Introduction

On the basis of the prevalence of cold environments in our solar system, the search for extraterrestrial life is focused largely on icy habitats. The McMurdo Dry Valleys (MDV) area is a polar desert with a mean annual temperature below freezing and extremely low humidity (Wharton et al.,1995) and thus offers a suitable earthly analog to our nearest exobiological candidate, Mars. Water is thought to have been abundant on Mars early in its geological history (Solomon et al., 2005; Squyres et al., 2006; Head and Marchant, this volume, Chapter 2) and perhaps may even have flowed across the surface more recently (Hauber et al., 2005; Head et al., 2005; Malin et al., 2006). Life as we know it requires the presence of liquid water to mediate biochemical reactions for energy as well as a reasonably stable environment in which to grow; therefore the search for extraterrestrial life has been largely a search for environments where liquid water can be maintained for some duration (e.g., Carr, 1983).

There is significant geomorphological evidence, and mounting physical evidence supporting the presence of paleolakes on ancient Mars (Squyres et al., 2006). This intrigues exobiologists because paleolakes would provide a suitable habitat for early martian life forms (Carr, 1983; Wharton et al., 1995; Doran et al., 2004). Lakes on the martian surface would have become progressively colder over geological time, developing seasonal and eventually perennial ice covers (Carr, 1983).

This lecture introduces the scientific motivation for integral field spectroscopy (IFS) and describes the results from this novel technique in Galactic studies as of 2005. The following chapter by Luis Colina then picks up on extragalactic studies and rounds out the picture giving an outlook to future integral field spectroscopic studies. Following the five one-hour lectures, this chapter is broken down into sections on:

• science motivation;

• the Galactic Centre black hole;

• the Galactic Centre stellar population;

• star formation; and

• the Solar System.

The publications discussed in this lecture are selected on the basis of their scientific impact as measured by the citation index and/or because they specifically qualify for a lecture from a didactic point of view.

Science motivation

Motivation for the development and application of integral field spectroscopy is manifold and is closely tied to personal interest and background. This is most obvious in the history of instrument development, outlined in the beginning of this section. The ‘ideal’ objects for integral field observations can be well characterized by their angular size, substructure and spectral characteristics. These qualifiers are used to identify the ideal objects on the distance ladder. The section ends comparing the apparent interest of the Galactic and extragalactic astronomical communities in integral field spectroscopy.

Motivation for integral field spectroscopy

The development of the first-generation integral field spectrographs was largely driven by an ‘experimental’ motivation.

In this chapter we review the challenges of, and opportunities for, 3D spectroscopy and how these have led to new and different approaches to sampling astronomical information. We describe and categorize existing instruments on 4 m and 10 m telescopes. Our primary focus is on grating-dispersed spectrographs. We discuss how to optimize dispersive elements, such as VPH gratings, to achieve adequate spectral resolution, high throughput, and efficient data packing to maximize spatial sampling for 3D spectroscopy. We review and compare the various coupling methods that make these spectrographs ‘3D’, including fibres, lenslets, slicers, and filtered multi slits. We also describe Fabry–Perot (FP) and spatial-heterodyne interferometers, pointing out their advantages as field-widened systems relative to conventional, grating-dispersed spectrographs. We explore the parameter space all these instruments sample, highlighting regimes open for exploitation. Present instruments provide a foil for future development. We give an overview of plans for such future instruments on today's large telescopes, in space and in the coming era of extremely large telescopes. Currently-planned instruments open new domains but also leave significant areas of parameter space vacant, beckoning further development.

Fundamental challenges and considerations

The detector limit I: six into two dimensions

Astronomical data exist within a six-dimensional hypercube sampling two spatial dimensions, one spectral dimension, one temporal dimension, and two polarizations. In contrast, high-efficiency, panoramic digital detectors today are only two-dimensional (with some limited exceptions).

In this set of lectures, I review recent observational progress on extragalactic studies using integral field spectroscopy (IFS) techniques, highlighting the importance of IFS for the study of the nuclear regions of nearby galaxies, of low-z active galactic nuclei (AGN) and massive star-forming galaxies, and of high-z galaxies, including lensed quasars, lensing galaxies and bright submillimetre galaxies. Emphasis is given to the study of (ultra)luminous infrared galaxies as examples of low-z systems where the physical processes relevant to the formation and evolution of galaxies can be investigated in more detail. Research projects involving future ground-based facilities and satellites are also briefly presented.

Introduction

The use of IFS for extragalactic studies has burgeoned over the past 10 years and is already becoming a standard observational technique used by several groups in many different areas. Most IFS systems (INTEGRAL, GMOS, PMAS, SAURON, SINFONI, VIMOS, etc.) allow us to simultaneously obtain spectra covering a wide spectral range over a wide field of view (up to 1 arcmin square for VIMOS). These instruments in their standard configurations provide low–intermediate spectral resolution (R of 1000 to 4000) with a relatively low angular resolution (0.5″ to 3.0″). In addition, a few IFS systems, such as OASIS on the William Herschel Telescope and SINFONI on the Very Large Telescope (VLT), can provide very high angular resolution (i.e. 0.1″) in the optical (OASIS) and near-infrared (SINFONI) when combined with adaptive optics (AO) systems.

3D spectroscopy has a relatively short history. Most of the present instrument concepts were developed in the 1980s and early 1990s. During those pioneering years a great deal of work was done in optical labs in an attempt to understand how the optical fibres, microlenses and image slicers behave. Only a few groups (often formed by one or two people) worked on this topic. Communications were not very good, which explains why virtually all the groups decided to refer to this technique by a different name. So we ended up with ‘spectral imaging’, ‘bidimensional spectroscopy’, ‘integral field spectroscopy’, ‘two-dimensional spectroscopy’, ‘3D spectroscopy’, etc.

During those years it was more than doubtful whether this technique was going to be useful at all. In fact, it looked like a kind of curiosity of limited practical interest to astronomy. However, in the 1990s the first scientific results were obtained and they immediately produced a change of perception.

In the last few years investment in this type of instrumentation has been enormous. Large telescopes all around the world are now equipped with integral field units. Two instruments of the future James Webb Space Telescope will also have integral field spectroscopic capabilities, etc. Instead of being based in the optical lab trying to characterize optical fibres or micro-lenses, more effort is dedicated nowadays to refining techniques for reducing, analysing and interpreting the data obtained with a new generation of 3D spectrographs.

Introduction

In this chapter, I give an introduction to observing with integral field units and performing basic reduction of the resulting data, prior to scientific analysis. After briefly considering the context of the lectures, I begin by discussing strategies for observing. This is followed by a short tutorial on sampling theory and its application to integral field unit (IFU) data, before continuing with an overview of the requirements for each stage of data reduction. I finish by considering the data reduction process as a whole, along with associated issues such as error propagation and file formats.

Background

Techniques for integral field spectroscopy (IFS) have been in development for at least two decades (Vanderriest, 1980). During the 1980s–1990s, numerous prototype IFUs and even a few public instruments were deployed at observatories and used for scientific work. Nevertheless, IFS has only become widely available at major telescopes during the past five years or so, following two centuries of slit spectroscopy. Experience in observing with IFUs and processing the data is just starting to become commonplace within the community, but will be spread more widely by the current generation of postdoctoral and student astronomers.

In terms of data reduction and analysis, IFS poses some non-trivial new requirements. The most obvious factor is the introduction of 3D datasets to mainstream optical and near-infrared (NIR) (as opposed to radio) astronomy. Although older scanning methods such as Fabry–Perot interferometry produce higher-dimensional datasets, these techniques are relatively specialized by comparison.

Introduction

Presentation and scope

This chapter contains the proceedings of the course on analysis of 3D spectrographic data given as part of the XVII Canary Island Winter School of Astrophysics. It provides an overview of some basic and generic analysis techniques for 3D spectrographic data.

It includes a description of an arbitrary selection of tasks with, whenever possible, examples on real data and a lot of discussion about noise and errors. To illustrate the examples, we will make a heavy use of tools that are part of the XOasis software developed at the Centre de Recherche Astrophysique de Lyon (CRAL) and of 3D datasets obtained using the TIGER and OASIS instruments.

This course is not limited to pure 3D analysis techniques as the core of the analysis of 3D datasets is either identical or similar to what is done for regular spectrographic data.

It has some obvious caveats and limitations:

• It is not exhaustive but contains a rather arbitrary selection of tasks and tools that we have considered as unavoidable.

• The methods and examples are biased toward extragalactic astronomy.

• It is limited to the data analysis techniques used in visible and near-infrared (NIR) astronomy.

• It does not address those used in the radio and X-ray communities (long-time users of 3D spectrography).

Data analysis

Before starting, we need to define better what data analysis is and, in particular, where it starts and stops in the process leading from raw data to ready-to-publish information (see Figure 4.1).

Introduction

Integral field spectroscopy (IFS) is a technique to obtain both spatial (x,y) and spectral (λ) information of a more or less continuous area of the sky simultaneously on the detector. Only a few instrumental concepts allow 3D information on 2D detectors to be obtained, and all of these are based on field splitters such as fibre bundles, lens array, or image slicers (see Figure 7.1) to sample the field of view. Each sampled element is then dispersed using a classic spectrograph and produces a spectrum on the detector. Depending on the field splitter used, the geometry of the spectra on the detectors may be very different. This diversity leads to the creation of very specific reduction techniques and/or packages, i.e. one per instrument built (e.g. P3d, Becker, 2001). Combined with the inherent complexity of 3D techniques, such software diversity has reduced the use of IFS for decades to a handful of specialists, mainly those involved in the teams building such instruments.

Conscious that this would be a handicap IFS specialists Walsh and Roth (2002) have started to standardize techniques and tools for integral field units (IFU). Recently, the Euro3D Research Training Network (RTN), whose aim was to promote 3D spectroscopy all over Europe (Walsh and Roth, 2002), made a great effort to create a standard data format (Kissler-Patig et al., 2004) for storing and exchanging 3D data, developing an application programming interface, API (Pécontal-Rousset et al., 2004), to ease the use of such a data format and creating a visualization tool (Sánchez, 2004) usable by any existing IFU.

Preface

The topic of the XVII IAC Winter School is ‘3D Spectroscopy’: a powerful astronomical observing technique, which has been in use since the early stages of the first prototype instruments about a quarter of a century ago. However, this technique is still not considered a standard common user tool among most present-day astronomers.

3D Spectroscopy (hereafter ‘3D’) is also called ‘integral field spectroscopy’ (IFS), sometimes ‘two-dimensional’ or even ‘area’ spectroscopy, and commonly also ‘three-dimensional’ spectroscopy; in other areas outside astronomy it is called ‘hyperspectral imaging’, and so forth. It is already this diversity in the nomenclature that perhaps reflects the level of confusion. For practical reasons, the organizers of this Winter School and the Euro3D network (which will be introduced below) have adopted the terminology ‘3D’, which is intuitively descriptive, but, as a caveat early on, is conceptually misleading if we restrict our imagination to the popular picture of the ‘datacube’ (Figure 1.1). Although this term will commonly be used throughout this book, we need to point out for the reasons given later in the first chapter that the idealized picture of an orthogonal cube with two spatial, and one wavelength, coordinate(s) is inappropriate in the most general case.

Whatever the terminology, it is the aim of this Winter School to help alleviate the apparent lack of insight into 3D instrumentation, its use for astronomical observations, the complex problems of data reduction and analysis, and to spread knowledge among a significant number of international young researchers at the beginning of their career.