An Hα nuclear spiral structure in the E0 active galaxy Arp 102B

Abstract

We report the discovery of a two-armed mini-spiral structure within the inner kiloparsec of the E0 LINER/Seyfert 1 galaxy Arp 102B. The arms are observed in Hα emission and located east and west of the nucleus, extending up to ≈1 kpc from it. We use narrow-band imaging from the Hubble Space Telescope Advanced Camera for Surveys, in combination with archival Very Large Array radio images at 3.6 and 6 cm to investigate the origin of the nuclear spiral. From the Hα luminosity of the spiral, we obtain an ionized gas mass of the order of 10 6 solar masses. One possibility is that the nuclear spiral represents a gas inflow triggered by a recent accretion event which has replenished the accretion disk, giving rise to the double-peaked emission-line profiles characteristic of Arp 102B. However, the radio images show a one-sided curved jet which correlates with the eastern spiral arm observed in the Hα image. A published milliarcsecond radio image also shows a one-sided structure at position angle ≈40º, approximately aligned with the inner part of the eastern spiral arm. The absence of a radio counterpart to the western spiral arm is tentatively interpreted as indicating that the jet is relativistic, with an estimated speed of 0.45c. Estimates of the jet kinetic energy and the ionizing luminosity of the active nucleus indicate that both are capable of ionizing the gas along the spiral arms.We conclude that, although the gas in the nuclear region may have originated in an accretion event, the mini spiral is most likely the result of a jet–cloud interaction rather than an inflowing stream.