A New Constituent of the Universe

The demands of war lead to technological advances that have consequences for pure research long after. The development of radar during World War II is one such example. The German radar dishes left behind in Holland at the end of the war enabled Oort to make a beginning for Dutch radio astronomy which, as we have seen, led to an understanding of the overall structure and kimenatics of the Milky Way Galaxy. Ewing and Purcell, the discoverers of the 21-cm line of neutral hydrogen, both had strong backgrounds in microwave radar technology, Ewing as a naval radar officer and Purcell as director of the MIT Radiation Laboratory. In the United Kingdom the young radio engineer Martin Ryle made a major contribution to the development of radar and thus to the British defense effort. But after the war, he applied these skills at Cambridge in the development of radio astronomy, in particular, the technique of radio interferometry.

In astronomy whenever a new wavelength window opens, major discoveries follow. This was certainly the case in the early 1950s, when crude radio telescopes began mapping the sky in continuum radiation and, in addition to smooth radiation from the Galaxy, discovered a number discrete sources scattered about the sky. Some were clearly Galactic in origin (they were in the plane of the Galaxy and associated with known objects such as supernova remnants) but others were more uniformly distributed outside of the Galactic plane.

Nearby, “Normal” Galaxies

In 1964 shortly after the discovery of quasars, George Field, then at Princeton, was impressed by the fact that the high redshift of quasars implied that they must be prevalent in the past – that very possibly they were associated with galaxy formation and not the end of a long process of galaxy evolution. He wrote a paper, now largely forgotten, entitled “Quasi-stellar radio sources as spherical galaxies in the process of formation.” This was before the popularity, or even the respectability, of the black hole scenario, so Field's picture was that a collapsing protogalaxy could form a dense spheroidal cloud in which star formation would be rapid. This would lead to a quasar luminosity and flux variations due to frequent supernovae. He even pointed out that the density of quasi-stellar sources could be consistent with the present observed density of spheroidal galaxies. In many ways this was a very prescient contribution, but now we know that the energy source is likely to be accretion onto black holes, and the black holes do not disappear. They remain in the center of most reasonably massive galaxies as a generally quiet quasar remnant (Lynden-Bell's idea), waiting there to flare occasionally when a morsel of food drifts by.

What Drives Cosmic Chemistry?

It is a relatively straightforward matter to use freely available computer codes and lists of chemical reactions to compute abundances of molecular species for many types of interstellar or circumstellar region. For example, the UDfA, Ohio, and KIDA websites (see Chapter 9) provide lists of relevant chemical reactions and reaction rate data. Codes to integrate time-dependent chemical rate equations incorporating these data are widely available and provide as outputs the chemical abundances as functions of time. For many circumstances, the codes are fast, and the reaction rate data (from laboratory experiments and from theory) have been assessed for accuracy. The required input data define the relevant physical conditions for the region to be investigated.

These codes and databases are immensely useful achievements that are based on decades of research. However, the results from this approach do not readily provide the insight that addresses some of the questions we posed in Chapter 1: What are the useful molecular tracers for observers to use, and how do these tracers respond to changes in the ‘drivers’ of the chemistry? Observers do not need to understand all the details of the chemical networks (which may contain thousands of reactions), but it is important to appreciate how the choice of the tracer molecule may be guided by, and depend on, the physical conditions in the regions they wish to study.

Why Are Molecules Important in Astronomy?

Molecules pervade the cooler, denser parts of the Universe. As a useful rule of thumb, cosmic gases at temperatures of less than a few thousand K and with number densities greater than one hydrogen atom per cm3 are likely to contain some molecules; even the Sun's atmosphere is very slightly molecular in sunspots (where the temperature – at about 3200 K – is lower than the average surface temperature). However, if the gas kinetic temperatures are much lower, say about 100 K or less, and gas number densities much higher, say more than about 1000 hydrogen atoms per cm3, the gas will usually be almost entirely molecular. The Giant Molecular Clouds (GMCs) in the Milky Way and in other spiral galaxies are clear examples of regions that are almost entirely molecular. The denser, cooler components of cosmic gas, such as the GMCs in the Milky Way Galaxy, contain a significant fraction of the nonstellar baryonic matter in the Galaxy. Counterparts of the GMCs in the Milky Way are found in nearby spiral galaxies (see Figure 1.1). Although molecular regions are generally relatively small in volume compared to hot gas in structures such as galactic jets or extended regions of very hot X-ray–emitting gas in interstellar space, their much higher density offsets that disparity, and so compact dense objects may be more massive than large tenuous regions.

Kapteyn's Universe

The “hondsrug” or “dog's back” passes for a mountain range in the Netherlands. It is a ridge of sand reaching an altitude of 30 m and stretching southeast to northwest from the German border through the wooded province of Drente into province of Groningen. In fact, the most northern point is the city of Groningen, which is certainly the reason why a city is there: it is the closest point to the North Sea that is still above sea level – that is to say, on a natural geological formation. To the north of the city there are ancient small villages built on “terps,” artificial small hills created over centuries from animal and human waste, usually with a church at the highest point. Long before the construction of dikes, local farmers would gather on these terps during storms or exceptionally high tides. It is a wet and grim climate: in winter low clouds hang over the flat green treeless landscape; sunny summer days can be disrupted by sudden downpours – soaking cyclists and sending them scurrying for bridges and highway overpasses.

Groningen is a large provincial town – the central market city of this rural region. It is, by Dutch standards, rather isolated – 200 km from Amsterdam and the other metropolises of crowded Holland. Basically it bears the same relation to the Netherlands as does Novo Sibersk to Russia; from the point of view of Holland, Groningen is in the far frozen and gloomy North. But because of this relative isolation, it has developed its own dialect and culture and bustling student life. For as unlikely as it might seem, the city has a university.

6.1 Introduction

It is appropriate to recall, in the context of this volume, that just over a century ago the first color-magnitude diagram (CMD) was published. The author of this landmark paper was not Ejnar Hertzsprung nor Henry N. Russell, but Hans O. Rosenberg, a colleague of Karl Schwarzschild at Göttingen. Rosenberg had been working since 1907 on getting spectral properties of stars by measuring plates obtained with the Zeiss objective prism camera (Hermann, 1994). To maximize the number of spectra per plate, he observed the Pleiades cluster and obtained spectra for about 60 of them, over 1907–1909, noting that their inferred effective temperatures correlated with their apparent magnitudes in the first ever published CMD (Rosenberg, 1910). His goal was to “make the most accurate determination of the spectral types of stars in the Pleiades” by using a “physiological blend” of the depth and width of the Ca II K line (393.37 nm) with the Balmer Hδ and Hζ lines. He excluded the Ca II H line at 396.9 nm as it was blended with H∈ in the very low dispersion spectra he used (1.9 mm from Hγ to Hζ). With an exposure time of 90 minutes he could measure spectra down to the 10th photographic magnitude, finding that for the actual members of the Pleiades “there is a strict relation between the brightness and the spectral type, with no exception in the interval from the 3rd to the 9th magnitude.”

4.1 Introduction

Our understanding of the cosmological world relies on two fundamental assumptions: (1) The validity of General Relativity, and (2) conservation of matter since the Big Bang. Both assumptions yield the standard cosmological model according to which dark matter structures form first and then accrete baryonic matter that fuels star formation in the emerging galaxies. One important way to test assumption one is to compare the phasespace properties of the nearest galaxies with the expectations of the standard cosmological model.

Although the possibility of the existence of dark matter (DM) was first evoked more than 85 years ago (Einstein, 1921; Oort, 1932; Zwicky, 1933) and has been under heavy theoretical and experimental scrutiny (Bertone et al., 2005) since the discovery of flat galactic rotation curves by Rubin and Ford (1970) and their verification and full establishment by Bosma (1981), the DM particle candidates still elude both direct and indirect detection (Lingenfelter et al., 2009; Latronico and for the Fermi LAT Collaboration, 2009). Indeed, it appears that also the cryogenic dark matter search (CDMS) experiment fails to find significant evidence for the existence of cold dark matter (CDMS II Collaboration et al., 2010). Favored today is dark matter made of non-relativistic (“cold”) particles (cold DM, CDM) as it allows the correct degree of large-scale structure formation. Less-massive particles can perhaps account for the observed structures as long as the particles are not too light, leading to Warm DM (WDM) models, while light, relativistic (“hot”) particles (Hot DM, HDM) are excluded because structures on galactic scales cannot form sufficiently rapidly.

7.1 Introduction: exercise goals

The main goal of this practical course is to build up a theoretical representation (N-body model) of the observed properties of the stellar stream associated to the globular cluster Palomar 5. Our priors are (i) a static (simplified) representation of the Milky Way potential, (ii) the position on the sky of the cluster remnant core, (iii) its heliocentric radial velocity, and (iv) its heliocentric distance.

We use the position of the stellar stream as detected in the Sloan Digital Sky Survey (SDSS) (see Grillmair and Dionatos, 2006) as observational constraints on the free-parameters of our models, which in this simplistic exercise correspond to the 2D-tangential components of the current velocity vector (i.e., proper motions) of Pal 5. Note that there are available measurements of Pal 5 proper motions. However, measuring those quantities for stellar systems as faint MV = —4.77 ± 0.20 and distant (D ≃ 21 kpc) as Pal 5 is subject to large observational uncertainties that translate into poorly constrained Galactocentric orbital parameters. To illustrate this issue, we adopt the Galactocentric proper motions of Pal 5 (μα,μδ) as free parameters that we derive from fitting the orientation of the stellar stream on the sky, and compare their values with measurements available in the literature. The second main goal of the exercise is thus to inspect the reliability of the existing proper motion measurements for Pal 5.

2.1 Somewhat historical: overview of the Local Group, dwarf galaxies, and their observed structures

Before taking on a discussion of the dynamics of Local Group (LG) galaxies and the contributing and competing effects of dark matter and tides, it is useful to have an understanding of the spatial distribution of these galaxies, the distribution of their types and masses, and their morphologies – all of which play critical roles in defining how dark matter and tides play out their dynamical tug-of-war. The most common types of galaxies – the dwarfs – which are the most dark matter dominated as well as those among LG galaxies to show the greatest evidence for tidal effects, are the primary focus of this chapter.

2.1.1 The Local Group in context

Large-scale galaxy redshift surveys over the past decades (e.g., Davis et al., 1982; Geller and Huchra, 1989; Shectman et al., 1996; York et al., 2000; Colless et al., 2001; Strauss et al., 2002; Abazajian et al., 2009; Jones et al., 2009) have revealed clearly the filamentary structure of the distribution of galaxies in the Universe. The nearest 100 Mpc shows vast voids but several large mass concentrations, such as the Perseus-Pisces, Pegasus, Pavo, Coma, Hydra-Centaurus, and Virgo Superclusters. The Milky Way (MW) and the LG of galaxies live on the outskirts of the Virgo Supercluster, whose center lies about 15 Mpc away.

Background

One of the unresolved problems of modern astrophysics is how the galaxies we observe today were formed. The Lambda–Cold Dark Matter paradigm predicts that large spiral galaxies like the Milky Way formed through the accretion and tidal disruption of satellite galaxies, a notion previously postulated on empirical grounds from the character of stellar populations found in our Galaxy. The Local Group galaxies are the best laboratory in which to investigate these galaxy formation processes as they can be studied with sufficiently high resolution to exhume the fossils of galactic evolution embedded in the spatial distribution, kinematics, and chemical abundances of their oldest stars.

Scientific rationale

This “Galactic archaeology” has recently undergone an unprecedented revolution, brought about by the spectacular increase in the quality and quantity of observations of Local Group galaxies using large-aperture ground-based telescopes and the Hubble Space Telescope, and with the advent of the first large-scale digital sky surveys (such as SLOAN and 2MASS) at the start of the twenty-first century.

The possibility of contrasting these observations with results on a small scale of cosmological simulations has drawn the attention of cosmologists towards the study of Local Group grand design galaxies and their satellites, thus giving rise to new lines of research that have involved numerous resources and a considerable observational and theoretical effort. The disagreement between the results of simulations and observations has also given rise to serious controversies among observers and theoretical cosmologists and is still the subject of active debate in the international community.

5.1 Introduction

Stellar streams represent the remnants of ancient accretion events into a galaxy, and are thus extremely important as tracers of the galaxy formation process. In recent years, it has been increasingly recognized that many of the clues to the fundamental problem of galaxy formation are preserved in fossil substructures (Freeman and Bland-Hawthorn, 2002), particularly in the outskirts of galaxies. Hierarchical formation models suggest that galaxy outskirts form by accretion of minor satellites, predominantly at early epochs when large disk galaxies were assembling for the first time. The size, metallicity, and amount of substructure in the faint outskirts of presentday galaxies are therefore directly related to issues such as the small-scale properties of the primordial power spectrum of density fluctuations and the suppression of star formation in small halos (Springel et al., 2005).

Remarkable progress has been made in recent years in understanding galaxy formation and evolution. High redshift observations have revealed the star formation history of the universe and the evolution of galaxy morphology (see, e.g., Bell et al., 2005; Ryan et al., 2008). However, the nature of look-back observations does not allow one to study the evolution of individual galaxies, and low-mass or small-scale structures remain out of reach in all but the nearest galaxies. Hence high spatial-resolution observations in nearby galaxies are required to complement the samples at cosmological distances to answer many of the big fundamental questions of galaxy formation such as how did the Milky Way build up, and how typical was this formation history?