M dwarfs are the most numerous stars in the Galaxy. They are characterized by strong magnetic activity. The ensuing high-energy emission is crucial for the evolution of their planets and the eventual presence of life on them. We systematically study the X-ray and ultraviolet emission of a subsample of M dwarfs from a recent proper-motion survey, selecting all M dwarfs within 10 pc to obtain a nearly volume-limited sample (∼ 90 % completeness). Archival ROSAT, XMM-Newton and GALEX data are combined with published spectroscopic studies of Hα emission and rotation to obtain a broad picture of stellar activity on M dwarfs. We make use of synthetic model spectra to determine the relative contributions of photospheric and chromospheric emission to the ultraviolet flux. We also analyse the same diagnostics for a comparison sample of young M dwarfs in the TW Hya association (∼ 10 Myrs). We find that generally the emission in the GALEX bands is dominated by the chromosphere but the photospheric component is not negligible in early-M field dwarfs. The surface fluxes for the Hα, near-ultraviolet, far-ultraviolet and X-ray emission are connected via a power law dependence. We present here for the first time such flux-flux relations involving broad-band ultraviolet emission for M dwarfs. Activity indices are defined as flux ratio between the activity diagnostic and the bolometric flux of the star in analogy to the Ca II R ′ HK index. For given spectral type these indices display a spread of 2 − 3 dex which is largest for M4 stars. Strikingly, at mid-M spectral types the spread of rotation rates is also at its highest level. The mean activity index for fast rotators, likely representing the saturation level, decreases from X-rays over the FUV to the NUV band and Hα, i.e. the fractional radiation output increases with atmospheric height. The comparison to the ultraviolet and X-ray properties of TW Hya members shows a drop of nearly three orders of magnitude for the luminosity in these bands between ∼ 10 Myr and few Gyrs age. A few young field dwarfs (< 1 Gyr) in the 10 pc sample bridge the gap indicating that the drop in magnetic activity with age is a continuous process. The slope of the age decay is steeper for the X-ray than for the UV luminosity.

We present the Galaxy Evolution Explorer (GALEX) far‐ultraviolet (FUV) and near‐ultraviolet (NUV) imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135 located in low‐density environments. The system of shells and fine structures visible in the optical is detected in the NUV image of NGC 2865 and in both NUV and FUV images of NGC 7135. The NUV image of NGC 5018 does not present shell structures. We detect absorption features in the nuclear region of all three galaxies. NGC 2865 has a nearly flat colour profile with (FUV−NUV) ≈ 2 throughout the whole galaxy. NGC 7135 is blue in the centre (FUV−NUV) ≈ 0 and as red as (FUV−NUV) ≈ 1.5 in the outskirts, including the faint shell‐like feature. The three shell galaxies are members of poor groups of galaxies. We compare GALEX NUV observations with available H i large‐scale measurements, and determine the UV magnitudes of likely companions. Most of the known (and possible) companions are gas‐rich late‐type galaxies, suggesting that our shell galaxies inhabit the ideal environment for hosting rejuvenating episodes driven by accretion events. We investigate the ability of the nuclearGALEX (FUV−NUV) colour to provide information about rejuvenation phenomena in the stellar populations of the shell galaxies. To this aim, we derive from theory the relationship between the Mg2, Hβ, HγA, HδA Lick line‐strength indices and the (FUV−NUV) colour. We extend the study to a sample of early‐type galaxies in low‐density environments which includes shell galaxies and/or galaxies with emission lines in their optical spectra. In the index versus (FUV−NUV) colour diagrams, most of the galaxies are well explained by passively evolving single stellar populations. On the average, ages and metallicities of the galaxies in our sample estimated from optical line‐strength indices are consistent with those inferred from the (FUV−NUV) colour. We note that the GALEX (FUV−NUV) and (UV−V) colours have different response to age and metallicity. In general, all the colours but for (FUV−NUV) and (FUV−V), become nearly age‐insensitive when 1–2 Gyr have elapsed from the last star‐forming event. Finally, considering composite stellar population models with a recent burst of star formation, we suggest that the positions of the NGC 7135 and NGC 2865 nuclei in the (FUV−NUV)–Hβ plane could be explained in terms of a recent rejuvenation episode.

Context. Understanding the astrophysical processes acting within galaxy groups and their effects on the evolution of the galaxy population is one of the crucial topics of modern cosmology, as almost 60% of galaxies in the Local Universe are found in groups. Aims. We aim at learning about galaxy evolution within nearby groups dominated by late-type galaxies, specifically by studying their ultraviolet-emitting stellar population. Methods. We imaged in the far (FUV, λ eff = 1539 Å) and near ultraviolet (NUV, λ eff = 2316 Å) with the Galaxy Evolution Explorer (GALEX) three nearby groups, namely LGG 93, LGG 127 and LGG 225. We obtained the UV galaxy surface photometry and, for LGG 225, the only group covered by the SDSS, the photometry in u, g, r, i, z bands. We discuss galaxy morphologies looking for interaction signatures and we analyze the spectral energy distribution of galaxies to infer their luminosity-weighted ages. The UV and optical photometry was also used to perform a luminosity-weighted kinematical and dynamical analysis of each group and to evaluate the stellar mass. Results. A few member galaxies in LGG 225 show a distorted UV morphology due to ongoing interactions. (FUV − NUV) colors suggest that spirals in LGG 93 and LGG 225 host stellar populations in their outskirts younger than that of M 31 and M 33 in the Local Group or with less extinction. The irregular interacting galaxy NGC 3447A has a significantly younger stellar population (a few Myr old) than the average of the other irregular galaxies in LGG 225 suggesting that the encounter triggered star formation. The early-type members of LGG 225, NGC 3457 and NGC 3522, have masses of the order of a few 10 9 M , comparable to the Local Group ellipticals. For the most massive spiral in LGG 225, we estimate a stellar mass of ≈4 × 10 10 M , comparable to M 33 in the Local Group. Ages of stellar populations range from a few to ≈7 Gyr for the galaxies in LGG 225. The kinematical and dynamical analysis indicates that LGG 127 and LGG 225 are in a pre-virial collapse phase, i.e. still undergoing dynamical relaxation, while LGG 93 is likely virialized. Both the photometric and the dynamical analyses suggest that LGG 225 is in a more active evolution phase than LGG 93 and LGG 127.

We investigate the effects of minor mergers between an S0 galaxy and a gas-rich satellite galaxy, by means of N-body/smoothed particle hydrodynamics simulations. The satellite galaxy is initially on a nearly parabolic orbit and undergoes several periapsis passages before being completely stripped. In most simulations, a portion of the stripped gas forms a warm dense gas ring in the S0 galaxy, with a radius of ∼6-13 kpc and a mass of ∼10 7 M . The ring is generally short-lived ( 3 Gyr) if it forms from prograde encounters, while it can live for more than 6 Gyr if it is born from counter-rotating or non-coplanar interactions. The gas ring keeps memory of the initial orbit of the satellite galaxy: it is corotating (counter-rotating) with the stars of the disc of the S0 galaxy, if it originates from prograde (retrograde) satellite orbits. Furthermore, the ring is coplanar with the disc of the S0 galaxy only if the satellite's orbit was coplanar, while it lies on a plane that is inclined with respect to the disc of the S0 galaxy by the same inclination angle as the orbital plane of the satellite galaxy. The fact that we form polar rings as long-lived and as massive as co-planar rings suggests that rings can form in S0 galaxies even without strong bar resonances. Star formation up to 0.01 M yr −1 occurs for >6 Gyr in the central parts of the S0 galaxy as a consequence of the interaction. We discuss the implications of our simulations for the rejuvenation of S0 galaxies in the local Universe.

We present GALEX far-ultraviolet (FUV, λ eff = 1538 Å) and near-ultraviolet (NUV, λ eff = 2316 Å) surface photometry of 40 early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs showing emission lines in their optical spectra. We derive FUV and NUV surface brightness profiles, (FUV−NUV) colour profiles and D 25 integrated magnitudes. We extend the photometric study to the optical r band from SDSS imaging for 14 of these ETGs. In general, the (FUV−NUV) radial colour profiles become redder with galactocentric distance in both rejuvenated (≤4 Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC 3489 and IC 5063 show rings and/or arm-like structures, bluer than the body of the galaxy, suggesting the presence of recent star formation. Although seven of our ETGs show shell systems in their optical image, only NGC 7135 displays shells in the UV bands. We characterize the UV and optical surface brightness profiles, along the major axis, using a Sersic law. The Sersic law exponent, n, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values of n (≤5). The Sersic law exponent n = 4 seems to be a watershed: ETGs with n > 4 tend to have [α/Fe] greater than 0.15, implying a short star-formation time-scale. We find a significant correlation between the FUV−NUV colour and central velocity dispersions σ , with the UV colours getting bluer at larger σ . This trend is likely driven by a combined effect of 'downsizing' and of the mass-metallicity relation.