We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.

Star Formation in the Milky Way and Nearby Galaxies

CO line emission represents the most accessible and widely used tracer of the molecular interstellar medium. This renders the translation of observed CO intensity into total H2 gas mass critical to understand star formation and the interstellar medium in our Galaxy and beyond. We review the theoretical underpinning, techniques, and results of efforts to estimate this CO-to-H2 “conversion factor,” XCO, in different environments. In the Milky Way disk, we recommend a conversion factor XCO = 2×10 20 cm −2 (K km s −1 ) −1 with ±30% uncertainty. Studies of other “normal galaxies” return similar values in Milky Way-like disks, but with greater scatter and systematic uncertainty. Departures from this Galactic conversion factor are both observed and expected. Dust-based determinations, theoretical arguments, and scaling relations all suggest that XCO increases with decreasing metallicity, turning up sharply below metallicity ≈ 1/3–1/2 solar in a manner consistent with model predictions that identify shielding as a key parameter. Based on spectral line modeling and dust observations, XCO appears to drop in the central, bright regions of some but not all galaxies, often coincident with regions of bright CO emission and high stellar surface density. This lower XCO is also present in the overwhelmingly molecular interstellar medium of starburst galaxies, where several lines of evidence point to a lower CO-to-H2 conversion factor. At high redshift, direct evidence regarding the conversion factor remains scarce; we review what is known based on dynamical modeling and other arguments. Subject headings: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM

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The CO-to-H2 Conversion Factor

We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we can estimate where galaxies of given properties live at a particular epoch. This allows us to calculate, in an a priori cosmological manner, where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare this with the observed mass functions, clustering, fractions as a function of halo and galaxy mass, and small-scale environments of mergers, and we show that this approach yields robust estimates in good agreement with observations and can be extended to predict detailed properties of mergers. Making the simple Ansatz that major, gas-rich mergers cause quasar activity (but not strictly assuming they are the only triggering mechanism), we demonstrate that this model naturally reproduces the observed rise and fall of the quasar luminosity density at -->z = 0–6, as well as quasar luminosity functions, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The recent observed excess of quasar clustering on small scales at -->z ~ 0.2–2.5 is a natural prediction of our model, as mergers will preferentially occur in regions with excess small-scale galaxy overdensities. In fact, we demonstrate that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where major mergers of ~L* gas-rich galaxies will be most efficient. We contrast this with a secular model in which quasar activity is driven by bars or other disk instabilities, and we show that, while these modes of fueling probably dominate the high Eddington ratio population at Seyfert luminosities (significant at -->z = 0), the constraints from quasar clustering, observed pseudobulge populations, and disk mass functions suggest that they are a small contributor to the -->z 1 quasar luminosity density, which is dominated by massive BHs in predominantly classical spheroids formed in mergers. Similarly, low-luminosity Seyferts do not show a clustering excess on small scales, in agreement with the natural prediction of secular models, but bright quasars at all redshifts do so. We also compare recent observations of the colors of quasar host galaxies and show that these correspond to the colors of recent merger remnants, in the transition region between the blue cloud and the red sequence, and are distinct from the colors of systems with observed bars or strong disk instabilities. Even the most extreme secular models, in which all bulge (and therefore BH) formation proceeds via disk instability, are forced to assume that this instability acts before the (dynamically inevitable) mergers, and therefore predict a history for the quasar luminosity density that is shifted to earlier times, in disagreement with observations. Our model provides a powerful means to predict the abundance and nature of mergers and to contrast cosmologically motivated predictions of merger products such as starbursts and active galactic nuclei.

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A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies. I. Galaxy Mergers and Quasar Activity

Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift We describe the physics behind these winds, discuss the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe, and consider the implications of energetic winds for the formation and evolution of galaxies and the intergalactic medium To inspire future research, we conclude with a set of observational and theoretical challenges

Galactic Winds

We study the relations between stellar mass, star formation history, size and internal structure for a complete sample of 122 808 galaxies drawn from the Sloan Digital Sky Survey We show that low-redshift galaxies divide into two distinct families at a stellar mass of 3 x 10 1 0 M O  Lower-mass galaxies have young stellar populations, low surface mass densities and the low concentrations typical of discs Their star formation histories are more strongly correlated with surface mass density than with stellar mass A significant fraction of the lowest-mass galaxies in our sample have experienced recent starbursts At given stellar mass, the sizes of low-mass galaxies are lognormally distributed with dispersion σ(In R 5 0 ) ∼05, in excellent agreement with the idea that they form with little angular momentum loss through cooling and condensation in a gravitationally dominant dark matter halo Their median stellar surface mass density scales with stellar mass as μ * M * 054, suggesting that the stellar mass of a disc galaxy is proportional to the three halves power of its halo mass All of this suggests that the efficiency of the conversion of baryons into stars in low-mass galaxies increases in proportion to halo mass, perhaps as a result of supernova feedback processes At stellar masses above 3 x 10 1 0 M O , there is a rapidly increasing fraction of galaxies with old stellar populations, high surface mass densities and the high concentrations typical of bulges In this regime, the size distribution remains lognormal, but its dispersion decreases rapidly with increasing mass and the median stellar mass surface density is approximately constant This suggests that the star formation efficiency decreases in the highest-mass haloes, and that little star formation occurs in massive galaxies after they have assembled

/pdf/the-dependence-of-star-formation-history-and-internal-1ptwh71cly.pdf

The dependence of star formation history and internal structure on stellar mass for 105 low‐redshift galaxies

Advancing Astrophysics with the Square Kilometre Array

The MMF technique is used to segment the cosmic web as seen in a cosmological N-body simulation into wall-like and filament-like structures. We find that the spins and shapes of dark matter halos are significantly correlated with each other and with the orientation of their host structures. The shape orientation is such that the halo minor axes tend to lie perpendicular to the host structure, be it a wall or filament. The orientation of the halo spin vector is mass-dependent. Low-mass halos in walls and filaments have a tendency to have their spins oriented within the parent structure, while higher mass halos in filaments have spins that tend to lie perpendicular to the parent structure.

Spin alignment of dark matter halos in filaments and walls

Aims. We present here a new method, MMF, for automatically segmenting cosmic structure into its basic components: clusters, fi laments, and walls. Importantly, the segmentation is scale independent, so all structures are identified without prejudice as to the ir size or shape. The method is ideally suited for extracting catalogues of clusters, wa lls, and filaments from samples of galaxies in redshift surve ys or from particles in cosmological N-body simulations: it makes no prior assumptions about the scale or shape of the structures. Methods. Our Multiscale Morphology Filter (MMF) method has been developed on the basis of visualization and feature extraction t echniques in computer vision and medical research. The density or intensity field of the sample is smoothed over a range of scales. Th e smoothed signals are processed through a morphology response filter whose for m is dictated by the particular morphological feature it see ks to extract, and depends on the local shape and spatial coherence of the intensity field. The morphology signal at each location is then defi ned to be the one with the maximum response across the full range of smoothing scales. The success of our method in identifying anisotropic features such as filaments and walls depends critically on the use of an optimally defined intensity field. This is accomplished by applying the DTFE re construction methodology to the sample particle or galaxy distribution. Results. We have tested our MMF Filter against a set of heuristic models of weblike patterns such as are seen in the Megaparsec cosmic matter distribution. To test its effectiveness in the context of more realistic configurations w e also present preliminary results from the MMF analysis of an N-body model. Comparison with alternative prescriptions for feature extraction shows that MMF is a remarkably strong structure finder

https://www.aanda.org/articles/aa/pdf/2007/40/aa7880-07.pdf

The Multiscale Morphology Filter: Identifying and Extracting Spatial Patterns in the Galaxy Distribution

Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891 reveal the presence of an H I halo extending up to at least 5 kpc from the plane This halo gas appears to rotate 25 to 100 km s(-1) more slowly than the gas in the plane If this velocity difference is due to the gradient in the gravitational potential, then it may serve to discriminate between disk and spheroidal mass models The classic picture of a large outer flare in the H I layer of NGC 891 may no longer be valid A correlation is seen between the distributions of H I, H alpha and radio continuum emission; this, in accordance with galactic fountain models, supports the picture of a substantial disk-halo circulation related to star formation activity in the disk of NGC 891 There is now also clear evidence for the presence of a rapidly rotating (upsilon(rot) similar or equal to 230 km s(-1)) disk or ring of H I in the central part of NGC 891

https://iopscience.iop.org/article/10.1086/304958/pdf

The H I Halo of NGC 891

Low surface brightness (LSB) galaxies are galaxies dominated by an exponential disc whose central surface brightness is much fainter than the value of mu(B)(0) = 21.65 +/- 0.30 mag arcsec(-2) found by Freeman. In this paper we present broadband photometry of a sample of 21 late-type LSB galaxies. The median central surface brightness of the sample is mu(B)(0) = 23.2 mag arcsec(-2) and the median scale length is 3.2 kpc, showing that LSB galaxies are normal-sized galaxies. We find colours that are comparable to or bluer than those of the more widely studied 'normal' high surface brightness (HSB) galaxies. LSB galaxies are therefore not faded discs that have no current star formation. The colours cannot on the other hand be ascribed entirely to metallicity effects, but we can explain them by assuming a sporadic star formation rate scenario. LSB galaxies hence appear to be unevolved and quiescent objects, which give us an insight into the evolution of galaxies in an unperturbed environment.

van der Thijs Hulst

Papers

Advancing Astrophysics with the Square Kilometre Array

Spin alignment of dark matter halos in filaments and walls

The Multiscale Morphology Filter: Identifying and Extracting Spatial Patterns in the Galaxy Distribution

The H I Halo of NGC 891

Surface photometry of low surface brightness galaxies