arXiv subjects

R. T. Schilizzi

Publications and source records attributed to R. T. Schilizzi.

At least 37 records · Page 2Linked to original sources

arXiv2002

A Parkes half-Jansky sample of GPS galaxies

This paper describes the selection of a new southern/equatorial sample of Gigahertz Peaked Spectrum (GPS) radio galaxies, and subsequent optical CCD imaging and spectroscopic observations using the ESO 3.6m telescope. The sample consists of 49 sources with -40<δ<+15 degrees, |b|>20 degrees, and S(2.7GHz)>0.5 Jy, selected from the Parkes PKSCAT90 survey. About 80% of the sources are optically identified, and about half of the identifications have available redshifts. The R-band Hubble diagram and evolution of the host galaxies of GPS sources are reviewed.

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arXiv2002

SS433: outflows here and there

We summarize the results of VLBA and global VLBI observations of SS433 between 1995 and 2000. With these observations we resolve the inner jet of the source and identify an absorption region (~25 AU), the "central radio gap". The radio gap is caused by free-free absorption of the jet radio emission by a flattened outflow from the binary system. Radio emission is detected on 100 AU scales perpendicular to the normal jets ("equatorial emission region"; "equatorial outflow"). At some epochs the emission is smooth but compact features are frequently detected. We suggest that equatorial outflows may be common in microquasars.

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arXiv2002

VLBI observations in Cluster-Cluster mode at 1.6 GHz

We describe, and present preliminary results from the analysis of 1.6 GHz VLBI observations, made in Cluster-Cluster mode between subgroups of antennas from the WSRT, VLA and MERLIN arrays. We observed for two 6-hour periods on consecutive days, simultaneously monitoring 4 compact radio sources with angular separations ranging from ~1 to 9 degrees. The data were recorded in a non-standard MkIII mode using special set-ups, and correlated at the MPIfR MkIIIa correlator. The preliminary results from standard and phase reference analyses are very encouraging. The experiment has provided a useful database for investigating the potential of this new observing technique at low frequencies.

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arXiv2002

The Equatorial Outflow of SS433

We present VLBI imaging results for SS433 from the period 1995-2000. An equatorial emission region is detected and confirmed in 7 experiments at 1.6 and 5 GHz. The structure of the region changes with time on scales of weeks to months. The nature of emission is unknown, but it is certainly non-thermal, maybe optically thin synchrotron. The spectrum however might be attenuated by a thermal population of electrons. There were previous indications for an equatorial outflow from the system. We do detect outward motion of a radio component, and estimate a speed of 1200/sin(i) km/s. Theoretical work and IR observations suggest that the mass-loss rate in SS433 is much higher in this region than in the jets. We suggest that the high-mass microquasars might all have equatorial outflows; in fact, it is the dominant form of mass-loss in these systems.

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arXiv2002

How is really decelerating the expansion of SN1993J?

SN1993J is to date the radio supernova whose evolution has been monitored in greatest detail and the one which holds best promise for a comprehensive theoretical-observational analysis. The shell-like radio structure of SN1993J has expanded in general accord with models of shock excited emission, showing almost circular symmetry for over 8 years, except for a bright feature at the south-eastern region of the shell that has been observed at every epoch. The spectrum of SN1993J has flattened from alpha =-1 to alpha =-0.67 (S_(ν) propto nu**(alpha)). The decelerated expansion can be modeled well with a single slope but apparently better with two slopes. There are also intriguing hints of structure in the expansion curve. The results by the two VLBI groups carrying out this research show general agreement, but also some differences. A comparison of the optical and VLBI results about the details of the deceleration show some discrepancies.

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arXiv2000

VLBI, MERLIN and HST observations of the giant radio galaxy 3C236

We present VLBI and MERLIN data at 1.66 and 4.99 GHz on the central component coincident with the nucleus of the giant radio galaxy, 3C236. The nuclear radio structure is composed of two complexes of emission which are resolved on scales from 1 milli-arcsec (mas) to 1 arcsec. Oscillations with an amplitude of about 5 degrees can be seen in the compact radio structure. Spectral index distributions are plotted at angular resolutions of 10 and 25 mas and allow us to identify the core component in the south-east emission complex. Re-examination of the HST WFPC-2 image of 3C236 by de Koff et al. (2000), shows that the normal to the dust disk in the nucleus is ~30 degrees from the plane of the sky and within 12 degrees of parallel to the overall orientation of the radio source. We suggest that the radio axis is also at an angle of ~30 degrees to the plane of the sky and that the north-west jet is on the approaching side. This orientation implies an overall size of 4.5 Mpc (H_o = 75 km/s/Mpc, q_o = 0.5) for 3C236. The coincidence of a dust feature and the south-east compact jet, within the astrometric errors, leads us to suggest that the dust may be in the form of a cloud encountered by the jet in the first ~400 pc of its journey out from the nucleus. One-sided emission at 5 GHz on 1 mas scales would suggest that the jets are ejected initially at <= 35 degrees to the line of sight, but this is difficult to reconcile with the obvious orientation stability of the jet system as a whole. Free-free absorption of the counter-jet may be an alternative explanation for the one-sideness.

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arXiv2000

Young radio-loud AGN: A new sample at low redshift

We report on the investigation of a complete sample of young radio-loud AGN at z<0.25, selected using the new comprehensive radio surveys and the optical APM/POSS-I catalog. This new sample will provide a unique opportunity for statistical studies of the early evolution of radio-loud AGN, free of cosmological evolution effects. In particular, their local luminosity function, linear-size and dynamical age distributions, may strongly constrain possible evolution scenarios. In addition, this new sample is well suited for statistical studies of other important properties, like their X-ray and infrared emissions, optical cluster environments, and neutral hydrogen in absorption towards the compact radio sources.

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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

The compact core-jet region of the superluminal quasar 3C216

Space very long baseline interferometry (VLBI) observations of the quasar 3C216 with the VLBI Space Observatory Programme (VSOP) reveal that the parsec scale structure of the source can be well described by compact jet models developed for interpreting the core region of radio-loud active galactic nuclei (AGN). The measured brightness temperature of T_(b) = 7.9*10^{11} K is comparable to the inverse Compton limit, from which we determine a lower limit of delta ~ 3.17 for the Doppler-boosting factor. The apparent transvers velocity of the superluminal component is beta_(app) = (3.0+-0.2) h^{-1} asuming a constant velocity, but deceleration of the jet material cannot be excluded from our data. A combination of the above values indicates that the viewing angle of the core-jet to the line-of-sight is less than 18.4 degrees, and the jet Lorentz-factor exceeds 3.16. The observed small size of the source is probably caused by both interaction with the interstellar medium, and a projection effect.

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arXiv2000

VSOP observation of the quasar PKS 2215+020: a new laboratory for core-jet physics at z=3.572

We report results of a VSOP (VLBI Space Observatory Programme) observation of a high redshift quasar PKS 2215+020 (z=3.572). The ~1 milliarcsecond resolution image of the quasar reveals a prominent `core-jet' structure on linear scales from 5/h to 300/h pc ($H_0=100*h km/(s*Mpc). The brightness temperatures and sizes of bright features identified in the jet are consistent with emission from relativistic shocks dominated by adiabatic energy losses. The jet is powered by the central black hole with estimated mass of ~4*10^9 solar masses. Comparisons with VLA and ROSAT observations indicate a possible presence of an extended radio/X-ray halo surrounding 2215+020.

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arXiv2000

ISO observations of a sample of Compact Steep Spectrum and GHz Peaked Spectrum Radio Galaxies

We present results from observations obtained with ISOPHOT, on board the ISO satellite, of a representative sample of seventeen CSS/GPS radio galaxies and of a control sample of sixteen extended radio galaxies spanning similar ranges in redshift (0.2 <= z <= 0.8) and radio luminosity (P_2.7 GHz >= 10^26 W/Hz). The observations have been performed at lambda = 60, 90, 174 and 200 microns. Seven of the CSS/GPS sources have detections >= 3 sigma at one or more wavelengths, one of which is detected at >= 5 sigma. By co-adding the data we have obtained average flux densities at the four wavelengths. We found no evidence that the FIR luminosities of the CSS/GPS sources are significantly different from those of the extended objects and therefore there is not any support for CSS/GPS sources being objects "frustrated" by an abnormally dense ambient medium. The two samples were then combined, providing FIR information on a new sample of radio galaxies at intermediate redshifts. We compare this information with what previously known from IRAS and discuss the average properties of radio galaxies in the redshift range 0.2 - 0.8. The FIR emission cannot be accounted for by extrapolation of the synchrotron radio spectrum and we attribute it to thermal dust emission. The average FIR luminosity is >= 6*10^11 L_sun. Over the observed frequency range the infrared spectrum can be described by a power law with spectral index alpha >~1.0 +/- 0.2. Assuming the emission to be due to dust, a range of temperatures is required, from >=80 K to \~25 K. The dust masses required to explain the FIR emission range from 5*10^5 M_sun for the hotter component up to 2*10^8 M_sun for the colder one. (abridged)

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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

A Thin HI Circumnuclear Disk in NGC4261

We report on high sensitivity, spectral line VLBI observations of the HI absorption feature in the radio galaxy NGC4261. Although absorption is only detectable on the most sensitive baseline, it can be unambiguously associated with the counterjet and is interpreted to originate in a thin atomic circumnuclear disk. This structure is probably a continuation of the dusty accretion disk inferred from HST imaging, which could be feeding the massive black hole. HI column densities in front of the counterjet of the order of 10^{21}(T_sp/100 K) cm^{-2} are derived, consistent with X-ray data and VLBI scale free-free absorption. The data presented here are the result of the first scientific project processed on the new EVN MkIV data processor.

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arXiv1999

The inner radio jet region and the complex environment of SS433

We present multi-frequency VLBA+VLA observations of SS433 at 1.6, 5 and 15 GHz. These observations provide the highest angular resolution radio spectral index maps ever made for this object. Motion of the components of SS433 during the observation is detected. In addition to the usual VLBI jet structure, we detect two radio components in the system at an anomalous position angle. These newly discovered radio emitting regions might be related to a wind-like equatorial outflow or to an extension of the accretion disk. We show that the radio core component is bifurcated with a clear gap between the eastern and western wings of emission. Modelfitting of the precessing jets and the moving knots of SS433 shows that the kinematic centre -- i.e. the binary -- is in the gap between the western and eastern radio core components. Spectral properties and observed core position shifts suggest that we see a combined effect of synchrotron self-absorption and external free-free absorption in the innermost AU-scale region of the source. The spatial distribution of the ionized matter is probably not spherically symmetric around the binary, but could be disk-like.

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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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arXiv1999

Parsec-scale radio structures in the nuclei of four Seyfert galaxies

We present 18-cm radio maps of four Seyfert nuclei, Mrk 1, Mrk 3, Mrk 231 and Mrk 463E, made with the European VLBI Network (EVN). Linear radio structures are present in three out of four sources on scales of ~100 pc to ~1 kpc, and the 20-mas beam of the EVN enables us to resolve details within the radio structures on scales of <10 pc. Mrk 3 was also imaged using MERLIN and the data combined with the EVN data to improve the sensitivity to extended emission. We find an unresolved flat-spectrum core in Mrk 3, which we identify with the hidden Seyfert 1 nucleus in this object, and we also see marked differences between the two highly-collimated radio jets emanating from the core. The western jet terminates in a bright hotspot and resembles an FRII radio structure, whilst the eastern jet has more in common with an FRI source. In Mrk 463E, we use the radio and optical structure of the source to argue that the true nucleus lies approximately 1 arcsec south of the position of the radio and optical brightness peaks, which probably represent a hotspot at the working surface of a radio jet. The EVN data also provide new evidence for a 100-pc radio jet powering the radio source in the Type 1 nucleus of Mrk 231. However, the Seyfert 2 galaxy Mrk 1 shows no evidence for radio jets down to the limits of resolution (~10 pc). We discuss the range of radio source size and morphology which can occur in the nuclei of Seyfert galaxies and the implications for Seyfert unification schemes and for radio surveys of large samples of objects.

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arXiv1999

Optical spectroscopy of faint gigahertz peaked spectrum sources

We present spectroscopic observations of a sample of faint Gigahertz Peaked Spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS). Redshifts have been determined for 19 (40%) of the objects. The optical spectra of the GPS sources identified with low redshift galaxies show deep stellar absorption features. This confirms previous suggestions that their optical light is not significantly contaminated by AGN-related emission, but is dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (probably) young radio sources as probes of galaxy evolution. The optical spectra of GPS sources identified with quasars are indistinguishable from those of flat spectrum quasars, and clearly different from the spectra of Compact Steep Spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples presented in the literature, peaking at z ~ 2-3. It is unlikely that a significant population of low redshift GPS quasars is missed due to selection effects in our sample. We therefore claim that there is a genuine difference between the redshift distributions of GPS galaxies and quasars, which, because it is present in both the radio-faint and bright samples, can not be due to a redshift-luminosity degeneracy. It is therefore unlikely that the GPS quasars and galaxies are unified by orientation, unless the quasar opening angle is a strong function of redshift. We suggest that the GPS quasars and galaxies are unrelated populations and just happen to have identical observed radio-spectral properties, and hypothesise that GPS quasars are a sub-class of flat spectrum quasars.

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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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