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M. A. Garrett

Publications and source records attributed to M. A. Garrett.

At least 127 records · Page 7Linked to original sources
arXiv2003

Changes in the measured image separation of the gravitational lens system, PKS 1830-211

We present eight epochs of 43 GHz, dual-polarisation VLBA observations of the gravitational lens system PKS 1830-211, made over fourteen weeks. A bright, compact ``core'' and a faint extended ``jet'' are clearly seen in maps of both lensed images at all eight epochs. The relative separation of the radio centroid of the cores (as measured on the sky) changes by up to 87 micro arcsec between subsequent epochs. A comparison with the previous 43 GHz VLBA observations (Garrett et al. 1997) made 8 months earlier show even larger deviations in the separation of up to 201 micro arcsec. The measured changes are most likely produced by changes in the brightness distribution of the background source, enhanced by the magnification of the lens. A relative magnification matrix that is applicable on the milliarcsecond scale has been determined by relating two vectors (the ``core-jet'' separations and the offsets of the polarised and total intensity emission) in the two lensed images. The determinant of this matrix, -1.13 +/-0.61, is in good agreement with the measured flux density ratio of the two images. The matrix predicts that the 10 mas long jet, that is clearly seen in previous 15 and 8.4 GHz VLBA observations (Garrett et al. 1997, Guirado et al. 1999), should correspond to a 4 mas long jet trailing to the south-east of the SW image. The clear non-detection of this trailing jet is a strong evidence for sub-structure in the lens and may require more realistic lens models to be invoked, e.g. Nair & Garrett (2000).

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arXiv2002

Deep Fields: The Faint sub-mJy and microJy Radio Sky - A VLBI Perspective

Until recently, VLBI targets have been drawn almost exclusively from the brightest and most compact radio sources in the sky, with typical flux densities well in excess of a few tens of mJy. These sources are predominantly identified with Active Galactic Nuclei (AGN), located at cosmological distances. In this lecture I will attempt to summarise what is currently known about the general properties of the faint sub-mJy and microJy radio source population, as determined from deep multi-wavelength studies of the HDF-N. In particular, I will try to provide a VLBI perspective, describing the first deep, wide-field, VLBI pilot observations of the HDF, together with a summary of the main results. The role VLBI can play in future high resolution studies of faint radio sources is also addressed.

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arXiv2002

Ground Based VLBI Facilities -- the European and Global VLBI Network

There are at least four well known Very Long Baseline Interferometer (VLBI) networks that provide open access to astronomers around the world. These include the EVN, VLBA, CMVA and APT/LBA. In this paper I have chosen to focus on a sub-set of these facilities, in particular the European VLBI Network, both in stand-alone mode but also as a major component of a Global VLBI Network, involving MERLIN and the VLBA.

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arXiv2002

Distant star forming galaxies, next generation radio telescopes and the radio universe before re-ionisation

I present the various capabilities of upgraded and next generation radio telescopes, in particular their ability to detect and image distant star forming galaxies. I demonstrate that e-MERLIN, EVLA and LOFAR can detect systems similar to Arp 220 out to cosmological distances. The SKA can detect such systems out to any reasonable redshift that they might be expected to exist. Employing very long integration times on the multiple-beam SKA will require the array to be extended beyond the current specification - simply to avoid confusion noise limitations at 1.4 GHz. Other arguements for extending the SKA baseline length are also presented. As well as going ``deeper'' all these instruments (especially LOFAR and the SKA) will also go ``wider'' - detecting many tens of thousands of galaxies in a single day's observing. I briefly comment on the prospects of detecting radio emission at much earlier epochs, just before the epoch of re-ionisation.

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arXiv2002

The FIR/Radio correlation of high redshift galaxies in the HDF-N

The correlation between the far-infrared (FIR) and radio emission is well established for nearby star forming galaxies. Many applications, in particular the radio-to-submm spectral index redshift indicator, tacitly assume that the relation holds well beyond our local neighbourhood, to systems located at cosmological distances. In order to test this assumption I have constructed a sample of 20 HDF-N galaxies, all with measured spectroscopic redshifts, and all detected by both ISO and the WSRT at 15 micron and 1.4 GHz respectively. The galaxies span a wide range of redshift with a median value of z ~ 0.7. The ISO 15 micron data were k-corrected and extrapolated to the FIR (60 and 100 micron) by assuming a starburst (M82) spectral energy distribution (SED) for the entire sample. An initial analysis of the data suggests that the correlation between the FIR and the radio emission continues to apply at high redshift with no obvious indication that it fails to apply beyond z ~ 1.3. The sample is ``contaminated'' by at least 1 distant (z=4.4), radio-loud AGN, VLA J123642+621331. This source has recently been detected by the first deep field VLBI observations of the HDF-N and is clearly identified as an outlier in the FIR/radio correlation.

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arXiv2002

2016+112: A Gravitationally Lensed Type-II Quasar

A single-screen model of the gravitational lens system 2016+112 is proposed, that explains recent Hubble Space Telescope} (HST) infrared (NICMOS-F160W) observations and new high-resolution European VLBI Network (EVN) 5-GHz radio observations, presented in this paper. In particular, we find that a massive `dark' structure at the lens position, previously suggested by X-ray, optical and spectroscopic observations of the field around 2016+112, is not necessarily required to accommodate the strong lensing constraints. A massive structure to the north-west of the lens system, suggested from a weak-lensing analysis of the field, is included in the model. The lensed source is an X-ray bright active galaxy at z=3.273 with a central bright optical continuum core and strong narrow emission lines, suggestive of a type-II quasar. The EVN 5-GHz radio maps show a radio-jet structure with at least two compact subcomponents. We propose that the diamond caustic crosses the counter-jet of the radio source, so that part of the counter-jet, host galaxy and narrow-line emission regions are quadruply imaged. The remainder of the radio source, including the core, is doubly imaged. Our lens model predicts a very high magnification (mu~300) at the bightness peaks of the inner two radio components of complex C. If the jet exhibits relativistic velocities on micro-arsecond scales, it might result in apparent hyperluminal motion. However, the lack of strong radio variability and the peaked radio spectrum imply that these motions need not be present in the source. Our model furthermore implies that the optical spectrum of C' can only show features of the AGN and its host galaxy.

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arXiv2001

Probing the pc- and kpc-scale environment of the powerful radio galaxy Hercules A

We present the kpc-scale behaviour of the powerful extragalactic radio source Hercules A and the behaviour of the intracluster gas in which the radio source is situated. We have found that Hercules A exhibits a strong Laing-Garrington effect. The X-ray observations have revealed an extended X-ray emission elongated along the radio galaxy axis. The estimated temperature of the cluster is kT = 2.45 keV and the central electron density is no~7.8 x 10^(-3) cm^(-3) which reveals a hot, dense environment in which Hercules A is situated. From the combined study of the radio and X-ray data we have estimated a central value of 3<Bo (muG)<9. We also present the most recent results from the analysis of the radio data on the pc-scale structure of the radio galaxy, observed at 18 cm by the EVN-MERLIN array. A faint but compact radio source, coincident with the optical centre of Hercules A was detected by the EVN at 18 mas resolution. The total flux density of the EVN core is 14.6 mJy. Its angular size is 18 x 7 mas with a position angle of ~139 degrees. There is also evidence for extended emission in the NW-SE direction, most probably from the eastern pc-scale jet. If this is true then there is a misalignment between the direction of the pc-eastern and the aligned kpc-scale jets of ~35 degrees.

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arXiv2001

The star-formation rate in the host of GRB 990712

We have observed the host galaxy of GRB 990712 at 1.4 GHz with the Australia Telescope Compact Array, to obtain an estimate of its total star-formation rate. We do not detect a source at the position of the host. The 2 sigma upper limit of 70 microJy implies that the total star-formation rate is lower than 100 Msun/yr, using conservative values for the spectral index and cosmological parameters. This upper limit is in stark contrast with recent reports of radio/submillimeter-determined star-formation rates of roughly 500 Msun/yr for two other GRB host galaxies. Our observations present the deepest radio-determined star-formation rate limit on a GRB host galaxy yet, and show that also from the unobscured radio point-of-view, not every GRB host galaxy is a vigorous starburst.

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arXiv2001

The FIR/Radio correlation of high redshift glaxies in the region of the HDF-N

The correlation between the far-infrared (FIR) and radio emission is well established for nearby star forming galaxies. Many applications, in particular the radio-to-submm spectral index redshift indicator, tacitly assume that the relation holds well beyond our local neighbourhood, to systems located at cosmological distances. In order to test this assumption I have constructed a sample of 22 HDF-N galaxies, all with measured spectroscopic redshifts, and all detected by both ISO and the WSRT at 15 micron and 1.4 GHz respectively. The galaxies span a wide range of redshift with a median value of z ~ 0.7. The ISO 15 micron data were k-corrected and extrapolated to the FIR (60 and 100 micron) by assuming a starburst (M82) spectral energy distribution (SED) for the entire sample. An initial analysis of the data suggests that the correlation between the FIR and the radio emission continues to apply at high redshift with no obvious indication that it fails to apply beyond z ~ 1.3. The sample is ``contaminated'' by at least 1 distant (z=4.4), radio-loud AGN, VLA J123642+621331. This source has recently been detected by the first deep field VLBI observations of the HDF-N and is clearly identified as an ``outlier'' in the FIR/radio correlation. I briefly comment on the impact upgraded and next generation radio instruments (such as e-MERLIN and the Square Km Array) can have in studies of star formation in the early Universe.

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arXiv2001

AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field

VLBI now has the capability of imaging relatively large fields-of-view (~ 5 arcmin sq) with sub-mJy detection limits. In principle such observations can distinguish between starburst and AGN activity in cosmologically distant sources. We present the first deep, wide-field European VLBI Network (EVN) 1.6 GHz observations of faint radio sources in the Hubble Deep Field (HDF) region. The EVN clearly (> 5sigma) detects 2 radio sources in a field that encompasses the HDF and part of the Hubble Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.01, low-luminosity FR-I radio source located within the HDF itself) and VLA J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system). A third radio source, J123646+621404, is detected at the 4sigma level. The observations presented here, suggest that the radio emission in the dusty starburst, VLA J123642+621331, arises mainly from AGN activity, suggesting that in this system, both AGN and starburst activity co-exist. The prospects for future VLBI deep fields are briefly discussed.

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arXiv2000

AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field

We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds. Above the 210 microJy/beam (5sigma) detection level, the EVN clearly detects two radio sources in a field that encompasses the HDF and part of the Hubble Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013, low-luminosity FR-I radio source located within the HDF itself) and VLA J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system). A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI detections of all three sources suggest that most of the radio emission of these particular sources (including the dusty starburst) is generated by an embedded AGN.

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arXiv2000

Global VLBI Observations of Compact Radio Sources in M82

Observations of the starburst galaxy, M82, have been made with a 20-station global VLBI array at $λ$18cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of $\sim$1 arcminute with 3 milliarcsecond resolution are discussed. A limit has been placed on the power law deceleration of the young SNR, 43.31+592 with an index greater than 0.73 $\pm$ 0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR. The latest global observations have also provided high resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s$^{-1}$. However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being due to the confinement of a supernova by a high density circumstellar medium. It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.

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arXiv2000

WSRT Observations of the Hubble Deep Field Region

We present deep WSRT 1.4 GHz observations of the Hubble Deep Field region. At the 5 sigma level, the WSRT clearly detects 85 regions of radio emission in a 10' x 10' field centred on the HDF. Eight of these regions fall within the HDF itself, four of these are sources that have not previously been detected at 1.4 GHz, although two of these are VLA detections at 8.5GHz. The two new radio sources detected by the WSRT are identified with relatively bright (I<21m) moderate redshift spiral and irregular type galaxies. In the full field, the WSRT detects 22 regions of emission that were not previously detected by the VLA at 1.4GHz. At least two of these are associated with nearby, extended star-forming galaxies.

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arXiv2000

A New Strategy for the Routine Detection & Imaging of Faint Radio Sources with VLBI

In this paper I outline a new strategy for the routine detection and imaging of faint (sub-mJy and microJy) radio sources with VLBI and SVLBI. The strategy relies on a combination of in-beam phase-referencing, wide-field VLBI imaging and simultaneous correlation of multiple field centres. A combination of these techniques, together with the steeply rising radio source counts observed at cm wavelengths, permit routine high resolution observations of radio sources previously considered too faint for conventional VLBI.

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arXiv2000

WSRT 1.4 GHz Observations of the Hubble Deep Field

We present WSRT 1.38 GHz observations of the Hubble Deep Field (and flanking fields). 72 hours of data were combined to produce the WSRT's deepest image yet, achieving an r.m.s. noise level of 8 microJy per beam. We detect radio emission from galaxies both in the HDF and HFF which have not been previously detected by recent MERLIN or VLA studies of the field.

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arXiv1999

Limitations of ad hoc "SKA+VLBI" configurations & the need to extend SKA to trans-continental dimensions

The angular resolution of the proposed Square Kilometre Array, SKA, must be extended towards the milliarcsecond scale if it is to resolve the distant starburst galaxies that are likely to dominate the radio source counts at micro and sub-microJy flux levels. This paper considers the best way of extending SKA's angular resolution towards the milliarcsecond scale. Two possible SKA-VLBI configurations have been investigated and simulated SKA and SKA-VLBI visibility data sets generated. The effects of non-uniform data weighting on the associated images are considered. The results suggest that the preferred option is for SKA to be extended to trans-continental dimensions. By retaining 50% of the array's collecting area within a region no larger than 50 km, the surface brightness sensitivity of the array at arcsec resolution is hardly compromised. In this configuration SKA's capabilities are impressive: in a single 12 hour run, between 100-1000 sources will be simultaneously detected and imaged with arcsecond, sub-arcsecond and milliarcsecond resolution.

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arXiv1999

Helical motions in the jet of blazar 1156+295

The blazar 1156+295 was observed by VLBA and EVN + MERLIN at 5 GHz in June 1996 and February 1997 respectively. The results show that the jet of the source has structural oscillations on the milliarcsecond scale and turns through a large angle to the direction of the arcsecond-scale extension. A helical jet model can explain most of the observed properties of the radio structure in 1156+295.

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