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

Abstract

We present results of long-slit and panoramic spectroscopy of extended gaseous disks in 18 nearby S0 galaxies, mostly in groups. The gas in our S0s is found to be often accreted from outside that is implied by its decoupled kinematics: at least 5 galaxies demonstrate strongly inclined large-scale ionized-gas disks smoothly coupled with their outer HI disks, 7 galaxies reveal circumnuclear polar ionized-gas disks, and in NGC 2551 the ionized gas though confined to the main galactic plane however counterrotates the stellar component. The ionized-gas excitation analysis reveals the gas ionization by young stars in 12 of 18 S0 galaxies studied here; the current star formation in these galaxies is confined to the ring-like zones coinciding with the UV-rings. The gas oxygen abundance estimates in the rings are closely concentrated around the value of 0.7 Z and do not correlate either with the ring radius nor with the metallicity of the underlying stellar population. By applying the tilted-ring analysis to the 2D velocity fields of the ionized gas, we have traced the orientation of the gas rotation-plane lines of nodes along the radius. We have found that current star formation proceeds usually just where the gas lies strictly in the stellar disk planes and rotates there circularly; the sense of the gas rotation does not matter (the counterrotating gas in NGC 2551 forms stars currently). In the galaxies without signs of current star formation the extended gaseous disks are either in steady-state quasi-polar orientation (NGC 2655, NGC 2787, or are acquired recently through the highly inclined external filaments provoking probably shock-like excitation (NGC 4026, NGC 7280). Our data imply crucial difference of the external-gas accretion regime in S0s with respect to spiral galaxies: the geometry of the gas accretion in S0s is typically off-plane.

Star formation in outer rings of S0 galaxies

“…In principle, it can be accretion of primeval gas from a cosmological filament because the current ratio of stellar-to-gas mass in the ring, which is approximately 8:1, places the chemical evolution of the ring of NGC 4513 in the "gas-depleted" stage (Zahid et al 2014;Ascasibar et al 2015), meaning that the current metallicity of the ionised gas must correspond to the saturation level, which is close to the solar value (Zahid et al 2014;Ascasibar et al 2015), independently of the initial gas metallicity. This may explain the proximity of the gas oxygen abundance in the ring of NGC 4513 to the common value found by us for a number of other outer rings in S0 galaxies (Sil'chenko et al 2019).…”

“…5. Comparison of the gas oxygen abundance in NGC 4513 to that of other outer rings of S0s (Proshina et al 2019;Sil'chenko et al 2019) reveals exactly the same metallicity: for a sample of a dozen gaseous outer rings in S0s, mostly with corotating kinematics (though in NGC 2551 the gas counter-rotates), we find [O/H] = −0.15 dex (Sil'chenko et al 2019).…”

“…This paper is the second in the series, which focuses on star formation A&A 634, A102 (2020) in the S0 rings; NGC 6534 and MCG 11-22-015, two galaxies with corotating detached outer rings, have previously been described by Sil'chenko et al (2018a). As we found in our previous study of the gas kinematics in the S0 rings (Sil'chenko et al 2019), the gas rotation in the plane coinciding with the stellar disc plane facilitates star formation. Indeed, in NGC 6534 and MCG 11-22-015 we measured SFRs of some 0.2 M per year, which appears relatively high for S0 galaxies.…”

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).…”

External gas accretion provides a fresh gas supply to the active S0 galaxy NGC 5077

“…Due to the dynamical configuration observed, with the gas significantly misaligned and in a polar configuration with respect to the major axis of the galaxy, the origin of the gas can be clearly identified as external, as also seen for several other galaxies in similar configurations (e.g. Sil'chenko et al 2019, Gnilka et al 2020, as secular processes are not able to explain large quantities of misaligned gas (Caldwell et al 1986, Kannappan & Fabricant 2001. The properties of NGC 5077 match rather well the predictions from numerical simulations by van de Voort et al (2015), where an early type galaxy can show a misaligned gas distribution caused by late-time gas accretion after a gas-rich major merger.…”