arXiv subjects

George Miley

Publications and source records attributed to George Miley.

45 records · Page 3Linked to original sources

arXiv2000

A study of young radio-loud AGN using space-VLBI

Gigahertz Peaked Spectrum (GPS) sources form a key element in the study of the onset and evolution of radio-loud AGN, since they are most likely the young counterparts of extended radio sources. Here we discuss space-VLBI observations of GPS sources, which enable us to obtain unprecedented angular resolution at frequencies near their spectral turnovers. Observed peak brightness temperatures of Log[T]=10.5-11 Kelvin indicate that synchrotron self absorption is responsible for their spectral turnovers. This is in close agreement with previous size - spectral turnover statistics for GPS sources. The combination of these new space-VLBI observations with ground-based VLBI observations taken at an earlier epoch, confirm the young ages for the most compact GPS galaxies of several hundred years.

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arXiv2000

A Sample of 669 Ultra Steep Spectrum Radio Sources to Find High Redshift Radio Galaxies

Since radio sources with Ultra Steep Spectra (USS; alpha <~ -1.30; S ~ nu^alpha) are efficient tracers of high redshift radio galaxies (HzRGs), we have defined three samples of such USS sources using the recently completed WENSS, TEXAS, MRC, NVSS and PMN radio-surveys. Our combined sample contains 669 sources with S_1400 > 10 mJy and covers virtually the entire sky outside the Galactic plane (|b|>15). For our 2 largest samples, covering declination > -35, we selected only sources with angular sizes Theta < 1'. For 410 sources, we present radio-maps with 0.3" to ~5" resolution from VLA and ATCA observations or from the FIRST survey, which allows the optical identification of these radio sources. We find that the spectral index distribution of 143,000 sources from the WENSS and NVSS consists of a steep spectrum galaxy and a flat spectrum quasar population, with the relative contribution of flat spectrum sources doubling from S_1400 >0.1 Jy to S_1400 >2.5 Jy. The identification fraction of our USS sources on the POSS (R <~ 20) is as low as 15%, independent of spectral index alpha < -1.30. We further show that 85% of the USS sources that can be identified with an X-ray source are probably contained in galaxy clusters, and that alpha < -1.6 sources are excellent Galactic pulsar candidates, because the percentage of these sources is four times higher in the Galactic plane. Our sample has been constructed to start an intensive campaign to obtain a large sample of high redshift objects (z>3) that is selected in a way that does not suffer from dust extinction or any other optical bias [abridged].

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arXiv2000

Limits on the Spatial Extent of AGN Measured with the Fine Guidance Sensors of the HST

The optical structure of several AGN has been studied using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope (HST). The FGSs are interferometric devices which can resolve structure on scales of 20 milliarcsecs or less and hence have the potential to improve on the resolution attainable by HST's cameras. The FGSs produce interferometric fringes known as S-curves which are related to the intensity profile of the object on the sky. These have been analyzed using a simple model for the radial intensity distribution and strength of the underlying background illumination of the observed objects. Eight different observations of six different AGN have been analyzed. No statistically significant differences from point sources are detected but significant upper limits of order 20 milliarcseconds are placed on any spatial extent. Systematic effects limiting the resolution are discussed and some simple conclusions about the physical size and luminosity densities of the emitting regions of the AGN implied by the data are given.

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arXiv1999

Ultra-Steep Spectrum Radio Galaxies at Hy Redshifts

Radio sources have traditionally provided convenient beacons for probing the early Universe. Hy Spinrad was among the first of the tenacious breed of observers who would attempt to obtain optical identifications and spectra of the faintest possible `radio galaxies' to investigate the formation and evolution of galaxies at hy redshift. Modern telescopes and instruments have made these tasks much simpler, although not easy, and here we summarize the current status of our hunts for hy redshift radio galaxies (HyZRGs) using radio spectral and near-IR selection.

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arXiv1999

VLT Spectroscopy of the z=4.11 Radio Galaxy TN J1338-1942

We present optical, infrared and radio data of the z=4.11 radio galaxy TN J1338-1942, including an intermediate resolution spectrum obtained with FORS1on the VLT Antu telescope. TN J1338-1942 was the first z>4 radio galaxy to be discovered in the southern hemisphere and is one of the most luminous Ly-alpha objects in its class. The Ly-alpha and rest-frame optical emission appear co-spatial with the brightest radio hotspot of this very asymmetric radio source, suggesting extremely strong interaction with dense ambient clouds. The VLT observations are sufficiently sensitive to detect the continuum flux both blue-ward and red-ward of the Ly-alpha emission, allowing us to measure the Ly-alpha forest continuum break (Ly-alpha 'discontinuity', D_A) and the Lyman limit. We measure a D_A=0.37+-0.1, which is ~0.2 lower than the values found for quasars at this redshift. We interpret this difference as possibly due to a bias towards large D_A introduced in high--redshift quasar samples that are selected on the basis of specific optical colors. If such a bias would exist in optically selected quasars, - and even in samples of Lyman break galaxies -, then the space density of both classes of object will be underestimated. Furthermore, the average HI column density along cosmological lines of sight as determined using quasar absorption lines would be overestimated. Because of their radio-based selection, we argue that z>4 radio galaxies are excellent objects for investigating D_A statistics. [abridged]

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arXiv1999

A Radio Galaxy at z=5.19

We report the discovery of the most distant known AGN, the radio galaxy TN J0924-2201 at z = 5.19. The radio source was selected from a new sample of ultra-steep spectrum (USS) sources, has an extreme radio spectral index alpha_365MHz^1.4GHz = -1.63, and is identified at near-IR wavelengths with a very faint, K = 21.3 +- 0.3 object. Spectroscopic observations show a single emission line at lambda ~ 7530A, which we identify as Ly-alpha. The K-band image, sampling rest frame U-band, shows a multi-component, radio-aligned morphology, typical of lower-redshift radio galaxies. TN J0924-2201 extends the near-IR Hubble, or K-z, relation for powerful radio galaxies to z > 5, and is consistent with models of massive galaxies forming at even higher redshifts.

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arXiv1998

The Highest Redshift Radio Galaxy Known in the Southern Hemisphere

We present the discovery of a z=4.13 galaxy TN J1338-1942, the most distant radio galaxy in the southern hemisphere known to date. The source was selected from a sample of Ultra Steep Spectrum (USS; alpha < -1.3) radio sources using the Texas and NVSS catalogs. The discovery spectrum, obtained with the ESO 3.6m telescope, shows bright extended Ly-alpha emission. The radio source has a very asymmetric morphology, suggesting a strong interaction with an inhomogeneous surrounding medium.

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arXiv1998

Very High Redshift Radio Galaxies

High redshift radio galaxies (HzRGs) provide unique targets for the study of the formation and evolution of massive galaxies and galaxy clusters at very high redshifts. We discuss how efficient HzRG samples are selected, the evidence for strong morphological evolution at near-infrared wavelengths, and for jet-induced star formation in the z=3.800 HzRG 4C41.17.

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arXiv1997

Searches for High Redshift Radio Galaxies

We have started a search for High Redshift Radio Galaxies (HZRGs) in an area covering 7 sr by selecting a sample of Ultra Steep Spectrum (USS) sources with a low flux density cut-off S(1400 MHz) > 10 mJy and a steep spectral index cut-off of alpha < -1.3 (S ~ nu**alpha) from the WENSS, NVSS and TEXAS surveys. Our first results for 27 sources show that we are almost twice as effective in finding HZRGs than than surveys of relatively bright radio sources with a spectral index cut-off of alpha < -1.0. The redshift distribution is consistent with an extension of the z - alpha relation to alpha < -1.3, but a large fraction of our sample (40%) consists of objects which are too faint to observe with 3-4m class telescopes. Our search is aimed at increasing the number of very high redshift radio galaxies for further detailed studies of the formation and evolution of massive galaxies and their environment.

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