arXiv subjects

K. I. Kellermann

Publications and source records attributed to K. I. Kellermann.

79 records · Page 5Linked to original sources

arXiv1999

The Nature of Microjansky Radio Sources and Implications for the Design of the Next Generation Very Sensitive Radio Telescopes

Deep radio surveys show a population of very red counterparts of microjansky radio sources, which are unidentified to I = 25 in ground based images and I = 28.5 in the Hubble Deep Field. This population of optically faint radio sources, which comprises about 20% of the microjansky radio samples, may be dust enshrouded starburst galaxies, extreme redshift or dust reddened AGN, or due to displaced radio lobes. Even deeper radio surveys, which will be made possible by next generation radio telescopes such as the Expanded VLA or the Square Kilometer Array, will reach to nanojansky levels which may be dominated by this new population, but only if special care is taken to achieve high angular resolution and dynamic range better than 60 dB. This will reuire array dimensions up to 1000 km to achieve confusion limited performance at 1.4 GHz and up to 10,000 km at 300 MHz. But, even then, the ability to study individual nanojanksy radio sources may be limited by the finite extent of the sources and consequential blending of their images.

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arXiv1999

VLBI imaging of extremely high redshift quasars at 5 GHz

We present very long baseline interferometry (VLBI) images of ten very high redshift (z>3) quasars at 5 GHz. The sources 0004+139, 0830+101, 0906+041, 0938+119 and 1500+045 were observed in September 1992 using a global VLBI array, while 0046+063, 0243+181, 1338+381, 1428+423 and 1557+032 were observed in October 1996 with the European VLBI Network and Hartebeesthoek, South Africa. Most of the sources are resolved and show asymmetric structure. The sample includes 1428+423, the most distant radio loud quasar known to date (z=4.72). It is barely resolved with an angular resolution of about 2.0*1.4 mas.

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arXiv1998

The ``angular size - redshift'' relation for compact radio structures in quasars and radio galaxies

We discuss the ``angular size - redshift'' relation for compact radio sources distributed over a wide range of redshifts 0.01 < z < 4.73. Our study is based on a sample of 330 5 GHz VLBI contour maps taken from the literature. Unlike extended source samples, the ``angular size - redshift'' relation for compact radio sources appears consistent with the predictions of standard Friedmann world models with q_o ~ 0.5 without the need to consider evolutionary or selection effects due to a ``linear size -- luminosity'' dependence. By confining our analysis to sources having a spectral index, -0.38 < alpha < 0.18, and a total radio luminosity greater than 10^{26} W/Hz, we are able to restrict the dispersion in the ``angular size -- redshift'' relation. The best fitting regression analysis in the framework of the Friedmann-Robertson-Walker model gives the value of the deceleration parameter q_o = 0.21 +- 0.30 if there are no evolutionary or selection effects due to a ``linear size -- luminosity'', ``linear size -- redshift'' or ``linear size -- spectral index'' dependence.

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arXiv1998

Radio Emission from Galaxies In The Hubble Deep Field

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 0.3" to 10". We detected 29 radio sources in a complete sample within 4.6' of the HDF center and above a flux density limit of 9.0 microjy (5 sigma). Seven of these sources are located within the HDF itself, while the remaining 22 sources are covered by the Hubble Flanking Fields (HFFs) 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. Three radio sources 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 (seven of which are contained in the HDF) with 6.3 microjy < S < 9.0 (3.5 sigma < S < 5 sigma) and and R < 25, but which are not included in the complete sample. At least 2/3 of the optical identifications of these microjansky radio sources are with disk systems (i.e., irregulars, mergers and spirals). We briefly discuss the possible physical mechnisms driving the strong evolution in the microjansky radio galaxy population. [truncated]

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arXiv1998

The Redshift Cutoff and the Quasar Epoch

We have obtained complete redshift information for a sample of 442 radio-loud quasars with flux densities S(11cm) >= 0.25Jy. These come from a completely-identified sample of 878 flat-spectrum radio sources, so there is no optical magnitude limit. With these quasars, therefore, we can map out the entire quasar epoch, unhindered by optical selection effects and any intervening dust. The rapid decline in true space density above redshifts of about 2.5 is confirmed. It agrees closely with that obtained from optically-selected samples, indicating that these are also not significantly affected by dust. The evolution of the quasar space density is similar to that deduced for the global star formation rate, implying that the quasar epoch may also be that of star formation. This sample of radio-selected quasars can now be used in a variety of other studies, including unbiased searches for damped Ly-alpha absorbers, studies of the phenomenon of ``red quasars'', and searches for high-redshift molecular absorption systems.

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arXiv1998

Sub-milliarcsecond Imaging of Quasars and AGN

We have used the VLBA at 15 GHz to image the structure of 132 strong compact AGN and quasars with a resolution better than one milliarcsecond and a dynamic range typically exceeding 1000 to 1. These observations were made as part of a program to investigate the sub-parsec structure of quasars and AGN and to study the changes in their structure with time. Many of the sources included in our study, particularly those located south of +35 degrees, have not been previously imaged with milliarcsecond resolution. Each of the sources has been observed at multiple epochs. In this paper we show images of each of the 132 sources which we have observed. For each source we present data at the epoch which had the best quality data. The milliarcsecond jets generally appear one-sided but two-sided structure is often found in lower luminosity radio galaxies and in high luminosity quasars with gigahertz peaked spectra. Usually the structure is unresolved along the direction perpendicular to the jet, but a few sources have broad plumes. In some low luminosity radio galaxies, the structure appears more symmetric at 2 cm than at long wavelengths. The apparent long wavelength symmetry in these sources is probably due to absorption by intervening material. A few sources contain only a single component with any secondary feature at least a thousand times weaker. We find no obvious correlation of radio morphology and the detection of gamma-ray emission by EGRET.

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arXiv1996

Detection of a Small Scale Cosmic Background Anisotropy at 3.6 cm

We report on the detection of a feature with negative radio flux in a sensitive, low resolution observation with the VLA. This morphologically peculiar feature is approximately 25" 65" in size with a peak central amplitude of about -0.25 mK compared to the brightness temperature of the background sky at 3.6 cm. Within about 1' of this microwave decrement, we also found two radio quiet quasars, both at 2.561, with a projected physical separation of about 1 Mpc ($q_o$ = 0.5, h = 0.5), suggestive of a possible galaxy cluster in the region. We are unable to account for this negative source by systematic phase fluctuations within our observations, sidelobe artifacts, or instrumental noise. We discuss possible physical origins of this microwave source, in particular the inverse-Compton scattering of the cosmic microwave background by hot gas in a distant cluster of galaxies.

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