The big problems

The history recounted in the preceding four chapters represents quite extraordinary progress in understanding the astrophysical origins and evolution of our Universe. The contrast between the apparently insuperable problems of determining precise values of cosmological parameters up till the 1990s and the era of precision cosmology of the early years of the twenty-first century is startling.

Yet, despite the undoubted success of the concordance model, it raises as many problems as it solves. The picture is incomplete in the sense that, within the context of the standard world models, the initial conditions listed in Tables 15.2 and 15.3 have to be put in by hand in order to create the Universe as we observe it today. How did these initial conditions arise? As the quality of the observations improved, a number of fundamental issues for astrophysical cosmology became apparent. The resolution of these problems will undoubtedly provide insight into the laws of physics under physical conditions which at the moment can only be studied by cosmological observations.

The horizon problem

This problem, clearly recognised by Robert Dicke in 1961, can be restated, ‘Why is the Universe so isotropic?’ (Dicke, 1961). At earlier cosmological epochs, the particle horizon r ~ ct encompassed less and less mass and so the scale over which particles could be causally connected became smaller and smaller.

The feedback effects of massive stars on their galactic and intergalactic environments can dominate evolutionary processes in galaxies and affect cosmic structure in the Universe. Only the Local Group offers the spatial resolution to quantitatively study feedback processes on a variety of scales. Lyman continuum radiation from hot, luminous stars ionizes H II regions and is believed to dominate production of the warm component of the interstellar medium (ISM). Some of this radiation apparently escapes from galaxies into the intergalactic environment. Supernovae and strong stellar winds generate shell structures such as supernova remnants, stellar wind bubbles, and superbubbles around OB associations. Hot (106 K) gas is generated within these shells, and is believed to be the origin of the hot component of the ISM. Superbubble activity thus is likely to dominate the ISM structure, kinematics, and phase balance in starforming galaxies. Galactic superwinds in starburst galaxies enable the escape of mass, ionizing radiation, and heavy elements. Although many important issues remain to be resolved, there is little doubt that feedback processes plays a fundamental role in energy cycles on scales ranging from individual stars to cosmic structure. This contribution reviews studies of radiative and mechanical feedback in the Local Group.

Introduction

The Local Group is especially suited as a laboratory for studying the effects of the massive star population on the galactic environment. There are three types of massive star feedback:

1. (a) Radiative feedback, i.e., ionizing emission, which results in photoionized nebulae and diffuse, warm (104 K) ionized gas;

2. (b) Mechanical feedback, predominantly from supernovae (SNe), resulting in supernova remnants (SNRs), superbubbles, and galactic superwinds; and

3. […]

The Galaxy's extended halo contains numerous satellites which are in the process of being disrupted. This paper discusses the stages of satellite accretion onto the Galaxy with a focus on the Magellanic Clouds and Sagittarius dwarf galaxy. In particular, a possible gaseous component to the stellar stream of the Sgr dwarf is presented that has a total neutral hydrogen mass between 4–10×106 M⊙ at the distance to the stellar debris in this direction (36 kpc). This gaseous stream was most likely stripped from the main body of the dwarf 0.2–0.3 Gyr ago during its current orbit after a passage through a diffuse edge of the Galactic disk with a density > 10−4 cm−3. This gas represents the dwarf's last source of star formation fuel and explains how the galaxy was forming stars 0.5–2 Gyr ago. This is consistent with the star formation history and H I content of the other Local Group dwarf galaxies.

Introduction

Our Galaxy has built itself up by accreting satellite galaxies. This process if evident today through the satellites currently found in the extended Galactic halo. There are nine satellite galaxies within 150 kpc interacting with our Galaxy at various levels. These are in order of distance (Grebel, Gallagher, & Harbeck 2003): the Fornax dSph (138 kpc), the Carina dSph (94 kpc), the Sculptor dSph (88 kpc), the Sextans dSph (86 kpc), the Draco dSph (79 kpc), the Ursa Minor dSph (69 kpc), the Small Magellanic Cloud (63 kpc), the Large Magellanic Cloud (50 kpc), and the closest example of a recognizable accreting satellite is the Sagittarius Dwarf (28 kpc; hereafter Sgr dwarf).

There is increasing observational evidence that hot, highly ionized interstellar and intergalactic gas plays a significant role in the evolution of galaxies in the local universe. The primary spectral diagnostics of the warm-hot interstellar/intergalactic medium are ultraviolet and X-ray absorption lines of O VI and O VII. In this paper, I summarize some of the recent highlights of spectroscopic studies of hot gas in the Local Group and low-redshift universe. These highlights include investigations of the baryonic content of low-z Ovi absorbers, evidence for a hot Galactic corona or Local Group medium, and the discovery of a highly ionized high velocity cloud system around the Milky Way.

Introduction

We live in a wonderful age of discovery and exploration of the universe. As we peer farther and farther back in time, it is becoming ever more important to make sure that we observe the local universe as well as possible. Observations of galactic systems and the intergalactic medium (IGM) in the low-redshift universe are required to study the universe as it has evolved over the last ∼5 billion years. They are essential for the interpretation of higher redshift systems, and they form a framework for studies of such key topics as galactic evolution, “missing mass,” and the distribution of dark matter. Studies of hot gas and its relationship to galaxies are shedding new light on these and other astronomical topics of interest today. In this review, I summarize some basic information about the elemental species and types of observations that can be used to study hot gas.

The primordial abundances of deuterium, helium-3, helium-4, and lithium-7 probe the baryon density of the Universe only a few minutes after the Big Bang. Of these relics from the early Universe, deuterium is the baryometer of choice. After reviewing the current observational status of the relic abundances (a moving target!), the baryon density determined by big bang nucleosynthesis (BBN) is derived. The temperature fluctuation spectrum of the cosmic background radiation (CBR), established several hundred thousand years later, probes the baryon density at a completely different epoch in the evolution of the Universe. The excellent agreement between the BBN- and CBR-determined baryon densities provides impressive confirmation of the standard model of cosmology, permitting the study of extensions of the standard model. In combination with the BBN- and/or CBR-determined baryon density, the relic abundance of 4He provides an excellent chronometer, constraining those extensions of the standard model which lead to a nonstandard early-Universe expansion rate.

Introduction

As the hot, dense, early Universe rushed to expand and cool, it briefly passed through the epoch of big bang nucleosynthesis (BBN), leaving behind as relics the first complex nuclei: deuterium, helium-3, helium-4, and lithium-7. The abundances of these relic nuclides were determined by the competition between the relative densities of nucleons (baryons) and photons and, by the universal expansion rate. In particular, while deuterium is an excellent baryometer, He provides an accurate chronometer.

Our Milky Way Galaxy is a typical large spiral galaxy, representative of the most common morphological type in the local Universe. We can determine the properties of individual stars in unusual detail, and use the characteristics of the stellar populations of the Galaxy as templates in understanding more distant galaxies. The star formation history and merging history of the Galaxy is written in its stellar populations; these reveal that the Galaxy has evolved rather quietly over the last ∼10 Gyr. More detailed simulations of galaxy formation are needed, but this result apparently makes our Galaxy unusual if ∧CDM is indeed the correct cosmological paradigm for structure formation. While our Milky Way is only one galaxy, a theory in which its properties are very anomalous most probably needs to be revised. Happily, observational capabilities of next-generation facilities should, in the foreseeable future, allow the acquisition of detailed observations for all galaxies in the Local Group.

Introduction: The fossil record

The origins and evolution of galaxies such as our own MilkyWay and of their associated dark matter haloes are among the major outstanding questions of astrophysics. Detailed study of the zero-redshift Universe provides complementary constraints on models of galaxy formation to those obtained from direct study of high-redshift objects. Stars of mass similar to that of the Sun live for essentially the present age of the Universe and nearby low-mass stars can be used to trace conditions in the high-redshift Universe when they formed, perhaps even the ‘First Light’ that ended the Cosmological Dark Ages.

The spatial resolution and multiwavelength capability of the Hubble Space Telescope has advanced greatly the study of spheroidal populations of galaxies. The main sequence turnoff point of the Galactic bulge has been clearly measured and the proper motions of stars used to separate the foreground disk from the bulge. The study of bulge globular clusters by HST shows that the bulge field population is coeval with the halo. In the obscured Galactic Center, infrared imaging with NICMOS reveals a continuous star formation history. NICMOS imaging of the nucleus of M31 has settled a long standing debate about whether luminous AGB stars (possibly indicative of an intermediate age stellar population) are present. Measurements of the composition from the ground, and of the age from HST, are consistent with a scenario in which spheroidal populations formed very early. HST spectroscopy has also revealed the presence of central black holes in many bulges and spheroids. The formation of the stellar populations and black holes in spheroids was probably one of the earliest events in galaxy formation.

Introduction

The bulge populations of the Local Group are a window in the nearby Universe to some of the most important aspects of galaxy formation: The age (formation epoch) of bulge populations, their metal content, and their coevolution and connection with nuclear black holes. Spheroidal populations (including bulges) account for more than half of the stellar mass at the present epoch (Fukugita, Hogan, & Peebles 1998) and also even at redshift 0.7 (Bell et al. 2004). The connection between nearby and distant bulge populations is not a new revelation.

I review what is known about the general trends of metallicity and element abundance ratios in Local Group galaxies, and some implications of the abundance trends for chemical evolution. The Local Group spirals show radial metallicity gradients and a mean metallicity that increases with luminosity. The composition gradients steepen with decreasing galaxy luminosity, but are roughly similar when the gradients are derived per unit disk scale length. This suggests that the evolution of the metallicity gradient is closely tied to the evolution of the baryon distribution. The M31 and MilkyWay bulges appear to have similar metallicity distributions. The high [α/Fe] in Galactic bulge stars indicates that the bulge formed rapidly. Metallicity distributions for M31 and Galactic halo stars are also similar, except that M31 has more globular clusters that are metal-rich, possibly related to its larger bulge. M33 is anomalous in that its halo clusters may be significantly younger than the Galactic halo. Local Group irregular galaxies are metal-poor, and their mean metallicity correlates with galaxy luminosity. They have low effective yields, as derived from a comparison of mean metallicity with gas fraction, and the effective yield is correlated with galaxy rotation speed (or mass). This is evidence that the irregulars have lost metals to the IGM, either through galactic winds or stripping. Dwarf ellipticals in the Local Group are also metal-poor, and follow a similar metallicity-luminosity relation. The fact that the dEs have no gas also points to loss of metals as a significant factor in their evolution.

The Space Telescope Science Institute Symposium on “The Local Group as an Astrophysical Laboratory” took place during 5–8 May 2003.

The Local Group is in some sense the universe in a nutshell. The processes of galaxy mergers and interactions are the bread and butter of hierarchical structure formation. These processes can be studied in unsurpassed detail in the Local Group. Starburst regions in the LMC provide spectacular local versions of their high-redshift counterparts. While black holes are believed to reside at the centers of most galaxies, the best determination of the mass of a central black hole has been achieved in our own Galaxy (through the orbits of individual stars). In addition, the Local Group provides a rich census of star formation histories and of stellar populations. In short, before we attempt to understand the Universe, understanding our own backyard is a good start.

These proceedings represent only a part of the invited talks that were presented at the symposium. We thank the contributing authors for preparing their manuscripts.

We thank Sharon Toolan of ST ScI for her help in preparing this volume for publication.

Status quo and perspectives of standard chemical evolution models of Local Group galaxies are summarized, and what we have learned from them is discussed, as well as what we have not learned yet, and what I think will be learned in the near future. Galactic chemical evolution models have shown that: i) stringent constraints on primordial nucleosynthesis can be derived from the observed Galactic abundances of the light elements; ii) the Milky Way has been accreting external gas from early epochs to the present time; and iii) the vast majority of Galactic halo stars have formed quite rapidly at early epochs. Chemical evolution models for the closest dwarf galaxies, although still uncertain, are expected to become extremely reliable in the immediate future, thanks to the quality of new generation photometric and spectroscopic data which are currently being acquired.

Introduction

The proximity of Local Group galaxies makes them the ideal benchmarks to study galaxy formation and evolution, because they are the only systems where the accuracy and the wealth of observational data allows us to understand them in a sufficiently reliable way. In fact, to understand the evolution of galaxies, astronomers must follow two distinct and complementary approaches: on one hand they must develop theoretical models of galaxy formation, of chemical and dynamical evolution, and on the other hand, they must collect accurate observational data to constrain the models. It is of particular importance to acquire reliable data on chemical abundances, masses and kinematics of galactic components (gas, stars, dark matter), star formation (SF) regimes, and stellar initial mass function (IMF)—quantities that are best derived in nearby systems.

I review our understanding of the structure and kinematics of the Large Magellanic Cloud (LMC), with a particular focus on recent results. This is an important topic, given the status of the LMC as a benchmark for studies of microlensing, tidal interactions, stellar populations, and the extragalactic distance scale. I address the observed morphology and kinematics of the LMC; the angles under which we view the LMC disk; its in-plane and vertical structure; the LMC self-lensing contribution to the total microlensing optical depth; the LMC orbit around the Milky Way; and the origin and interpretation of the Magellanic Stream. Our understanding of these topics is evolving rapidly, in particular due to the many large photometric and kinematic datasets that have become available in the last few years. It has now been established that: the LMC is considerably elongated in its disk plane; the LMC disk is thicker than previously believed; the LMC disk may have warps and twists; the LMC may have a pressure-supported halo; the inner regions of the LMC show unexpected complexities in their vertical structure; and precession and nutation of the LMC disk plane contribute measurably to the observed line-of sight velocity field. However, many open questions remain and more work is needed before we can expect to converge on a fully coherent structural, dynamical and evolutionary picture that explains all observed features of the LMC.

Planetary Nebulae (PNs) in the Magellanic Clouds offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, in an environment that is free of the distance scale bias and the differential reddening that hinder the observations of the Galactic sample. The study of LMC and SMC PNs also offers the direct comparison of stellar populations with different metallicity. The relative proximity of the Magellanic Clouds allows detailed spectroscopic analysis of the PNs therein, while the Hubble Space Telescope (HST) is necessary to obtain their spatially-resolved images. In this paper we discuss the history and evolution of this relatively recent branch of stellar astrophysics by reviewing the pioneering studies, and the most recent ground- and space-based achievements. In particular, we present the results from our recent HST surveys, including the metallicity dependence of PN identification (and, ultimately, the metallicity dependence of PN counts in galaxies); the morphological analysis of Magellanic PNs, and the correlations between morphology and other nebular properties; the relations between morphology and progenitor mass and age; and the direct analysis of Magellanic central stars and their importance to stellar evolution. Our morphological results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNs have on average larger masses than the progenitors of symmetric PNs, without any assumption or relation to binarity of the stellar progenitors.

Introduction

Planetary Nebulae (PNs) are the gaseous relics of the envelopes ejected by low- and intermediate-mass stars (1 < M < 8 M⊙) at the tip of the asymptotic giant branch (AGB), thus they are important probes of stellar evolution, stellar populations, and cosmic recycling.

Recent progress on the astrophysics of globular clusters is discussed. Highlights are (a) developments in color-magnitude survey work, (b) globular cluster structures and the “fundamental plane,” and (c) the relation between globular clusters and the halo field stars in the same host galaxy.

Color-Magnitude studies: The beginning and end of an era

Above almost all other types of astronomical systems, globular clusters offer the chance to take a broad historical perspective. Much of the history of astrophysics in the twentieth century—stellar structure, stellar evolution, the distance scale, galactic structure and evolution, stellar populations, variable stars, high-energy sources—was driven by the need to understand the complex array of phenomena taking place inside these dense stellar systems.

No review of this kind should fail to mention the continuing efforts to understand the stellar content of these elegant systems through color-magnitude studies (CMDs), which are penetrating to ever-greater detail and depth. The very first such studies (see, for example, Shapley & Davis 1920) barely showed the red-giant stars and brightest horizontal-branch stars for the nearest clusters. This past year, a watershed in this kind of basic color-magnitude survey work was reached with the publication of the monumental survey project of Piotto et al. (2002), who used the WPFC2 camera on board HST to obtain (B, V) CMDs for 74 globular clusters, very nearly half of the entire Milky Way globular cluster population. The objects range from nearby, high-latitude clusters with beautifully precise, classic CMD sequences, down to sparse objects deeply embedded in the heavy field contamination and differential reddening of the Galactic bulge.

I present the preliminary results of a program to measure the star formation history in the halo of the Andromeda galaxy. Using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, we obtained the deepest optical images of the sky to date, in a field on the southeast minor axis of Andromeda, 51′ (11 kpc) from the nucleus. The resulting color-magnitude diagram (CMD) contains approximately 300,000 stars and extends more than 1.5 mag below the main sequence turnoff, with 50% completeness at V = 30.7 mag. We interpret this CMD using comparisons to ACS observations of five Galactic globular clusters through the same filters, and through χ2-fitting to a finely-spaced grid of calibrated stellar population models. We find evidence for a major (∼30%) intermediate-age (6–8 Gyr) metal-rich ([Fe/H]> −0.5) population in the Andromeda halo, along with a significant old metal-poor population akin to that in the Milky Way halo. The large spread in ages suggests that the Andromeda halo formed as a result of a more violent merging history than that in our own Milky Way.

Introduction

One of the primary quests of observational astronomy is understanding the formation history of galaxies. An impediment to this research is the relative paucity of galaxies in the Local Group, which contains no giant ellipticals, and only two giant spirals-our own Milky Way and Andromeda. Fortunately, Andromeda (M31, NGC 224) is well situated for studying the formation of giant spiral halos, due to its proximity (770 kpc; Freedman & Madore 1990), small foreground reddening (EB-V = 0.08 mag; Schlegel, Finkbeiner, & Davis 1990), and low inclination (i ≍ 12.50°; de Vaucouleurs 1958).

The galaxies of the Local Group that are currently forming stars can serve as our laboratories for understanding star formation and the evolution of massive stars. In this talk I will summarize what I think we've learned about these topics over the past few decades of research, and briefly mention what I think needs to happen next.

Introduction

My talk today will be restricted to giving a brief introduction to the study of massive stars in the Local Group; I'll begin by discussing why I think the subject is important, and giving you a few of the complications and caveats. I'll spend most of my time then talking about what I think we've learned, first about star formation (stories of star formation, the initial mass function, and the upper mass cut-off), and second about the evolution of massive stars (including Luminous Blue Variables, Wolf-Rayet stars, and red supergiants). Finally I'll conclude with a brief discussion of what I think we need to do next. This talk is based in large part on an Annual Reviews of Astronomy & Astrophysics paper that I have coming out in October (Massey 2003), and the reader is referred there for a more in-depth analysis. I have used this opportunity to update some of the figures and thoughts from that, so hopefully the two will be somewhat complementary.