Catching evolution of galaxies in nearby groups

Catching evolution of galaxies in nearby groups

Groups of galaxies contain about 60% of the galaxies in the Local Universe. Furthermore, the fraction of star-forming galaxies in groups is midway between the field and clusters environments, i.e. potentially groups hide the key of the puzzle of the galaxy evolution. Some physical mechanisms would transform actively star forming, gas rich, spiral galaxies in the filed into cluster-like galaxies, i.e. gas poor, mostly passively evolving early-type (Es+S0s) galaxies. Groups, as a whole, are driven from an ``active" (e.g. star forming) phase, typical of field, to a more ``passive" phase, typical of clusters, during the formation/virialization of groups, through a quenching of the star formation and a morphological transformation.

 

What mechanisms drives and what are the phases of this transformation?

Since the velocity of individual galaxies within groups, generated by the gravitational attraction are much smaller than within clusters, galaxy-galaxy merging and interactions are more favoured in groups than in clusters. Mergers can transform spiral galaxies into ellipticals and may also quench future star formation by ejecting the interstellar medium via starburst, AGN or shock-driven winds. Simulations of groups evolution suggest that the quenching of star formation does not happen suddenly! Galaxies has already transformed into ellipticals before the star formation stops. This happen when the groups is nearly assembled and as a consequence of the quenching of gas accretion and stripping (to a lesser degree). The same fate happens to disk galaxies that, as they join the group, are turned into red-and-dead disks. Together with galaxy-galaxy interaction and merging, ``ram-pressure'' stripping seems to have a role in removing the hot gas from the halo around galaxies. This mechanism, called strangulation, could in a long period of time (> 1Gyr) to reduce also the cold gas replenishment up to a complete cutoff of the star.r

Understanding the co-evolution of groups and their members then requires a considerable observational effort, starting from the group definition, e.g. considering the presence of possible substructures, and a correct galaxy morphological (and kinematical) classification. Multi-wavelength surface photometric and 2D kinematical studies are necessary to investigate the effect of galaxy-galaxy and galaxy-ICM interaction i.e.  the galaxy transforming mechanisms.

Looking for variation in the star formation properties of galaxies we are pushed to use observations sensible even to small variation of such properties like the far UV. Nearby groups are the more suitable targets for a detailed study of the galaxies but we are forced to use wide field instruments since group members may cover sky areas of the order of few square degrees. In this context we are using Galaxy Evolution Explorer (GALEX) imaging combined with Digital Sky Survey (SDSS) images for a set of groups showing a different type of galaxy population. We are considering from loose group dominated by spirals up to to more compact configurations rich of elliptical and S0 galaxies. The Far UV photometry can map accurately the star formation properties of the galaxies in the groups. We show here images coming from recent studies of groups similar to the Local Group where is located our galaxy the Milky Way (Marino et al. 2010) and of USGC U268 and USGC U376 groups in the Leo cloud (Marino et al. 2013).

People: Roberto Rampazzo, Daniela Bettoni, Lucio Buson and Paola Mazzei

Collaborations: Antonietta Marino and Giuseppe Galletta (Univ. di Padova); Bianchi Luciana and David Thilker (JHU, Baltimore, USA)


Recent publications: Marino et al. (2013, MNRAS, 428, 476); Marino et al. (2011, ApJ, 736, 154); Rampazzo et al. (2011, Ap&SS, 335, 243)

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