69 research outputs found
How galaxies gain and lose their angular momentum
Spiral, fast-rotating galaxies like the Milky Way are the most common type in the Universe. One of the most pressing challenges faced by current models of galaxy formation is the origin of their angular momentum and disk. According to the standard tidal-torque theory the galactic spin is originated by tidal interactions between dark halos around galaxies and neighboring structures in the expanding Universe. We use a large cosmological N-body simulation to study the origin of possible correlations between the merging history and spin of cold dark matter halos. In particular, we examine claims that remnants of major mergers tend to have higher-than-average spins, and find that the effect is driven largely by unrelaxed systems: equilibrium dark matter halos show no significant correlation between spin and merging history. Out-of-equilibrium halos have, on average, higher spin than relaxed systems, suggesting that the virialization process leads to a net decrease in the value of the spin parameter. We present also high-resolution N-body/SPH cosmological simulations including cold gas and dark matter to investigate the processes by which gas loses its angular momentum during the protogalactic collapse phase, leading to simulated disk galaxies that are too compact with respect to the observations. We show that the gas and the dark matter have similar specific angular momenta until a merger event occurs at redshift 2. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and falls quickly into the center. Dynamical friction by small infalling substructures plays a minor role, in contrast to previous claim
Detectability of Free Floating Planets in Open Clusters with JWST
Recent observations have shown the presence of extra-solar planets in
Galactic open stellar clusters, as in the Praesepe (M44). These systems provide
a favorable environment for planetary formation due to the high heavy-element
content exhibited by the majority of their population. The large stellar
density, and corresponding high close-encounter event rate, may induce strong
perturbations of planetary orbits with large semimajor axes. Here we present a
set of N-body simulations implementing a novel scheme to treat the tidal
effects of external stellar perturbers on planetary orbit eccentricity and
inclination. By simulating five nearby open clusters we determine the rate of
occurrence of bodies extracted from their parent stellar system by
quasi-impulsive tidal interactions. We find that the specific free-floating
planet production rate (total number of free-floating planets per unit of time,
normalized by the total number of stars) is proportional to the stellar density
of the cluster, with a constant of proportionality equal to (23 +/- 5)10^-6
pc^3 Myr^-1. For the Pleiades (M45) we predict that about 26% of stars should
have lost their planets. This raises the exciting possibility of directly
observing these wandering planets with the James Webb Space Telescope in the
NIR band. Assuming a surface temperature of the planet of 500 K, a
free-floating planet of Jupiter size inside the Pleiades would have a specific
flux @4.4 micron of approximately 400 nJy, which would lead to a very clear
detection (S/N of order 100) in only one hour of integration.Comment: Accepted for publication in ApJ Letters on 4 November 201
The Halo Density Profiles with Non-Standard N-body Simulations
We propose a new numerical procedure to simulate a single dark halo of any
size and mass in a hierarchical framework coupling the extended Press-Schechter
formalism (EPSF) to N-body simulations. The procedure consists of assigning
cosmological initial conditions to the particles of a single halo with a EPSF
technique and following only the dynamical evolution using a serial N-body
code. The computational box is fixed with a side of Mpc. This
allows to simulate galaxy cluster halos using appropriate scaling relations, to
ensure savings in computing time and code speed. The code can describe the
properties of halos composed of collisionless or collisional dark matter. For
collisionless Cold Dark Matter (CDM) particles the NFW profile is reproduced
for galactic halos as well as galaxy cluster halos. Using this numerical
technique we study some characteristics of halos assumed to be isolated or
placed in a cosmological context in presence of weak self-interacting dark
matter: the soft core formation and the core collapse. The self-interacting
dark matter cross section per unit mass is assumed to be inversely proportional
to the particle collision velocity: .Comment: Accepted for publication in MNRAS (2 figures added
Discovery of new stellar groups in the Orion complex
We test the ability of two unsupervised machine learning algorithms,
\textit{EnLink} and Shared Nearest Neighbour (SNN), to identify stellar
groupings in the Orion star-forming complex as an application to the
5-dimensional astrometric data from \textit{Gaia} DR2. The algorithms represent
two distinct approaches to limiting user bias when selecting parameter values
and evaluating the relative weights among astrometric parameters.
\textit{EnLink} adopts a locally adaptive distance metric and eliminates the
need of parameter tuning through automation. The original SNN relies only on
human input for parameter tuning so we modified SNN to run in two stages. We
first ran the original SNN 7,000 times, each with a randomly generated sample
according to within-source co-variance matrices provided in \textit{Gaia} DR2
and random parameter values within reasonable ranges. During the second stage,
we modified SNN to identify the most repeating stellar groups from 25,798 we
obtained in the first stage. We reveal 21 spatially- and kinematically-coherent
groups in the Orion complex, 12 of which previously unknown. The groups show a
wide distribution of distances extending as far as about 150 pc in front of the
star-forming Orion molecular clouds, to about 50 pc beyond them where we find,
unexpectedly, several groups. Our results expose to view the wealth of
sub-structure in the OB association, within and beyond the classical Blaauw
Orion OBI sub-groups. A full characterization of the new groups is of the
essence as it offers the potential to unveil how star formation proceeds
globally in large complexes such as Orion. The data and code that generated the
groups in this work as well as the final table can be found at \protect\url{
https://github.com/BoquanErwinChen/GaiaDR2_Orion_Dissection}.Comment: 9 pages, 4 figures. Accepted by A&A. Comments welcom
The Magellanic Group and the Seven Dwarfs
The Magellanic Clouds were the largest members of a group of dwarf galaxies
that entered the Milky Way (MW) halo at late times. This group, dominated by
the LMC, contained ~4% of the mass of the Milky Way prior to its accretion and
tidal disruption, but ~70% of the known dwarfs orbiting the MW. Our theory
addresses many outstanding problems in galaxy formation associated with dwarf
galaxies. First, it can explain the planar orbital configuration populated by
some dSphs in the MW. Second, it provides a mechanism for lighting up a subset
of dwarf galaxies to reproduce the cumulative circular velocity distribution of
the satellites in the MW. Finally, our model predicts that most dwarfs will be
found in association with other dwarfs. The recent discovery of Leo V
(Belokurov et al. 2008), a dwarf spheroidal companion of Leo IV, and the nearby
dwarf associations supports our hypothesis.Comment: Contributed talk to IAU Symposium 256: "The Magellanic System: Stars,
Gas, and Galaxies
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