Parsec-scale morphology and spectral index distribution in faint high frequency peakers

We investigate the parsec-scale structure of 17 high frequency peaking radio sources from the faint HFP sample. VLBA observations were carried out at two adjacent frequencies, 8.4 and 15.3 GHz, both in the optically-thin part of the spectrum, to obtain the spectral index information. We found that 64% of the sources are resolved into subcomponents, while 36% are unresolved even at the highest frequency. Among the resolved sources, 7 have a morphology and a spectral index distribution typical of young radio sources, while in other 4 sources, all optically associated with quasars, the radio properties resemble those of the blazar population. The equipartition magnetic field of the single components are a few tens milliGauss, similar to the values found in the hotspots of young sources with larger sizes. Such high magnetic fields cause severe radiative losses, precluding the formation of extended lobe structures emitting at centimeter wavelengths. The magnetic fields derived in the various components of individual source are usually very different, indicating a non self-similar source evolution, at least during the very first stages of the source growth.Comment: 14 pages, 5 figures, accepted for publication in MNRA

Spectral variability in faint high frequency peakers

We present the analysis of simultaneous multi-frequency Very Large Array (VLA) observations of 57 out of 61 sources from the ``faint'' high frequency peaker (HFP) sample carried out in various epochs. Sloan Digital Sky Survey (SDSS) data have been used to identify the optical counterpart of each radio source. From the analysis of the multi-epoch spectra we find that 24 sources do not show evidence of spectral variability, while 12 objects do not possess a peaked spectrum anymore at least in one of the observing epochs. Among the remaining 21 sources showing some degree of variability, we find that in 8 objects the spectral properties change consistently with the expectation for a radio source undergoing adiabatic expansion. The comparison between the variability and the optical identification suggests that the majority of radio sources hosted in galaxies likely represent the young radio source population, whereas the majority of those associated with quasars are part of a different population similar to flat-spectrum objects, which possess peaked spectra during short intervals of their life, as found in other samples of high-frequency peaking objects. The analysis of the optical images from the SDSS points out the presence of companions around 6 HFP hosted in galaxies, suggesting that young radio sources resides in groups.Comment: 16 pages, 5 figures; accepted for publication in MNRA

Physical properties in young radio sources. VLBA observations of high frequency peaking radio sources

Multifrequency Very Long Baseline Array (VLBA) observations were performed to study the radio morphology and the synchrotron spectra of four high frequency peaking radio sources. They are resolved in several compact components and the radio emission is dominated by the hotspots/lobes. The core region is unambiguously detected in J1335+5844 and J1735+5049. The spectra of the main source components peak above 3 GHz. Assuming that the spectral peak is produced by synchrotron self-absorption, we estimate the magnetic field directly from observable quantities and in half of the components it agrees with the equipartition field, while in the others the difference exceeds an order of magnitude. By comparing the physical properties of the targets with those of larger objects we found that the luminosity increases with the linear size for sources smaller than a few kpc, while it decreases for larger objects. The asymmetric sources J1335+5844 and J1735+5049 suggest that the ambient medium is inhomogeneous and is able to influence the evolution of the radio emission even during its first stages. The core luminosity increases with the linear size for sources up to a few kpc, while it seems constant for larger sources suggesting an evolution independent from the source total luminosity.Comment: 16 pages, 10 figures, accepted for publication in MNRA

High Frequency Peakers: The Faint Sample

We present a sample of sources with convex radio spectra peaking at frequencies above a few GHz, known as "High Frequency Peakers" (HFPs). A "bright" sample with a flux density limit of 300 mJy at 5 GHz has been presented by Dallacasa et al. (2000). Here we present the "faint" sample with flux density between 50 and 300 mJy at 5GHz, restricted to the area around the North Galactic Cap, where the FIRST catalogue is available. The candidates have been observed with the VLA at several frequencies ranging from 1.4 to 22 GHz, in order to derive a simultaneous radio spectrum. The final list of confirmed HFP sources consists of 61 objects.Comment: 3 pages. Accepted for publication in Astronomische Nachrichte

Proper motion and apparent contraction in J0650+6001

We present a multi-epoch and multi-frequency VLBI study of the compact radio source J0650+6001. In VLBI images the source is resolved into three components. The central component shows a flat spectrum, suggesting the presence of the core, while the two outer regions, with a steeper spectral index, display a highly asymmetric flux density. The time baseline of the observations considered to derive the source expansion covers about 15 years. During this time interval, the distance between the two outer components has increased by 0.28+/-0.13 mas, that corresponds to an apparent separation velocity of 0.39c+/-0.18c and a kinematic age of 360+/-170 years. On the other hand, a multi-epoch monitoring of the separation between the central and the southern components points out an apparent contraction of about 0.29+/-0.02 mas, corresponding to an apparent contraction velocity of 0.37c+/-0.02c. Assuming that the radio structure is intrinsically symmetric, the high flux density ratio between the outer components can be explained in terms of Doppler beaming effects where the mildly relativistic jets are separating with an intrinsic velocity of 0.43c+/-0.04c at an angle between 12 and 28 degrees to the line of sight. In this context, the apparent contraction may be interpreted as a knot in the jet that is moving towards the southern component with an intrinsic velocity of 0.66c+/-0.03c, and its flux density is boosted by a Doppler factor of 2.0.Comment: 7 pages, 5 pages. Accepted for publication in MNRA

Radio properties of Compact Steep Spectrum and GHz-Peaked Spectrum radio sources

Compact steep spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources represent a large fraction of the extragalactic objects in flux density-limited samples. They are compact, powerful radio sources whose synchrotron peak frequency ranges between a few hundred MHz to several GHz. CSS and GPS radio sources are currently interpreted as objects in which the radio emission is in an early evolutionary stage. In this contribution I review the radio properties and the physical characteristics of this class of radio sources, and the interplay between their radio emission and the ambient medium of the host galaxy.Comment: 9 pages, 4 figures. Accepted for publication in Astronomische Nachrichten. Review presented at the 5th Workshop on CSS and GPS radio sources, held in Rimini (Italy) in May 201