The infrared space observatory camera

At the focal plane of the ISO 60 cm telescope, ISOCAM will take images of the sky in the wavelength range 2.5 to 17 m. It features two independent channels, containing each a 32×32 array detector. The long wavelength (4–17 m) detector, developed by LIR-LETI specifically for ISOCAM, is a Si:Ga array bonded with indium bumps to a DV readout circuit. The short wavelength (2.5–5.5 m) detector, developed by SAT, is an InSb array with a CID readout.Only one channel operates at a time. The channel selection is obtained with two field mirrors supported by a wheel. The field mirror in operation is in the focal plane of the telescope, covering the 3 arcmin field of view, and reflecting the light into one or into the other channel.In each channel, 10 or 12 discrete band pass filters and CVFs with resolution 45 are mounted on a wheel. A second wheel supports a set of lenses, which reimage the focal plane of the telescope on the array, with a pixel field of view of 1.5, 3, 6 or 12 arcsec.Two integrating spheres, illuminated through small holes by black body sources, are mounted on the selection wheel, for flat fielding and calibration purposes.The flight model of ISOCAM, fully tested and calibrated, has been, delivered to ESA and is ready to be integrated in the satellite, whose expected launch date is September 1995.     

The global distribution of sulfur dioxide ice on Io, observed with OSIRIS on the W.M. Keck telescope

We present ground based observations of Io taken with a high spatial resolution imaging spectrometer on 1 and 2 June 2006. We mapped the 1.98 and 2.12 μm absorptions of SO₂ frost, across Io's surface. We analyze these data with surface reflectance modeling using the Hapke method to determine the general frost distribution. This analysis also determined a lower limit of 700 μm on the grain size for the areas of strongest absorption. We incorporate our findings of a predominantly equatorial distribution of SO₂ frost, with the maps of Carlson et al. [Carlson, R.W., Smythe, W.D., Lopes-Gautier, R.M.C., Davies, A.G., Kamp, L.W., Mosher, J.A., Soderblom, L.A., Leader, F.E., Mehlman, R., Clark, R.N., Fanale, F.P., 1997. Geophys. Res. Lett. 24, 2479-2482], McEwen [McEwen, A.S., 1988. Icarus 73, 385-426] and Douté et al. [Douté, S., Schmitt, B., Lopes-Gautier, R., Carlson, R., Soderblom, L., Shirley, J., and The Galileo NIMS Team, 2001. Icarus 149, 107-132] to produce a self consistent explanation of the global distribution of SO₂. We propose that the differences between the above maps is attributable, in part, to the different bands that were studied by the investigators.     

The Ooty synthesis radio telescope: First results

A 4-km synthesis radio telescope has recently been commissioned at Ootacamund, India for operation at 327 MHz. It consists of the Ooty Radio Telescope (530 m × 30 m) and 7 small antennas which are distributed over an area of about 4 km × 2 km. It has a coverage of about ± 40‡ in declination δ. The beam-width is about 40 arcsec × 90 arcsec at δ = 0‡ and about 40 arcsec × 50 arcsec at δ = 40‡. The sensitivity attained for a 5:1 signal-to-noise ratio is about 15 m Jy after a 10-hour integration. The observational programmes undertaken and some of the results obtained recently are summarized. The radio halo around the edge-on spiral NGC 4631 is found to have a larger scale-height at 327 MHz than is known at higher frequencies. Mapping of interesting radio galaxies at 327 MHz is being carried out; preliminary results for 0511-305 (∼2 Mpc) and 1333-337 (∼750 kpc) are summarized. The very-steep-spectrum radio source in the Abell cluster A85 is found to be resolved; since it has no obvious optical counterpart, it is conceivable that it is a remnant of past activity of a galaxy that has drifted away in about 10⁹ years.     

Speckle imaging of volcanic hotspots on Io with the Keck telescope

Using speckle imaging techniques on the 10-m W.M. Keck I telescope, we observed near-infrared emission at 2.2 μm from volcanic hotspots on Io in July-August 1998. Using several hundreds of short-exposure images we reconstructed diffraction-limited images of Io on each of three nights. We measured the positions of individual hotspots to ±0.004″ or better, corresponding to a relative positional error of ∼20 km on Io's surface. The sensitivity of normal ground-based images of Io is limited by confusion between overlapping sources; by resolving these multiple points we detected up to 17 distinct hotspots, the largest number ever seen in a single image.During the month-long span of our 1998 observations, several events occurred. Loki was at the end of a long brightening, and we observed it to fade in flux by a factor of 2.8 over the course of one month. At the 3-sigma level we see evidence that Loki's position shifts by ∼100 km. This suggests that the brightening may not have been located at the “primary” Loki emission center but at a different source within the Loki caldera. We also see a bright transient source near Loki. Among many other sources we detect a dim source on the limb of Io at the latitude of Pele; this source is consistent with 2.7% of the thermal emission from the Pele volcano complex being scattered by the Pele plume, which would be the first detection of a plume through scattered infrared hotspot emission.     

An excess of damped Lyman α galaxies near quasi-stellar objects

We present a sample of 33 damped Lyman α systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS) whose absorption redshifts ( z _abs) are within 6000 km s⁻¹ of the quasi-stellar object's (QSO) systemic redshift ( z _sys). Our sample is based on  731 2.5 < z _sys < 4.5  non-broad absorption line (non-BAL) QSOs from Data Release 3 (DR3) of the SDSS. We estimate that our search is ≈100 per cent complete for absorbers with N (H  i )  ≥ 2 × 10²⁰ cm⁻²  . The derived number density of DLAs per unit redshift, n ( z ), within  Δ v < 6000 km s⁻¹  is higher (3.5σ significance) by almost a factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e. there is evidence for an overdensity of galaxies near the QSOs. This provides a physical motivation for excluding DLAs at small velocity separations in surveys of intervening 'field' DLAs. In addition, we find that the overdensity of proximate DLAs is independent of the radio-loudness of the QSO, consistent with the environments of radio-loud and radio-quiet QSOs being similar.     

Keck observations of near-Earth asteroids in the thermal infrared

We present the results of thermal-infrared observations of 20 near-Earth asteroids (NEAs) obtained in the period March 2000-February 2002 with the 10-m Keck-I telescope on Mauna Kea, Hawaii. The measured fluxes have been fitted with thermal-model emission continua to determine sizes and albedos. This work increases the number of NEAs having measured albedos by 35%. The spread of albedos derived is very large (p_v=0.02−0.55); the mean value is 0.25, which is much higher than that of observed main-belt asteroids. In most cases the albedos are in the ranges expected for the spectral types, although some exceptions are evident. Our results are consistent with a trend of increasing albedo with decreasing size for S-type asteroids with diameters below 20 km. A number of objects are found to have unexpectedly low apparent color temperatures, which may reflect unusual thermal properties. However, the results from our limited sample suggest that high thermal-inertia, regolith-free objects may be uncommon, even amongst NEAs with diameters of less than 1 km. We discuss the significance of our results in the light of information on these NEAs taken from the literature and the uncertainties inherent in applying thermal models to near-Earth asteroids.     

Quasars from the 7C survey – I. Sample selection and radio maps

We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151-MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10^h 28^m, Dec.+41° and covers ≈0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting '7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S ₁₅₁>100 mJy; POSS-I E -plate magnitude E ≈ R <20; POSS-I colour ( O E )<1.8; the effective area of the survey drops significantly below S ₁₅₁≈200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al., that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars.     

The 6C** sample of steep-spectrum radio sources – II. Redshift distribution and the space density of high-redshift radio galaxies

We use the 6C** sample to investigate the comoving space density of powerful, steep-spectrum radio sources. This sample, consisting of 68 objects, has virtually complete K -band photometry and spectroscopic redshifts for 32 per cent of the sources. In order to find its complete redshift distribution, we develop a method of redshift estimation based on the K – z diagram of the 3CRR, 6CE, 6C* and 7CRS radio galaxies. Based on this method, we derive redshift probability density functions for all the optically identified sources in the 6C** sample. Using a combination of spectroscopic and estimated redshifts, we select the most radio luminous sources in the sample. Their redshift distribution is then compared with the predictions of the radio luminosity function of Jarvis et al. We find that, within the uncertainties associated with the estimation method, the data are consistent with a constant comoving space density of steep-spectrum radio sources beyond z ≳ 2.5, and rule out a steep decline.     

Nifte: The near infrared faint-object telescope experiment

The high sensitivity of large format InSb arrays can be used to obtain deep images of the sky at 3–5 m. In this spectral range cool or highly redshifted objects (e.g. brown dwarfs and protogalaxies) which are not visible at shorter wavelengths may be observed. Sensitivity at these wavelengths in ground-based observations is severely limited by the thermal flux from the telescope and from the earth's atmosphere. The Near Infrared Faint-Object Telescope Experiment (NIFTE), a 50 cm cooled rocket-borne telescope combined with large format, high performance InSb arrays, can reach a limiting flux < 1 Jy (1) over a large field-of-view in a single flight. In comparison, ISO will require days of observation to reach a sensitivity more than one order of magnitude worse over a similar area of the sky. The deep 3–5 m images obtained by the rocket-borne telescope will assist in determining the nature of faint red objects detected by ground-based telescopes at 2 m, and by ISO at wavelengths longer than 5 m.     

The central engines of radio-quiet quasars

Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), either represents emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers ∼ 10³ times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high-resolution (∼pc-scale) radio-imaging observations of a sample of 12 RQQs using the Very Long Baseline Array (VLBA). We find strong evidence for jet-producing central engines in eight members of our sample.     

The radio properties of radio-quiet quasars

Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S  ∝ ν^−α), and high brightness temperatures ( T _B ≥ 10⁵ K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.     

The LBDS Hercules sample of mJy radio sources at 1.4 GHz – I. Multicolour photometry

The results are presented of an extensive programme of optical and infrared imaging of radio sources in a complete subsample of the Leiden–Berkeley Deep Survey. The LBDS Hercules sample consists of 72 sources observed at 1.4 GHz, with flux densities S _1.41.0 mJy, in a 1.2 deg² region of Hercules. This sample is almost completely identified in the g , r , i and K bands, with some additional data available at J and H . The magnitude distributions peak at r ≃22 mag, K ≃16 mag and extend down to r ≃26 mag, K ≃21 mag. The K -band magnitude distributions for the radio galaxies and quasars are compared with those of other radio surveys. At S _1.4 GHz≲1 Jy, the K -band distribution does not change significantly with radio flux density. The sources span a broad range of colours, with several being extremely red ( r − K ≳6). Though small, this is the most optically complete sample of mJy radio sources available at 1.4 GHz, and is ideally suited for studying the evolution of the radio luminosity function out to high redshifts.     

Studies of the quasar OH 471

Based on the 1964–1996 observations in the frequency range 0.325–90 GHz, we study peculiarities of the variability of the quasar OH 471 (z=3.4). The double-humped spectrum had peaks at frequencies near 1 and 20 GHz. The flux density of the low-frequency component first decreased and then began to increase. The high-frequency component rose to 2.5 Jy in the late 1970s. The peak frequencies were virtually constant. VLBI observations at 1.6 GHz revealed a core-jet structure with the jet extended eastward to 5 mas. The object is a powerful quasar.     

WSRT observations of the quasar OH 471

The 325-MHz observations of the quasar OH 471 (z=3.4) in 1985–1996 revealed variability of its radio emission. Over this period, its radio flux density increased by a factor of 1.6. A steep-spectrum radio source was identified $2_.^\prime 5$ north of the quasar. Its radio flux density is 1.46 mJy at 5 GHz and 14.5 mJy at 350 MHz.     

A model for the molecular accretion disk and H2O maser in the nucleus of the galaxy NGC 4258

A model is developed for the H₂O maser source observed in the circumnuclear region of the galaxy NGC 4258. The maser emission originates at distances of 0.15–0.29 pc from the center in a thin, cold accretion disk (gas-dust torus) rotating around a supermassive black hole of mass 4 × 10⁷ M _⊙. The conditions for the emergence of an inverse population of the signal 6₁₆?5₂₃ levels in ortho-H₂O working molecules are simulated numerically. The complex line profile, which includes both central and high-velocity components, is calculated. A comparison of the calculations with radiointerferometric and spectrophotometric observations allowed the physical conditions in the emitting region to be determined.     

The near infrared spectrophotometer for the 182 cm Asiago telescope

A new near infrared spectrophotometer has been built for the Asiago 182 cm telescope. The instrument performs both large band JHKLM photometry and 1–5 spectrophotometry; a tracking system on a field star allows closed loop mapping. The design and performances are discussed.     

Compact radio sources variable at 151 MHz – II. High-resolution observations of an unbiased sample

High‐resolution observations, made with the Very Large Array (VLA) at 330 MHz, 1.4 GHz and 8.4 GHz and with the Ryle Telescope at 15 GHz, are presented of a sample of 23 sources which are variable at 151 MHz, concluding the observations of an unbiased sample of 40 such sources. The 8.4 GHz emission of almost all of the sources is dominated by structure on a scale ≲0.1 arcsec – the spectra of these compact components are such that they will also dominate the emission at 151 MHz; the number of sources for which this is not the case is consistent with the number of spurious variables expected to be found in the sample. About two-thirds of the sources have the self-absorbed, or flat, spectra expected from their size. The majority (∼75 per cent) of the sources belong to the compact steep-spectrum (CSS) class, but are generally more compact than other CSS sources; the CSS variables also appear to exhibit more spectral ageing than typical CSS sources, which may indicate a difference in the nature of the sources.     

A 23\mathop .\limits^d 2 period in the ultrahigh-energy gamma-ray flux from the Mk 501 active galactic nucleus

Observations at the Crimean Astrophysical Observatory are used to show that the ultrahigh-energy gamma-ray flux from the galaxy Mk 501 varies with a $23\mathop .\limits^d 2$ period. The period was determined by the HEGRA team from the 1997 observations.