Summary

This Thesis deals with the exploitation of space infrared extragalactic surveys as a powerful tool for astronomical investigation. More precisely, it deals with the development of a new method (LARI Method) for the reduction and analysis of data obtained by an infrared satellite (ISO), the application of this method to data obtained within the most ambitious extragalactic survey carried out with this satellite (ELAIS), the first scientific results obtained through this application, and finally the possible applications of such technical and scientific contributions to an infrared satellite which has recently started operations (Spitzer) as well as to future infrared missions. As a testimony to the particularly heterogeneous nature of the skills that are necessary in order to realize a successful space project, the Thesis stands at the boundary between several significantly different disciplines, such as detector physics, signal analysis and image processing, software engineering, galaxy formation and evolution and observational cosmology. Although focusing on a particular mission (ISO), throughout an attempt was made at putting the work into an "historical" perspective, with a keen eye both for the efforts of the "pioneers" of infrared astronomy and for the exciting prospects that space missions will offer to this discipline in the years to come.

Chapter 1 - Galaxies in the Infrared

This Chapter describes the infrared spectral region and the most important phenomenological properties of galaxies when observed at these wavelengths. The infrared spectral range, its division into near-, mid- and far-infrared and the issues of atmospheric emission and absorption at these wavelengths are introduced. The origin of infrared emission in galaxies, its relation with their underlying physical properties, the phenomenon of starburst and AGN galaxy activity and the properties of Luminous Infrared Galaxies are discussed. Finally, the main results obtained by ISO in the characterization of the properties of single galaxies are introduced.

Chapter 2 - ISO Extragalactic Surveys

This Chapter details the motivation and properties of the vast array of extragalactic surveys which were undertaken by ISO, and briefly describes their main results as far as statistical properties of infrared galaxies are concerned. The relevance of the discovery of the Cosmic Infrared Background Radiation for infrared galaxy surveys is outlined. Observational parameters and scientific rationale of all ISO extragalactic surveys are reviewed. Statistical properties of galaxies as arising from these studies are discussed in terms of source number counts and cosmic star formation history from optical and infrared surveys. The relevance of ELAIS as a bridge between IRAS all sky survey and deeper ISO surveys is highlighted.

Chapter 3 - The ISO Satellite : Mission and Instrumentation

This Chapter sets the technical framework of the Thesis, discussing the most relevant aspects of the ISO mission, including the satellite, its instrumentation and the use of its ISOCAM instrument for the execution of imaging observations. The overall mission design and operations, the spacecraft, the telescope, its four focal plane instruments (the two spectrometers ISOSWS and ISOLWS and the two cameras ISOCAM and ISOPHOT) and their performance are briefly described. The ISOCAM instrument, its detector arrays, its operation and the observing modes available for imaging observations are detailed.

Chapter 4 - The LARI Method for ISO-CAM/PHOT Data Reduction and Analysis

This Chapter introduces the LARI Method, a new data reduction and analysis method for ISO imaging data, and details the techniques and tools that were developed within its framework. The space environment the ISO spacecraft operated in, the associated high cosmic ray impact rate, related problems in data reduction and different previous attempts at the development of a reliable method for the reduction of ISO imaging observations are summarized. The need for a new fully interactive data reduction and analysis method providing reliable and complete source catalogues from ISO surveys of different depths is thus outlined. The new approach adopted within the LARI Method to deal with cosmic ray impacts and glitches appearing in data is then introduced. The LARI Method describes the sequence of readouts, or time history, of each pixel of ISO-CAM/PHOT detectors in terms of a mathematical model of detectors' behaviour and of the charge release towards the contacts. Such a model is based on the assumption of the existence, in each pixel, of two charge reservoirs, a short-lived one and a long-lived one, evolving independently with a different time constant and fed by both the photon flux and the cosmic rays. Such a model is fully conservative, meaning that no charge decay is considered except towards the contacts and thus contributing to the signal, so that the observed signal is related to the incident photon flux and to the accumulated charges through a simple differential equation describing the evolution of the two aforementioned charge reservoirs. As part of this Thesis the LARI Method has been substantially refined, extended so as to make it possible to apply it to datasets taken at different wavelengths and to observations of different depths, and turned into a powerful but easy-to-use software tool for the detection of faint sources in ISO-CAM/PHOT observations. Most importantly, and differently from other methods, the method is fully interactive by its very nature, allowing to easily check at will the reliability of detected sources. The principles underlying the method, the new techniques employed to overcome the several "ancillary" issues in ISO data, the developed software and its use are described. Finally, the engineering, testing and packaging process of the various software implementing the LARI Method and its "ancillary" routines, which was undertaken as part of this Thesis, is presented. This process included the production of a prototype software package, the LARI Package, and related draft user's manual. As a result, employed techniques and developed software were extensively validated and can now be confidently applied to any ISO imaging data of particular interest. Reduction of similar data from other present and future infrared missions may certainly benefit from these as well. Their possible application to ongoing and future surveys carried out by Spitzer is briefly outlined.

Chapter 5 - ELAIS : The European Large Area ISO Survey

This Chapter introduces ELAIS, the most ambitious ISO extragalactic survey, and its unique role within other surveys carried out by the satellite. This survey covers the flux gap in the infrared spectral range existing between IRAS all sky survey and deeper ISO surveys, and is thus a fundamental ingredient for a full understanding of the cosmic star formation history. Science objectives, choice of survey wavelengths, depths and fields, planning and execution of ISO observations, early issues in data reduction and the extensive multi-wavelength follow-up program undertaken within the ELAIS consortium to try and make sense of the nature of infrared-detected populations are described. Thanks to such an extensive multi-wavelength coverage, the ELAIS fields have now become among the best studied sky areas of their size, and natural targets of on-going or planned large-area surveys with the most powerful ground- and space-based facilities. Thus the need of reducing ISO data with the uttermost care and provide the community with agreed-upon results from the ELAIS project.

Chapter 6 - Final Analysis of ELAIS 15 $\mu$m Observations

This Chapter describes the application of the LARI Method to ELAIS 15 $\mu$m observations obtained with the ISOCAM instrument on board ISO, a process known as ELAIS 15 $\mu$m Final Analysis, quantifying the performance achieved in data reduction and detailing the properties of the resulting catalogue. First the main properties of ELAIS 15 $\mu$m fields and observations are reviewed. Then the application of the software implementing the LARI Method, the massive work of data reduction and interactive analysis, the production of the source catalogue and the evaluation of its quality, that were all carried out as part of this Thesis, are described. The ELAIS 15 $\mu$m Final Analysis Catalogue contains 1923 sources detected with $S/N > 5$ in the 0.5 - 100 mJy flux range over an area of 10.85 deg$^2$, and is thus the largest extragalactic catalogue provided to date by any single ISO project. Robust optical counterparts to 15 $\mu$m sources were determined for $\sim 85\,\%$ of the sample, on the basis of a variety of observational material. Achieved performance in data reduction and analysis was evaluated by carrying out an extensive set of simulations and applying the LARI Method itself to simulations. Reliability, completeness, astrometric and photometric accuracy of the resulting catalogue were thus estimated. Estimated astrometric accuracy is of order 1 arcsec in both RA and Dec for $S/N > 10$, while it increases up to about 2 arcsec in both RA and Dec for $S/N \sim 5$, being only marginally better for sources detected in higher-redundancy regions. Photometric accuracy is estimated to be better than 25% over the whole range of fluxes and redundancy levels probed by our catalogue, and better than 15% for $S/N > 10$ sources. The comparison of measured stellar fluxes with fluxes estimated on the basis of stellar atmosphere models calibrated on IRAS data and on near-infrared photometry allowed to achieve an IRAS/ISO relative photometric calibration. An IRAS/ISO relative calibration factor of $1.0974 \pm 0.0121$ was determined, shedding doubts on the goodness of the two independently determined calibrations at the 10% level. For lack of a simple way to identify error sources in IRAS and/or ISO calibration process, it was decided to put our catalogue on the more commonly used IRAS flux scale. However, while this choice was taken for the sake of compatibility of our results with studies adopting the IRAS flux scale (and particularly IRAS-based source counts and luminosity functions), this is not to indicate that IRAS calibration is more secure that ISO's.

Chapter 7 - Multi-Wavelength Studies of ELAIS 15 $\mu$m Final Analysis Sources

This Chapter describes the results of the multi-wavelength identification process of ELAIS 15 $\mu$m Final Analysis sources and a first scientific analysis of these results, which I was involved in as part of this Thesis. The process of optical identification and star-galaxy discrimination, whose reliability is critical to later carry out studies of well-defined samples of extragalactic sources, is presented for the N1 and N2 fields, i.e. the only ELAIS fields where deep multi-colour imaging from the Isaac Newton Telescope Wide Field Survey was available. Identification of optical counterparts to 15 $\mu$m sources made use of the likelihood ratio method, quantitatively evaluating the probability of each possible counterpart to a given source being the correct one and thus allowing automated decisions in this respect. Through this sophisticated process, $\sim 92\,\%$ of 15 $\mu$m sources is assigned a reliable counterpart, while $\sim 8\,\%$ of them are classified as blank fields. Building on the results of this optical identification process, which was extended to ELAIS radio sources, the ELAIS Band-Merged Catalogue is presented, integrating the various ELAIS follow-up observations into a coherent multi-wavelength catalogue and providing a precious legacy from the ELAIS project to the astronomical community. The production of the catalogue is described, with emphasis on the band-merging process and adopted search radii, and the format of its 3762 entries is described. Discussion of the physical properties of the extragalactic populations appearing in the catalogue is carried out through analysis of the sample's colour-colour diagrams, redshift distributions, spectral energy distributions and number of rare objects. For extragalactic sources observed in 3 or more infrared bands, colour-colour diagrams are presented and discussed in terms of the contributing infrared populations. Spectral energy distributions of selected sources are compared with cirrus, M82 and Arp220 starburst, and AGN dust torus models. Spectroscopic redshifts are tabulated, where available. For the N1 and N2 areas, the Isaac Newton Telescope Wide Field Survey deep multi-colour imaging allows photometric redshifts to be estimated for galaxies and quasars. These agree well with the spectroscopic redshifts, within the uncertainty of the photometric method ($\sim 10$% in $(1+z)$). The redshift distribution is given for sources detected in different bands and resulting diagrams are discussed. There is a high proportion of Ultraluminous Infrared Galaxies in the catalogue ($> 10 \%$ of 15 $\mu$m sources), many with Arp220-like colours. The 8% of the 15 $\mu$m sources which are genuine optically blank fields must have high infrared-to-optical ratios and probably have $z > 0.2$, so are high luminosity dusty starbursts or Type 2 AGN. 9 Hyperluminous Infrared Galaxies are found in the survey. The large numbers of Ultraluminous Infrared Galaxies imply very strong evolution in the star formation rate between $z=0$ and 1. 9 Extremely Red Objects are also detected, whose mid-infrared and optical fluxes are consistent with all 9 objects being highly extinguished starbursts like Arp220 at $z \sim 1$. Finally, 9 objects characterized by extremely high mid-to-near infrared flux ratio ( $f_\nu(15\,\mu\mathrm{m}) / f_\nu(2.2\,\mu\mathrm{m}) > 25$) are detected. These newly-dubbed Extreme Mid-to-Near Infrared Objects (EMNOs) may belong to a new extragalactic population of obscured AGN and very strong dusty starbursts at $z > 1$ which was not detected in deeper ISO surveys because of their small covered area. If this tentative discovery were confirmed, Spitzer surveys such as SWIRE would be bound to uncover large numbers of these objects, which would provide a useful window into the high-redshift obscured nuclear activity and star formation phenomena in a similar way the discovery of Extremely Red Objects has been used to define samples of high-redshift early-type galaxies.

Chapter 8 - Spitzer Extragalactic Surveys

This Chapter reviews some of the main characteristics of ISO's natural successor, the Spitzer Space Telescope, and underlines the relevance of the results obtained as part of this Thesis for its mission. The mission and instrumentation are reviewed and the science objectives that will be pursued with it within the so called Spitzer Legacy Science Program are discussed. The largest portion of this latter program, the Spitzer Wide-area InfraRed Extragalactic (SWIRE) survey, resembles ELAIS both ideally, being the most ambitious and largest-area extragalactic survey carried out by Spitzer, and practically, observing three of four ELAIS main fields. Both technical tools and scientific results arising from this Thesis will prove useful in dealing with the new challenges in extragalactic infrared astronomy opened up by Spitzer's observations.

Appendices

The two Appendices contain material that did not find a suitable collocation within the Thesis main body, but that was nevertheless believed to be useful to the comprehension of its results. Appendix A reproduces the draft User's Manual to the LARI Package, realized as part of this Thesis. The User's Manual describes the purpose, contents, installation and documentation of the package, and gives some information on its basic usage. Appendix B reviews the most important past and future infrared extragalactic survey projects. Pre-ISO attempts at characterizing the infrared sky and post-Spitzer projects building upon and furthering our present knowledge are presented. Their respective strengths and limits are underlined, with an emphasis on the employed instrumentation.