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I. Scientific Description
1.1. The Stellar Matter Cycle: Star Formation, Stellar Evolution and Mass Return
The overwhelming part of the luminous matter in galaxies like e.g. in our Milky Way Galaxy (MWG) exists in at least two different states: firstly, in the form of tenuous gas partly containing small solid particles of sizes of the visual wavelengths, called dust, and, secondly, as hot dense stellar plasma. Stars are embedded into the Interstellar Medium (ISM) that is structured and determined by the stellar energy deposit and the dynamical action of stellar mass loss. Even the larger aggregates built by gas and stars, galaxies, are surrounded by tenuous gas, the Intergalactic Medium (IGM).
Matter construction in the very early Universe allowed the formation of baryonic matter up to only light elements as Li, B, and Be. Hydrogen and Helium have contributed more than 99%.The production of heavier elements happens in the nucleosynthesis manufactory of stars, which supply their luminosity from the energy release of fusion processes, and by neutron capture combined with a beta decay. By this, the fractions of produced elements up to Fe and Ni can be well determined and their enrichment to the ISM depends on their delivery by stellar mass loss. Since still today H and He in our MWG contributes 98% of baryonic mass, all elements heavier than both are laxly denoted as metals.
1.1.1. Star Formation
Depending on the stellar energy impact the ISM takes different states (phases
) determined by temperature and density. If gas is shielded against the stellar radiation field or is condensed by the dynamics of the ISM, it can cool to form the coolest phase of the ISM with highly complex dense molecular clumps that are embedded into larger molecular clouds (MCs). Within these clumps stars are formed in groups of dozens to several hundred thousands members.
Already within their parent clouds (even before their central hydrogen burning starts) young stars begin immediately to affect their environment by energy and matter exchange due to different processes: during their early stages the multiple low-mass stars develop strong winds and bipolar outflows as jets.
While one has developed a certain understanding of the star formation (SF) of the low-mass stars, this is by far not the case for massive stars. The reason is that their feedback to the mature MC happens by various processes and self-regulates the SF process itself on different scales as it determines the stellar mass distribution, called initial mass function (IMF) (and as visible, also the survival of proto-planetary disks), the star-formation efficiency, cloud destruction, and further more.
While in our local Universe the SF produces obviously the same IMF and this even over a significant range of different metallicity, this could have been totally different under the unique conditions of the Early Universe. In the absence of molecules and dust as well as of stellar heating, the ISM should have been structured differently and the first generation of stars should have formed in a different manner. Their internal structure, element production, and stellar mass loss must have varied significantly from present-day conditions. Explorations are tackling this first-stars' problem from both sides: observations of extreme metal-poor stars in our MWG and of abundances in the earliest galaxies are complemented by numerical simulations. Nevertheless, no decisive picture is yet feasible.
1.1.2. Stellar Structure, Evolution, and Mass Loss
By different stellar nucleosynthesis processes and by stellar gas return a continuous enrichment of the ISM with heavy elements occurred continuously. Particular abundance enhancements can be determined on all Cosmic scales and provide a detailed insight into the stellar element production, their timescale of release and their mixing with existing gas and their transport through structures in the Universe, i.e. an insight into the Cosmic Matter Circuit.
During their advanced stages the stars expand to Giants while their surface cools and drive a slow wind to enrich the ISM with specific heavy elements like C, N, etc. As importantly, these elements condense to solid particles, so-called dust, with sizes of the optical wavelengths. Intermediate-mass stars in binary systems can experience mass transfer that would lead in the case of a White Dwarf to SN explosions of type Ia with the exclusive production of Fe.
In contrast, massive stars are very hot (10000 K to above 50000 K), because of their exceedingly large energy production shortly living (only a few million years), and energizing their surroundings much more efficiently. After photo-dissociation the radiation field of a massive star subsequently form a HII region that expands into the neutral ambient medium because of its overpressure. In addition, massive stars expel stellar matter by a highly energetic wind with expansion velocities of several 1000 km/s depending on metallicity and evolutionary stage. In the Wolf-Rayet stage massive stars peel off their outermost layers down to nuclear burning shells in which He, C, N, and O are enriched and thus released. Finally, massive stars explode as supernovae type II (SNeII) and deliver heavy elements processed in their very interiors.
Massive stars play an overwhelming role in the evolutionary course of galaxies. They are not only the primary sources of gas and metal return but also dominate the turbulent energy input into the ISM by stellar winds, radiation, and supernova explosions.
Although stellar nucleosynthesis products are primarily deposited by winds and SNe into the hot tenuous gas and since cooling would last too long, processes like e.g. turbulence, shock fronts, and heat conduction enhance its condensation, mixing, and cooling so that MCs can contain parts of these fresh elements and can incorporate them into the next generation of stars. This closes the stellar matter cycle on the smallest scale, for which stars are the driving engines. Although stars return most of their matter in different forms described before, long-living low-mass stars and stellar remnants lock substantial amounts of baryonic matter and, by this, withdraw it from the cycle.
1.1.3. The Interstellar Medium
The understanding of the processes executed in the framework of this matter cycle in the ISM serves as the basis of the comprehension in various fields of Astrophysics, like e.g. star and planet formation (see section before), evolution of galaxies, the formation of galaxy morphologies, the Cosmic chemical evolution and the plasma-physical evolution of structures in the early Universe. Moreover, because of its extreme conditions with respect to terrestrial ones the ISM serves as an ideal laboratory for the Physics of extremely tenuous hot as well as cool plasmas, for chemical processes under extreme conditions, for Atomic and Molecular Physics, but also for Solid-state Physics (e.g. dust). Also for many other research fields in natural sciences ISM studies stimulate new developments, like e.g. the growing interest in Astrobiology.
The different phases
of the ISM can be reached by the same gas due to transitions from one to the other due to cooling vs. external heating or are directly fed by the released stellar matter. The timescales of these processes affect the evolution of a galaxy as a whole. The same importance is valid for the solid particles in the ISM, the dust. The preconditioning for forming dust from gas particles requires dense and cool gas. Since such conditions occur in winds of giant stars (and even in hot Wolf-Rayet winds), the production of dust particles also links the ISM directly with stellar ejecta.
From extinction measurements within our Galaxy one can derive that dust grains include 50% of the metals but distributed over particular elements.
Although the ISM in galaxies contributes only the minor fraction of baryonic matter in the present Universe, it is still the basis of SF, it is carrying the stellar metal products, and it determines the evolution of galaxies. Since in the early Universe after the decoupling of matter and radiation, baryonic matter existed only as gas until SF commenced, the investigation of present-day and nearby observable processes of the gas cycles will allow to better understand the evolution of the early Universe, e.g. by energy release to the re-ionization era as self-regulation of baryonic structure formation and by gas mixing of different phases to account for the observed early Cosmic metal enrichment.
1.2. The Galactic Matter Cycle
1.2.1. Cosmic Rays and Magnetic Fields
Explosions of massive stars are not only contributing by radiation and hot gas, the shocks of SN bubbles also accelerate electrons and ions to extremely high energies. These particles, together with γ-rays called Cosmic Rays (CR), pervade the Galaxy and heat it over large galactic scales because of their small collisional cross sections. In addition, they transfer momentum and energy to the gas by resonant scattering of MHD waves propagating along magnetic fields, which are frozen into the interstellar plasma.
As an additional component, also affecting the dynamics of the ISM and ubiquitous in the Universe, B-fields play an important role on all scales. Because of their charges the CR particles are closely coupled to the magnetic field structure and their diffusion through a galaxy and the intergalactic space interconnects intimately with the magnetic field structure.
While CRs thus support the energy distribution throughout a galaxy, magnetic fields imply more or less a steering mechanism of CR spreadening.
1.2.2. The disk-halo connection
Since massive stars are formed simultaneously in associations and cannot leave their birthplaces due to their short lifetimes, their explosions as SNeII occur in short time intervals and spatially correlated. Since the hot gas bubbles expand vehemently on larger timescales they overlap and accumulate to form a so-called superbubble.
The density stratification of gas in a galactic disk like our MWG facilitates the expansion of superbubbles in vertical direction out of the disk. Because of its low density this hot superbubble gas cools slowly so that its expansion continues into the halo of galaxies. There it forms a hot coronal-gas envelope around the visual part of galaxies as is observable in X-rays and by non-thermal radio emission of edge-on spiral galaxies. If it is still gravitationally bound, cooling of this hot gas and inhomogeneities lead to a delayed fall-back to the disk, in combination with the lift-off from the disk denoted as Galactic Fountain. This closes the Galactic Matter Cycle. This gas as carrier of metals is therefore withdrawn from the small-scale stellar matter cycle but migrates to the next range, on which the reorganization of matter on a galactic scale happens, still driven by the stellar engines.
Indeed, HI clouds falling towards the Galactic plane are observed. A significant part of their velocities range up to escape velocity and are divided into low- and intermediate-velocity clouds (LVCs and IVCs, respectively). Consequently, by the outflow of hot metal-rich gas into the halo and its delayed return to the gas disk but to places different from its launch site, it supports the mixing and chemical homogenization within the gas disk. Although the distance to LVC/IVCs is hardly determined they seem to belong to the MWG, but surprisingly contain lower metal abundances than the Galactic ISM. That these infalling clouds also exist as high-velocity clouds (HVCs) close to and above escape velocity gives a clear hint that at least a significant fraction of LVCs/IVCs can represent a continuation of HVCs to lower velocities due to frictional deceleration by the hot halo gas.
1.2.3. The Galaxy Formation Process
During the formation of galaxies they are not well structured and their ISM is probably extremely irregularly distributed and, by this, also the star-forming regions. This leads, on the one hand, to local inhomogeneities also in the metal enrichment and the energetics caused by the local dominance of individual stellar cycles. This is visible by the abundance variations in the stellar halo within a galaxy and e.g. by the local metal enrichment of the subsequently formed galactic disk from distant star-forming regions like the bulge. On the other hand, it allows hot gas and ionizing radiation to propagate over large distances. Both latter processes have the effect that the first structures forming the first stars are intensively coupling with their environment, leading e.g. to the re-ionization of the Universe, discernible by the Gunn-Peterson trough, and to a delayed formation of dwarf galaxies as well as to an early and unexpectedly high metal enrichment of young galaxies. In the early Universe because of the smaller separations of objects the galactic matter cycle leaves galactic dimensions and transits into a global one.
1.3. The Global Matter Cycle: Galaxies and their Environment
From different kinematical facts the present-day picture of the Universe must include a mass component of Dark Matter (DM) that dominates the total mass in galaxies as well as on large scales but does probably not consist of common baryonic substance. Since spatial distribution of DM halos and sizes of DM substructures are not directly accessible, this information must be derived from kinematical and energetic influences on the visible matter. Details of the Cosmic matter circuit in particular on large scales will provide an insight into this hidden substrate. The understanding of the processes determining the Global matter cycle are therefore highly important and have also substantial impact on cosmological simulations of structure formation.
1.3.1. High-velocity Clouds and Gas Infall
The significant fraction of infalling clouds with velocities larger than the binding energy and denoted as (ultra-)HVCs cannot stem from our Galaxy. Reasonably, we know that gas is disrupted from the Magellanic Clouds, dwarf irregular galaxies that orbit around the Milky Way in a distance of about 50-60 kpc. This gas that also descends to our Galactic disk has definitely extragalactic origin. The standard cosmological concept requires that the low-mass building blocks, halos of DM, should acquire gas so that a significant quantity of intergalactic clouds of cosmological origin should exist. However, several problems arise for our understanding ranging from the much lower number of intergalactic HI clouds than requested, the absence of stars to the missing DM substructures and HVC close to the Galactic disk. For these respects also the role of e.g. the intergalactic radiation field and the dynamical and thermal interactions between different gas phases has to be considered.
Observations of dwarf irregular galaxies provide growing evidence that all those objects suffering starbursts, i.e. extremely high SF rates, are experiencing gas infall from HI envelopes.
Since these intergalactic HVCs are metal poor, they trigger not only SF but also accomplish the refreshment of the ISM with pristine material. The search for further low-mass HI clouds around the Andromeda Nebula, our closest spiral galaxy neighbor, is yet unsuccessful like other attempted detections in the local Universe with increasing detector sensitivity have proven. The exploration of the HI Universe is therefore of great importance as a necessary (corner)stone in the mosaic of our cosmological framework.
1.3.2. Galaxies and their Environment
The optical appearance of galaxies mostly obtrudes the picture of almost generally isolated islands in the Universe. Statistical surveys analyzing structural and internal differences between galaxies in different environments have convincingly revealed that galaxy evolution is strongly, if not at all, coupled with its environment. Galaxies are connected by gas bridges, are fed with gas by the gravitational exhaustion of neighboring galaxies.
As in the cosmological standard picture, smaller aggregates, like e.g. dwarf galaxies, cold DM units, etc. cluster to larger structures, galaxies assemble to Galaxy Groups and Clusters. These mass concentrations attract the entire material in their environment, i.e. IGM and galaxies, respectively, that has to fall into the continuously deepening gravitational potentials. This structural evolution in the Universe has various consequences, a few of them also affecting the Cosmic Matter Circuit: not all the gas from the Big Bang has already settled into galaxies, but is still available in the Universe and concentrates towards Galaxy Clusters. In energy balance with the cluster's gravitation its IGM becomes very hot.
In clusters and groups the encounter rate of galaxies is much larger because of their on average smaller separation. Recent evidence that galaxies in groups possess stronger central SF activity than isolated systems has yet to be investigated theoretically. In close encounters of galaxies the SF is obviously enhanced and bursts, plausibly, in galaxy mergers, where the ISM is extremely condensed. Merger objects in the local Universe are partly only visible in the Infrared (IR) due to their dust content and are because of their high luminosity in this spectral range denoted as Ultra-luminous IR Galaxies (ULIRGs). Such dust-enshrouded extremely IR-luminous merger objects plausibly also existed in the early Universe after dust could have formed from the first stellar metal products, so that their detection with respect to their redshift is searched in the sub-mm wavelength range.
Material is e.g. liberated from galaxies in galaxy mergers and dispersed into the IGM, when tidal tails dissolve from the galactic potentials. Moreover, there is now observational evidence that galactic material may be recycled in the space between galaxies, forming a new generation of objects: Tidal Dwarf Galaxies are young galaxies assembled from the gas and stars that are ejected from interacting galaxies.
1.3.3. Galactic Gas Removal
If the above-mentioned superbubble expansion contains more energy than is necessary to bind the gas is directly expelled from galaxies as galactic wind. This means that galaxies are not only accreting matter from their surroundings and that this is not necessarily of cosmological origin, but that also the active coupling of galaxies with the surroundings becomes perceivable. Since sensitive X-ray observations are available, galactic winds are observed as an almost common phenomenon and related to the local SF rate in galaxies.
Simultaneously, with a better understanding of galaxy evolution, our knowledge of the cluster gas is greatly improved and allows us to determine its physical state. Its relatively high abundances of elements as stellar products and the enrichment of the IGM on cosmological scales suggest, in general, that pollution by galactic gas loss is responsible. Indeed, in addition to galactic winds, further processes can account for the IGM replenishment and its metal enrichment. Even if the gas can be kept bound in more massive galaxies, tidal interactions by galaxy encounters can remove galaxy gas as well as the relative motion of galaxies through the surrounding IGM by means of ram-pressure stripping (RPS).
This picture of galactic gas removal but the equivalence of IGM accretion and the gravitational contest among galaxies for the IGM justify expecting a strong dependence of galaxy evolution, even down-scaled to star formation, on this Global Matter Cycle.
1.4. The Cosmic Matter Circuit as a Challenge in Modern Astrophysics
On the one hand, various forms of matter cycles are directly observable and can be explored in our local cosmic vicinity. On the other hand, these matter cycles must have been initiated immediately when the first stellar objects have formed and ceased their lives, i.e. at a stage of the early Universe whose study becomes now available by the aid of most modern astronomical instruments. The Cosmic Matter Circuit is thus the most important ingredient of Cosmic evolution, i.e. from cosmological structure formation and evolution via galaxy evolution and the chemical enrichment to SF. For this, it has combined on a highly sophisticated level the astrophysical research fields dealing with SF, stellar evolution, Physics of the ISM, (magneto-)hydrodynamics, dust formation, galaxy formation and evolution, and cosmic structure formation. Consequently, the exploration and understanding of the matter cycles in the local Universe that allows the close comparison of observations and theory, their coupling within the course of Cosmic evolution and their extrapolation to the early stages of the Universe, the issues of which become slowly discernible but not in such resolutions as closeby, is therefore one of the biggest challenges in modern Astrophysics. Indeed, the complexity of processes that contribute to the matter cycles requires a large variety of different investigations and this in a broad context of different fields. The central role of this research field is also emphasized in recent White Books of national Astronomical communities and research agencies, like e.g. the German Science Foundation, the American National Academy of Sciences, and others.
Its growing relevance over the last decade becomes visible by the increasing number of conferences addressing exactly questions of both kinds, which interactions are perceivable and how the acting processes can be understood, respectively. E.g. in 2002, besides other conferences, the Astronomische Gesellschaft dedicated its annual meeting in Berlin to The Cosmic Circuit of Matter
. The recent largest conference on matter cycles was entitled Recycling Interstellar Matter and Intergalactic Matter
and took place in July 2003 in Sydney with more than 240 participants as an international symposium embedded into the General Assembly of the IAU. In August 2004 another international conference on The Environments of Galaxies: from Kiloparsecs to Mega-parsecs
was held on Crete. Also within the last year a growing number of meetings have addressed particular aspects of the cosmic matter cycles. The links between the closeby understandable processes of these cycles and their cosmological effects are also topics of conferences in 2006, like e.g. the IAU Symp. 235 in Prague on Galaxy Evolution across the Hubble time
where particular emphasis is given to the star-gas cycle and environmental effects on galaxy evolution as well as the CRAL-Conference this July in Lyon on Chemodynamics: from first stars to local galaxies
.