The unusual interacting pair of galaxies IC 3481 and IC 3481A: An optical-NIR photometric and spectroscopic analysis

We present the first optical and near infrared (NIR) photometric and spectroscopic analysis of the interacting pair of galaxies IC 3481 (lenticular) and IC 3481A (elliptical). These galaxies have a tidal bridge connecting them, and an unusual long filament in the shape of an ellipse extending to the SE of IC 3481A. Surface brightnesses and colors were calculated for regions such as galaxy centers, bridges, filaments and optical knots. The optical colors and underlying continuum spectra of the galaxies are consistent with old populations dominated by K-type stars, but the optical-NIR colors imply more intermediate ages. IC 3481 has very weak emission lines with ratios implying it is a borderline AGN-H II region case, but IC 3481A has strong emission lines implying a Hα luminosity of about 1.3×1042erg s-1 with H II-region-like line ratios, and an SFR of ∼11 M⊙yr-1. Interacting early-type galaxies, and early-type galaxies in pairs and groups have generally been found to have very low SFRs, and it may be that this unusually active pair represents a phase in the merger history of a compact group. One possibility is that one or both galaxies may be the end result of a merger involving a late-type galaxy, providing sufficient gas to drive the circumnuclear starburst in IC 3481A, although the implied ages of the underlying stellar populations in both galaxies are against this hypothesis. The ellipse-like filament is possibly formed out of the remains of an accreted small disk galaxy.