The Impact of Surveys

Abstract

Since the most ancient times astronomers felt the need to collect and list in atlases and catalogs all the visible objects in the sky the first stellar catalog known in the western world being the one of Hipparchus (II century BC). We have to wait until Charles Messier at the end of the XVIII century to have the first incidental catalog of nebulae, i.e. including a mixture of fuzzy objects, nebulæ, that telescopes of the epoch could detect. In Chapter 1 we have already discussed the atlases and catalogs that soon after the discovery of galaxies appeared in the literature describing the properties of the nearby galaxies, in particular their morphologies in Chapter 3. The subject of extragalactic papers, during the photographic plate era, were one or few galaxies, whose properties were carefully scrutinized looking at all details resolved by telescopes in the optical waveband. This happens also today, of course, basically at all wavelengths. However, the impact of new technologies permits in the last two decades to tackle survey programs addressed to the study of specific extragalactic problems considering millions of galaxies. These systematic investigations of enormous galaxy samples requires dedicated observing, reduction and storing/retrieval facilities provided, in the Big Science era, by international consortia. Data are treated in a statistically way, combining the multi-wavelength information coming from several instruments, scanning galaxy properties as a function of the redshift and of the environment. The big teams of scientists and engineers that has planned and built the instrumental facilities, including sometimes the telescopes themselves, are also charged of providing a nearly immediate access to the data, as well as their maintenance and calibration. The data are easy available through simple queries on the web. Usually one of the main step of these large teams is to build a catalog of the detected sources, providing the first measures typically obtained from automatic softwares of analysis. These data are then cross-correlated with other data available for the same sources in the web databases, possibly refined with better calibration. Relations between entries are evaluated and discussed at the light of theoretical models and simulations by members of the same team. Observations have the strong tendency to produce facts so models are refined and data-set revisited and/or integrated with new observations. This Chapter is dedicated to galaxy surveys. It is even difficult to offer a complete overview of the enormous number of galaxy surveys today available and/or ongoing. We interviewed scientists asking them to present the most significant surveys, in their view, in their area of scientific interest and emphasized the main results obtained by the surveys that have seen their active collaboration. Before entering the core of such discussion George Paturel will provide us in Section 5.2 a much clear idea of the efforts that are necessary to build a good galaxy catalogue and to homogenize the entries in a database. The information about the single galaxies are indeed so wide that a new concept of data managing has been developed: the Virtual Observatory (see Section 9.9). Since the beginning of the astronomical use of photographic techniques, big plates archives are preserved basically in all observatories. Thanks to their large field of view and easy storability, astronomers often re-used this patrimony over the time, e.g. to follow time variable phenomena or to inspect several different objects in the field of view of the plates. Along this line Virtual Observatories have been conceived for providing an easy retrieval of the multiple types of data available for individual objects. In Section 5.3 Alessandro Boselli and Laura Ferrarese will characterize the galaxy environments that are coming out from the various sky surveys, dealing in particular with the Virgo and Fornax clusters, our closer biggest associations of galaxies. Then, in Section 5.3.1 Bianca Poggianti will draw the picture emerging from WINGS, the WIde field Galaxy cluster Survey, dedicated to the study of the nearby (z < 0.07) galaxy clusters. Nils Bergvall (Section 5.3.2) and Valentina Karachentseva (Section 5.3.2) will present the properties of galaxies inhabiting the low density environments up to isolation. Martha Haynes and Riccardo Giovanelli will discuss the HI surveys in Section 5.4 and related subsections. Luciana Bianchi will introduce the UV surveys, with a particular emphasis for the results obtained by GALEX in Section 5.5.1. The most important IR and X ray surveys will be discussed by Alessandro Boselli and Ginevra Trinchieri respectively in Section 5.5.2 and 5.5.3. Finally, in Section 5.6 Jonathan Bland-Hawthorn and Bianca Poggianti will give a panoramic sketch of the main spectroscopic surveys.