arXiv subjects

D. Cseh

Publications and source records attributed to D. Cseh.

22 records · Page 2Linked to original sources

arXiv2010

High-resolution images of five radio quasars at early cosmological epochs

Context: Until now, there have only been seven quasars at z>4.5 whose the high-resolution radio structure had been studied in detail with Very Long Baseline Interferometry (VLBI) imaging. Aims: We almost double the number of VLBI-imaged quasars at these high redshifts with the aim of studying their redshift-dependent structural and physical properties in a larger sample. Methods: We observed five radio quasars (J0813+3508, J1146+4037, J1242+5422, J1611+0844, and J1659+2101) at 4.5<z<5 with the European VLBI Network (EVN) at 1.6 GHz on 29 October 2008 and at 5 GHz on 22 October 2008. The angular resolution achieved ranges from 1.5 to 25 milli-arcseconds (mas), depending on the observing frequency, the position angle in the sky, and the source's celestial position. Results: The sources are all somewhat extended on mas scales, but compact enough to be detected at both frequencies. With one exception of a flat-spectrum source (J1611+0844), their compact emission is characterised by a steep radio spectrum. We found no evidence of Doppler-boosted radio emission in the quasars in our sample. The radio structure of one of them (J0813+3508) is extended to ~7", which corresponds to 43 kpc projected linear size. Many of the highest redshift compact radio sources are likely to be young, evolving objects, far-away cousins of the powerful gigahertz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources that populate the Universe at lower redshifts.

astro-ph.CO↗

arXiv2010

Radio interferometric observations of two core-dominated triple radio sources at z>3

Aims. We selected two radio quasars (J1036+1326 and J1353+5725) based on their 1.4-GHz radio structure, which is dominated by a bright central core and a pair of weaker and nearly symmetric lobes at ~10" angular separation. They are optically identified in the Sloan Digital Sky Survey (SDSS) at spectroscopic redshifts z>3. We investigate the possibility that their core-dominated triple morphology can be a sign of restarted radio activity in these quasars, involving a significant repositioning of the radio jet axis. Methods. We present the results of high-resolution radio imaging observations of J1036+1326 and J1353+5725, performed with the European Very Long Baseline Interferometry (VLBI) Network (EVN) at 1.6 GHz. These data are supplemented by archive observations from the Very Large Array (VLA).We study the large- and small-scale radio structures and the brightness temperatures, then estimate relativistic beaming parameters. Results. We show that the central emission region of these two high-redshift, core-dominated triple sources is compact but resolved at ~10 milli-arcsecond resolution. We find that it is not necessary to invoke large misalignment between the VLBI jet and the large-scale radio structure to explain the observed properties of the sources.

astro-ph.CO↗

arXiv2010

Radio observations of NGC 6388: an upper limit on the mass of its central black hole

We present the results of deep radio observations with the Australia Telescope Compact Array (ATCA) of the globular cluster NGC 6388. We show that there is no radio source detected (with a r.m.s. noise level of 27 uJy) at the cluster centre of gravity or at the locations of the any of the Chandra X-ray sources in the cluster. Based on the fundamental plane of accreting black holes which is a relationship between X-ray luminosity, radio luminosity and black hole mass, we place an upper limit of 1500 M_sun on the mass of the putative intermediate-mass black hole located at the centre of NGC 6388. We discuss the uncertainties of this upper limit and the previously suggested black hole mass of 5700 M_sun based on surface density profile analysis.

astro-ph.HE↗

arXiv2008

A close look at compact triple radio sources

We show an archival 1.4-GHz Very Large Array (VLA) image of the core-dominated triple (CDT) radio source J1628+3906, along with a recent high-resolution 5-GHz Very Long Baseline Interferometry (VLBI) image of its core. This core is resolved into two nearly symmetric components at ~10 milli-arcsecond (mas) angular scale. The inner structure is markedly misaligned with respect to the large-scale structure. This morphology can be interpreted as a possible signature of once ceased and then restarted activity of the galactic nucleus, coupled with a rapid repositioning of the central radio jet axis. In a search for compact (<5"), optically identified, and highly redshifted (z>3) quasars in the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) and the Sloan Digital Sky Survey (SDSS) catalogues, we have also found two quasars associated with weaker two-sided radio lobes at ~10" scale. We present archival 1.4-GHz VLA images of these quasars (J1036+1326 and J1353+5725). The compact cores of both of these CDT sources are likely to be detectable at mas scales. Future VLBI observations at the highest resolution could reveal if their inner radio structure is similar to that of the Compact Symmetric Objects (CSOs). The characteristic directions of their arcsecond-scale and mas-scale structures are probably misaligned which could also support the "restarting and rapid repositioning" scenario.

astro-ph↗