Star-forming Rings in Lenticular Galaxies: Origin of the Gas^∗

Abstract

Rings in S0s are enigmatic features which can however betray the evolutionary paths of particular galaxies. We have undertaken long-slit spectroscopy of five lenticular galaxies with UV-bright outer rings. The observations have been made with the Southern African Large Telescope (SALT) to reveal the kinematics, chemistry, and the ages of the stellar populations and the gas characteristics in the rings and surrounding disks. Four of the five rings are also bright in the Hα emission line, and the spectra of the gaseous rings extracted around the maxima of the Hα equivalent width reveal excitation by young stars betraying current star formation in the rings. The integrated level of this star formation is 0.1-0.2 M ⊙ per year, with the outstanding value of 1 M ⊙ per year in NGC 7808. The difference of chemical composition between the ionized gas of the rings which demonstrate nearly solar metallicity and the underlying stellar disks which are metal poor implies recent accretion of the gas and star formation ignition; the star formation history estimated by using different star formation indicators implies that the star formation rate decreases with e-folding time of less than 1 Gyr. In NGC 809 where the UV-ring is well visible but the Hα emission line excited by massive stars is absent, the star formation has already ceased.

Double-peak emission line galaxies in the SDSS catalogue

“…Molaeinezhad et al 2019). In Katkov et al (2014Katkov et al ( , 2015, Proshina et al (2019), andSil'chenko et al (2019), the authors discuss the origin of this counter-rotating gas and argue that S0s might accrete gas probably from filaments or from minor mergers. In parallel, numerous works have shown that S0s can be produced by galaxy mergers including major mergers (Bekki 1998;Querejeta et al 2015;Tapia et al 2017;Eliche-Moral et al 2018) as well as minor mergers (Bournaud et al 2005;Bekki & Couch 2011).…”

The Gas Kinematics, Excitation, and Chemistry, in Connection with Star Formation, in Lenticular Galaxies

“…It is a rather low-luminosity S0 galaxy for which no SDSS data were found. We have made our own gri photometry which results are to be presented elsewhere (Proshina et al 2019); in the same paper we analyze also the long-slit kinematics. Here we note only a well-sampled ionized-gas velocity field ( Fig.…”

“…At least, starforming regions demonstrate quite recognizable emission-line spectra; and the models allow to determine the chemical composition -mainly oxygen abundances -by using the flux ratios of strong emission lines just for the case of the gas excited by young stars. We have checked the gas excitation in our S0 galaxies by studying the emission lines in the long-slit spectra obtained with the SCORPIO/BTA; for NGC 774 and NGC 3106 we have used the public spectral datacubes from the CALIFA survey (Sánchez et al 2016), for NGC 2697 and NGC 4324 -additional spectral data obtained earlier at the 11m SALT of the SAAO with the long-slit spectrograph RSS (Proshina et al 2019) have been involved into our present analysis. Weak and rather homogeneously distributed along the slit emission lines related to the gaseous disks with strongly decoupled rotation -those in NGC 2655, NGC 2787, NGC 3414, and NGC 7280, -have all demonstrated shock-like excitation: in their spectra the emission line [NII]λ6583 is comparable or even stronger than the Hα everywhere along the slit (see 2D spectra obtained along major axes of the mentioned galaxies in Fig.…”

“…15 demonstrate a surprising homogeneity of the gas metallicities within the starforming rings: they are all concentrated around the slightly subsolar value, −0.15 dex, and do not correlate neither with the radius of the ring nor with the metallicity of the underlying stellar population estimated here from the Lick indices measured in our long-slit spectra and confronted to the evolutionary synthesis models from Thomas, Maraston, and Bender (2003). The metallicities of the stellar populations are either taken from our earlier works (Ilyina et al 2014;Proshina et al 2019), or calculated particularly for this analysis from the spectra obtained with the SCORPIO and SCORPIO-2; for the latter galaxies we show the index-index diagrams in Fig. 14. The details of the stellar population analysis are analogous to our earlier works and can be inspected in Ilyina et al (2014).…”

Possible formation of ring galaxies by torus-shaped magnetic wormholes

“…Such approach is developed, for example, by Wu and Jiang [8]. Also there are another models which may explain the origin of outer stellar rings by the accretion of a cold gas from outside of the galaxy, e.g., see discussions in [9,10] and references therein.…”