have undertaken a survey of dwarf galaxies discovered from the Second Palomar Sky Survey. Of 278 candidate objects from 1400 deg2 of plate material, 145 were detected at Arecibo in HI emission, essential for determination of a redshift distance for these small, faint objects. Optical properties have also been measured for 101 of these gas-rich field dwarf galaxies. These galaxies have low surface brightnesses and are relatively distant. They have bluer V - I colors than either active star-forming giant galaxies or low metallicity globular clusters, implying that these dwarfs have both low metallicities and little past star formation. These galaxies are extremely gas rich. Overall, the properties of this sample of dwarf galaxies point towards their being quiescent objects that have undergone little star formation over the age of the Universe, rather than being faded objects. In fact, they may be going through one of their first periods of weak star formation.
Scodeggio, Giovanelli & Haynes have employed Arecibo HI spectra taken between 1990 and 1994 of Sb to Sd galaxies in the cluster of galaxies, A2634, for a study which uses the Tully-Fisher (TF) and Fundamental Plane (FP) relations to obtain independent estimates of the relative distances between A2634 and the Coma cluster. Previous studies of A2634 have showed a large discrepancy between the distance estimates obtained with the TF and the Dn - sigma (FP) relations, questioning the reliability of z-independent distances from these relations. The new distance estimates derived to A2634 are based on much larger samples, selected by rigorous cluster-membership criteria. After the appropriate corrections for sample incompleteness have been taken into account, they find that the TF and FP distance estimates are in good agreement, both indicating that A2634 has a negligibly small peculiar velocity with respect to the Cosmic Microwave Background reference frame. The results suggest that the ratio of the distances to A2634 and to Coma is 1.26 +/-0.03 pc. Given the high accuracy with which the two distance estimates have been obtained, their agreement strongly supports the universality of the TF and FP relations, and therefore their reliability for the estimation of z-independent distances.
Figure 4. Spectrum of MG0201+113 showing the region of 21-cm absorption line at 323.764 MHz. The solid line shows the fitted Gaussian to the absorption line using the linear baseline fitted to the entire 2.5 MHz spectrum (after RFI editing), while the dashed line has a baseline fitted to the portion of the spectrum shown in this plot.
Briggs, Brinks & Wolfe have used the Arecibo telescope to produce a tentative detection of the lambda 21-cm line of HI at zabs = 3.38716 +/-0.00007 in absorption against the radio continuum of the QSO MG 0201+113 (zem = 3.61, Fig 4). This redshift corresponds roughly with that determined earlier by the Westerbork Synthesis Radio Telescope. However, they find different results for the width and optical depth of the line. If the detection holds, it provides evidence for a high column density of neutral gas that is confined to a cold layer with velocity dispersion ~10 kms-1. Although the interpretation is uncertain due to a lack of detailed knowledge of the extended radio structure of the background quasar, and the relative quantities of neutral gas in the cold and turbulent components, the observations specify high spin temperatures, Ts >= 1000 K for both this lambda 21-cm line absorbing cloud and the turbulent component, which together have NHI ~ 1021.4 cm-2. Results of optical spectroscopy require the additional presence of metal-enriched clouds of still broader velocity dispersion than the lambda 21-cm line, creating a picture which is consistent with this system being a young disk galaxy which has evolved sufficiently by z = 3.4 to have collapsed to a flattened system and produced a population of stars that have aged to pollute a turbulent halo. The observations constrain the neutral gas mass of a possible lambda 21-cm line emitter associated with the intervening absorber to be ( <=DeltaV / 100 kms-1) x 1013 Mo for velocity widths DeltaV in the range 200=1200 kms-1 (H0 = 100 kms-1 Mpc-1, Omega0 =1).