We report on sensitive radio observations made with the VLA at 8.5 GHz centered on the Hubble Deep Field (HDF). We collected data in the A, CnB, C, DnC, and D configurations corresponding to angular ...resolutions ranging from 3 to 10 arcsec. We detected 29 radio sources in a complete sample within 46 of the HDF center and above a flux density limit of 9.0 micro-Jy. Seven of these sources are located within the HDF itself, while the remaining 22 sources are covered by the Hubble flanking fields or ground-based optical images. All of the sources in the HDF are identified with galaxies with a mean magnitude R = 21.7, while the mean magnitude of the identifications outside the HDF is R = 22.1. One radio source in the HDF, which is just below our formal completeness level but is confirmed by independent 1.4 GHz observations, has no optical counterpart above the HDF limit of R = 29. Three radio sources outside the HDF have no optical counterparts to R = 27. Based on a radio and optical positional coincidence, we detected an additional 19 radio sources in this field with S(v) not less than 6.3 micro-Jy and less than 9.0 micro-Jy and R = 25 or less but which are not included in the complete sample. The microjansky radio sources are distributed over a wide range of redshifts and have a typical monochromatic luminosity of about 10 exp 23 W/Hz. The majority of the optical identifications are with luminous galaxies at modest redshifts, many with evidence for recent star formation. (Author)
We discuss multiple Very Long Baseline Interferometry (VLBI) continuum and spectral line imaging observations and Westerbork Synthesis Radio Telescope spectroscopy of the compact variable nuclear ...radio jet source in the elliptical galaxy NGC 1052. Absorption and emission signatures reveal ionised, atomic, and molecular components of the surrounding medium. Ten epochs of Very Long Baseline Array (VLBA) data at 15 GHz, spanning almost six years, show bi-symmetric jets, in which multiple sub-parsec scale features display outward motions of typically $v_{\rm app}\sim0.26c$ (${\rm H}_0=65$ km s-1 Mpc-1) on each side. The jets are most likely oriented near the plane of the sky. Multi-frequency VLBA observations at seven frequencies between 43 and 1.4 GHz show free-free absorption in the inner parsec around the nucleus, probably together with synchrotron self-absorption. The free-free absorption is apparently due to a structure which is geometrically thick and oriented roughly orthogonal to the jets, but which is patchy. The western jet is covered more deeply and extensively, and hence is receding. Hi spectral line VLBI observations reveal atomic gas in front of the approaching as well as the receding jet. There appear to be three velocity systems. Broad, shallow absorption asymmetrically straddles the systemic velocity spanning -35 to 85 km s-1. This gas could be local to the AGN environment, or distributed on galactic scales. Superimposed in the range 25 to 95 km s-1 are several sharper (3–15 km s-1) features, each detectable over a few tenths of a pc at various places along the inner 2 pc of the approaching jet. The third, deepest system is at “high velocities", which is receding by 125 to 200 km s-1 with respect to the systemic velocity of NGC 1052. It may have a continuous velocity gradient across the nucleus of some 10 km s-1 pc-1. This atomic gas seems restricted to a shell 1–2 pc away from the core, within which it might be largely ionised. Westerbork Synthesis Radio Telescope spectroscopy has revealed the 18 cm OH main lines (1667 and 1665 MHz) in absorption along the full velocity span of -35 to 200 km s-1, with their line ratio varying roughly from 1:1 to 2:1. They are deepest in the high velocity system, where the OH profiles are similar to Hi, suggesting co-location of that atomic and molecular gas, and leaving unclear the connection to the H2O masing gas seen elsewhere. In the high velocity system we have also detected the 18 cm OH satellite lines: 1612 MHz in absorption, and 1720 MHz in emission. The conjugate behaviour of the satellite line profiles, and the variable main line ratio resemble the situation in Cen A and NGC 253.
We discuss the X-ray properties of the radio sources detected in a deep 1.4 and 5 GHz VLA Radio survey of the Extended Chandra Deep Field-South (E-CDFS). Among the 266 radio sources detected, we find ...89 sources (1/3 of the total) with X-ray counterparts in the catalog of the 1 Ms exposure of the central 0.08 deg2 or in the catalog of the 250 ks exposure of the 0.3 deg2 E-CDFS field. For 76 (85%) of these sources, we have spectroscopic or photometric redshifts, and therefore we are able to derive their intrinsic properties from X-ray spectral analysis, namely intrinsic absorption and total X-ray luminosities. We find that the population of submillijansky radio sources with X-ray counterparts is composed of a mix of roughly 1/3 star-forming galaxies and 2/3 active galactic nuclei (AGNs). The distribution of intrinsic absorption among X-ray-detected radio sources is different from that of the X-ray-selected sample. Namely, the fraction of low-absorption sources is at least 2 times larger than that of X-ray selected sources in the CDFS. This is mostly due to the larger fraction of star-forming galaxies present among the X-ray-detected radio sources. If we investigate the distribution of intrinsic absorption among sources with L X > 1042 erg s-1 in the hard 2-10 keV band (therefore in the AGN luminosity regime), we find agreement between the X-ray population with and without radio emission. In general, radio-detected X-ray AGNs are not more heavily obscured than the non-radio-detected AGN. This argues against the use of radio surveys as an efficient way to search for the missing population of strongly absorbed AGNs. For the radio sources without cataloged X-ray counterparts, we measure their average photometric properties in the X-ray bands with stacking techniques. We detect emission with very high confidence level in the soft band and marginally in the hard band. Given their redshift distribution, the average X-ray luminosity of these sources is consistent with being powered by star formation. We note that on average, the spectral shape of our radio sources is soft with HR ~ -0.5 and constant in different bins of radio flux. This result shows that the statistics do not indicate a significant trend in the average X-ray spectral properties, but it is consistent with the radio source population being dominated by star-forming galaxies below 100 is a subset of Jy, as shown by our morphological and multiwavelength analysis presented in Mainieri et al. and Padovani et al..
Issue Title: Proceedings of the Fifth Stromlo Symposium: Disks, Winds and Jets - From Planets to Quasars We discuss results from a decade long program to study the fine-scale structure and the ...kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its proximity, is observed with excellent spatial resolution. Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The data are best fit with a distribution of Lorentz factors extending up to γ30 and intrinsic luminosity up to 10^sup 26^ WHz^sup -1^. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed brightness temperature near the base of the jet extend up to 5×10^sup 13^ K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic energy. However, more typically, the observed brightness temperatures are 2×10^sup 11^ K, i.e., closer to equipartition. PUBLICATION ABSTRACT
I review the scientific and technical history of the Search for Extraterrestrial Intelligence (SETI), discuss the impact of the political involvement, and speculate on the nature of a successful ...detection and its potential social and cultural impact. Emphasis is on the development of SETI in the United States and the complementary progress in the Former Soviet Union.
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