Fraction Of Disk Mass Research Articles

The Rapid Onset of Stellar Bars in the Baryon-dominated Centers of Disk Galaxies

Recent observations of high-redshift galactic disks (z ≈ 1–3) show a strong negative trend in the dark-matter (DM) fraction f DM with increasing baryon surface density. For this to be true, the inner baryons must dominate over DM in early massive galaxies, as observed in the Milky Way today. If disks are dominant at early times, we show that stellar bars form promptly within these disks, leading to a high bar fraction. New James Webb Space Telescope observations provide the best evidence for mature stellar bars in this redshift range. The disk mass fraction f disk within R s = 2.2 R disk is the dominant factor determining how rapidly a bar forms. Using 3D hydro simulations of halo-bulge-disk galaxies, we confirm the “Fujii relation” for the exponential dependence of the bar formation time τ bar as a function of f disk. For f disk > 0.3, the bar formation time declines exponentially fast with increasing f disk. Instead of Fujii's arbitrary threshold for when a bar appears, for the first time, we exploit the exponential growth timescale associated with the positive feedback cycle as the bar emerges from the underlying disk. A modified, mass-dependent trend is observed for halos relevant to systems at cosmic noon (), where the bar onset is slower for higher-mass halos at a fixed f disk. If baryons dominate over DM within R ≈ R s , we predict that a high fraction of bars will be found in high-redshift disks long before z = 1.

WALLABY Pilot Survey: The Diversity of Ram Pressure Stripping of the Galactic H i Gas in the Hydra Cluster

Abstract This study uses H i image data from the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) pilot survey with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, covering the Hydra cluster out to 2.5r 200. We present the projected phase–space distribution of H i-detected galaxies in Hydra, and identify that nearly two-thirds of the galaxies within 1.25 r 200 may be in the early stages of ram pressure stripping. More than half of these may be only weakly stripped, with the ratio of strippable H i (i.e., where the galactic restoring force is lower than the ram pressure in the disk) mass fraction (over total H i mass) distributed uniformly below 90%. Consequently, the H i mass is expected to decrease by only a few 0.1 dex after the currently strippable portion of H i in these systems has been stripped. A more detailed look at the subset of galaxies that are spatially resolved by WALLABY observations shows that, while it typically takes less than 200 Myr for ram pressure stripping to remove the currently strippable portion of H i, it may take more than 600 Myr to significantly change the total H i mass. Our results provide new clues to understanding the different rates of H i depletion and star formation quenching in cluster galaxies.

Estimating carbon loss due to internal decay in living trees using tomography: implications for forest carbon budgets

The world’s forests sequester and store vast amounts of atmospheric carbon, playing a crucial role in climate change mitigation. Internal stem decay in living trees results in the release of stored carbon back into the atmosphere, constituting an important, but poorly understood, countervailing force to carbon sequestration. The contribution of internal decay to estimates of forest carbon stocks, though likely significant, has yet to be quantified, given that an accurate method for the non-destructive quantification of internal decay has been lacking. To that end, we present here a novel and potentially transformative methodology, using sonic and electrical resistance tomography, for non-destructively quantifying the mass of stored carbon lost to internal decay in the boles of living trees. The methodology was developed using 72 northern hardwood trees (Fagus grandifolia, Acer saccharum and Betula alleghaniensis) from a late-successional forest in northwestern Connecticut, USA. Using 105 stem disks corresponding to tomographic scans and excised from 39 of the study’s trees, we demonstrate the accuracy with which tomography predicts the incidence and severity of internal decay and distinguishes active decay from cavities. Carbon mass fractions and densities, measured and calculated from 508 stem disk wood samples corresponding to density categories, as predicted by sonic tomography, were used with stem disk volumes to generate indirect estimates of stem disk carbon mass accounting for decay, CSD, or assuming no decay, CND; these indirect estimates were compared with direct estimates calculated using stem disk mass, Cmass, and carbon mass fraction data. A comparison of three linear regression models with Cmass as the response variable and CSD or CND as the predictor variable ( R2 = 0.9733, Model 1; Cmass ∼ CND, ) demonstrates the accuracy with which CSD predicts Cmass. Forcing the regression through the origin resulted in improved metrics ( Model 2) for which a null hypothesis that y = x (Model 3) could not be rejected (). For each of the study’s 72 trees, two estimates of lower bole carbon mass—Cbole, accounting for decay, and assuming no decay—were obtained using all three models, with the difference between Cbole and used to estimate the proportion of the lower bole’s carbon lost to decay, Overall, tomography identified decay in 47 of the 72 trees, with values ranging from 0.13% to 36.7%. No decay was detected by tomography in the remaining 25 trees. The combined uncertainty due to both measurement error and model prediction error was ±2.1% for all three models. These results demonstrate the efficacy of the proposed methodology in non-destructively quantifying the carbon loss associated with internal decay in the boles of living trees, and its applicability to studies aimed at measuring internal decay rates, and more accurately quantifying forest carbon stocks.

Global N-body simulation of galactic spiral arms

The origin of galactic spiral arms is one of the fundamental problems in astrophysics. Based on the local analysis, Toomre (1981) proposed the swing amplification mechanism in which the self-gravity forms spiral arms as leading waves of stars rotate to trailing ones due to galactic shear. The structure of spiral arms is characterized by their number and pitch angle. We perform global N-body simulations of spiral galaxies to investigate the dependence of the spiral structure on disc parameters and compare the simulation results with the swing amplification model. We find that the spiral structure in the N-body simulations agrees well with that predicted by the swing amplification for the wide range of parameters. The pitch angle decreases with increasing the shear rate and is independent of the disc mass fraction. The number of spiral arms decreases with both increasing the shear rate and the disc mass fraction. If the disc mass fraction is fixed, the pitch angle increases with the number of spiral arms.

The dynamics of stellar discs in live dark-matter haloes

Recent developments in computer hardware and software enable researchers to simulate the self-gravitating evolution of galaxies at a resolution comparable to the actual number of stars. Here we present the results of a series of such simulations. We performed $N$-body simulations of disk galaxies with between 100 and 500 million particles over a wide range of initial conditions. Our calculations include a live bulge, disk, and dark matter halo, each of which is represented by self-gravitating particles in the $N$-body code. The simulations are performed using the gravitational $N$-body tree-code Bonsai running on the Piz Daint supercomputer. We find that the time scale over which the bar forms increases exponentially with decreasing disk-mass fraction and that the bar formation epoch exceeds a Hubble time when the disk-mass fraction is $\sim0.35$. These results can be explained with the swing-amplification theory. The condition for the formation of $m=2$ spirals is consistent with that for the formation of the bar, which is also an $m=2$ phenomenon. We further argue that the non-barred grand-design spiral galaxies are transitional, and that they evolve to barred galaxies on a dynamical timescale. We also confirm that the disk-mass fraction and shear rate are important parameters for the morphology of disk galaxies. The former affects the number of spiral arms and the bar formation epoch, and the latter determines the pitch angle of the spiral arms.

A study of the effect of bulges on bar formation in disc galaxies

We use N-body simulations of bar formation in isolated galaxies to study the effect of bulge mass and bulge concentration on bar formation. Bars are global disk instabilities that evolve by transferring angular momentum from the inner to outer disks and to the dark matter halo. It is well known that a massive spherical component such as halo in a disk galaxy can make it bar stable. In this study we explore the effect of another spherical component, the bulge, on bar formation in disk galaxies. In our models we vary both the bulge mass and concentration. We have used two sets of models, one that has a dense bulge and high surface density disk. The second model has a less concentrated bulge and a lighter disk. In both models we vary the bulge to disk mass fraction from 0 to 0.7. Simulations of both the models show that there is an upper cutoff in bulge to disk mass ratio M b /M d above which bars cannot form; the cutoff is smaller for denser bulges( M b /M d = 0.2) compared to less denser ones (M b /M d = 0.5). We define a new criteria for bar formation in terms of bulge to disk radial force ratio (F b /F d ) at the disk scale lengths above which bars cannot form. We find that if F b /F d > 0.35, a disk is stable and a bar cannot form. Our results indicate that early type disk galaxies can still form strong bars in spite of having massive bulges.

Simulating a slow bar in the low surface brightness galaxy UGC 628

We present a disc-halo N-body model of the low surface brightness galaxy UGC 628, one of the few systems that harbours a "slow" bar with a ratio of corotation radius to bar length of $\mathcal{R} \equiv R_c/a_b \sim 2$. We select our initial conditions using SDSS DR10 photometry, a physically motivated radially variable mass-to-light ratio profile, and rotation curve data from the literature. A global bar instability grows in our submaximal disc model, and the disc morphology and dynamics agree broadly with the photometry and kinematics of UGC 628 at times between peak bar strength and the onset of buckling. Prior to bar formation, the disc and halo contribute roughly equally to the potential in the galaxy's inner region, giving the disc enough self gravity for bar modes to grow. After bar formation there is significant mass redistribution, creating a baryon dominated inner and dark matter dominated outer disc. This implies that, unlike most other low surface brightness galaxies, UGC 628 is not dark matter dominated everywhere. Our model nonetheless implies that UGC 628 falls on same the relationship between dark matter fraction and rotation velocity found for high surface brightness galaxies, and lends credence to the argument that the disc mass fraction measured at the location where its contribution to the potential peaks is not a reliable indicator of its dynamical importance at all radii.

Suppression of gravitational instabilities by dominant dark matter halo in low-surface-brightness galaxies

The low surface brightness galaxies are gas-rich and yet have a low star formation rate, this is a well-known puzzle. The spiral features in these galaxies are weak and difficult to trace, although this aspect has not been studied much. These galaxies are known to be dominated by the dark matter halo from the innermost regions. Here we do a stability analysis for the galactic disc of UGC 7321, a low surface brightness, superthin galaxy, for which the various observational input parameters are available. We show that the disc is stable against local, linear axisymmetric and non-axisymmetric perturbations. The Toomre Q parameter values are found to be large (>> 1) mainly due to the low disc surface density and the high rotation velocity resulting due to the dominant dark matter halo, which could explain the observed low star formation rate. For the stars-alone case, the disc shows finite swing amplification but the addition of dark matter halo suppresses that amplification almost completely. Even the inclusion of the low-dispersion gas which constitutes a high disc mass fraction does not help in causing swing amplification. This can explain why these galaxies do not show strong spiral features. Thus the dynamical effect of a halo that is dominant from inner regions can naturally explain why star formation and spiral features are largely suppressed in low surface brightness galaxies, making these different from the high surface brightness galaxies.

Disc evolution and the relationship between Laccand L*in T Tauri stars

We investigate the evolution of accretion luminosity Lace and stellar luminosity L. in pre-main-sequence stars. We make the assumption that when the star appears as a Class II object, the major phase of accretion is long past and the accretion disc has entered its asymptotic phase. We use an approximate stellar evolution scheme for accreting pre-main-sequence stars based on Hartmann, Cassen & Kenyon. We show that the observed range of values k = L acc /L* between 0.01 and 1 can be reproduced if the values of the disc mass fraction M disc /M* at the start of the T Tauri phase lie in the range 0.01-0.2, independent of stellar mass. We also show that the observed upper bound of Lace ∼ L* is a generic feature of such disc accretion. We conclude that as long as the data uniformly fill the region between this upper bound and observational detection thresholds, then the degeneracies between age, mass and accretion history severely limit the use of this data for constraining possible scalings between disc properties and stellar mass.

An N-body/SPH study of isolated galaxy mass density profiles

We investigate the evolution of mass density profiles in secular disk galaxy models, paying special attention to the development of a two-component profile from a single initial exponential disk free of cosmological evolution (i.e., no accretion or interactions). As the source of density profile variations, we examine the parameter space of the spin parameter, halo concentration, virial mass, disk mass and bulge mass, for a total of 162 simulations in the context of a plausible model of star formation and feedback. The isolated galaxy models are based on the method of Springel & White (1999) and were evolved using the N-body/SPH code GADGET-2. The initially pure exponential disks have a minimum of 1.4 million particles and most models were evolved over a period of 10 Gyr. We find that the slope of the outer density profile is in close agreement with that of the initial profile and remains stable over time, whereas the inner density profile slope evolves considerably as a result of angular momentum redistribution. The evolution of the galaxy mass density profile, including the development of a two-component profile with an inner and outer segment, is controlled by the ratio of the disk mass fraction, md, to the halo spin parameter, λ. The location of the break between the two components and speed at which it develops is directly proportional to md/λ; the amplitude of the transition between the inner and outer regions is however controlled by the ratio of halo concentration to virial velocity. The location of the divide between the inner and outer profile does not change with time. The condition for a two-component profile is roughly md/λ > 1. While the development of a two-component density profile is coupled to bar formation, not all barred galaxies develop a two-component profile. A galaxy model showing a clear minimum Toomre Q, normally linked to a double exponential in the stellar profile, may never exhibit any two-component feature thus yielding a fully evolved pure exponential disk.

A Revised Model for the Formation of Disk Galaxies: Low Spin and Dark Halo Expansion

We use observed rotation velocity-luminosity (VL) and size-luminosity (RL) relations to single out a specific scenario for disk galaxy formation in the ΛCDM cosmology. Our model involves four independent lognormal random variables: dark halo concentration c, disk spin λgal, disk mass fraction mgal, and stellar mass-to-light ratio ϒI. A simultaneous match of the VL and RL zero points with adiabatic contraction requires low-c halos, but this model has V2.2 ~ 1.8Vvir (where V2.2 and Vvir are the circular velocity at 2.2 disk scale lengths and the virial radius, respectively), which will be unable to match the luminosity function (LF). Similarly models without adiabatic contraction but standard c also predict high values of V2.2/Vvir. Models in which disk formation induces an expansion rather than the commonly assumed contraction of the dark matter halos have V2.2 ~ 1.2Vvir, which allows a simultaneous fit of the LF. This may result from nonspherical, clumpy gas accretion, where dynamical friction transfers energy from the gas to the dark matter. This model requires low λgal and mgal values, contrary to naive expectations. However, the low λgal is consistent with the notion that disk galaxies predominantly survive in halos with a quiet merger history, while a low mgal is also indicated by galaxy-galaxy lensing. The smaller than expected scatter in the RL relation and the lack of correlation between the residuals of the VL and RL relations, respectively, imply that the scatter in λgal and in c needs to be smaller than predicted for ΛCDM halos, again consistent with the idea that disk galaxies preferentially reside in halos with a quiet merger history.

Disc galaxy evolution models in a hierarchical formation scenario: structure and dynamics

We predict the internal structure and dynamics of present-day disc galaxies using galaxy evolution models within a hierarchical formation scenario. The halo mass aggregation histories, for a flat cold dark matter model with cosmological constant, were generated and used to calculate the virialization of dark matter haloes. A diversity of halo density profiles were obtained, the most typical one being close to that suggested by Navarro, Frenk & White. We modelled the way in which discs in centrifugal equilibrium are built within the evolving dark haloes, using gas accretion rates proportional to the halo mass aggregation rates, and assuming detailed angular momentum conservation. We calculated the gravitational interactions between halo and disc — including the adiabatic contraction of the halo due to disc formation — and the hydrodynamics, star formation and evolution of the galaxy discs. We find that the slope and zero-point of the Tully-Fisher (TF) relation in the infrared bands may be explained as a direct consequence of the cosmological initial conditions. This relation is almost independent of the assumed disc mass fraction, when the disc component in the rotation curve decomposition is non-negligible. Thus, the power spectrum of fluctuations can be normalized at galaxy scales through the TF relation independently of the disc mass fraction assumed. The rms scatter of the model TF relation originates mainly from the scatter in the dark halo structure and, to a minor extension, from the dispersion of the primordial spin parameter λ. The scatter obtained from our models does not disagree with the observational estimates. Our models allow us to understand why the residuals of the TF relation do not correlate significantly with disc size or surface brightness. We can also explain why low and high surface brightness galaxies have the same TF relation; the key point is the dependence of the star formation efficiency on the disc surface density. The correlations between gas fraction and surface brightness, and between scalelength and Vmax obtained with our models agree with those observed. Discs formed within the growing haloes, where λ is assumed to be time independent, have nearly exponential surface density distributions. The shape of the rotation curves changes with disc surface brightness and is nearly flat for most cases. The rotation curve decompositions show a dominance of dark matter down to very small radii, in conflict with some observational inferences. The introduction of shallow cores in the dark halo attenuates this difficulty and produces haloes with slightly smaller rotation velocities. Other features of our galaxy models are not strongly influenced by the shallow core.

The G‐dwarf problem in the solar neighbourhood: a Lagrangian picture. II

AbstractThe current paper investigates how the empirical, G‐dwarf metallicity distribution constrains simple, comoving models of chemical evolution. In doing this, the application of the models to a data sample, performed in a previous paper, is refined and extended. The key idea is that (i) different star formation rates with different mass spectra take place in different phases of evolution, i.e. contraction and equilibrium, and (ii) disk formation begins at a time t = Td and ends at t = Tc, which marks the transition from contraction to equilibrium. In this view, the lowest‐metallicity point of the empirical, differential distribution, consistent with a linear fit, is related to the beginning of disk formation, and an apparent discontinuity point to the transition from contraction to equilibrium. In addition, different linear fits hold on the left (early distribution) and on the right (late distribution) of the discontinuity point. Models consistent with the empirical, G‐dwarf metallicity distribution are related to linear fits on the early and late side. Homologous solutions during the equilibrium phase are analysed in detail with respect to changes in Tc and Ta, the age of the Galaxy. Then we are left with a single free parameter which is relevant to the chemical evolution, i.e. the mass spectrum exponent during the equilibrium phase. The allowed values for the other parameters, thought as a function of the above mentioned one, are plotted for each case. A Salpeter mass spectrum exponent, p = −2.35, is ruled out by the theoretical, lower stellar mass limit, contrary to a Scalo mass spectrum exponent, p = −2.90, in contrast with previous literature. The reasons for this discrepancy are discussed. Our results are marginally consistent with a same initial mass function during the contraction and equilibrium phase, but in this case the disk mass fraction is of the is same order, or less, than the halo mass fraction. It is also investigated how the empirical age‐metallicity relation constrains the duration of the contraction phase, for a reasonable upper limit of Ta. Keeping in mind that the empirical, G‐dwarf metallicity distribution has not been corrected for the large cosmic scatter shown by the empirical, age‐metallicity relation, we find a duration of disk formation, Tc — Td = 1.07–1.5 Gyr, by a factor 3–5 less than it is found by use of simple infall models. The reasons of this difference are explained. The idea of a massive, white dwarf halo, which seems to be indicated by microlensing experiments, is ruled out by the empirical, G‐dwarf metallicity distribution, in the light of the current model and provided the solar neighbourhood is a typical region of the Galaxy. More refined models involving e.g., the relax of instantaneous recycling would change our results, but the trend is expected to be only slightly altered.

The G‐dwarf problem in the solar neighbourhood: a Lagrangian picture

AbstractRecent data on the empirical metallicity distribution of G dwarfs in the disk solar neighbourhood are fitted in two different ways. We use an extended Poisson distribution in the limit where the probability of star formation is small, and a Gauss distribution in the limit where a large number of physical variables is required to determine stellar metal abundance. Both are found to reproduce the data at the same (acceptable) extent, with a slight preference for the former. The emprirical, differential metallicity distribution of G dwarfs in the disk solar neighbourhood is compared with its theoretical counterpart, in the picture of a closed, comoving model of chemical evolution. The limits of the currently used infall models are discussed and a scenario of galactic formation and evolution is presented. The Galactic history is thought as made of two main phases: contraction (which produces the extended component) and equilibrium (which gives the disk). In this view, the stars observed within the solar cylinder did not necessarily arise from the primordial gas which later collapsed into the disk solar neighbourhood. It is found that the G‐dwarf problem is strongly alleviated, with the possible exception of the low‐metallicity and high‐metallicity tail of the distribution. The best choice of parameters implies: (i) a metal yield in the contraction phase which is larger by a factor of about 5 with respect to the equilibrium phase; (ii) a model halo mass fraction of about 0.3; (iii) a model disk mass fraction of about 0.6. It provides additional support to the idea of a generalized Schmidt star formation law, which is different in different phases of evolution. The model, cumulative, G‐dwarf metallicity distribution in the disk solar neighbourhood is found to predict too may low‐metallicity stars with respect to its empirical counterpart, related to a Poissonian or Gaussian fit. The main resons for the occurrence of a G‐dwarf problem are discussed. Finally, a stochastic process of star formation, related to a Poisson distribution, is briefly outlined.

Chemical evolution of two-component galaxies

In this paper, we try to insert into a single evolutionary scheme — in dealing with chemical evolution of galaxies — two different viewpoints that (at least in not too much complicated models) have been treated separately: namely, theS models, allowing mass conservation; andI models, allowing initial zero masses and no mass conservation due to gas inflow. The true evolution of a real proto-galaxy (after reaching the state of maximum expansion) is simulated as follows: A spheroidal gas mass continued to collapse and form stars until a flat configuration is reached after a timeT c has elapsed, while a given amount of gas flows in on a time-scale τ. According to this scheme, the basic equations of chemical evolution are derived and models which simulate the history of solar neighborhood, other regions and Galactic spheroid component are built up, in the whole range between theS-limit (mass conservation) and theI-limit (zero initial mass and subsequent accretion due to inflowing gas). Concerning the solar neighbourhood, we find that neither the occurrence of gas inflow nor inflow on time-scales τ≈2–3 109 yr are necessary in order to reproduce the temporal behaviour and the empirical distribution of metal content, as pointed out by some authors. On the contrary, the constraint on the lower mass limit for stars formed,m mf≳0.01, allows only models with τ≈T c (i.e. inflow time-scale of the order of the contraction time), while the constraint on the disk mass fraction,R D(T a)≲0.75, rules out the cases near theI-limit forT c≈0.55 but permits all cases forT c≈2.75. Concerning other regions, models are built up which roughly simulate elliptical, spiral and irregular galaxies, and all less extended regions resembling such systems. If the stellar birthrate function is assumed to be an universal law, the chemical evolution of the Galactic disk may be understood in terms of different zones (that might be thought as concentric and coaxial rings) the total density of which decreases monotonically, owing to a corresponding decrease in total mass and/or increase in volume, when passing from the center to the border of the disk. The constraintsm mf≳0.01 andR D(T a)≲0.75 for different regions of the Galactic disk would also rule out all models well beyond theS-limit, but further results are required in order to confirm this conclusion. Finally, concerning the Galactic spheroid component, it is found that onlyS models with massive halos (R D(T a)≈0.01) are able to reproduce in an acceptable way the empirical metal abundance distribution. In order to obtain a complete fit, a spheroid component has to be assumed, with a steeper mass spectrum exponent in the stellar birthrate function, and a lower yield of metallicity, in respect to the disk component. According to this last model, a mean value of disk metal content (with respect to spatial distribution) of the order of the solar value also results.

Chemical evolution with inhibited star formation rate

In this paper a simple model of chemical evolution is proposed, according to which active phases of star formation occured, each followed by a quiescent phase where the formation of stars withm≲1m⊙ was totally inhibited. The contraction of the gaseous component of the system was not stopped during quiescent phases, according to observed distributions of chemical and dynamical parameters in objects of different populations. Building up a detailed model we assume that: (i) star formation is inhibited during a given active phase, at a rate which is connected to the number of supernovae born at that time; (ii) total mass is conserved in the unit comoving volume; (iii) instantaneous recycling approximation holds. According to these hypothesis, theoretical metal abundance distributions as function of a single parameter\(\bar \mu _f \) may be derived and then compared with empirical distributions related to different kinds of populations. It is found that theoretical curves systematically overestimate the objects of lower metallicity and underestimate the objects of higher metallicity in respect to observations, as a possible consequence of the last two assumptions reported above. If we leave the detailed explanation of this effect to more refined models and taking other observational data into account, the chemical evolution of both Galactic spheroid component and solar neighbourhood may be derived and the results are to be considered as essentially qualitative. If three active phases of star formation occurred during the history of the Galactic spheroid component, the yield of metals,\(\hat p\), had to increase passing from an active phase to the next even if\(\hat p\,\alpha \,Z\) (metal content) and the disk mass fraction,RD, had not to exceed a value of about 0.4 when\(\hat p\, \approx 0.01\) is assumed as a characteristic value for the disk component and the main part of this last is thought to have formed at the end of the second active phase. If a single active (and no quiescent) phase occurred during the history of the solar neighbourhood, the yield did not increase appreciably up to to-day and no more than half (if any) of the initial mass could escape from the system in form of galactic wind. A disk mass fractionRD≲0.2 is further derived, if a birth-rate stellar function smoothly dependent on the gas mass fraction and a ratio of mean past to present star formation rate of the order of unity are to be reproduced, in agreement with recent observations.