The X-ray view of giga-hertz peaked spectrum radio galaxies

¹ European Space Astronomy Centre of ESA, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain e-mail: Matteo.Guainazzi@sciops.esa.int
² Institute of Technology, University of Linköping, 581 83 Linköping, Sweden
³ Harvard-Smithsonian Centre for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁴ Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia (CSIC), Madrid, Spain
⁵ H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
⁶ Istituto di Astrofisica Spaziale e Fisica Cosmica-Bologna, INAF, via Gobetti 101, 40129 Bologna, Italy
⁷ Osservatorio Astronomico di Roma (INAF), via Frascati 33, 00040 Monteporzio Catone, Roma, Italy

Received: 3 November 2008
Accepted: 11 March 2009

Abstract

Context. This paper presents the X-ray properties of a flux- and volume-limited complete sample of 16 giga-hertz peaked spectrum (GPS) galaxies.

Aims. This study addresses three basic questions in our understanding of the nature and evolution of GPS sources: a) What is the physical origin of the X-ray emission in GPS galaxies? b) Which physical system is associated with the X-ray obscuration? c) What is the “endpoint” of the evolution of compact radio sources?

Methods. We discuss in this paper the results of the X-ray spectral analysis, and compare the X-ray properties of the sample sources with radio observables.

Results. We obtain a 100% (94%) detection fraction in the 0.5–2 keV (0.5–10 keV) energy band. GPS galaxy X-ray spectra are typically highly obscured ( cm^-2;  dex). The X-ray column density is larger than the HI column density measured in the radio by a factor 10 to 100. GPS galaxies lie well on the extrapolation to high radio powers of the correlation between radio and X-ray luminosity known in low-luminosity FR I radio galaxies. On the other hand, GPS galaxies exhibit a comparable X-ray luminosity to FR II radio galaxies, notwithstanding their much larger radio luminosity.

Conclusions. The X-ray to radio luminosity ratio distribution in our sample is consistent with the bulk of the high-energy emission being produced by the accretion disk, as well as with dynamical models of GPS evolution where X-rays are produced by Compton upscattering of ambient photons. Further support to the former scenario comes from the location of GPS galaxies in the X-ray to O[iii] luminosity ratio versus N_H plane. We propose that GPS galaxies are young radio sources, which would reach their full maturity as classical FR II radio galaxies. However, column densities 10²² cm^-2 could lead to a significant underestimate of dynamical age determinations based on the hotspot recession velocity measurements.