FIRST-based survey of Compact Steep Spectrum sources, II. MERLIN and VLA observations of Medium-sized Symmetric Objects

TL;DR

This paper presents new radio observations of a sample of weaker Compact Steep Spectrum sources, revealing diverse morphologies and evolutionary stages, including potential 'dying' sources, contributing to understanding CSS source evolution.

Contribution

It introduces a new sample of weaker CSS sources and provides detailed multi-frequency radio observations, highlighting their varied morphologies and evolutionary states.

Findings

Six sources exhibit FR II-like morphology but are not edge-brightened.

One source may be a prematurely 'dying' MSO or 'fader'.

Some CSS sources may transition early to a coasting phase, fading before becoming large-scale objects.

Abstract

A new sample of candidate Compact Steep Spectrum (CSS) sources that are much weaker than the CSS source prototypes has been selected from the VLA FIRST catalogue. MERLIN `snapshot' observations of the sources at 5 GHz indicate that six of them have an FR II-like morphology, but are not edge-brightened as is normal for Medium-sized Symmetric Objects (MSOs) and FR IIs. Further observations of these six sources with the VLA at 4.9 GHz and MERLIN at 1.7 GHz, as well as subsequent full-track observations with MERLIN at 5 GHz of what appeared to be the two sources of greatest interest are presented. The results are discussed with reference to the established evolutionary model of CSS sources being young but in which not all of them evolve to become old objects with extended radio structures. A lack of stable fuelling in some of them may result in an early transition to a so-called coasting phase so that they fade away instead of growing to become large-scale objects. It is possible that one of the six sources (1542+323) could be labelled as a prematurely `dying' MSO or a `fader'.