The eventful life of galaxies in low density environments

Figure 1:Top: X-ray isointensity contours of RR 143 (NGC 2305/2307: left) and RR 242 (NGC5090/5091: right)
observed with XMM-Newton superimposed to WFI R-band images. The emission is centered on the bright E member.
Bottom: Surface brightness profile of the X-ray emission (adapted from Grutzbauch et al. 2007).

 Interaction, accretion and merging events along the Hubble time play a key role galaxies. Several studies suggest that this events are effective not only in clusters but also in the poor galaxy aggregates of low density environments (LDE). If typical galaxy aggregates in LDE, such as groups, today contain a substantial fraction of the mass of the Universe, the local environment strongly affects the evolution of most galaxies. In this context very poor galaxy aggregates and nearly isolated galaxies, in particular early-type galaxies (ETGs), deserve a special study. The former include the simplest systems where the effect of the ongoing interaction on the galaxy evolution can be directly tested, while ETGs are widely considered the fossil evidence of galaxy evolution.We adopt the so called archeological approach, i.e. the subject of our investigations are nearby objects, where detailed, multi-wavelength studies of single galaxies, of the groups structure and of the inter-galactic medium (IGM) can be successfully performed. We investigate several very poor galaxy systems, often identified as pairs, containing ETGs. We combine the analysis of the optical photometric and kinematic properties of the bright member galaxies including the study of their hot [Figure 1], warm and cold gas phases with the search for a faint galaxy population possibly associated to the system.

Figure 2:GALEX (FUV - NUV) colour vs. Hβ within an aperture of re/8 radius. In this plane, the aging path of a galaxy stellar population is indicated by the arrow. The position of a 10 Gyr old galaxy with solar chemical composition is indicated by the large star located in the top left part of the plot. A burst of star formation, superposed on such an old stellar component, produces the closed paths traced in the plot by dotted, long-dashed, short-dashed and solid lines according to the percentage of the mass involved in the burst and of the different metallicity in the newly born stars. The paths shown consider the percentages of 10% and 20% of mass engaged in star formation, with a solar and twice solar metallicities. The time elapsed from the beginning of the burst is indicated along the solid line, tracing the path of a burst involving 20% of the mass with twice solar chemical abundance. In this context, the time elapsed from the burst of star formation in NGC 7135 appears shorter than that of NGC 2865 and in NGC 1553 (adapted from Rampazzo et al. 2007).

At odds with their optical, kinematics, and environmental similarities, the ETGs in our poor systems have remarkably different X-ray properties. Our studies suggest that X-ray luminous poor systems are probably already evolved groups. Although only a few luminous members are associated with these systems, they seems to posses a significant population of faint members. In X-ray faint systems, ETGs are likely the dominant members of evolving groups. The cold gas rich Arp 227 system is a prototypical example of this class of poor groups. We suggest that the different X-ray properties are explained by either the present evolutionary phase of the system, or the past history of the ETGs. Recent optical studies suggest that, on the average, field ETGs are younger than their cluster counterparts. This is likely to be a consequence of accretion/merging episodes and their signature in a younger stellar population. We address nearly isolated ETGs with fine structure performing a pilot far UV analysis of shell galaxies. We investigate how the GALEX (FUV-NUV) color provides information about rejuvenation phenomena in their stellar population. To this aim, we derive from theory the relationship between Mg2, Hβ, HγA, HδA Lick line-strength indices and the (FUV-NUV) color. Considering composite stellar population models with a recent burst of star formation, we suggest that shell galaxies in the (FUV-NUV) - Hβ plane could be explained by a rejuvenation episode. Future work will continue to address the analysis of stellar populations of ETGs in LDEs showing emission lines in their spectra. We are analysing both IRS-SPITZER (PI. R. Rampazzo Cycle 3) and GALEX (P.I. R. Rampazzo Cycle 3) data to enlarge the ETGs Spectral Energy Distribution to the Mid Infrared and the Far UV. We aim at breaking the age-metallicity degeneracy affecting the optical band, to obtain an accurate estimate of the star formation as well as an estimate of the mass involved in the of star formation burst. Furthermore, the combined use of our sample in LDE and the successful IRS observations of cluster galaxies allow us to explore the ETGs evolution across a large range of local galaxy density. This will quantify the role of the environment in the ETGs star formation history.

People: R. Rampazzo, D. Bettoni, A. Bressan, L.M. Buson, M. Clemens, E.V. Held
Collaboration: A. Buzzoni (INAF OA Bologna), P. Focardi (Bologna Univ.), C. Chiosi, G. Galletta, R. Tantalo (Padova Univ.), M. Longhetti, G. Trinchieri (INAF OA Brera), G. Granato, L. Silva (INAF OA Trieste), L. Danese (SISSA), P. Alexander (Cavendish Lab., Cambridge, UK), F. Annibali (StScI, Baltimore USA), L. Bianchi, A. Marino (JHU, Baltimore, USA), P. Amram, J. Boulesteix, M. Marcelin (OAstron., Marseille, France), C. Carignan (Univ. de Montreal, Canada), R. Gruetzbauch, W.W. Zeilinger (IfA, Univ. Wien, Austria), B. Nikolic (NRAO, Charlottesville, USA), P. Panuzzo (CEA, Saclay, France), H. Plana (Univ. Estadual de S. Cruz, BA, Brazil), R.M. Rich (UCLA, Los Angeles, USA), L. Rizzi (Univ. Hawaii, USA), M. Rosado (UNAM, Mexico DF, Mexico), K. Sheth (Caltech, Pasadena, USA), J.W. Sulentic (Univ. Alabama, Tuscallosa, USA), J.R. Valdes, O. Vega (INAOE, Puebla, Mexico)
Recent Publications: Trinchieri et al. (2008), A&A, 489,85; Grützbauch et al. (2007), AJ, 133,220; Rampazzo et al. (2007), MNRAS 381,245
Link: More information about this research