arXiv subjects

George Miley

Publications and source records attributed to George Miley.

At least 37 records · Page 2Linked to original sources

arXiv2013

Probing the nature of compact ultra-steep spectrum radio sources with the e-EVN and e-MERLIN

We present first results from electronic Multi-Element Remotely Linked Interferometer Network (e-MERLIN) and electronic European VLBI Network (e-EVN) observations of a small sample of ultra-steep spectrum radio sources, defined as those sources with a spectral index alpha < -1.4 between 74 MHz and 325 MHz, which are unresolved on arcsecond scales. Such sources are currently poorly understood and a number of theories as to their origin have been proposed in the literature. The new observations described here have resulted in the first detection of two of these sources at milliarcsecond scales and show that a significant fraction of ultra-steep spectrum sources may have compact structures which can only be studied at the high resolution available with very long baseline interferometry (VLBI).

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arXiv2012

The Mid-Infrared Environments of High-Redshift Radio Galaxies

Taking advantage of the impressive sensitivity of Spitzer to detect massive galaxies at high redshift, we study the mid-infrared environments of powerful, high-redshift radio galaxies at 1.2 -0.1 (AB), in the fields of 48 radio galaxies at 1.2 1.2. Using a counts-in-cell analysis, we identify a field as overdense when 15 or more red IRAC sources are found within 1arcmin (i.e.,~0.5Mpc at 1.2 1.2.

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arXiv2011

LOFAR and APERTIF surveys of the radio sky: probing shocks and magnetic fields in galaxy clusters

At very low frequencies, the new pan-European radio telescope LOFAR is opening the last unexplored window of the electromagnetic spectrum for astrophysical studies. The revolutionary APERTIF phased arrays that are about to be installed on the Westerbork radio telescope (WSRT) will dramatically increase the survey speed for the WSRT. Combined surveys with these two facilities will deeply chart the northern sky over almost two decades in radio frequency from \sim 15 up to 1400 MHz. Here we briefly describe some of the capabilities of these new facilities and what radio surveys are planned to study fundamental issues related the formation and evolution of galaxies and clusters of galaxies. In the second part we briefly review some recent observational results directly showing that diffuse radio emission in clusters traces shocks due to cluster mergers. As these diffuse radio sources are relatively bright at low frequencies, LOFAR should be able to detect thousands of such sources up to the epoch of cluster formation. This will allow addressing many question about the origin and evolution of shocks and magnetic fields in clusters. At the end we briefly review some of the first and very preliminary LOFAR results on clusters.

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arXiv2010

Galaxy protocluster candidates at 1.6<z<2

We present a study of protoclusters associated with high redshift radio galaxies. We imaged MRC1017-220 (z=1.77) and MRC0156-252 (z=2.02) using the near-infrared wide-field (7.5'x7.5') imager VLT/HAWK-I in the Y, H and Ks bands. We present the first deep Y-band galaxy number counts within a large area (200 arcmin2). We then develop a purely near-infrared colour selection technique to isolate galaxies at 1.6<z<3 that may be associated with the two targets, dividing them into (i) red passively evolving or dusty star-forming galaxies or (ii) blue/star-formation dominated galaxies with little or no dust. Both targeted fields show an excess of star-forming galaxies with respect to control fields. No clear overdensity of red galaxies is detected in the surroundings of MRC1017-220 although the spatial distribution of the red galaxies resembles a filament-like structure within which the radio galaxy is embedded. In contrast, a significant overdensity of red galaxies is detected in the field of MRC0156-252, ranging from a factor of 2-3 times the field density at large scales (2.5Mpc, angular distance) up to a factor of 3-4 times the field density within a 1Mpc radius of the radio galaxy. Half of these red galaxies have colours consistent with red sequence models at z~2, with a large fraction being bright (Ks<21.5, i.e. massive). In addition, we also find a small group of galaxies within 5" of MRC0156-252 suggesting that the radio galaxy has multiple companions within ~50 kpc. We conclude that the field of MRC0156-252 shows many remarkable similarities with the well-studied protocluster surrounding PKS1138-262 (z=2.16) suggesting that MRC0156-252 is associated with a galaxy protocluster at z~2.

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arXiv2008

HST NIR Snapshot Survey of 3CR Radio Source Counterparts II: An Atlas and Inventory of the Host Galaxies, Mergers and Companions

We present the second part of an H-band (1.6 microns) atlas of z<0.3 3CR radio galaxies, using the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (HST NICMOS2). We present new imaging for 21 recently acquired sources, and host galaxy modeling for the full sample of 101 (including 11 archival) -- an 87% completion rate. Two different modeling techniques are applied, following those adopted by the galaxy morphology and the quasar host galaxy communities. Results are compared, and found to be in excellent agreement, although the former breaks down in the case of strongly nucleated sources. Companion sources are tabulated, and the presence of mergers, tidal features, dust disks and jets are catalogued. The tables form a catalogue for those interested in the structural and morphological dust-free host galaxy properties of the 3CR sample, and for comparison with morphological studies of quiescent galaxies and quasar host galaxies. Host galaxy masses are estimated, and found to typically lie at around 2*10^11 solar masses. In general, the population is found to be consistent with the local population of quiescent elliptical galaxies, but with a longer tail to low Sersic index, mainly consisting of low-redshift (z<0.1) and low-radio-power (FR I) sources. A few unusually disky FR II host galaxies are picked out for further discussion. Nearby external sources are identified in the majority of our images, many of which we argue are likely to be companion galaxies or merger remnants. The reduced NICMOS data are now publicly available from our website (http://archive.stsci.edu/prepds/3cr/)

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arXiv2008

Distant Radio Galaxies and their Environments

We review the properties and nature of luminous high-redshift radio galaxies (HzRGs, z > 2) and the environments in which they are located. HzRGs have several distinct constituents which interact with each other - relativistic plasma, gas in various forms, dust, stars and an active galactic nucleus (AGN). These building blocks provide unique diagnostics about conditions in the early Universe. We discuss the properties of each constituent. Evidence is presented that HzRGs are massive forming galaxies and the progenitors of brightest cluster galaxies in the local Universe. HzRGs are located in overdense regions in the early Universe and are frequently surrounded by protoclusters. We review the properties and nature of these radio-selected protoclusters. Finally we consider the potential for future progress in the field during the next few decades. A compendium of known HzRGs is given in an appendix.

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arXiv2007

The Massive Hosts of Radio Galaxies Across Cosmic Time

We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1 60% for ~75% the high redshift radio galaxies. As expected from unified models of AGN, the stellar fraction of the rest-frame H-band luminosity has no correlation with redshift, radio luminosity, or rest-frame mid-IR (5um) luminosity. Additionally, while the stellar H-band luminosity does not vary with stellar fraction, the total H-band luminosity anti-correlates with the stellar fraction as would be expected if the underlying hosts of these radio galaxies comprise a homogeneous population. The resultant stellar luminosities imply stellar masses of 10^{11-11.5}Msun even at the highest redshifts. Powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGN, despite the stellar contribution to their mid-IR SEDs at shorter-wavelengths. The mid-IR luminosities alone classify most HzRGs as LIRGs or ULIRGs with even higher total-IR luminosities. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly-accreting AGN. We find a weak correlation of stellar mass with radio luminosity.

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arXiv2007

Metal enriched gaseous halos around distant radio galaxies: Clues to feedback in galaxy formation

We present the results of an optical and near-IR spectroscopic study of giant nebular emission line halos associated with three z > 3 radio galaxies, 4C 41.17, 4C 60.07 and B2 0902+34. Previous deep narrow band Ly-alpha imaging had revealed complex morphologies with sizes up to 100 kpc), possibly connected to outflows and AGN feedback from the central regions. The outer regions of these halos show quiet kinematics with typical velocity dispersions of a few hundred km/s, and velocity shears that can mostly be interpreted as being due to rotation. The inner regions show shocked cocoons of gas closely associated with the radio lobes. These display disturbed kinematics and have expansion velocities and/or velocity dispersions >1000 km/s. The core region is chemically evolved, and we also find spectroscopic evidence for the ejection of enriched material in 4C 41.17 up to a distance of approximately 60 kpc along the radio-axis. The dynamical structures traced in the Ly-alpha line are, in most cases, closely echoed in the Carbon and Oxygen lines. This shows that the Ly-alpha line is produced in a highly clumped medium of small filling factor, and can therefore be used as a tracer of the dynamics of high-z radio galaxies (HzRGs). We conclude that these HzRGs are undergoing a final jet-induced phase of star formation with ejection of most of their interstellar medium before becoming "red and dead" Elliptical galaxies.

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arXiv2006

An optical-IR jet in 3C133

We report the discovery of a new optical-IR synchrotron jet in the radio galaxy 3C133 from our HST/NICMOS snapshot survey. The jet and eastern hotspot are well resolved, and visible at both optical and IR wavelengths. The IR jet follows the morphology of the inner part of the radio jet, with three distinct knots identified with features in the radio. The radio-IR SED's of the knots are examined, along with those of two more distant hotspots at the eastern extreme of the radio feature. The detected emission appears to be synchrotron, with peaks in the NIR for all except one case, which exhibits a power-law spectrum throughout.

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arXiv2005

Very high-resolution radio observations of HzRGs

We report on first results of an ongoing effort to image a small sample of high-redshift radio galaxies (HzRGs) with milliarcsecond (mas) resolution, using very-long-baseline interferometry (VLBI) techniques. Here, we present 1.7 and 5.0 GHz VLBA observations of B3 J2330+3927, a radio galaxy at z=3.087. Those observations, combined with 8.4 GHz VLA-A observations, have helped us interpret the source radio morphology, and most of our results have already been published (Perez-Torres & De Breuck 2005). In particular, we pinpointed the core of the radio galaxy, and also detected both radio lobes, which have a very asymmetric flux density ratio, R>11. Contrary to what is seen in other radio galaxies, it is the radio lobe furthest from the nucleus which is the brighest. Almost all of the Ly-alpha emission is seen between the nucleus and the furthest radio lobe, which is also unlike all other radio galaxies. The values of radio lobe distance ratio, and flux density ratio, as well as the fraction of core emission make of B3 J2330+3927 an extremely asymmetric source, and challenges unification models that explain the differences between quasars and radio galaxies as due to orientation effects.

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arXiv2004

Clustering and Proto-Clusters in the Early Universe

The clustering properties of clusters, galaxies and AGN as a function of redshift are briefly discussed. It appears that extremely red objects at z ~ 1, and objects with J-K > 1.7 and photometric redshifts 2 < z_phot < 4 are highly clustered, indicating that a majority of these objects constitutes the progenitors of nearby ellipticals. Similarly clustered seem luminous radio galaxies at z\sim 1, indicating that these objects comprise a short lived phase in the lifetime of these red objects. The high level of clustering furthermore suggests that distant powerful radio galaxies (e.g. z>2) might be residing in the progenitors of nearby clusters -- proto-clusters. A number of observational projects targetting fields with distant radio galaxies, including studies of Lya and Ha emitters, Lyman break galaxies and (sub)mm and X-ray emitters, all confirm that such radio galaxies are located in such proto-clusters. Estimates of the total mass of the proto-clusters are similar to the masses of local clusters. If the total star formation rate which we estimate for the entire proto-clusters is sustained up to z~1, the metals in the hot cluster gas of local clusters can easily be accounted for.

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arXiv2004

A multi-wavelength study of the proto-cluster surrounding the z=4.1 radio galaxy TN J1338-1942

We present a 1.2 mm (250 GHz) map obtained with MAMBO on the IRAM 30m telescope of the central 25 arcmin^2 of the proto-cluster surrounding the z=4.1 radio galaxy TN J1338-1942. The map reaches a 1sigma sensitivity of 0.6 mJy in the central area, increasing to 1.2 mJy at the edges. We detect 10 candidate mm sources, of which 8 are also detected in a deep VLA 1.4 GHz map and/or a VLT R-band image. Three sources have a flux density S_{1.2 mm}>4.0 mJy, representing a 7sigma overdensity compared to random field surveys, which predict only 1 such source in our map area. We obtained SCUBA/JCMT 850 um and 450 um photometry of six radio/optically identified MAMBO sources, confirming 5 of them with S/N>4. Radio-to-mm and mm-to-submm redshift estimators cannot put strong constraints on the redshifts of these MAMBO sources, but 9 of them are consistent within the uncertainties (mean Delta z=+2.6) with z=4.1. One faint MAMBO source is possibly identified with an extremely red object (R-K=6.1) at a likely spectroscopic redshift z=1.18. The four brightest MAMBO sources are all located north of the radio galaxy, while the densest area of companion Ly-alpha excess and Lyman break galaxies is to the southeast. None of the 14 spectroscopically confirmed Ly-alpha emitters in the MAMBO field are detected at 1.2 mm; their average 1.2 mm flux density is =0.25+-0.24 mJy. If the mm sources lie at z=4.1, none of them show excess Ly-alpha emission in our narrow-band images. Both populations thus show no apparent overlap, possibly due to dust quenching the Ly-alpha emission. If the mm sources are part of the proto-cluster, our results suggest that galaxies with star formation rates of a few 1000 M_{Sun} yr^{-1} could be spread throughout the proto-cluster over projected scales of at least 2 Mpc.

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arXiv2003

Proto-clusters associated with radio galaxies from z = 2 to z = 4

We have carried out narrow band imaging targeted at the Lya line of emitters associated with radio galaxies at 2 < z < 5. Subsequent spectroscopy has confirmed the identity of > 120 Lya emitters and led to the discovery of six proto-clusters up to z = 4.1 with velocity dispersions between 300 and 1000 km/s. The number of emission line galaxies in the observed fields is four to fifteen times higher than in blank fields. These results strongly support the idea that high redshift radio galaxies are the progenitors of central brightest cluster galaxies located in the progenitors of clusters of galaxies.

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arXiv2003

Giant Lya nebulae associated with high redshift radio galaxies

We report deep Keck narrow-band Lya images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100-200 kpc scale emission line nebulae, centered on these galaxies, which exhibit a wealth of morphological structure, including extended low surface brightness emission in the outer regions, radially directed filaments, cone-shaped structures and (indirect) evidence for extended Lya absorption. We discuss these features within a general scenario where the nebular gas cools gravitationally in large Cold Dark Matter (CDM) halos, forming stars and multiple stellar systems. Merging of these ``building'' blocks triggers large scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds super-massive black holes which produce the powerful radio jets and lobes. The radio sources, starburst superwinds and AGN radiation then disrupt the accretion process limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lya nebulae.

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arXiv2001

Optical and Near-IR Imaging of Ultra Steep Spectrum Radio Sources - The K-z diagram of radio and optically selected galaxies

We present optical and/or near-IR images of 128 ultra steep spectrum (USS) radio sources. Roughly half of the objects are identified in the optical images (R <~ 24), while in the near-IR images, >94% are detected at K<~ 22. The mean K-magnitude is K=19.26 within a 2" diameter aperture. The distribution of R-K colors indicates that at least 1/3 of the objects observed have very red colors (R-K>5). The major axes of the identifications in K-band are preferentially oriented along the radio axes, with half of them having compact morphologies. The 22 sources with spectroscopic redshifts and K-band magnitudes follow the K-z relation found from previous radio samples, but with a larger scatter. We argue that this may be due to a dependence of K-magnitude on the radio power, with the highest radio power sources inhabiting the most massive host galaxies. We present a composite K-z diagram of radio-loud and radio-quiet galaxies, selected from the HDF-North and the Hawaii surveys. Out to z <~ 1, the radio-loud galaxies trace the bright envelope of the radio quiet galaxies, while at z >~ 1, the radio-loud galaxies are >~ 2 magnitudes brighter. We argue that this is not due to a contribution from the AGN or emission lines. This difference strongly suggests that radio galaxies pinpoint the most massive systems out to the highest known redshifts, probably due to the mutual correlation of the mass of the galaxy and the radio power on the mass of the central black hole.

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arXiv2000

Spectroscopy of Ultra Steep Spectrum Radio Sources

We present optical spectroscopy of 62 objects selected from several samples of ultra steep spectrum (USS) radio sources. 46 of these are from our primary catalog, consisting of 669 sources with radio spectral indices alpha < -1.30 (S_nu ~ nu^alpha); this first spectroscopic sub-sample was selected on the basis of their faint optical and near-IR identifications. Most are identified as narrow-lined radio galaxies with redshifts ranging from z=0.25 to z=5.19. Ten objects are at z>3, nearly doubling the number of such sources known to date. Four of the USS radio sources are identified with quasars, of which at least three have very red spectral energy distributions. The source TN J0936-2242 is identified with an extremely red object (ERO, R-K>5); both it and a close companion are at z=1.479. The spectrum of the ERO closely resembles that of previously discovered radio galaxies at z~1.5. Five sources show continuum emission, but fail to show any clear emission or absorption features, despite integrations of ~1 h with the Keck telescope. We suggest that these objects could be (i) radio galaxies with faint emission lines in the ``redshift desert'' at 1.5 <~ z <~ 2.3, (ii) radio galaxies with an obscured AGN, which are dominated by a stellar continuum observed with insufficient S/N, or (iii) pulsars. Three radio sources identified with faint objects in the K-band images remain undetected in 50-90 min spectroscopic integrations with the Keck telescope, and are possible z>7 candidates.

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arXiv2000

A Statistical Study of Emission Lines from High Redshift Radio Galaxies

We have compiled a sample of 165 radio galaxies from the literature to study the properties of the extended emission line regions and their interaction with the radio source over a large range of redshift 0<z<5.2. For each source, we have collected radio (size, lobe distance ratio and power) and spectroscopic parameters (luminosity, line width and equivalent width) for the four brightest UV lines. We also introduce a parameter A_{Ly-alpha} measuring the asymmetry of the Ly-alpha line. Using these 18 parameters, we examine the statistical significance of all 153 mutual correlations, and find the following significant correlations: (i) Ly-alpha asymmetry A_{Ly-alpha} with radio size and redshift, (ii) line luminosity with radio power, (iii) line luminosities of UV lines with each other, and (iv) equivalent widths of UV lines with each other. Using line-ratio diagnostic diagrams, we examine the ionization mechanism of the extended emission line regions in HzRGs. The high ionization lines seem to confirm previous results showing that AGN photo-ionization provides the best fit to the data, but are inconsitent with the CII/CIII ratio, which favour the highest velocity shock ionization models. We note that the CII line is 5 times more sensitive to shock ionization than the high ionization UV lines, and show that a combination of shock and photo-ionization provides a better overall fit to the integrated spectra of HzRGs. Because most HzRGs have radio sizes <~150 kpc, their integrated spectra might well contain a significant contribution from shock ionized emission. [abridged]

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arXiv2000

On the evolution of young radio-loud AGN

This paper describes an investigation of the early evolution of extragalactic radio sources using samples of faint and bright Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio galaxies. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. In addition, the data seem to suggest that the sources are in equipartition while they evolve. If GPS sources evolve to large size radio sources, their redshift dependent birth-functions should be the same. Therefore, since the lifetimes of radio sources are thought to be short compared to the Hubble time, the observed difference in redshift distribution between GPS and large size sources must be due to a difference in slope of their luminosity functions. We argue that this slope is strongly affected by the luminosity evolution of the individual sources. A scenario for the luminosity evolution is proposed in which GPS sources increase in luminosity and large scale radio sources decrease in luminosity with time. This evolution scenario is expected for a ram-pressure confined radio source in a surrounding medium with a King profile density. In the inner parts of the King profile, the density of the medium is constant and the radio source builds up its luminosity, but after it grows large enough the density of th e surrounding medium declines and the luminosity of the radio source decreases. A comparison of the local luminosity function (LLF) of GPS galaxies with that of extended sources is a good test for this evolution scenario [abridged].

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