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A Complete CEnsus of Star-formation and nuclear activity in the Shapley super-cluster

Final Report Summary - ACCESS (A Complete CEnsus of Star-formation and nuclear activity in the Shapley super-cluster)

Project context and objectives

The environment that a galaxy inhabits has a profound impact upon its properties and evolution. Strong support to this idea is provided by the morphology-density relation and the star formation-density relation, according to which non star-forming and gas-poor 'early-type' galaxies dominate the densest environments (namely the cores of galaxy clusters), while gas-rich, actively star-forming spirals are predominantly found in isolated, less dense environments or in the filaments connecting the clusters. This suggests that galaxies lose their gas, and subsequently have their star formation inhibited, by some mechanism related to the environment when they pass from the loose environments into the clusters.

The aim of the project is to study the transformations of galaxies caused by their interaction with the environment.

The Shapley supercluster core (SSC) is the ideal target for this study because it resides at the centre of the most massive cosmic structure in the nearby universe. It is also the most dynamically active, with its clusters currently in the process of merging. This makes it possible to also study the transformations of galaxies in relation to the assembly process of cosmic structures.

ACCESS is a collaborative project carried out by INAF-OAC (Italy), the Australian National University, and the universities of Birmingham and Durham (United Kingdom). Two other institutions joined the project: the Monash University of Melbourne (Australia) and the University of Innsbruck (Austria).

Work performed

The first step to studying the mechanisms responsible for quenching the star formation (SF) is to conduct a complete census of SF in all the environments. Since most of the radiation produced by SF is absorbed by the dust within and surrounding the star-forming regions and is re-emitted in the infra-red, we obtained far-infra-red (24 and 70 microns) images of the SSC with the Spitzer satellite. Some of the radiation also escapes in the ultraviolet (UV), as soon as the newly formed stars move away from the dust clouds. To measure this radiation we observed the SSC with the GALEX UV satellite.

Over the entire SSC region, we determined a total SF rate of over 300 solar masses per year, of which as much as 80 % is only observable in the far-infra-red.

The next step was to search for galaxies whose SF is being affected by the environment. We analysed the ratio of the far-infra-red emission at 24 microns to the near-infra-red (2.2 microns) (f_24/f_K), which is a measure of the so-called 'specific SF rate': the rate of SF normalised to the mass of the galaxy. By plotting this flux ratio as a function of the far-infra-red flux, it became apparent that there is a class of 'transitional' galaxies, with properties between the star-forming and the non-star-forming ('passive') galaxies. We interpret this class of 'transitional' galaxies as those in the process of having their SF quenched by the interaction of the environment.

Our infra-red data allowed us to identify a family of galaxies with exceptionally 'cold' infra-red emission, whose emission at 70 microns exceeds that at 24 microns by more than a factor of 25. Such an excess cannot be explained by current theoretical models, and cannot be caused by star formation activity.

The 'infra-red cold' galaxies all have an S0 morphology, without signs of spiral arms. They are massive and with very little, if any, star formation. The fact that these galaxies reside preferentially in dense environments (near the centres of the clusters) shows that they are affected by the processes responsible for the quenching of star formation in the cluster environment.

A sub-sample of galaxies were targeted with integral-field spectroscopy with the Wide Field Spectrograph (WiFeS) attached to the 2.3 m telescope of the Australian National University at Siding Spring, Australia.

WiFeS is an integral-field spectrograph, and is able to take 1900 spectra simultaneously, covering a whole galaxy at the distance of the Shapley supercluster in one shot. This instrument allowed us to get a wealth of information on the physical state of the gas being affected by the environment.

One main mechanism quenching star formation in galaxies is 'ram-pressure stripping'. Clusters of galaxies are filled with hot, tenuous gas, called the intra-cluster medium. When a galaxy enters a cluster coming from the outer regions, the intra-cluster medium acts like a wind that exerts a pressure on the interstellar matter of the galaxy. Depending on a number of physical conditions, this pressure may sweep away part or all of the interstellar matter. This process is called ram-pressure stripping. Since new stars in galaxies form from the collapsing clouds of this interstellar material, ram-pressure stripping may therefore stop the star formation.

The data collected with the WiFeS allowed us to analyse the whole phenomenology of ram-pressure stripping in an early phase, in which we observed the gas being swept out of the galaxy disc.

To understand this complex phenomenology, we took advantage of numerical simulations based on theoretical models, which are crucial to constrain the geometry and physics behind the observations.

Although ram-pressure stripping is a primary mechanism for the suppression of star formation, it might induce enhanced star formation in an initial phase due to the compression of the gas on the leading edge of the galaxy. This phenomenon had been predicted for a long time but had never been observed. For the first time, the WiFeS data allowed us to relate an episode of intense star formation to the onset of ram-pressure stripping, thus adding a crucial piece of information to this complex mechanism.

The ACCESS project was initially based on deep optical photometry (B and R wavebands) in the inner 2.5 square degrees of the Shapley supercluster core, which was collected from the European Southern Observatory's (ESO) 2.2 m Wide Field Imager archive. Near-infra-red imaging of the same region was collected with the United Kingdom’s Infra-red Telescope and ~450 spectra from the AAOmega spectrograph at the Anglo-Australian telescope. We then obtained ultra-violet and infra-red data with space-based instruments (GALEX and Spitzer).

During the second half of the project, we started a new deep photometric survey with the ESO VLT Survey Telescope (VST), named VST-ACCESS. With this survey, which started in January 2012, we have been collecting high-quality imaging of 23 square degrees in the centre of the Shapley supercluster in four optical wavebands (u, g, r and i; wavelength range 0.35-1 micron). This data, thanks to the superior optical imaging of the instrument and the observing site, will allow us to obtain morphological classification of galaxies and consequently the ability to identify those galaxies that are undergoing transformations and, for this reason, present disturbed morphologies.

Due to a successful proposal submitted to the European OPTICON consortium, we will obtain ~3000 new redshifts at the AAOmega spectrograph, which will allow us to define the 3-D positions of the galaxies belonging to the supercluster.

These spectroscopic redshifts, together with the data from the VST-ACCESS survey and the infra-red data available from the (space-based) WISE survey, will in turn allow us to obtain photometric redshifts for a further ~200 000 galaxies.

For a selected sub-sample of 'transforming' galaxies we will continue to collect data with WiFeS. This data produces detailed information on the physics of the gas involved in the transformation of the galaxies, allowing us to obtain deep insights into the involved phenomena.

More information on the ACCESS project and its more recent results can be found on the project website: http://www.oacn.inaf.it/ACCESS/