Although thick stellar discs are detected in nearly all edge-on disc galaxies, their formation scenarios still remain a matter of debate. Due to observational difficulties, there is a lack of information about their stellar populations. Using the Russian 6m telescope BTA we collected deep spectra of thick discs in three edge-on S0-a disc galaxies located in different environments: NGC 4111 in a dense group, NGC 4710 in the Virgo cluster, and NGC 5422 in a sparse group. We see intermediate age (4−5 Gyr) metal rich ([Fe/H] ∼ −0.2 . . . 0.0 dex) stellar populations in NGC 4111 and NGC 4710. On the other hand, NGC 5422 does not harbour young stars, its disc is thick and old (10 Gyr), without evidence for a second component, and its α-element abundance suggests a 1.5 − 2 Gyr long formation epoch implying its formation at high redshift.Our results suggest the diversity of thick disc formation scenarios.
We present high resolution Hi 21cm Giant Meterwave Radio Telescope (GMRT) observations of the superthin galaxy FGC1540 with a spatial resolution of 10 × 8 and a spectral resolution of 1.73 kms −1 and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial Hi surface density profile by fitting a 3-dimensional tilted ring model directly to the Hi data cubes by using the publicly-available software, Fully Automated Tirrific (FAT). We also present the rotation curve of FGC1540 derived from its optical spectroscopy study using the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We use the rotation curve, the Hi surface density profile together with Spitzer 3.6 µm and the SDSS i-band data to construct the mass models for FGC1540. We find that both the Pseudo-isothermal (PIS), as well as Navarro-Frenk-White (NFW) dark matter (DM) halos, fit the observed rotation curve equally well. The PIS model indicates a compact dark matter halo (R C /R D < 2), with the best-fitting core radius (R C ) approximately half the exponential stellar disc scale length (R D ), which is in agreement with the mass models of superthin galaxies studied earlier in the literature. Since the vertical thickness of the galactic stellar disc is determined by a balance between the net gravitational field and the velocity dispersion in the vertical direction, the compact dark matter halo may be primarily responsible in regulating the superthin vertical structure of the stellar disc in FGC1540 as was found in case of the superthin galaxy UGC7321.
Aims. We revisit large spectroscopic data sets for field stars from the literature to derive the upper Li envelope in the high metallicity regime in our Galaxy. Methods. We take advantage of Gaia EDR3 data and state-of-the-art stellar models to precisely determine the position of the sample dwarf stars in the Hertzsprung-Russell diagram. Results. The highest Li abundances are found in field metal-rich warm dwarfs from the GALAH survey, located on the hot side of the Li-dip. Their mean Li value agrees with what was recently derived for warm dwarfs in metal-rich clusters, pointing towards a continuous increase of Li up to super-solar metallicity. However, if only cool dwarfs are considered in GALAH, as done in the other literature surveys, it is found that the upper Li envelope decreases at super-solar metallicities, blurring the actual Li evolution picture. We confirm the suggestion that field and open cluster surveys that found opposite Li behaviour in the high metallicity regime do not sample the same types of stars: The first ones, with the exception of GALAH, miss warm dwarfs that can potentially preserve their original Li content. Conclusions. Although we can discard the bending of the Li upper envelope at high metallicity derived from the analysis of cool star samples, we still need to evaluate the effects of atomic diffusion on warm, metal-rich early-F and late-A type dwarfs before deriving the actual Li abundance at high metallicity.
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