Lobe advance velocities in the extragalactic compact symmetric object 4C 31.04

¹ Istituto di Radioastronomia del CNR, via Gobetti 101, 40129 Bologna, Italy
² Dipartimento di Astronomia dell'Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
⁴ Onsala Space Observatory, 439 92 Onsala, Sweden
⁵ Dpto. Fisica Teorica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
⁶ Instituto de Astrofisica de Andalucia (CSIC), Apdo. 3004, 18080 Granada, Spain

Corresponding author: M. Giroletti, giroletti@ira.cnr.it

Received: 15 October 2002
Accepted: 6 December 2002

Abstract

We report on the results of a two epoch study of the low power Compact Symmetric Object 4C 31.04. Observations performed with the VLBA at 5 GHz in 1995 and 2000 have yielded images of this source at milliarcsecond angular resolution. A central core is detected, with bright compact hot spots and extended lobes on both sides. Model-fitting and other analysis of the data (brightness profile, difference map) clearly indicate that the source is expanding. We estimate the velocity of this expansion to be (0.085 ± 0.016) mas/yr, i.e. (0.33 ± 0.06) in both hot spots. Assuming a constant expansion velocity, we estimate the kinematic age of the source at 550 yrs. We also study the spectral index using VLBA observations at 1.3 GHz and MERLIN at 22 GHz. The derived spectral age is 3000–5000 years in equipartition conditions. The two estimates are discussed and found to be in agreement, given present uncertainties.