Scenarios For Galaxy Evolution

The combination of fossil evidence in the nearest galaxies and the pieces of galaxy evolution that we can see at substantial redshifts provide hints as to how galaxies might have formed, and suggest what we might be looking for as we investigate these early stages of cosmic evolution. They broadly echo the pictures derived from the ages and composition of stars in the Milky Way, taking in extreme cases a monolithic collapse of a single enormous gas cloud to form each galaxy, or the gradual...

Cooling The

Like star formation, galaxy formation is more than a gravitational process. In order to collapse and, in particular, in order for the baryonic material we see to collapse to a more compact configuration than dark matter the initially uniform and warm gas must be able to cool. Virtually all mechanisms for cooling available to atomic and molecular gas are collisional, taking place only during two-body interactions. Thus, the cooling rate has a basic dependence on the collision rate, which scales...

Introduction

Galaxies do not exist in isolation, either from one another or from their gaseous environments. Unless galaxy formation was implausibly efficient, we might expect some ordinary matter to be left over, still filling the vast intergalactic spaces. Something closely analogous happens when stars are formed. Outflows, whether collimated into narrow jets or in expansive winds, are important to the process, and it appears that the mass of material ejected from a star-forming cloud is roughly equal to...

C 368 z1132

This montage of Hubble images shows radio galaxies at a range of redshift, all to the same angular scale and all in the emitted near-ultraviolet. Low-redshift objects are normal ellipticals, while high-redshift examples are less symmetric and preferentially aligned with the radio source axes. (Data from the NASA ESA Hubble Space Telescope archive, with observations originally obtained by P. Best and M. Longair.) might suspect this process in the early Universe, so that AGN would...

Passive And Active Evolution

One of the easiest ways to look for the evolution of galaxies is through their composite populations of stars, as reflected in the overall spectrum of each galaxy. Ignoring for the moment such complications as internal absorption by dust grains and the changing masses and temperatures of co-evolving close binary stars, the spectrum of a galaxy will be simply the sum of the spectra of its constituents. The galaxy spectrum G(A) can be expressed as where the index i reflects various kinds of stars...

Primordial Nucleosynthesis

The initial chemistry for galaxies, which defined how material could cool and what the raw material for the first stars would be, was set long before recombination, starting after the first three minutes. In the first seconds of cosmic history, the temperature and density (of both matter and radiation) were high enough to maintain the population of protons and neutrons at the number ratio set by their masses as long as the rates for the three n p reactions are fast compared with the expansion...

The Universe Accelerates

A remarkable refinement of the paradigm in cosmology came with the adoption of evidence for a nonzero cosmological constant. The cosmological constant is a notion dating back to Einstein's original work on a cosmic picture consistent with general relativity, in which he found that maintaining a static Universe required introducing a new term A in his field equations. This term represents a phenomenon whose effect is cosmic repulsion. The idea was dropped soon thereafter as Hubble made the case...

Galaxy interactions and mergers

Peculiar galaxies have long attracted special attention as extreme, unusual, and difficult to explain. Most of the objects in Vorontsov-Velyaminov's (1977) Atlas and Catalog of Interacting Galaxies and Arp's Atlas of Peculiar Galaxies display asymmetries, tails, plumes, and other kinds of structures which are now interpreted as the result of tidal encounters with other galaxies, sometimes leading to a merger of the galaxies involved. It was by no means obvious circa 1970 that gravitational...

Farinfrared And Submillimeter Observations From Space

Another occupant of the increasingly popular L2 region will be the European Space Agency's Herschel (Figure 11.5), originally known as the Far Infrared Space Observatory or FIRST. Herschel will carry a 3.5-meter passively cooled telescope, launched on the same Ariane V rocket as Planck. It will carry imaging cameras and spectrometers operating over the 60-670 micron range. This includes the range which is proving a key to understanding much of the early star-forming history of galaxies,...

Cosmological dimming

In an expanding Universe, the inverse-square law for propagation of radiation breaks down increasingly over long spacetime intervals, because the surface area encompassed by an expanding wavefront emitted at a particular time t0 is no longer simply the surface of a static sphere of radius c(t t0). One consequence of this, seen in the relations from Chapter 1, is that the surface brightness of an extended source is not redshift-independent, as it would be in a static and Euclidean situation....

After Hubble The James Webb Space Telescope Jwst

The three formation questions compared at the beginning of this volume the origins of planets, stars, and galaxies all require a similar approach for the next step in our understanding. In each case, though for different reasons, high resolution and sensitivity are required to work farther into the infrared than the Hubble Space Telescope has been able to do. Observations with Hubble have reached several fundamental limits that can only be improved with such a new instrument. HST was not...

LYMAN a Emitters

Emission-line objects can be more easily detected against the sky background than continuum objects, so any population with strong emission lines is attractive simply because they can be more easily identified at particular redshifts. In the ultraviolet range that shifts into the optical band for high redshifts, the only strong emission line produced by galaxies other than AGN is Lyman a, emitted at 1216 A. There has been a long history of searches for this line from galaxies at high redshift,...

The Lyman a forest

Lyman a has a special role in spectroscopy. It is the strongest spectral line arising from the ground state (i.e., a resonance transition) of the most abundant chemical element, hydrogen. At low temperatures (below about 3000 K), most hydrogen will be in this state, so this line is the most sensitive probe for cold hydrogen when suitably back-illuminated. The UV-bright continuum of quasars thus gives us the ability to probe neutral hydrogen in interstellar space with exquisite sensitivity. As...

A cosmological cartoon

While many observational programs have ended up (unavoidably) mixing the results of galaxy evolution with effects of cosmology, I will try to separate them here as far as practical. The basic problem in doing so is that we may trace changes in the luminosity or size of galaxies, which can be mimicked by the properties of space itself through departures from the familiar inverse-square law for light propagation. Unless we have other means of tracing the behavior of size, distance, and light in...