Emission lines are powerful means to detect faint objects and to study their composition and physical properties. Detecting and studying objects ranging from galactic sources to the most distant galaxies is made possible by using these lines. The aim of the XVIII Winter School is to give a thorough introduction to this emission-line Universe from both theoretical and observational points of view. For this reason, the Winter School contents include not only classical lectures, but also tutorials on data reduction and analysis. This structure enables young researchers to participate actively in current and future research projects, while serving also as a reference book for experienced researchers.

The subject of this School was motivated by the upcoming advent of a new generation of wide-field instruments for large telescopes, specifically optimized for observing emission-line objects in two dimensions. These instruments will boost the study of these kinds of objects by providing large amounts of data, whose digestion will require a theoretical basis as well as specific data-reduction techniques. These powerful facilities will enable the study of very faint emission lines of nearby objects, or conspicuous lines of very distant targets. The former will provide finer details on the chemical composition and characteristics of the gas, while the latter will furnish insight on structure formation and its evolution via scanning of large proper volumes of Universe.

Most cosmological surveys have been based on the continuum emission of the objects of the Universe via broadband imaging and their spectroscopic follow-up.

Introduction

Emission lines are observed almost everywhere in the Universe, from the Earth's atmosphere (see Wyse & Gilmore 1992 for a summary) to the most-distant objects known (quasars and galaxies), on all scales and at all wavelengths, from the radio domain (e.g. Lobanov 2005) to gamma rays (e.g. Diehl et al. 2006). They provide very efficient tools to explore the Universe, measure the chemical composition of celestial bodies and determine the physical conditions prevailing in the regions where they are emitted.

The subject is extremely vast. Here, we will restrict ourselves in wavelength, being mostly concerned with the optical domain, with some excursions to the infrared and ultraviolet domains and, occasionally, to the X-ray region.

We will mainly deal with the mechanisms of line production and with the interpretation of line intensities in various astrophysical contexts. We will discuss neither quasars and Seyfert galaxies, since those are the subject of Chapter 5, nor Lyman-α galaxies, which are extensively covered in Chapter 4 of this book. However, we will discuss diagnostic diagrams used to distinguish active galaxies from other emission-line galaxies and will mention some topics linked with H Lyα. Most of our examples will be taken from recent literature on planetary nebulae, H II regions and emission-line galaxies. Emission-line stars are briefly described in Chapter 7 and a more detailed presentation is given in the book The Astrophysics of Emission Line Stars by Kogure & Leung (2007).

What do we mean by primeval? According to the Webster dictionary “Primeval: adj. [primaevus, from: primus first + aevum age] of or relating to the earliest ages (as of the world or human history)”. We will follow this definition and mostly discuss topics related to galaxies in the “early” Universe, whose limit we somewhat arbitrarily define at redshifts z ≳ 6, corresponding approximately to the first billion years (Gyr) after the Big Bang. In contrast the frequently employed adjective “primordial”, defined as “Primordial: adj. [primordialis, from primordium origin, from primus first + ordiri to begin] a) first created or developed b) existing in or persisting from the beginning (as of a solar system or universe) c) earliest formed in the growth of an individual or organ”, should not be used synonymously, for obvious reasons. Luckily “primeval” encompasses more than “primordial”, otherwise there would not be much in the way of observational aspects to discuss (now in 2006–2007) in these lectures!

If we follow the history of discoveries of quasars and galaxies over the last few decades it is indeed impressive to see how progress has been made in detecting ever-more-distant objects, increasing samples at a given redshift and their analysis and interpretation. During the last decade, approximately since the pioneering observations of the Hubble Deep Field in 1996 (Williams et al. 1996) and the spectroscopic studies of a large sample of star-forming galaxies at redshift 3 by Steidel and collaborators (Steidel et al. 1996), the observational limits have continuously been pushed further, reaching now record redshifts of z ∼ 7 (secure) (Iye et al. 2006) but maybe up to ∼ 10 (Pelló et al. 2004; Richard et al.

Introduction

Ultimately, the overwhelming majority of emission-line sources in the Universe are “galactic sources” - meaning discrete objects located within a particular galaxy (rather than some global property of a galaxy or some source not located in a galaxy). However, the most common of these, HII regions, are so ubiquitous that they are being covered elsewhere in this volume as the “baseline” source of emission lines. In addition, most of the other chapters are devoted to line emission either integrated over entire galaxies (or significant portions thereof) or from active galactic nuclei.

Given that coverage, I will focus this chapter primarily on “stellar” sources of line emission in the Milky Way other than HII regions - including young stellar objects, massive and/or evolved stars, and stellar remnants (planetary nebulae, supernova remnants, and accreting compact objects in binary systems). I will also put considerable emphasis on emission lines with rest wavelengths in the near-infrared waveband, due to the importance of this waveband for probing the dusty planar regions of the Milky Way where most of these sources are to be found.

In the sections below, I will begin with a review of important diagnostic optical emission lines and a more-detailed overview of key (rest-wavelength) infrared emission lines. I will then move on to “nebular” sources of emission lines (omitting HII regions, but including planetary nebulae and supernova remnants).

Introduction

Charles Fabry, who was born in 1867, specialized in optics and devised methods for the accurate measurement of interference effects. He worked with Alfred Pérot, during 1896–1906, on the design and uses of a device now known as the Fabry-Pérot interferometer, which was specifically designed for high-resolution spectroscopy, and is composed of two thinly silvered glass plates placed in parallel, producing interference due to multiple reflections.

In 1899 they described the Fabry-Pérot interferometer which enabled high-resolution observation of spectral features (Fabry & Pérot 1899). It was a significant improvement over the Michelson interferometer. The difference between the two lies in the fact that in the Fabry-Pérot design multiple rays of light reflected by the two plane surfaces are responsible for the creation of the observed interference patterns. The last sentence of the article reads We must emphasize the simplicity of the apparatus used and the ease with which it can be mounted at the telescope. When the silvering has been carefully selected, the interference apparatus does not cause the loss of much light and permits the study of objects of very feeble brightness.

Definition of a Fabry-Pérot interferometer

Basically, a Fabry-Pérot interferometer or etalon (from the French étalon, meaning “measuring gauge” or “standard”) is typically made of a transparent plate with two reflecting surfaces, or two parallel highly reflecting mirrors (technically the former is an etalon and the latter is an interferometer, but the terminology is often used inconsistently).