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The specific angular momenta (j ≡ J / M) of stars (j ⋆), gas (j gas), baryons as a whole (j b) and dark matter haloes (j h) contain clues of vital importance about how galaxies form and evolve. Using one of the largest samples of disc galaxies (S0–BCD) with high-quality rotation curves and near-infrared surface photometry, we perform a detailed comparative analysis of j that stretches across a variety of galaxy properties. Our analysis imposes tight constraints on the 'retained' fractions of specific angular momentum (j ⋆/ j h, j H  i / j h, and j b/ j h), as well as on their systematic trends with mass fraction and galaxy morphology, thus on how well specific angular momentum is conserved in the process of disc galaxy formation and evolution. In particular, one of the most innovative results of our analysis is the finding that galaxies with larger baryon fractions have also retained larger fractions of their specific angular momentum. Furthermore, our analysis demonstrates how challenging it is to characterize barred galaxies from a gravitational instability point of view. This is true not only for the popular Efstathiou, Lake & Negroponte bar instability criterion, which fails to separate barred from non-barred galaxies in about 55 per cent of the cases, but also for the mass-weighted Toomre parameter of atomic gas, 〈 Q H  i 〉, which succeeds in separating barred from non-barred galaxies, but only in a statistical sense. [ABSTRACT FROM AUTHOR]

The stellar-to-halo mass relation (SHMR) is not only one of the main sources of information we have on the connection between galaxies and their dark matter haloes, but also an important indicator of the performance of galaxy formation models. Here, we use one of the largest sample of galaxies with both high-quality rotation curves and near-infrared surface photometry, and perform a detailed comparative analysis of the SHMR. Our analysis shows that there are significant statistical differences between popular forms of the SHMR, and illustrates the predictive power of a new physically motivated scaling relation, which connects the stellar mass fraction (M ⋆/ M h) to the stellar specific angular momentum (j ⋆) and the stellar radial velocity dispersion (σ⋆) via disc gravitational instability. Making use of such a relation, we demonstrate (i) how challenging it is to reproduce the efficiency of galaxy formation even for state-of-the-art cosmological hydrodynamical simulations, and (ii) that the evolution of the stellar mass fraction is regulated by disc gravitational instability: when M ⋆/ M h varies, j ⋆ and σ⋆ also vary as predicted by our scaling relation, thus erasing the memory of such evolution. This implies that the process of disc gravitational instability is intriguingly uniform across disc galaxies of all morphological types: from lenticulars to blue compact dwarfs. In particular, the cosmic variance of Toomre's Q is 0.2 dex, a universal value for both stars and atomic gas. [ABSTRACT FROM AUTHOR]

We present a new set of galaxy scaling relations for the relative mass content of atomic gas, molecular gas, and stars. Such relations are driven by disc gravitational instability, and originate from the low galaxy-to-galaxy variance of Toomre's Q stability parameter. We test such relations using more than 100 galaxies, from massive spirals to dwarf irregulars, thus spanning several orders of magnitude in stellar mass (⁠|$M_{\star }\approx 10^{6\rm {-}11}\, \mbox{M}_{\odot }$|⁠) and atomic gas mass (⁠|$M_{\rm{H\, \small {I}}}\approx 10^{7\rm {-}10.5}\, \mbox{M}_{\odot }$|⁠). Such tests demonstrate (i) that our scaling relations are physically motivated and tightly constrained, (ii) that the mass-averaged gravitational instability properties of galaxy discs are remarkably uniform across the sequence Sa–dIrr, and (iii) that specific angular momentum plays an important role in such a scenario. Besides providing new insights into a very important topic in galaxy evolution, this work provides a simple formula (equation 5) that one can use for generating other galaxy relations driven by disc instability. We explain how to do that, mention a few possible applications, and stress the importance of testing our approach further. [ABSTRACT FROM AUTHOR]

The stellar velocity dispersion, σ, is a quantity of crucial importance for spiral galaxies, where it enters fundamental dynamical processes such as gravitational instability and disc heating. Here we analyse a sample of 34 nearby spirals from the Calar Alto Legacy Integral Field Area (CALIFA) spectroscopic survey, deproject the line-of-sight σ to σ R , and present reliable radial profiles of σ R as well as accurate measurements of 〈σ R 〉, the radial average of σ R over one effective (half-light) radius. We show that there is a trend for σ R to increase with decreasing R , that 〈σ R 〉 correlates with stellar mass (M ⋆), and tested correlations with other galaxy properties. The most significant and strongest correlation is the one with M ⋆: |$\langle \sigma _{R}\rangle \propto M_{\star }^{0.5}$|⁠. This tight scaling relation is applicable to spiral galaxies of type Sa–Sd and stellar mass M ⋆ ≈ 109.5–1011.5 M⊙. Simple models that relate σ R to the stellar surface density and disc scale length roughly reproduce that scaling, but overestimate 〈σ R 〉 significantly. [ABSTRACT FROM AUTHOR]

We introduce a new diagnostic for exploring the link between angular momentum and local gravitational instability in galaxy discs. Our diagnostic incorporates the latest developments in disc instability research, is fully consistent with approximations that are widely used for measuring the stellar specific angular momentum, j ⋆ = J ⋆/ M ⋆, and is also very simple. We show that such a disc instability diagnostic hardly correlates with j ⋆ or M ⋆, and is remarkably constant across spiral galaxies of any given type (Sa–Sd), stellar mass (⁠|$M_{\star }=10^{9.5}\mbox{--}10^{11.5}\, \mbox{M}_{\odot }$|⁠), and velocity dispersion anisotropy (σ |$z$| ⋆/σ R ⋆ = 0–1). The fact that M ⋆ is tightly correlated with star formation rate, molecular gas mass (M mol), metallicity (12 + log O/H), and other fundamental galaxy properties thus implies that nearby star-forming spirals self-regulate to a quasi-universal disc stability level. This not only proves the existence of the self-regulation process postulated by several star formation models, but also raises important caveats. [ABSTRACT FROM AUTHOR]

The velocity dispersion of cold interstellar gas, σ, is one of the quantities that most radically affect the onset of gravitational instabilities in galaxy discs, and the quantity that is most drastically approximated in stability analyses. Here we analyse the stability of a large sample of nearby star-forming spirals treating molecular gas, atomic gas and stars as three distinct components, and using radial profiles of σCO and σHI derived from HERA CO-Line Extragalactic Survey (HERACLES) and The HI Nearby Galaxy Survey (THINGS) observations. We show that the radial variations of σCO and σHI have a weak effect on the local stability level of galaxy discs, which remains remarkably flat and well above unity, but is low enough to ensure (marginal) instability against non-axisymmetric perturbations and gas dissipation. More importantly, the radial variation of σCO has a strong impact on the size of the regions over which gravitational instabilities develop, and results in a characteristic instability scale that is one order of magnitude larger than the Toomre length of molecular gas. Disc instabilities are driven, in fact, by the self-gravity of stars at kiloparsec scales. This is true across the entire optical disc of every galaxy in the sample, with a few exceptions. In the linear phase of the discinstability process, stars and molecular gas are strongly coupled, and it is such a coupling that ultimately triggers local gravitational collapse/fragmentation in the molecular gas. [ABSTRACT FROM AUTHOR]

We explore the role that gravitational instability plays in NGC 1068, a nearby Seyfert galaxy that exhibits unusually vigorous starburst activity. For this purpose, we use the Romeo-Falstad disc instability diagnostics and data from the BIMA Survey of Nearby Galaxies, the Sloan Digital Sky Survey and the Spectrographic Areal Unit for Research on Optical Nebulae. Our analysis illustrates that NGC 1068 is a gravitationally unstable 'monster'. Its starburst disc is subject to unusually powerful instabilities. Several processes, including feedback from the active galactic nucleus and starburst activity, try to quench such instabilities from inside out by depressing the surface density of molecular gas across the central kpc, but they do not succeed. Gravitational instability 'wins' because it is driven by the stars via their much higher surface density. In this process, stars and molecular gas are strongly coupled, and it is such a coupling that ultimately triggers local gravitational collapse/fragmentation in the molecular gas. [ABSTRACT FROM AUTHOR]

Gravitational instabilities play a primary role in shaping the clumpy structure and powering the star formation activity of gas-rich high-redshift galaxies. Here, we analyse the stability of such systems, focusing on the size and mass ranges of unstable regions in the disc. Our analysis takes into account the mass–size and linewidth–size scaling relations observed in molecular gas, originally discovered by Larson. We show that such relations can have a strong impact on the size and mass of star-forming clumps, as well as on the stability properties of the disc at all observable scales, making the classical Toomre parameter a highly unreliable indicator of gravitational instability. For instance, a disc with Q = 1 can be far from marginal instability, while a disc with Q ≪ 1 can be marginally unstable. Our work raises an important caveat: if clumpy discs at high redshift have scale-dependent surface densities and velocity dispersions, as implied by the observed clump scaling relations, then we cannot thoroughly understand their stability and star formation properties unless we perform multiscale observations. This will soon be possible thanks to dedicated Atacama Large Millimeter/submillimeter Array surveys, which will explore the physical properties of supergiant molecular clouds at the peak of cosmic star formation and beyond. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the gravitational instability of galactic discs, treating stars and cold interstellar gas as two distinct components, and taking into account the phenomenology of turbulence in the interstellar medium (ISM), i.e. the Larson-type scaling relations observed in the molecular and atomic gas. Besides deriving general properties of such systems, we analyse a large sample of galaxies from The H i Nearby Galaxy Survey (THINGS), and show in detail how interstellar turbulence affects disc instability in star-forming spirals. We find that turbulence has a significant effect on both the inner and the outer regions of the disc. In particular, it drives the inner gas disc to a regime of transition between two instability phases and makes the outer disc more prone to star-dominated instabilities. [ABSTRACT FROM AUTHOR]

We explore the gravitational instability of clumpy and turbulent gas discs, taking into account the Larson-type scaling laws observed in giant molecular clouds (GMCs) and H i, as well as more general scaling relations. This degree of freedom is of special interest in view of the coming high- z interstellar medium surveys and is thus potentially important for understanding the dynamical effects of turbulence at all epochs of galaxy evolution. Our analysis shows that turbulence has a deep impact on the gravitational instability of the disc. It excites a rich variety of stability regimes, several of which have no classical counterpart. Among other diagnostics, we provide two useful tools for observers and simulators: (1) the stability map of turbulence, which illustrates our stability scenario and relates it to the phenomenology of interstellar turbulence: GMC/H i observations, simulations and models; (2) a Toomre-like stability criterion, , which applies to a large class of clumpy/turbulent discs. We make specific predictions about GMC and cold-H i turbulence and point out the implications of our analysis for high- z galaxy surveys. [ABSTRACT FROM AUTHOR]