2012MNRAS.427.2772F

Interactions between galaxies are very common. There are special types of interactions that produce systems called polar ring galaxies (PRGs), composed of a lenticular, elliptical or spiral host galaxy, surrounded by a ring of stars and gas, orbiting in an approximately polar plane. In this paper, we study AM 2020-504, a PRG with an elliptical host galaxy, and a narrow and well-defined ring, probably formed by accretion of material from a donor galaxy, collected by the host galaxy. Our observational study is based on BVRI broad-band imagery as well as long-slit spectroscopy in the wavelength range 4100–8600 Å, performed at the 1.6-m telescope at the Observatório do Pico dos Dias, Brazil. We have estimated a redshift of z = 0.01683, corresponding to a heliocentric radial velocity of 5045±23 km/s. The (B - R) colour map shows that the ring is bluer than the host galaxy, which indicates that the ring is a younger structure. Standard diagnostic diagrams have been used to classify the main ionizing source of selected emission-line regions (nucleus, host galaxy and ring). We find that the ring regions are mainly ionized by massive stars, while the nucleus presents the characteristics of an active galactic nucleus. Using two empirical methods, we have found oxygen abundances for the HII regions located in the ring in the range 12+log(O/H) = 8.3–8.8 dex. We have also found the presence of an oxygen gradient across the ring, and it appears that AM 2020-504 follows the metallicity–luminosity relation of spiral galaxies. These results support the accretion scenario for this object and they rule out cold accretion as the source for the HI gas in the polar ring.