ABSTRACT
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.
In this paper, we propose a Q stability parameter that is more realistic than those commonly used, and is easy to evaluate (see equation 19). Using our
parameter, you can take into account several ...stellar and/or gaseous components as well as the stabilizing effect of disc thickness, you can predict which component dominates the local stability level, and you can do all that simply and accurately. To illustrate the strength of
, we analyse the stability of a large sample of spirals from The H i Nearby Galaxy Survey (THINGS), treating stars, H i and H2 as three distinct components. Our analysis shows that H2 plays a significant role in disc (in)stability even at distances as large as half the optical radius. This is an important aspect of the problem, which was missed by previous (two-component) analyses of THINGS spirals. We also show that H i plays a negligible role up to the edge of the optical disc and that the stability level of THINGS spirals is, on average, remarkably flat and well above unity.
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
The morphology of gas-rich disc galaxies at redshift $\sim 1\!-\!3$ is dominated by a few massive clumps. The process of formation or assembly of these clumps and their relation to molecular ...clouds in contemporary spiral galaxies are still unknown. Using simulations of isolated disc galaxies, we study how the structure of the interstellar medium and the stability regime of the discs change when varying the gas fraction. In all galaxies, the stellar component is the main driver of instabilities. However, the molecular gas plays a non-negligible role in the interclump medium of gas-rich cases, and thus in the assembly of the massive clumps. At scales smaller than a few 100 pc, the Toomre-like disc instabilities are replaced by another regime, especially in the gas-rich galaxies. We find that galaxies at low gas fraction (10 per cent) stand apart from discs with more gas, which all share similar properties in virtually all aspects we explore. For gas fractions below $\approx 20{{\ \rm per\ cent}}$, the clump-scale regime of instabilities disappears, leaving only the large-scale disc-driven regime. Associating the change of gas fraction to the cosmic evolution of galaxies, this transition marks the end of the clumpy phase of disc galaxies, and allows for the onset of spiral structures, as commonly found in the local Universe.
We study the impact of stellar feedback in shaping the density and velocity structure of neutral hydrogen (H i) in disc galaxies. For our analysis, we carry out ∼4.6 pc resolution N-body+adaptive ...mesh refinement hydrodynamic simulations of isolated galaxies, set up to mimic a Milky Way and a Large and Small Magellanic Cloud. We quantify the density and velocity structure of the interstellar medium using power spectra and compare the simulated galaxies to observed H i in local spiral galaxies from THINGS (The H i Nearby Galaxy Survey). Our models with stellar feedback give an excellent match to the observed THINGS H i density power spectra. We find that kinetic energy power spectra in feedback-regulated galaxies, regardless of galaxy mass and size, show scalings in excellent agreement with supersonic turbulence (E(k) ∝ k
−2) on scales below the thickness of the H i layer. We show that feedback influences the gas density field, and drives gas turbulence, up to large (kpc) scales. This is in stark contrast to density fields generated by large-scale gravity-only driven turbulence. We conclude that the neutral gas content of galaxies carries signatures of stellar feedback on all scales.
Using hydrodynamical simulations of entire galactic discs similar to the Milky Way (MW), reaching 4.6 pc resolution, we study the origins of observed physical properties of giant molecular clouds ...(GMCs). We find that efficient stellar feedback is a necessary ingredient in order to develop a realistic interstellar medium, leading to molecular cloud masses, sizes, velocity dispersions, and virial parameters in excellent agreement withMWobservations. GMCscaling relations observed in the MW, such as the mass-size (M-R), velocity dispersion-size (σ-R), and the σ-RΣ relations, are reproduced in a feedback-driven ISM when observed in projection, with M∝R2.3 and σ∝R0.56.When analysed in 3D, GMC scaling relations steepen significantly, indicating potential limitations of our understanding of molecular cloud 3D structure from observations. Furthermore, we demonstrate how a GMC population's underlying distribution of virial parameters can strongly influence the scatter in derived scaling relations. Finally, we show that GMCs with nearly identical global properties exist in different evolutionary stages, where a majority of clouds being either gravitationally bound or expanding, but with a significant fraction being compressed by external ISM pressure, at all times.
ABSTRACT
The specific angular momenta (j ≡ J/M) of stars (j⋆), gas (jgas), baryons as a whole (jb) and dark matter haloes (jh) 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⋆/jh, jH i/jh, and jb/jh), 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, 〈QH i〉, which succeeds in separating barred from non-barred galaxies, but only in a statistical sense.
Although recommender systems (RSs) play a crucial role in our society, previous studies have revealed that the performance of RSs may considerably differ between groups of individuals with different ...characteristics or from different demographics. In this case, a RS is considered to be unfair when it does not perform equally well for different groups of users. Considering the importance of RSs in the distribution and consumption of musical content worldwide, a careful evaluation of fairness in the context of music RSs is crucial. To this end, we first introduce LFM-2b, a novel large-scale real-world dataset of music listening records, comprising a subset to investigate bias of RSs regarding users’ demographics. We then define a notion of fairness based on the performance gap of a RS between the users with different demographics, and evaluate a variety of collaborative filtering algorithms in terms of accuracy and beyond-accuracy metrics to explore the fairness in the RS results toward a specific gender group. We observe the existence of significant discrepancies (unfairness) between the performance of algorithms across male and female user groups. Based on these discrepancies, we explore to what extent recommender algorithms lead to intensifying the underlying population bias in the final results. We also study the effect of a resampling strategy, commonly used as debiasing method , which yields slight improvements in the fairness measures of various algorithms while maintaining their accuracy and beyond-accuracy performance.
•Novel large-scale real-world dataset of music listening records.•Debiasing yields slight improvements of fairness recommendation algorithms.•Formalizing and measuring the extent of compounding data biases by recommendation algorithms.
ABSTRACT
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 σR, and present reliable radial profiles of σR as well as accurate measurements of 〈σR〉, the radial average of σR over one effective (half-light) radius. We show that there is a trend for σR to increase with decreasing R, that 〈σR〉 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 σR to the stellar surface density and disc scale length roughly reproduce that scaling, but overestimate 〈σR〉 significantly.
ABSTRACT
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⋆/Mh) 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⋆/Mh 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.
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