The modern science of networks has brought significant advances to our understanding of complex systems. One of the most relevant features of graphs representing real systems is community structure, or clustering, i. e. the organization of vertices in clusters, with many edges joining vertices of the same cluster and comparatively few edges joining vertices of different clusters. Such clusters, or communities, can be considered as fairly independent compartments of a graph, playing a similar role like, e. g., the tissues or the organs in the human body. Detecting communities is of great importance in sociology, biology and computer science, disciplines where systems are often represented as graphs. This problem is very hard and not yet satisfactorily solved, despite the huge effort of a large interdisciplinary community of scientists working on it over the past few years. We will attempt a thorough exposition of the topic, from the definition of the main elements of the problem, to the presentation of most methods developed, with a special focus on techniques designed by statistical physicists, from the discussion of crucial issues like the significance of clustering and how methods should be tested and compared against each other, to the description of applications to real networks.

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Community detection in graphs

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

The large quantity and high quality of modern radio and infrared line observations require efficient modeling techniques to infer physical and chemical parameters such as temperature, density, and molecular abundances. We present a computer program to calculate the intensities of atomic and molecular lines produced in a uniform medium, based on statistical equilibrium calculations involving collisional and radiative processes and including radiation from background sources. Optical depth effects are treated with an escape probability method. The program is available on the World Wide Web at http://www.sron.rug.nl/~vdtak/radex/index.shtml . The program makes use of molecular data files maintained in the Leiden Atomic and Molecular Database (LAMDA), which will continue to be improved and expanded. The performance of the program is compared with more approximate and with more sophisticated methods. An Appendix provides diagnostic plots to estimate physical parameters from line intensity ratios of commonly observed molecules. This program should form an important tool in analyzing observations from current and future radio and infrared telescopes. Note: Accepted by AA 18 A4 pages, 11 figures;

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A computer program for fast non-LTE analysis of interstellar line spectra

Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

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Complex Organic Interstellar Molecules

Flattened, rotating disks of cool dust and gas extending for tens to hundreds of astronomical units are found around almost all low-mass stars shortly after their birth. These disks generally persist for several million years, during which time some material accretes onto the star, some is lost through outflows and photoevaporation, and some condenses into centimeter- and larger-sized bodies or planetesimals. Through observations mainly at IR through millimeter wavelengths, we can determine how common disks are at different ages; measure basic properties including mass, size, structure, and composition; and follow their varied evolutionary pathways. In this way, we see the first steps toward exoplanet formation and learn about the origins of the Solar System. This review addresses observations of the outer parts, beyond 1 AU, of protoplanetary disks with a focus on recent IR and (sub)millimeter results and an eye to the promise of new facilities in the immediate future.

http://www.ifa.hawaii.edu/users/cieza/WC2011_ARAA.pdf

Protoplanetary Disks and Their Evolution

Carbon, and molecules made from it, have already been observed in the early Universe. During cosmic time, many galaxies undergo intense periods of star formation, during which heavy elements like carbon, oxygen, nitrogen, silicon and iron are produced. Also, many complex molecules, from carbon monoxide to polycyclic aromatic hydrocarbons, are detected in these systems, like they are for our own Galaxy. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surprisingly high number of molecules that are used in contemporary biochemistry on the Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites and interplanetary dust particles. Large quantities of extra-terrestrial material were delivered via comets and asteroids to young planetary surfaces during the heavy bombardment phase. Monitoring the formation and evolution of organic matter in space is crucial in order to determine the prebiotic reservoirs available to the early Earth. It is equally important to reveal abiotic routes to prebiotic molecules in the Earth environments. Materials from both carbon sources (extra-terrestrial and endogenous) may have contributed to biochemical pathways on the Earth leading to life's origin. The research avenues discussed also guide us to extend our knowledge to other habitable worlds.

The evolution of organic matter in space

We present ISO-LWS and ground-based SEST observations which probe the Photo Dissociation Region (PDR) of W49N. The ISO-LWS observations consist of low resolution (similar to 200) grating spectra between 43 mum and 197 mum, and high resolution (similar to 10(4)) Fabry-Perot spectra of the [CII] 158 mum and [OI] 63 and 145 mum lines. The SEST observations consist of (CO)-O-16 J = 1 -->0 and (CO)-O-18 J = 2 -->1 maps of the region. Using all these observations and with the help of a detailed PDR model, we have determined the physical and chemical structure of the PDR of W49N. We find that only a tiny fraction, 2.5%, of the cloud gas is in the atomic form. This gas has an average density n=10(4) cm(-3) and a temperature T similar to 130 K. The PDR is created by a FUV field G(0) = 3x10(5) times brighter than the average FUV interstellar field, which is emitted by the stars associated with the several HII regions inside the cloud. The photoelectric heating efficiency is very low, similar to 10(4), relative to other known PDRs, probably because of the relatively high G(0)=n ratio. Overall, W49N seems to be one of the youngest giant molecular clouds actively forming stars in the Galaxy, since most of the cloud material is still molecular and dense and not yet dispersed by the several embedded newly formed massive protostars.

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The physical conditions in the PDR of W49N

The conditions that affect the formation of stars in radiatively and mechanically active environments are quite different from the conditions that apply to our local interstellar neighborhood. In these galactic environments, a variety of feedback processes can play a significant role in shaping the initial mass function (IMF). Here, we present a numerical study on the effects of an accreting black hole and the influence of nearby massive stars to a collapsing, 800 M-circle dot, molecular cloud at 10 pc distance from the black hole. Our work focusses on the star-forming ISM in the centers of (ultra-)luminous infrared galaxies ((U)LIRGS). We therefore assume that this region is enshrouded by gas and dust and that most of the UV and soft X-ray radiation from the broad line region (BLR) is attenuated along the line of sight to the model cloud. We then parametrize and study radiative feedback effects of hard X-rays emanating from the black hole BLR, increased cosmic ray rates caused by supernovae in starbursts, and strong UV radiation produced by nearby massive stars. We also investigate the importance of shear from the supermassive, 10(6)-10(8) M-circle dot, black hole as the star-forming cloud orbits around it. A grid of 42 models is created and calculated with the hydrodynamical code FLASH. We find that thermal pressure from X-rays compresses the cloud, which induces a high star-formation rate early on, but reduces the overall star-formation efficiency (SFE) to about 7% through gas depletion by evaporation. We see that the turn-over mass of the IMF increases up to a factor of 2.3, M-turn = 1-1.5 M-circle dot, for the model with the highest X-ray flux (160 erg s(-1) cm(-2)), while the high-mass slope of the IMF becomes Gamma greater than or similar to -1 (Gamma(Salpeter) = -1.35). This results in more high-mass stars and a non-Salpeter IMF. Cosmic rays penetrate deeply into the cloud and increase the gas temperature to about 50 K for rates that are roughly 100 times Galactic and 200 K for 3000 times Galactic, which leads to a reduced formation efficiency of low-mass stars. While the shape of the mass function is preserved, high cosmic ray rates increase the average mass of stars, thereby shifting the turn-over mass to higher values, i.e., up to several solar masses. Owing to this process, the onset of star formation is also delayed. We find that UV radiation plays only a minor role. Because UV photons cannot penetrate a dense, n >= 10(5) cm(-3), cloud deep enough, they only affect the late time accretion by heating the medium where the cloud is embedded in. When we increase the black hole mass, for a cloud that is at 10 pc distance, the turbulence caused by shearing effects reduces the SFE slightly. Furthermore, shear weakens the effect of the other parameters on the slope of the IMF as well as the turn-over mass. The run with the most massive black hole, however, causes so much shear that the hydrodynamics is completely dominated by this effect and it severely inhibits star formation. We conclude that the IMF inside active galaxies is different from the one obtained from local environments. We also find that the combined effects of X-rays, cosmic rays, UV, and shear tend to drive toward a less pronounced deviation from a Salpeter IMF.

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Star formation near an obscured AGN - Variations in the initial mass function

We model the spectra and surface brightness distributions for the Ly radiation expected from protogalaxies that are caught in the early stages of their assembly. We use the results of a companion paper to characterize the radiation emerging from spherically collapsing gas clouds. We then modify these spectra to incorporate the effect of subsequent resonant scattering in the IGM. Using these models, we interpret a number of recent observations of extended Ly blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, relatively shallow surface brightness profiles, and double-peaked spectra, that several of these LABs may be associated with collapsing protogalaxies. We suggest two follow-up observations to diagnose the presence of gas infall. High-S/N spectra of LABs should reveal a preferential flattening of the surface brightness profile at the red side of the line. Complementary imaging of the blobs at redshifted H wavelengths should reveal the intrinsic Ly emissivity and allow its separation from radiative transfer effects. We show that Ly scattering by infalling gas can reproduce the observed spectrum of the Steidel et al. LAB2 as accurately as a recently proposed outflowmodel. Finally, we find similar evidence for infall in the spectra of pointlike Ly emitters. The presence of scattering by the infalling gas implies that the intrinsic Ly luminosities and derived quantities, such as the star formation rate, in these objects may have been underestimated by about an order of magnitude. Subject headingg s: cosmology: theory — galaxies: formation — galaxies: halos — intergalactic medium — quasars: general — radiative transfer Online material: color figures

Lyman Alpha Radiation From Collapsing Protogalaxies II: Observational Evidence for Gas Infall

Radiative feedback from populations II stars played a vital role in early structure formation. Particularly, photons below the Lyman limit can escape the star forming regions and produce a background ultraviolet (UV) flux which consequently may influence the pristine halos far away from the radiation sources. These photons can quench the formation of molecular hydrogen by photo-detachment of $\rm H^{-}$. In this study, we explore the impact of such UV radiation on fragmentation in massive primordial halos of a few times $\rm 10^{7}$~M${_\odot}$. To accomplish this goal, we perform high resolution cosmological simulations for two distinct halos and vary the strength of the impinging background UV field in units of $\rm J_{21}$. We further make use of sink particles to follow the evolution for 10,000 years after reaching the maximum refinement level. No vigorous fragmentation is observed in UV illuminated halos while the accretion rate changes according to the thermal properties. Our findings show that a few 100-10, 000 solar mass protostars are formed when halos are irradiated by $\rm J_{21}=10-500$ at $\rm z>10$ and suggest a strong relation between the strength of UV flux and mass of a protostar. This mode of star formation is quite different from minihalos, as higher accretion rates of about $\rm 0.01-0.1$ M$_{\odot}$/yr are observed by the end of our simulations. The resulting massive stars are the potential cradles for the formation of intermediate mass black holes at earlier cosmic times and contribute to the formation of a global X-ray background.

Marco Spaans

Papers

The evolution of organic matter in space

The physical conditions in the PDR of W49N

Star formation near an obscured AGN - Variations in the initial mass function

Lyman Alpha Radiation From Collapsing Protogalaxies II: Observational Evidence for Gas Infall

The formation of massive primordial stars in the presence of moderate UV backgrounds