The Galaxy Evolution Explorer UV emission in shell galaxies: tracing galaxy ‘rejuvenation’ episodes

Abstract

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.

Quenching or Bursting: Star Formation Acceleration—A New Methodology for Tracing Galaxy Evolution

“…Nevertheless, those papers assume a simplistic model of star formation histories in which galaxies move single-handedly from blue to red sequence with exponentially declining star formation rates. We do know, however, that some galaxies of intermediate color are actually bursting, getting temporarily bluer perhaps due to a sudden inflow of gas and subsequent star formation episode (e.g., Rampazzo et al 2007;Thilker et al 2010;Thomas et al 2010;Fang et al 2012;Salim et al 2012).…”

Galaxy evolution in Local Group analogs

“…At odds with cluster counterparts, ETGs in LDEs frequently show significant signatures of relatively recent activity as indicated by the presence of distorted morphologies (Reduzzi et al 1996;Colbert et al 2001) and of kinematical sub-components (Emsellem et al 2004, and reference therein). The analysis of line-strength indices (Trager et al 2000;Longhetti et al 2000;Kuntschner 2000;Thomas et al 2003;Denicoló et al 2005;Annibali et al 2007;Rampazzo et al 2007) shows that ETGs in LDE have a large dispersion in their luminosity weighted age, from a Hubble time to ≈1 Gyr. About 40% of ETGs in the Annibali et al (2007) sample have a luminosity weighted age ≤ 6 Gyr, and 10% ≤ 3 Gyr.…”

Diffuse X-Ray-emitting Gas in Major Mergers

“…It was identified as a merger remnant by Hau, Carter, & Balcells (1999), who noted a kinematically distinct core. The UV morphology was studied by Rampazzo et al (2007). For the current study, we assume NGC 2865 is a stage 7 major merger, although there is some uncertainty as to the mechanism that produces shell galaxies.…”