Flat Central Density Research Articles

Gravity-driven filamentary flow in molecular clouds

ABSTRACT We present a numerical study of the gravity-driven filamentary flow arising in the presence of elongated perturbations embedded in a globally gravitationally unstable medium. We perform idealized simulations of the gravitational collapse of a moderate filamentary perturbation with a central enhancement (a core) embedded in either a uniform or a stratified background. Both simulations maintain the filamentary structure during the collapse, developing a hierarchical accretion flow from the cloud to the filament, and from the filament to the core. Only the stratified simulation produces a flat central density profile of filaments, best matching the observed Plummer-like profiles, supporting suggestions that molecular clouds may be preferentially flattened. The flow changes direction smoothly from the cloud to the filament, with no density divergence nor a shock developing at the filament’s axis during the prestellar evolution. The drainage of material by the filament-to-core accretion slows down the growth of the filament, causing the ratio of the core’s central density to the filament’s axial density to increase in time, and to diverge at the time when a singularity (protostar) forms in the core. We argue that the system should evolve towards a stationary state in which the filament-to-core accretion balances the cloud-to-filament one, and search for it in the simulations, but find no unambiguous evidence. However, we find that, after a period of accelerated increase, the filament’s linear mass density reaches a linear growth rate. The stationary state may be approached during the protostellar stage.

EXACT SOLUTION TO FINITE TEMPERATURE SFDM: NATURAL CORES WITHOUT FEEDBACK

Recent high-quality observations of low surface brightness (LSB) galaxies have shown that their dark matter (DM) halos prefer flat central density profiles. However, the standard cold dark matter model simulations predict a more cuspy behavior. One mechanism used to reconcile the simulations with the observed data is the feedback from star formation. While this mechanism may be successful in isolated dwarf galaxies, its success in LSB galaxies remains unclear. Additionally, the inclusion of too much feedback in the simulations is a double-edged sword—in order to obtain a cored DM distribution from an initially cuspy one, the feedback recipes usually require one to remove a large quantity of baryons from the center of the galaxies; however, some feedback recipes produce twice the number of satellite galaxies of a given luminosity and with much smaller mass-to-light ratios from those that are observed. Therefore, one DM profile that produces cores naturally and that does not require large amounts of feedback would be preferable. We find both requirements to be satisfied in the scalar field dark matter model. Here, we consider that DM is an auto-interacting real scalar field in a thermal bath at temperature T with an initial Z2 symmetric potential. As the universe expands, the temperature drops so that the Z2 symmetry is spontaneously broken and the field rolls down to a new minimum. We give an exact analytic solution to the Newtonian limit of this system, showing that it can satisfy the two desired requirements and that the rotation curve profile is no longer universal.

Flat central density profile and constant dark matter surface density in galaxies from scalar field dark matter

The scalar field dark matter (SFDM) model proposes that galaxies form by condensation of a scalar field (SF) very early in the universe forming Bose-Einstein Condensates (BEC) drops, i.e., in this model haloes of galaxies are gigantic drops of SF. Here big structures form like in the LCDM model, by hierarchy, thus all the predictions of the LCDM model at big scales are reproduced by SFDM. This model predicts that all galaxies must be very similar and exist for bigger redshifts than in the LCDM model. In this work we show that BEC dark matter haloes fit high-resolution rotation curves of a sample of thirteen low surface brightness galaxies. We compare our fits to those obtained using a Navarro-Frenk-White and Pseudo-Isothermal (PI) profiles and found a better agreement with the SFDM and PI profiles. The mean value of the logarithmic inner density slopes is -0.27 +/- 0.18. As a second result we find a natural way to define the core radius with the advantage of being model-independent. Using this new definition in the BEC density profile we find that the recent observation of the constant dark matter central surface density can be reproduced. We conclude that in light of the difficulties that the standard model is currently facing the SFDM model can be a worthy alternative to keep exploring further.

The gravitational collapse of Plummer protostellar clouds

The growing observational evidence that main-sequence and pre-main-sequence protostars are found in binary and multiple systems suggests that they are formed by a fragmentation of collapsing molecular cloud cores. In this paper we present the results of a set of numerical simulations aimed to study the gravitational collapse and fragmentation of a centrally condensed cloud with a nearly flat central density region and a surrounding gas envelope. In order to describe this cloud structure, we use the Plummer radial density profile which satisfies the observed fact that protostellar clouds have a flat central density profile in their innermost region. We consider the cloud to be made of molecular hydrogen and describe the thermodynamics with a single barotropic equation of state, which includes a critical density ρcrit as a unique free parameter that determines a thermodynamical change on the collapsing gas: from an isothermal to an adiabatic regime. In this paper we consider four different values for the initial radius Rc of the cloud, ranging from 2675 to 19 913 AU. In the models, for each ratio Rc/R0 of the cloud to core radius, we use two critical density values: ρcrit = 5.0 × 10 −14 and 5.0 × 10 −12 gr cm −3 . When the adiabatic change regime starts earlier, we find interesting gas structures as a result of the collapse, although these structures are different according to the initial mass content of the envelope and the initial angular velocity of the cloud. When the thermodynamical change occurs later, i.e., for ρcrit = 5.0 × 10 −12 gr cm −3 , we observe that the previously found structure is almost erased to give place instead to a single clump of gas without any adorning spiral arms. In general, we find that as the extension of the envelope mass increases, the possibility of a model to produce a multiple system decreases. This is a result of the initial configuration of our models, namely that with bigger envelopes their cores have a lower ratio of rotational to gravitational energy β0, a lower ratio of thermal plus rotational to gravitational energy α0 + β0, and a lower angular velocity Ω0 ,w hich induces a stronger collapse which in turn contributes to the destruction of the structure that is formed during the initial phases of the collapse. Thus in a sufficient quantity rotational energy is crucial for the fragmentation to occur and survive.

The Structure of Dark Matter Halos in Dwarf Galaxies

Recent observations indicate that dark matter haloes have flat central density profiles. Cosmological simulations with non-baryonic dark matter predict however self similar haloes with central density cusps. This contradiction has lead to the conclusion that dark matter must be baryonic. Here it is shown that the dark matter haloes of dwarf spiral galaxies represent a one parameter family with self similar density profiles. The observed global halo parameters are coupled with each other through simple scaling relations which can be explained by the standard cold dark matter model if one assumes that all the haloes formed from density fluctuations with the same primordial amplitude. We find that the finite central halo densities correlate with the other global parameters. This result rules out scenarios where the flat halo cores formed subsequently through violent dynamical processes in the baryonic component. These cores instead provide important information on the origin and nature of dark matter in dwarf galaxies.

FORMATION OF A GIANT GALACTIC GASEOUS HALO: METAL-ABSORPTION LINES AND HIGH-VELOCITY CLOUDS

FORMATION OF A GIANT GALACTIC GASEOUS HALO: METAL-ABSORPTION LINES AND HIGH-VELOCITY CLOUDS