Rotating Disk Galaxy Research Articles

How does dark matter stabilize disc galaxies?

ABSTRACT The study presents a theoretical framework for understanding the role of dark matter on the stability of the galactic disc. We model the galaxy as a two-component system consisting of stars and gas in equilibrium with an external dark matter halo. We derive the equations governing the growth of perturbations and obtain a stability criterion that connects the potential of the dark matter halo and the gas fraction with the stability levels of the galaxy. We find that a two-component disc is more susceptible to the growth of gravitational instabilities than individual components, particularly as gas fractions increase. However, the external field, due to the dark matter halo, acts as a stabilizing agent and increases the net stability levels even in the presence of a cold gas component. We apply the stability criterion to models of the Milky Way, low surface brightness galaxies, and baryon-dominated cold rotating disc galaxies observed in the early universe. Our results show that the potential due to the dark matter halo plays a significant role in stabilizing nearby galaxies, such as the Milky Way, and low surface brightness galaxies, which would otherwise be prone to local gravitational instabilities. However, we find that the baryon-dominated cold disc galaxies observed in the early universe remain susceptible to the growth of local gravitational instabilities despite the stabilizing effect of the dark matter halo.

Structure and kinematics of a massive galaxy at z ∼ 7

Context. Observations of the rest-frame UV emission of high-redshift galaxies suggest that the early stages of galaxy formation involve disturbed structures. Imaging the cold interstellar medium (ISM) can provide a unique view of the kinematics associated with the assembly of galaxies. Aims. In this paper, we analyze the spatial distribution and kinematics of the cold ionized gas of the normal star-forming galaxy COS-2987030247 at z = 6.8076, based on new high-resolution observations of the [C II] 158 μm line emission obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Methods. The analysis of these observations allowed us to: compare the spatial distribution and extension of the [C II] and rest-frame UV emission, model the [C II] line data-cube using the 3DBAROLO code, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions. Results. The system is found to be composed of a main central source, a fainter north extension, and candidate [C II] companions located 10-kpc away. We find similar rest-frame UV and [C II] spatial distributions, suggesting that the [C II] emission emerges from the star-forming regions. The agreement between the UV and [C II] surface brightness radial profiles rules out diffuse, extended [C II] emission (often called a [C II] halo) in the main galaxy component. The [C II] velocity map reveals a velocity gradient in the north-south direction, suggesting ordered motion, as commonly found in rotating-disk galaxies. However, higher resolution observations would be needed to rule out a compact merger scenario. Our model indicates an almost face-on galaxy (i ∼ 20°), with a average rotational velocity of 86 ± 16 km s−1 and a low average velocity dispersion, σ < 30 km s−1. This result implies a dispersion lower than the expected value from observations and semi-analytic models of high redshift galaxies. Furthermore, our measurements indicate that COS-2987030247 and its individual regions systematically lie within the local L[CII]-SFR relationship, yet slightly below the local Σ[CII]-ΣUV relation. Conclusions. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will possibly become perturbed at later times by accreting sources.

Fundamental Properties of the Dark and the Luminous Matter from the Low Surface Brightness Discs

Dark matter (DM) is one of the biggest mystery in the Universe. In this review, we start reporting the evidences for this elusive component and discussing about the proposed particle candidates and scenarios for such phenomenon. Then, we focus on recent results obtained for rotating disc galaxies, in particular for low surface brightness (LSB) galaxies. The main observational properties related to the baryonic matter in LSBs, investigated over the last decades, are briefly recalled. Next, these galaxies are analyzed by means of the mass modelling of their rotation curves both individual and stacked. The latter analysis, via the universal rotation curve (URC) method, results really powerful in giving a global or universal description of the properties of these objects. We report the presence in LSBs of scaling relations among their structural properties that result comparable with those found in galaxies of different morphologies. All this confirms, in disc systems, the existence of a strong entanglement between the luminous matter (LM) and the dark matter (DM). Moreover, we report how in LSBs the tight relationship between their radial gravitational accelerations g and their baryonic components gb results to depend also on the stellar disk length scale and the radius at which the two accelerations have been measured. LSB galaxies strongly challenge the ΛCDM scenario with the relative collisionless dark particle and, alongside with the non-detection of the latter, contribute to guide us towards a new scenario for the DM phenomenon.

A cold, massive, rotating disk galaxy 1.5 billion years after the Big Bang.

Massive disk galaxies like the Milky Way are expected to form at late times in traditional models of galaxy formation1,2, but recent numerical simulations suggest that such galaxies could form as early as a billion years after the Big Bang through the accretion of cold material and mergers3,4. Observationally, it has been difficult to identify disk galaxies in emission at high redshift5,6 in order to discern between competing models of galaxy formation. Here we report imaging, with a resolution of about 1.3kiloparsecs, of the 158-micrometre emission line from singly ionized carbon, the far-infrared dust continuum and the near-ultraviolet continuum emission from a galaxy at a redshift of 4.2603, identified by detecting its absorption of quasar light. These observations show that the emission arises from gas inside a cold, dusty, rotating disk with a rotational velocity of about 272kilometres per second. The detection of emission from carbon monoxide in the galaxy yields a molecular mass that is consistent with the estimate from the ionized carbon emission of about 72 billion solar masses. The existence of such a massive, rotationally supported, cold disk galaxy when the Universe was only 1.5 billion years old favours formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations7,8.

Rotating Disk Galaxies without Dark Matter Based on Scientific Reasoning

The most cited evidence for (non-baryonic) dark matter has been an apparent lack of visible mass to gravitationally support the observed orbital velocity of matter in rotating disk galaxies, yet measurement of the mass of celestial objects cannot be straightforward, requiring theories derived from the known physical laws along with some empirically established semi-quantitative relationship. The most reliable means for determining the mass distribution in rotating disk galaxies is to solve a force balance equation according to Newton’s laws from measured rotation curves, similar to calculating the Sun’s mass from the Earth’s orbital velocity. Another common method to estimate galactic mass distribution is to convert measured brightness from surface photometry based on empirically established mass-to-light ratio. For convenience, most astronomers commonly assumed a constant mass-to-light ratio for estimation of the so-called “luminous” or “visible” mass, which would not likely be accurate. The mass determined from a rotation curve typically exhibits an exponential-like decline with galactrocentric distance, qualitatively consistent with observed surface brightness but often with a larger disk radial scale length. This fact scientifically suggests variable mass-to-light ratio of baryonic matter in galaxies without the need for dark matter.

The angular momentum of disc galaxies at z = 1

We investigate the relation between stellar mass (M⋆) and specific stellar angular momentum (j⋆), or “Fall relation”, for a sample of 17 isolated, regularly rotating disc galaxies at z ∼ 1. All galaxies have rotation curves determined from Hα emission-line data; HST imaging in optical and infrared filters; and robust determinations of their stellar masses. We use HST images in f814w and f160w filters, roughly corresponding to rest-frames B and I bands, to extract surface-brightness profiles for our systems. We robustly bracket j⋆ by assuming that rotation curves beyond the outermost Hα rotation point stay either flat or follow a Keplerian fall-off. By comparing our measurements with those determined for disc galaxies in the local universe, we find no evolution in the Fall relation in the redshift range 0 < z < 1, regardless of the band used and despite the uncertainties in the stellar rotation curves at large radii. This result holds unless stellar masses at z = 1 are systematically underestimated by ≳50%. Our findings are compatible with expectations based on a ΛCDM cosmological framework and support a scenario where both the stellar Tully–Fisher and mass-size relations for spirals do not evolve significantly in this redshift range.

Testing modified Newtonian dynamics through statistics of velocity dispersion profiles in the inner regions of elliptical galaxies

Modified Newtonian dynamics (MOND) proposed by Milgrom provides a paradigm alternative to dark matter (DM) that has been successful in fitting and predicting the rich phenomenology of rotating disc galaxies. There have also been attempts to test MOND in dispersion-supported spheroidal early-type galaxies, but it remains unclear whether MOND can fit the various empirical properties of early-type galaxies for the whole ranges of mass and radius. As a way of rigorously testing MOND in elliptical galaxies we calculate the MOND-predicted velocity dispersion profiles (VDPs) in the inner regions of ∼2000 nearly round Sloan Digital Sky Survey elliptical galaxies under a variety of assumptions on velocity dispersion (VD) anisotropy, and then compare the predicted distribution of VDP slopes with the observed distribution in 11 ATLAS3D galaxies selected with essentially the same criteria. We find that the MOND model parametrized with an interpolating function that works well for rotating galaxies can also reproduce the observed distribution of VDP slopes based only on the observed stellar mass distribution without DM or any other galaxy-to-galaxy varying factor. This is remarkable in view that Newtonian dynamics with DM requires a specific amount and/or profile of DM for each galaxy in order to reproduce the observed distribution of VDP slopes. When we analyse non-round galaxy samples using the MOND-based spherical Jeans equation, we do not find any systematic difference in the mean property of the VDP slope distribution compared with the nearly round sample. However, in line with previous studies of MOND through individual analyses of elliptical galaxies, varying MOND interpolating function or VD anisotropy can lead to systematic change in the VDP slope distribution, indicating that a statistical analysis of VDPs can be used to constrain specific MOND models with an accurate measurement of VDP slopes or a prior constraint on VD anisotropy.

Giant disc galaxies: where environment trumps mass in galaxy evolution

We identify some of the most HI massive and fastest rotating disk galaxies in the local universe with the aim of probing the processes that drive the formation of these extreme disk galaxies. By combining data from the Cosmic Flows project, which has consistently reanalyzed archival galaxy HI profiles, and 3.6$\mu$m photometry obtained with the Spitzer Space Telescope, with which we can measure stellar mass, we use the baryonic Tully-Fisher (BTF) relationship to explore whether these massive galaxies are distinct. We discuss several results, but the most striking is the systematic offset of the HI-massive sample above the BTF. These galaxies have both more gas and more stars in their disks than the typical disk galaxy of similar rotational velocity. The "condensed" baryon fraction, $f_C$, the fraction of the baryons in a dark matter halo that settle either as cold gas or stars into the disk, is twice as high in the HI-massive sample than typical, and almost reaches the universal baryon fraction in some cases, suggesting that the most extreme of these galaxies have little in the way of a hot baryonic component or cold baryons distributed well outside the disk. In contrast, the star formation efficiency, measured as the ratio of the mass in stars to that in both stars and gas, shows no difference between the HI-massive sample and the typical disk galaxies. We conclude that the star formation efficiency is driven by an internal, self-regulating process, while $f_C$ is affected by external factors. We also found that the most massive HI detected galaxies are located preferentially in filaments. We present the first evidence of an environmental effect on galaxy evolution using a dynamical definition of a filament.

Density wave formation in differentially rotating disk galaxies: Hydrodynamic simulation of the linear regime

Most rapidly and differentially rotating disk galaxies, in which the sound speed (thermal velocity dispersion) is smaller than the orbital velocity, display graceful spiral patterns. Yet, over almost 240yr after their discovery in M51 by Charles Messier, we still do not fully understand how they originate. In this first paper of a series, the dynamical behavior of a rotating galactic disk is examined numerically by a high-order Godunov hydrodynamic code. The code is implemented to simulate a two-dimensional flow driven by an internal Jeans gravitational instability in a nonresonant wave–“fluid” interaction in an infinitesimally thin disk composed of stars or gas clouds. A goal of this work is to explore the local and linear regimes of density wave formation, employed by Lin, Shu, Yuan and many others in connection with the problem of spiral pattern of rotationally supported galaxies, by means of computer-generated models and to compare those numerical results with the generalized fluid-dynamical wave theory. The focus is on a statistical analysis of time-evolution of density wave structures seen in the simulations. The leading role of collective processes in the formation of both the circular and spiral density waves (“heavy sound”) is emphasized. The main new result is that the disk evolution in the initial, quasilinear stage of the instability in our global simulations is fairly well described using the local approximation of the generalized wave theory. Certain applications of the simulation to actual gas-rich spiral galaxies are also explored.

Fitting the Lin–Shu-type density-wave theory for our own Galaxy★

The presence of spiral structure in rapidly and differentially rotating disc galaxies is currently attributed to the phenomenon of unstable (that is to say, growing) Lin–Shu compression-type waves, or density waves, rotating at a constant angular velocity around the system's centre. It is important that when a density-wave structure is present in a galaxy, the gravitational field of the spiral arms will systematically deflect the motion of gas and young stars away from their mean circular rotation, and point masses in such a model react by streaming motions that are of spiral shape too. We examine the kinematics of Milky Way's 233 Cepheid stars on the assumption that the system is subject to moderately growing spiral density waves by taking into account small-amplitude perturbations of the Galactic gravitational potential. Using Cepheid line-of-sight velocities, we propose new estimates of the parameters of solar motion and Galactic rotation corrected for the effects of density waves, the radial and azimuthal components of systematic stellar motion due to the spiral arms as well as the dynamical parameters of the waves. A basis is given for preferring the dominant one-armed spiral structure in the solar neighbourhood of the Galaxy.

Stability of galactic discs: finite arm-inclination and finite-thickness effects★

A modified theory of the Lin–Shu density waves, studied in connection with the problem of spiral pattern of rapidly and differentially rotating disc galaxies, is presented for both the axisymmetric and non-axisymmetric structures in highly flattened galaxies resulted from the classical Jeans instability of small gravity perturbations (e.g. those produced by a spontaneous disturbance). A new method is provided for the analytical solution of the self-consistent system of the gas-dynamic equations and the Poisson equation describing the stability of a three-dimensional galactic disc composed of stars or gaseous clouds. In order to apply the method, the modifications introduced for the properties of the gravitationally unstable, that is to say, amplitude-growing density waves are considered by removing the often used assumptions that the gravity perturbations are axisymmetric and the disc is infinitesimally thin. In contrast to previous studies, in this paper these two effects – the non-axial symmetry effect and the finite thickness effect – are simultaneously taken into account. We show that nonaxisymmetric perturbations developing in a differentially rotating disc are more unstable than the axisymmetric ones. We also show that destabilizing self-gravity is far more ‘dangerous’ in thin discs than in thick discs. The primary effect of small but finite thickness is a reduction of the growth rate of the gravitational Jeans instability and a shift in the threshold of instability towards a longer wavelength (and larger wavelength will include more mass). The results of this paper are in qualitative agreement with previous analytical and numerical estimations of the effects. The extent to which our results on the disc’s stability can have a bearing on observable spiral galaxies, including the Milky Way, is also discussed.

A taxonomic algorithm for bar-building orbits

Recently it has been realized that the major structures observed in rotating disc galaxies, i.e. bars and spirals, can be supported by regular as well as by chaotic orbits. The fact that the building of a structure cannot be attributed just to quasipe riodic orbits associated with a single orbital family, creates the need to classify the traject ories of the particles in structuresupporting and structure-non-supporting within a time interval of interest. Our goal is to present a simple algorithm that detects and classifies the or bits which reinforce the rectangularity of the outer envelope of a bar, independently of their regular or chaotic character. Our bar is a 2D response bar, formed when an external potential estimated from near-infrared observations of an early type barred-spiral galaxy, is impo sed to a set of initial conditions. For this purpose we use a method based on tracing patterns in sequences of characters, which indicate sign changes of the (Cartesian) coordinates. These sign changes occur when as integrate an orbit for a time t and we follow it in 2D subspaces of the phase space ((x, y), ( ˙, ˙ y), etc.). A sign change indicates crossing of an axis during the integration. In the case we describe in our paper the bar in the inner parts is supported by regular orbit s following the x1 flow, while the outer envelope of the bar is supported mainly by chaotic orbits at higher energies. With our method, at a given Jacobi constant, first we assess the contri bution to the local surface density of a large number of orbital segments. This is done by integrating their initial conditions for 10 pattern rotations. We depict this information on grey scale maps. We have realized, that this contribution is independent of the regular or chaotic chara cter of the integrated orbits. Then, by analyzing the arrays of sign changes we have registered during the orbital integrations, we separate the trajectories that shape the outer structure of the bar in two classes. They follow mainly boxy or/and diamond-like morphologies within the time of integration. By repeating the method at several Jacobi constants (EJ) we find that the majority of the orbits that support

The Primordial Origin Model of Magnetic Fields in Spiral Galaxies

We propose a primordial-origin model for composite configurations of global magnetic fields in spiral galaxies. We show that a uniform tilted magnetic field wound up into a rotating disk galaxy can evolve into composite magnetic configurations comprising bisymmetric spiral (S $=$ BSS), axisymmetric spiral (A $=$ ASS), plane-reversed spiral (PR), and/or ring (R) fields in the disk, and vertical (V) fields in the center. By MHD simulations we show that these composite galactic fields are indeed created from a weak primordial uniform field, and that different configurations can co-exist in the same galaxy. We show that spiral fields trigger the growth of two-armed gaseous arms. The centrally accumulated vertical fields are twisted and produce a jet toward the halo. We found that the more vertical was the initial uniform field, the stronger was the formed magnetic field in the galactic disk.

High star formation rates as the origin of turbulence in early and modern disk galaxies

Observations of star formation and kinematics in early galaxies at high spatial and spectral resolution have shown that two-thirds are massive rotating disk galaxies, with the remainder being less massive non-rotating objects. The line-of-sight-averaged velocity dispersions are typically five times higher than in today's disk galaxies. This suggests that gravitationally unstable, gas-rich disks in the early Universe are fuelled by cold, dense accreting gas flowing along cosmic filaments and penetrating hot galactic gas halos. These accreting flows, however, have not been observed, and cosmic accretion cannot power the observed level of turbulence. Here we report observations of a sample of rare, high-velocity-dispersion disk galaxies in the nearby Universe where cold accretion is unlikely to drive their high star formation rates. We find that their velocity dispersions are correlated with their star formation rates, but not their masses or gas fractions, which suggests that star formation is the energetic driver of galaxy disk turbulence at all cosmic epochs.

Spatially resolved dynamics of high-z star forming galaxies

AbstractI report on two major programs to study the kinematic properties of galaxies at z ~ 1.5 − 3 with spatially resolved spectroscopy for the first time. Using the adaptive optics assisted, integral field spectrometer SINFONI on the ESO VLT, we have observed more than 70 galaxies and find compelling evidence for large, geometrically thick (turbulent), rotating disk galaxies in a majority of the objects that we can spatially resolve. It appears that these star forming disks are driven by continuous, rapid accretion of gas from their dark matter halos, and that their evolution is strongly influenced by internal, secular evolution. In contrast to the 20 submillimeter galaxies that we have investigated with the IRAM Plateau de Bure millimetre interferometer we find strong evidence for compact, major mergers. I discuss the impact of these new observations on our understanding of galaxy evolution in the early Universe.For the SINS survey we have carried out Hα integral field spectroscopy of well-resolved, UV/optically selected star-forming galaxies at z ~ 2 with SINFONI on the ESO VLT. The SINS sample is representative of the majority of massive (M* > a few 1010M⊙) star-forming galaxies at that redshift. Our data obtained with laser guide star assisted adaptive optics in good seeing show the presence of turbulent, rotating star-forming rings/disks in at least a third of the sample, plus central bulge/inner disk components in some of the best cases, whose mass fractions relative to total dynamical mass appears to scale with [NII]/Hα flux ratio and ‘star formation’ age. Another third of the SINS galaxies show clear signs of kinematic perturbations by a merger, while the last third appear to be compact, ‘dispersion’ limited systems.Our interpretation of these data is that the buildup of the central disks and bulges of massive galaxies at z ~ 2 can be driven by the early secular evolution of gas-rich ‘proto’-disks. High-redshift disks exhibit large random motions. This turbulence may in part be stirred up by the release of gravitational energy in the rapid ‘cold’ accretion flows along the filaments of the cosmic web. As a result, dynamical friction and viscous processes proceed on a time scale of < 1 Gyr, at least an order of magnitude faster than in disk galaxies at z ~ 0. Early secular evolution thus drives gas and stars into the central regions and can build up exponential disks and massive bulges, even without major mergers. Secular evolution along with increased efficiency of star formation at high surface densities may also help to account for the short time scales of the stellar buildup observed in massive galaxies at z ~ 2.

Ring Galaxies: The Dynamical Effect of the Halo and the Role of Shear Viscosity and Pressure Gradient Forces

The peculiar ring galaxies are formed as a result of a cosmic interaction. An intruder galaxy plunges through the center of a rotating disk galaxy triggering radially expanding density waves inside the disk. In this paper we exploited SPH simulations with the aim to examine the role of a “live” halo and/or bulge in driving the morphology of the interacting galaxy. Moreover we explore the effects of different implementations of the shear viscosity and of the pressure gradient in the code.

The rapid formation of a large rotating disk galaxy three billion years after the Big Bang

Observations and theoretical simulations have established a framework for galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark-matter haloes; mergers of haloes and galaxies then led to the hierarchical build-up of galaxy mass. It remains unclear, however, over what timescales galaxies were assembled and when and how bulges and disks--the primary components of present-day galaxies--were formed. It is also puzzling that the most massive galaxies were more abundant and were forming stars more rapidly at early epochs than expected from models. Here we report high-angular-resolution observations of a representative luminous star-forming galaxy when the Universe was only 20% of its current age. A large and massive rotating protodisk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole. The high surface densities of gas, the high rate of star formation and the moderately young stellar ages suggest rapid assembly, fragmentation and conversion to stars of an initially very gas-rich protodisk, with no obvious evidence for a major merger.

The effect of rigid rotation in the formation of giant molecular clouds by aggregation

The formation of giant molecular clouds (GMCs) by aggregation in a rigidly rotating disk galaxy is studied. The result shows that the GMCs so formed consist mainly of individual clouds located close to one another from inelastic collision and self-gravitation under a velocity dispersion. The clouds so formed are not very massive. If there is differential rotation, then there is a greater chance of more massive clouds being formed.

Stellar Dynamics and the Implications on the Merger Evolution in NGC 6240

We report near-infrared integral field spectroscopy of the luminous merging galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by 16 are the central parts of inclined, rotating disk galaxies with equal mass bulges. The dynamical masses of the nuclei are much larger than the stellar mass derived from the K-band light, implying that the progenitor galaxies were galaxies with massive bulges. The K-band light is dominated by red supergiants formed in the two nuclei in starbursts, triggered ≈2 × 107 yr ago, possibly by the most recent perigalactic approach. Strong feedback effects of a superwind and supernovae are responsible for a short duration burst (≈5 × 106 yr) that is already decaying. The two galaxies form a prograde-retrograde rotating system and from the stellar velocity field it seems that one of the two interacting galaxies is subject to a prograde encounter. Between the stellar nuclei is a prominent peak of molecular gas (H2, CO). The stellar velocity dispersion peaks there indicating that the gas has formed a local, self-gravitating concentration decoupled from the stellar gravitational potential. NGC 6240 has previously been reported to fit the paradigm of an elliptical galaxy formed through the merger of two galaxies. This was based on the near-infrared light distribution, which follows a r1/4-law. Our data cast strong doubt on this conclusion: the system is by far not relaxed, rotation plays an important role, as does self-gravitating gas, and the near-infrared light is dominated by young stars.

Damped Lyα absorbers from dwarf galaxy ejecta

ABSTRA C T It is argued that the formation of a dwarf galaxy causes a massive burst of star formation, resulting in the ejection of most of the available gas from the galaxy as a weakly collimated wind. The ejected gas can give rise to a damped Lya absorber (DLA). Weakly collimated outflows naturally explain the asymmetric profiles seen in low-ionization absorption lines caused by heavy elements associated with DLAs, where absorption is strongest at one edge of the absorption feature. The shape of the distribution of column densities in the model agrees reasonably well with observations. In particular, the break in slope is caused by external photoionization of the wind. A semi-analytical model for galaxy formation is used to show that, for currently acceptable cosmological parameters, dwarf galaxy outflows can account for the majority of DLA systems and their distribution with redshift. This model also predicts a correlation between velocity structure and metallicity of DLA systems, in qualitative agreement with observations. DLAs do not require many large, rapidly rotating disc galaxies to have formed early on, as in other models for their origin.