Ruth Grutzbauch

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

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. n n n nThe 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. n n n nThe three shell galaxies are members of poor groups of galaxies. We compare GALEX NUV observations with available Hxa0i 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. n n n nWe 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.

Small-Scale Systems of Galaxies. III. X-Ray-detected Elliptical+Spiral Galaxy Pairs in Low-Density Environments

We present a comprehensive study of the local environments of four elliptical+spiral (E+S) galaxy pairs with the main goal of investigating their formation/evolution histories. We obtained new XMM-Newton data for two pairs (RR 143 and RR 242) that complement existing ROSAT data for the other two (RR 210 and RR 216). The new observations reveal diffuse X-ray emission in both pairs with luminosities of LX ~ 3 × 1041 ergs s-1 (0.5-2.0 keV). The emission is asymmetric in both cases and extends out to ~500 (120 kpc) and 700 (160 kpc) in RR 143 and RR 242, respectively. The nucleus of RR 242 hosts a low-luminosity (LX ~ 1041 ergs s-1 [2.0-10 keV]) mildly absorbed (NH ~ 1022 cm-2) active galactic nucleus. We find that the early-type components of pairs with diffuse hot gas appear to be relaxed objects, while those in RR 210 and RR 216, where no diffuse emission has been found, display unambiguous signatures of ongoing interaction. New wide-field V- and R-band observations are used to study the photometric properties of the early-type components and to search for candidate faint galaxy populations around each of the pairs. While no diffuse optical light is found for any of the pairs, all of the early-type members show very extended and concentric luminous envelopes. We identify a faint galaxy sample in each field, and we consider whether they could be physically associated with the luminous pairs based on (V - R) colors and photometric properties. We find that the distribution of re and MR for the candidates are similar in three of the fields (RR 143, 216, and 242). The same selection criteria applied to the field of RR 210 suggest a fainter and more compact population, possibly suggesting a larger background fraction than in the other fields.

Optical properties of the NGC 5328 group of galaxies

We present the results of a photometric and spectroscopic study of seven members of the NGC 5328 group of galaxies, a chain of galaxies spanning over 200 kpc (H 0 = 70 km s -1 Mpc -1 ). We analyse the galaxy structure and study the emission-line properties of the group members looking for signatures of star formation and active galactic nucleus activity. We finally attempt to infer, from the modelling of line-strength indices, the stellar population ages of the early-type members. We investigate also the presence of a dwarf galaxy population associated with the bright members. The group is composed of a large fraction of early-type galaxies including NGC 5328 and 5330, two bona fide ellipticals at the centre of the group. In both galaxies no recent star formation episodes are detected by the Hβ versus MgFe line-strength indices of these galaxies. 2MASX J 13524838-2829584 has extremely boxy isophotes which are believed to be connected to a merging event: line-strength indices suggest that this object probably had a recent star formation episode. A warped disc component emerges from the model subtracted image of 2MASX J13530016-2827061, which is interpreted as a signature of an ongoing interaction with the rest of the group. Ongoing star formation and nuclear activity are present in the projected outskirts of the group. The two early-type galaxies 2MASX J13523852-2830444 and 2MASX J13525393-2831421 show spectral signatures of star formation, while a Seyfert 2 type nuclear activity is detected in MCG-5-33-29.

Small-Scale Systems of Galaxies. II. Properties of the NGC 4756 Group of Galaxies

This paper is part of a series that focuses on investigating galaxy formation and evolution in small-scale systems of galaxiesinlow-densityenvironments.WepresentresultsfromastudyoftheNGC4756group,whichisdominatedby the elliptical galaxy NGC 4756. The characteristics of the group are investigated through (1) the detailed investigation of the morphological, photometric, and spectroscopic properties of nine galaxies among the dominant members of thegroup; (2) thedetermination ofthe photometric parameters of thefaint galaxy population inanarea of34 0 ; 34 0 centered on NGC 4756; and (3) an analysis of the X-ray emission in the area based on archival data. The nine member galaxies are located in the core part of the NGC 4756 group (a strip � 300 kpc in diameter, H0 ¼ 70 km s � 1 Mpc � 1 ), which has a very loose configuration. The central part of the NGC 4756 group contains a significant fraction of early-type galaxies. Three new group members with previously unknown systemic velocities are identified, one of which is type dE. At about 7A5 southwest of NGC 4756 a substructure of the group is detected, including IC 829, MCG � 2-33-35, MCG � 2-33-36, and MCG � 2-33-38, that meets the Hickson criteria for being a compact group. Most of the galaxies in this substructure show interaction signatures. We do not detect apparent fine structureandsignaturesofrecentinteractioneventsintheearly-typegalaxypopulation,withtheexceptionofastrong dust lane in the elliptical galaxy MCG � 2-33-38. However, this galaxy displays signatures of nuclear activity. Strong

XMM-Newton X-ray and optical monitor far UV observations of NGC 7070A and ESO 2400100 shell galaxies

Context. Shell galaxies are considered to be the debris of recent accretion/merging episodes. Their high frequency in low density environments suggests that these episodes could drive the secular evolution of at least some fraction of the early-type galaxy population. Aims. We present XMM-Newton X-ray observations of two shell galaxies, NGC 7070A and ESO 2400100, and far UV observations obtained with the optical monitor for these and for an additional shell galaxy, NGC 474, for which we also have near and far UV data from GALEX. We attempt to gain insight into the overall evolution as traced by their star formation history and hot gas content. Methods. The X-ray and far UV data are used to derive their X-ray spatial and spectral characteristics and their UV luminosity profiles. We use ad hoc models developed to investigate the age of the most recent episode of star formation from the (UV – optical) colors and line strength indices. Results. The X-ray spatial and spectral analysis show significant differences in the two objects. A low-luminosity nuclear source is the dominant component in NGC 7070A (log LX = 41. 7e rg s −1 in the 2–10 keV band). In ESO 2400100, the X-ray emission is due to a low temperature plasma with a contribution from the collective emission of individual sources. In the optical monitor image ESO 2400100 shows a double nucleus, one bluer than the other. This is probably due to a very recent star formation event in the northern nuclear region. The extension of the UV emission is consistent with the optical extent of all galaxies, at different degrees of significance in different filters. The presence of the double nucleus, corroborated by the (UV – optical) colors and line strength indices analysis, suggests that ESO 2400100 is accreting a faint companion. We explore the evolution in the X-ray luminosity during accretion processes with time. We discuss the link between the presence of gas and age, since gas is detected either before coalescence or several Gyr (>3) after.