Abstract
Accurately predicting the demographics of dark matter (DM) substructure is of paramount importance for many fields of astrophysics, including gravitational lensing, galaxy evolution, halo ...occupation modelling, and constraining the nature of DM. Because of its strongly non-linear nature, DM substructure is typically modelled using N-body simulations, which reveal that large fractions of DM subhaloes undergo complete disruption. In this paper, we use both analytical estimates and idealized numerical simulations to investigate whether this disruption is mainly physical, due to tidal heating and stripping, or numerical (i.e. artificial). We show that, contrary to naive expectation, subhaloes that experience a tidal shock ΔE that exceeds the subhalo's binding energy, |Eb|, do not undergo disruption, even when ΔE/|Eb| is as large as 100. Along the same line, and contrary to existing claims in the literature, instantaneously stripping matter from the outskirts of a DM subhalo also does not result in its complete disruption, even when the instantaneous remnant has positive binding energy. In addition, we show that tidal heating due to high-speed (impulsive) encounters with other subhaloes (‘harassment’) is negligible compared to the tidal effects due to the host halo. Hence, we conclude that, in the absence of baryonic processes, the complete physical disruption of CDM substructure is extremely rare and that most disruption in numerical simulations therefore must be artificial. We discuss various processes that have been associated with numerical overmerging and conclude that inadequate force softening is the most likely culprit.
ABSTRACT The classical picture of a star-forming filament is a near-equilibrium structure with its collapse dependent on its gravitational criticality. Recent observations have complicated this ...picture, revealing filaments to be a mess of apparently interacting subfilaments with transsonic internal velocity dispersions and mildly supersonic intra-subfilament dispersions. How structures like this form is unresolved. Here, we study the velocity structure of filamentary regions in a simulation of a turbulent molecular cloud. We present two main findings. First, the observed complex velocity features in filaments arise naturally in self-gravitating hydrodynamic simulations of turbulent clouds without the need for magnetic or other effects. Second, a region that is filamentary only in projection and is in fact made of spatially distinct features can display these same velocity characteristics. The fact that these disjoint structures can masquerade as coherent filaments in both projection and velocity diagnostics highlights the need to continue developing sophisticated filamentary analysis techniques for star formation observations.
Studies by Lada et al. and Heiderman et al. have suggested that star formation mostly occurs above a threshold in gas surface density capital sigma of capital sigma c ~ 120 M sub(middot in circle) pc ...super(-2) (A sub(K) ~ 0.8). Heiderman et al. infer a threshold by combining low-mass star-forming regions, which show a steep increase in the star formation rate per unit area capital sigma sub(SFR) with increasing capital sigma , and massive cores forming luminous stars which show a linear relation. We argue that these observations do not require a particular density threshold. The steep dependence of capital sigma sub(SFR), approaching unity at protostellar core densities, is a natural result of the increasing importance of self-gravity at high densities along with the corresponding decrease in evolutionary timescales. The linear behavior of capital sigma sub(SFR) versus capital sigma in massive cores is consistent with probing dense gas in gravitational collapse, forming stars at a characteristic free-fall timescale given by the use of a particular molecular tracer. The low-mass and high-mass regions show different correlations between gas surface density and the area A spanned at that density, with A ~ capital sigma super(-3) for low-mass regions and A ~ capital sigma super(-1) for the massive cores; this difference, along with the use of differing techniques to measure gas surface density and star formation, suggests that connecting the low-mass regions with massive cores is problematic. We show that the approximately linear relationship between dense gas mass and stellar mass used by Lada et al. similarly does not demand a particular threshold for star formation and requires continuing formation of dense gas. Our results are consistent with molecular clouds forming by galactic hydrodynamic flows with subsequent gravitational collapse.
ABSTRACT We study the stability of filaments in equilibrium between gravity and internal as well as external pressure using the grid-based AMR code RAMSES. A homogeneous, straight cylinder below a ...critical line mass is marginally stable. However, if the cylinder is bent, such as with a slight sinusoidal perturbation, an otherwise stable configuration starts to oscillate, is triggered into fragmentation, and collapses. This previously unstudied behavior allows a filament to fragment at any given scale, as long as it has slight bends. We call this process "geometrical fragmentation." In our realization, the spacing between the cores matches the wavelength of the sinusoidal perturbation, whereas up to now, filaments were thought to be only fragmenting on the characteristic scale set by the mass-to-line ratio. Using first principles, we derive the oscillation period as well as the collapse timescale analytically. To enable a direct comparison with observations, we study the line-of-sight velocity for different inclinations. We show that the overall oscillation pattern can hide the infall signature of cores.
State-of-the-art integral field surveys like ATLAS3D, SLUGGS, CALIFA, SAMI, and MaNGA provide large data sets of kinematical observations of early-type galaxies (ETGs), yielding constraints on the ...formation of ETGs. Using the cosmological hydrodynamical Magneticum Pathfinder simulations, we investigate the paradigm of fast- and slow-rotating ETGs in a fully cosmological context. We show that the ETGs within the Magneticum simulation are in remarkable agreement with the observations, revealing fast and slow rotators quantified by the angular momentum proxy λR and the flattening ɛ with the observed prevalence. Taking full advantage of the three-dimensional data, we demonstrate that the dichotomy between fast- and slow-rotating galaxies gets enhanced, showing an upper and lower population separated by an underpopulated region in the edge-on λ _{R_{1/2}}-ɛ plane. We show that the global anisotropy parameter inferred from the λ _{R_{1/2}}-ɛ edge-on view is a very good predictor of the true anisotropy of the system. This drives a physically based argument for the location of fast rotators in the observed plane. Following the evolution of the λ _{R_{1/2}}-ɛ plane through cosmic time, we find that, while the upper population is already in place at z = 2, the lower population gets statistically significant below z = 1 with a gradual increase. At least 50{{ per cent}} of the galaxies transition from fast to slow rotators on a short time scale, in most cases associated to a significant merger event. Furthermore, we connect the M*-j* plane, quantified by the b-value, with the λ _{R_{1/2}}-ɛ plane, revealing a strong correlation between the position of a galaxy in the λ _{R_{1/2}}-ɛ plane and the b-value. Going one step further, we classify our sample based on features in their velocity map, finding all five common kinematic groups, also including the recently observed group of prolate rotators, populating distinct regions in the λ _{R_{1/2}}-b plane.
ABSTRACT
Post-starburst (PSB) galaxies belong to a short-lived transition population between star-forming (SF) and quiescent galaxies. Deciphering their heavily discussed evolutionary pathways is ...paramount to understanding galaxy evolution. We aim to determine the dominant mechanisms governing PSB evolution in both the field and in galaxy clusters. Using the cosmological hydrodynamical simulation suite Magneticum Pathfinder, we identify 647 PSBs with z ∼ 0 stellar mass $M_* \ge 5 \times 10^{10} \, \mathrm{M_{\odot }}$ . We track their galactic evolution, merger history, and black hole activity over a time-span of $3.6\,$ Gyr. Additionally, we study cluster PSBs identified at different redshifts and cluster masses. Independent of environment and redshift, we find that PSBs, like SF galaxies, have frequent mergers. At z = 0, $89{{\ \rm per\ cent}}$ of PSBs have experienced mergers and $65{{\ \rm per\ cent}}$ had at least one major merger within the last $2.5\,$ Gyr, leading to strong star formation episodes. In fact, $23{{\ \rm per\ cent}}$ of z = 0 PSBs were rejuvenated during their starburst. Following the mergers, field PSBs are generally shutdown via a strong increase in active galactic nucleus (AGN) feedback (power output $P_{\rm AGN,PSB} \ge 10^{56}\,$ erg Myr−1). We find agreement with observations for both stellar mass functions and z = 0.9 line-of-sight phase space distributions of PSBs in galaxy clusters. Finally, we find that z ≲ 0.5 cluster PSBs are predominantly infalling, especially in high-mass clusters and show no signs of enhanced AGN activity. Thus, we conclude that the majority of cluster PSBs are shutdown via an environmental quenching mechanism such as ram-pressure stripping, while field PSBs are mainly quenched by AGN feedback.
3D shape of Orion A from Gaia DR2 Großschedl, Josefa E.; Alves, João; Meingast, Stefan ...
Astronomy and astrophysics (Berlin),
11/2018, Letnik:
619
Journal Article
Recenzirano
Odprti dostop
We use the Gaia DR2 distances of about 700 mid-infrared selected young stellar objects in the benchmark giant molecular cloud Orion A to infer its 3D shape and orientation. We find that Orion A is ...not the fairly straight filamentary cloud that we see in (2D) projection, but instead a cometary-like cloud oriented toward the Galactic plane, with two distinct components: a denser and enhanced star-forming (bent) Head, and a lower density and star-formation quieter ∼75 pc long Tail. The true extent of Orion A is not the projected ∼40 pc but ∼90 pc, making it by far the largest molecular cloud in the local neighborhood. Its aspect ratio (∼30:1) and high column-density fraction (∼45%) make it similar to large-scale Milky Way filaments (“bones”), despite its distance to the galactic mid-plane being an order of magnitude larger than typically found for these structures.
ABSTRACT Large-scale gaseous filaments with lengths up to the order of 100 pc are on the upper end of the filamentary hierarchy of the Galactic interstellar medium (ISM). Their association with ...respect to the Galactic structure and their role in Galactic star formation are of great interest from both an observational and theoretical point of view. Previous "by-eye" searches, combined together, have started to uncover the Galactic distribution of large filaments, yet inherent bias and small sample size limit conclusive statistical results from being drawn. Here, we present (1) a new, automated method for identifying large-scale velocity-coherent dense filaments, and (2) the first statistics and the Galactic distribution of these filaments. We use a customized minimum spanning tree algorithm to identify filaments by connecting voxels in the position-position-velocity space, using the Bolocam Galactic Plane Survey spectroscopic catalog. In the range of , we have identified 54 large-scale filaments and derived mass ( ), length (10-276 pc), linear mass density (54-8625 pc−1), aspect ratio, linearity, velocity gradient, temperature, fragmentation, Galactic location, and orientation angle. The filaments concentrate along major spiral arms. They are widely distributed across the Galactic disk, with 50% located within 20 pc from the Galactic mid-plane and 27% run in the center of spiral arms. An order of 1% of the molecular ISM is confined in large filaments. Massive star formation is more favorable in large filaments compared to elsewhere. This is the first comprehensive catalog of large filaments that can be useful for a quantitative comparison with spiral structures and numerical simulations.
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