Though S0 galaxies are usually thought to be `red and dead, they demonstrate often star formation organized in ring structures. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. The moderate-lu
minosity nearby S0 galaxy, NGC 4513, is studied here. By applying long-slit spectroscopy along the major axis of NGC 4513, we have measured gas and star kinematics, Lick indices for the main body of the galaxy, and strong emission-line flux ratios in the ring. After inspecting the gas excitation in the ring using the line ratios diagnostic diagrams and have assured that it is ionized by young stars, we have determined the gas oxygen abundance by using popular strong-line calibration methods. We have estimated star formation rate (SFR) in the outer ring by using the archival Galaxy Evolution Explorer (GALEX) ultraviolet images of the galaxy. The ionized gas counterrotates the stars over the whole extension of NGC 4513 so being accreted from outside. The gas metallicity in the ring is slightly subsolar, [O/H]=-0.2 dex, matching the metallicity of the stellar component of the main galactic disc. However the stellar component of the ring is much more massive than can be explained by the current star formation level in the ring. We conclude that probably the ring of NGC 4513 is a result of tidal disruption of a massive gas-rich satellite, or it may be a consequence of a long star-formation event provoked by a gas accretion from a cosmological filament having started some 3 Gyr ago.
Though S0 galaxies are usually thought to be `red and dead, they often demonstrate star formation organized in ring structures. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. Two early-type
galaxies with outer rings, NGC 6534 and MCG 11-22-015, are selected to be studied. The ionized gas is excited by young stars in the ring of NGC 6534 and partly by shocks -- in MCG 11-22-015. The oxygen abundances in the HII regions of the rings are close to solar. We estimate the star formation rates (SFR) in the two outer rings of the galaxies by using several SFR indicators derived from narrow-band photometry in the H-alpha emission line and archival GALEX ultraviolet images of the galaxies. The derived SFRs allow to qualitatively restore star formation histories (SFH) in the rings: in NGC 6534 the SFH is flat during the last 100-200 Myr, and in MCG 11-22-015 the star formation has started only a few Myr ago. We suggest that the rings in NGC 6534 and MCG 11-22-015 have different natures: the former is a resonant one supplied with gas perhaps through tidal effects, and the latter has been produced by a satellite accretion. Recent outer gas accretion is implied in both cases.
Many galaxies contain magnetic fields supported by galactic dynamo action. However, nothing definitive is known about magnetic fields in ring galaxies. Here we investigate large-scale magnetic fields in a previously unexplored context, namely ring ga
laxies, and concentrate our efforts on the structures that appear most promising for galactic dynamo action, i.e. outer star-forming rings in visually unbarred galaxies. We use tested methods for modelling $alpha-Omega$ galactic dynamos, taking into account the available observational information concerning ionized interstellar matter in ring galaxies. Our main result is that dynamo drivers in ring galaxies are strong enough to excite large-scale magnetic fields in the ring galaxies studied. The variety of dynamo driven magnetic configurations in ring galaxies obtained in our modelling is much richer than that found in classical spiral galaxies. In particular, various long-lived transients are possible. An especially interesting case is that of NGC 4513 where the ring counter-rotates with respect to the disc. Strong shear in the region between the disc and the ring is associated with unusually strong dynamo drivers for the counter-rotators. The effect of the strong drivers is found to be unexpectedly moderate. With counter-rotation in the disc, a generic model shows that a steady mixed parity magnetic configuration, unknown for classical spiral galaxies, may be excited, although we do not specifically model NGC 4513. We deduce that ring galaxies constitute a morphological class of galaxies in which identification of large-scale magnetic fields from observations of polarized radio emission, as well as dynamo modelling, may be possible. Such studies have the potential to throw additional light on the physical nature of rings, their lifetimes and evolution.
