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Register a new userStar formation in outer rings of S0 galaxies. I. NGC 6534 and MCG 11-22-015
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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.
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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-luminosity 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 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 luminous S0 galaxy with a large ring, UGC 5936, is studied here. By applying long-slit spectroscopy along the major axis of UGC 5936, 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 line ratios diagnostic diagrams and having ensured that it is ionized mostly by young stars, we have determined the gas oxygen abundance by using popular strong-line methods. Also we have proved the spatial proximity of the south-eastern dwarf satellite to UGC 5936 and have measured its gas metallicity. The ionized gas of the ring is excited by young stars and has solar metallicity. Star formation in the ring is rather prolonged, and its intensity corresponds to the current HI content of UGC 5936 (to the Kennicutt-Schmidt relation). The whole morphology of the HI distribution implies current accretion of the cold gas from the satellite onto the outer disc of UGC 5936; due to the satellite location and rotation in the plane of the stellar disc of the host galaxy, the accretion is smooth and laminar providing the favorable condition for star formation ignition.
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