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Aspects techniques et performances astronomiques de ce nouvel interferometre a raie spectrale de 21 cm, operationnel a l'observatoire d'Arecibo

The abundance of 21-cm absorption lines seen in surveys at high latitudes can be translated into a line of sight abundance of clouds vs. column density using an empirical relationship between temperature and optical depth. As VLA surveys of 21-cm absorption at low latitudes are now becoming available, it is possible to study the variation of this function with galactic radius. It is interesting to compare the abundance of these diffuse atomic clouds (with temperatures of 50 to 100 K and masses of 1 to 10 M⊙) to the abundance of molecular clouds. To do the latter we must make assumptions about cloud cross-sections in order to convert the line of sight abundance of diffuse clouds into a number per unit volume, and to convert from cloud column density to mass. The spectrum of diffuse clouds matches fairly well the spectrum of molecular clouds, although observationally there is a gap of several orders of magnitude in cloud mass. Optical absorption studies also agree well with the 21-cm results for clouds of column density a few times 1020 M⊙.

Nineteen spiral and irregular galaxies with bright continuum nuclei were observed with the VLA to search for absorption at 21 cm by cool atomic hydrogen in or in front of the nuclei. Fifteen absorption lines were detected, and six tentatively identified, in 15 of these galaxies. Equivalent widths are much larger than for a spiral disk like that of the Milky Way; it is suggested that absorption arises not in the disks but in circumstellar clouds, or in some cases in an intervening system falling toward the active nucleus. Simple order-of-magnitude calculations suggest accretion rates of about 1 solar mass of H I per year onto the nucleus. 85 references.

A continuum VLA survey of the Galactic plane has been performed at 1.5 GHz, with most fields complete to about 30 mJy peak flux density. Measurements of position, peak, and total flux density have been obtained for 471 compact sources. An excess of sources above that predicted from extragalactic source counts is found for longitudes less than 40 deg at all flux-density intervals. With the exception of 86 compact sources that lie within 2.4 arcsec of a source in the radio recombination-line survey of Lockman (1988), source counts are shown to agree with extragalactic source counts for all flux-density and longitude intervals. 21 references.

The preliminary results of a search for infrared counterparts to compact continuum radio sources found within 0.5 deg of the Galactic plane are presented. Out of 75 positions searched, nine very red sources, each with a probability of less than 0.01 of being a random field star, were found. Most of the nine sources are characterized by deep silicate absorption features at 10 microns and a red energy distribution that continues to rise beyond 25 microns. Six of the sources are best explained as late O or early B stars exciting a compact H II region. The radio emission is close to that expected for the rate of emission of ionizing photons for a normal main-sequence star. Although these H II regions are compact, they are not sufficiently dusty in the immediate environment of the star to significantly reduce the ionizing flux. Three of the sources show considerably more emission than expected for a main-sequence star, indicating they are either hotter than a main-sequence star with the same luminosity or the radio emision is nonthermal and coming from a more exotic object.

The paper presents 21 cm observations made with the NRAO 43 m (140 foot) telescope of 20 randomly selected intermediate and high galactic latitude regions. The data are examined for evidence of the neutral gas clumping required by models in which a substantial fraction of the diffuse soft X-ray background (hv = 0.1 - 0.284 keV) originates outside the galactic disk and is absorbed by interstellar gas. No such evidence is found, and it is concluded that the degree of clumping required by such models must, if it exists, have characteristic angular scales less than 14 arcmin. Furthermore, an analysis of other data indicates that the required clumping does not exist on smaller size scales. It is therefore unlikely that a significant fraction of the X-ray flux originates in a galactic corona, unless some other explanation of the anomalously small apparent absorption can be found.

Observations are reported of radio emission from the transient X-ray source A0620-00. Data obtained over the period 1975 August 15 to August 22 at frequencies between 1400 and 2695 MHz indicate a decaying radio event. The radio event appears to be quite different from those associated with recent galactic novae and is difficult to reconcile with a standard thermal model. A nonthermal event similar to those associated with Cygnus X-3, however, is consistent with the observations. (AIP)

Spiral galaxies with strong nuclear radio-continuum emission have been surveyed for H I absorption at 21 cm wavelength. Eight galaxies show clear or tentative absorption lines. The absorption line centers are offset from the galaxy systemic velocities by up to 300 km s/sup -1/; for the entire sample, there is an overall preference for infall of gas toward the nulceus rather than outflow (+98 km s/sup -1/ median relative velocity). Cases with both positive and negative realtive velocities are apparent. The presence of absorption appears to be correlated with the inclination of the galaxies: edge-on systems are more likely to show absorption lines. For the detected lines, the total absorption (equivalent width) is much greater than would be expected from the disk of our Galaxy.

We present results of a search for small scale H I structure at high galactic latitudes using the NRAO 140 foot telescope, which has a 21' beam at 21 cm. We examined randomly selected 4° x 5° regions, as well as regions of particularly low total column density. The amount of apparent structure is small in directions with total column densities of a few times 1020 cm-2.

On a effectue un balayage en continuum proche de 1,4 Ghz avec la configuration D du VLA centree sur l'amas Hercules de galaxies (A 2151). On donne les positions et les flux pour 65 sources 21-25 sources sont associees a des galaxies (principalement des galaxies brillantes) et la separation entre les positions radio et optique est ≤S 11 dans la plupart des cas

A new set of 21 cm H I absorption spectra were obtained toward 21 compact continuum sources in the Galactic plane is presented. The 21 cm line velocity-averaged absorption coefficient as a function of Galactocentric distance is calculated. The result for distances within 2 kpc of the sun agrees with the local value found from absorption toward pulsars of 5-7 km/s/kpc. Overall, the absorption coefficient decreases to about half of its local value inside a Galactocentric radius of about 4 kpc. This decrease is shown to be primarily due to an increase in the mean line-of-sight distance between absorbing atomic clouds. Thus, the cool phase of the atomic gas is less abundant in the inner Galaxy than at the solar circle. The absorption spectra are similar in appearance to existing (C-12)O spectra. The spectral regions which show H I absorption also show CO emission.

The development of galaxies catalogs from the Palomar Sky Survey using the Minnesota Automated Plate Scanner (APS) is examined. The design and operation of APS are described. The algorithm used to distinguish stars from galaxies is discussed, and the completeness and confusion of the algorithm as a function of magnitude are estimated. The quality of the photometric calibration for galaxy magnitudes and surface-brightness thresholds is evaluated. The relation between the distribution of colors and morphological types is studied. The luminosity functions for Hercules galaxies are calculated and presented. The catalog generated using the APS is 80 percent complete; photometry good to 0.23 mag rms is possible; and J-F colors are good to 0.17 mag rms.

We have observed the interstellar medium in the directions of 27 extragalactic sources at high and intermediate galactic latitudes (vertical-barb/sup ii/vertical-bar>5/sup 0/) at 21 cm wavelength with the upgraded Arecibo telescope (HPBW=3'.2, gainapprox.6 K Jy/sup -1/). The small beam greatly reduces the uncertainty in the derived on-source emission profile and increases confidence that it corresponds to the same material as the absorption spectrum. Each region has been observed for approximately 2 hours to tetect optical depths as low as 10/sup -2/ in most cases. The data allow the derivation of optical depths, harmonic mean spin temperatures, and column densities for the neutral hydrogen as a function of velocity in the range -200