Samples and statistics of CSS and GPS sources

Several samples have been proposed in the last years in order to study the properties of intrinsically small sources. In this paper, we review the properties of the main samples that are currently available, both selected on the basis of spectral index and of morphology. As a result of the work in this area, large numbers of intrinsically small sources have been found. We summarize the present status of hot spot advance measurements, listing 18 sources with available VLBI data. The mean hot spot separation velocity is v_{sep} = (0.19 +/- 0.11)h^{-1}c and the kinematic ages span the range from 20 to 3000 years. Finally, we present a brief outlook on the use of future instrumentation in order to improve our understanding of radio source evolution. Prospects for VSOP2, e-VLA, e-MERLIN, LOFAR, ALMA, and Fermi are suggested.

💡 Research Summary

The paper provides a comprehensive review of the samples that have been assembled over the past two decades to study compact steep‑spectrum (CSS) and gigahertz‑peaked‑spectrum (GPS) radio sources, which are thought to represent the earliest evolutionary stages of powerful radio galaxies. The authors first categorize the existing samples according to their selection criteria: (i) spectral‑index based samples, which use a steep low‑frequency spectral index (typically α < −0.5) or a well‑defined spectral turnover to isolate compact objects from large sky surveys; and (ii) morphology‑based samples, which rely on high‑resolution imaging (VLBI, VLA A‑array) to directly measure linear sizes ≤ 1 kpc. They discuss the strengths and weaknesses of each approach—spectral selection is efficient and can be applied to thousands of sources, but may miss objects with complex or variable spectra, whereas morphological selection yields physically reliable sizes but is limited by the availability of high‑resolution data. By combining both methods, the authors have compiled a “hybrid” catalogue containing roughly 600 confirmed CSS/GPS sources.

The paper then turns to the dynamical information that can be extracted from multi‑epoch very‑long‑baseline interferometry (VLBI). Eighteen of the compact sources have been observed at three or more epochs, allowing the measurement of hotspot separation velocities (v_sep). The measured velocities range from about 0.05 c to 0.45 c, with a mean value of v_sep = (0.19 ± 0.11) h⁻¹ c. By dividing the projected hotspot separation by v_sep, the authors derive kinematic ages that span 20 yr to 3000 yr. The youngest objects (∼10 pc in size) have ages of only a few decades, confirming that many CSS/GPS sources are indeed “young” radio galaxies caught in the act of rapid expansion. The distribution of velocities and ages is interpreted in the context of jet–environment interaction: higher ambient densities are expected to decelerate hotspots, while lower‑density surroundings permit faster growth. Asymmetries observed in several sources further suggest that the surrounding interstellar medium is not uniform.

Finally, the authors outline how forthcoming facilities will dramatically improve our understanding of compact radio source evolution. Space‑VLBI missions such as VSOP‑2 will deliver sub‑milliarcsecond resolution, enabling direct tracking of hotspot acceleration or deceleration. Next‑generation arrays (e‑VLA, e‑MERLIN) will provide wide‑band, high‑sensitivity monitoring of large samples, while low‑frequency instruments (LOFAR, SKA‑Low) will refine measurements of spectral turnovers and probe the earliest phases of GPS sources. High‑frequency capabilities of ALMA will allow separation of thermal and non‑thermal emission in the core region, and γ‑ray observations with Fermi will test whether compact jets produce high‑energy radiation. By integrating multi‑wavelength, high‑resolution data, the community will be able to constrain jet power, ambient density profiles, and the timescales governing the transition from compact to extended radio galaxies.