220 research outputs found
A theoretical explanation for the Central Molecular Zone asymmetry
It has been known for more than thirty years that the distribution of
molecular gas in the innermost 300 parsecs of the Milky Way, the Central
Molecular Zone, is strongly asymmetric. Indeed, approximately three quarters of
molecular emission comes from positive longitudes, and only one quarter from
negative longitudes. However, despite much theoretical effort, the origin of
this asymmetry has remained a mystery. Here we show that the asymmetry can be
neatly explained by unsteady flow of gas in a barred potential. We use
high-resolution 3D hydrodynamical simulations coupled to a state-of-the-art
chemical network. Despite the initial conditions and the bar potential being
point-symmetric with respect to the Galactic Centre, asymmetries develop
spontaneously due to the combination of a hydrodynamical instability known as
the "wiggle instability" and the thermal instability. The observed asymmetry
must be transient: observations made tens of megayears in the past or in the
future would often show an asymmetry in the opposite sense. Fluctuations of
amplitude comparable to the observed asymmetry occur for a large fraction of
the time in our simulations, and suggest that the present is not an exceptional
moment in the life of our Galaxy.Comment: Accepted for publication in MNRAS. Videos of the simulations are
available at http://www.ita.uni-heidelberg.de/~mattia/download.htm
Polaron effects in electron channels on a helium film
Using the Feynman path-integral formalism we study the polaron effects in
quantum wires above a liquid helium film. The electron interacts with
two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one
dimension (1D) by an harmonic potential. The obtained results are valid for
arbitrary temperature (), electron-phonon coupling strength (), and
lateral confinement (). Analytical and numerical results are
obtained for limiting cases of , , and . We found the
surprising result that reducing the electron motion from 2D to quasi-1D makes
the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Dynamically driven inflow onto the galactic center and its effect upon molecular clouds
Funding: ERC via the ERC Synergy Grant āECOGALā (grant 855130) (M.C.S., R.G.T., S.C.O.G., and R.S.K). R.J.S. gratefully acknowledges an STFC Ernest Rutherford fellowship (grant ST/N00485X/1).The Galactic bar plays a critical role in the evolution of the Milky Way's Central Molecular Zone (CMZ), driving gas toward the Galactic Center via gas flows known as dust lanes. To explore the interaction between the CMZ and the dust lanes, we run hydrodynamic simulations in arepo, modeling the potential of the Milky Way's bar in the absence of gas self-gravity and star formation physics, and we study the flows of mass using Monte Carlo tracer particles. We estimate the efficiency of the inflow via the dust lanes, finding that only about a third (30% Ā± 12%) of the dust lanes' mass initially accretes onto the CMZ, while the rest overshoots and accretes later. Given observational estimates of the amount of gas within the Milky Way's dust lanes, this suggests that the true total inflow rate onto the CMZ is 0.8 Ā± 0.6 Mā yrā1. Clouds in this simulated CMZ have sudden peaks in their average density near the apocenter, where they undergo violent collisions with inflowing material. While these clouds tend to counter-rotate due to shear, co-rotating clouds occasionally occur due to the injection of momentum from collisions with inflowing material (ā¼52% are strongly counter-rotating, and ā¼7% are strongly co-rotating of the 44 cloud sample). We investigate the formation and evolution of these clouds, finding that they are fed by many discrete inflow events, providing a consistent source of gas to CMZ clouds even as they collapse and form stars.Peer reviewe
Is the molecular KS relationship universal down to low metallicities?
Funding: . RJS gratefully acknowledges an STFC Ernest Rutherford fellowship (grant ST/N00485X/1). SCOG, RT, MCS, and RSK acknowledge funding from the European Research Council via the ERC Synergy Grant āECOGAL ā Understanding our Galactic ecosystem: From the disc of the Milky Way to the formation sites of stars and planets (project ID 855130).In recent years, it has been speculated that in extreme low-metallicity galactic environments, stars form in regions that lack H2. In this paper, we investigate how changing the metallicity and ultraviolet (UV) field strength of a galaxy affects the star formation within, and the molecular gas KennicuttāSchmidt (KS) relation. Using extremely high-resolution AREPO simulations of isolated dwarf galaxies, we independently vary the metallicity and UV field to between 1 perācent and 10 perācent solar neighbourhood values. We include a non-equilibrium, time-dependent chemical network to model the molecular composition of the interstellar medium and include the effects of gas shielding from an ambient UV field. Crucially, our simulations directly model the gravitational collapse of gas into star-forming clumps and cores and their subsequent accretion using sink particles. In this first publication, we find that reducing the metallicity and UV field by a factor of 10 has no effect on star formation and minimal effect on the cold, dense star-forming gas. The cold gas depletion times are almost an order of magnitude longer than the molecular gas depletion time due to the presence of star formation in HāI dominated cold gas. We study the H2 KS relationship that arises naturally within the simulations and find a near-linear power-law index of N = 1.09 Ā± 0.014 in our fiducial 10 per cent solar metallicity model. As the metallicity and UV field are reduced, this becomes moderately steeper, with a slope of N = 1.24 Ā± 0.022 for our 1 per cent solar metallicity and 1 per cent solar UV field model.Peer reviewe
Magnetic fields do not suppress global star formation in low metallicity dwarf galaxies
Funding: DJW is grateful for support through a STFC Doctoral Training Partnership. RJS gratefully acknowledges an STFC Ernest Rutherford fellowship (grant ST/N00485X/1). DJW, SCOG, RT, JDS, and RSK acknowledge funding from the European Research Council via the ERC Synergy Grant āECOGAL ā Understanding our Galactic ecosystem: from the disc of the Milky Way to the formation sites of stars and planetsā (project ID 855130).Many studies concluded that magnetic fields suppress star formation in molecular clouds and Milky Way like galaxies. However, most of these studies are based on fully developed fields that have reached the saturation level, with little work on investigating how an initial weak primordial field affects star formation in low metallicity environments. In this paper, we investigate the impact of a weak initial field on low metallicity dwarf galaxies. We perform high-resolution AREPO simulations of five isolated dwarf galaxies. Two models are hydrodynamical, two start with a primordial magnetic field of 10-6 Ī¼G and different sub-solar metallicities, and one starts with a saturated field of 10-2 Ī¼G. All models include a non-equilibrium, time-dependent chemical network that includes the effects of gas shielding from the ambient ultraviolet field. Sink particles form directly from the gravitational collapse of gas and are treated as star-forming clumps that can accrete gas. We vary the ambient uniform far ultraviolet field, and cosmic ray ionization rate between 1 perācent and 10 perācent of solar values. We find that the magnetic field has little impact on the global star formation rate (SFR), which is in tension with some previously published results. We further find that the initial field strength has little impact on the global SFR. We show that an increase in the mass fractions of both molecular hydrogen and cold gas, along with changes in the perpendicular gas velocity dispersion and the magnetic field acting in the weak-field model, overcome the expected suppression in star formation.Peer reviewe
Simulations of the Milky Way's central molecular zone -- I. Gas dynamics
We use hydrodynamical simulations to study the Milky Way's central molecular
zone (CMZ). The simulations include a non-equilibrium chemical network, the gas
self-gravity, star formation and supernova feedback. We resolve the structure
of the interstellar medium at sub-parsec resolution while also capturing the
interaction between the CMZ and the bar-driven large-scale flow out to R\sim
5\kpc. Our main findings are as follows: (1) The distinction between inner
(~pc) and outer (~pc) CMZ that is
sometimes proposed in the literature is unnecessary. Instead, the CMZ is best
described as single structure, namely a star-forming ring with outer radius
~pc which includes the 1.3 complex and which is directly
interacting with the dust lanes that mediate the bar-driven inflow. (2) This
accretion can induce a significant tilt of the CMZ out of the plane. A tilted
CMZ might provide an alternative explanation to the -shaped structure
identified in Herschel data by Molinari et al. 2011. (3) The bar in our
simulation efficiently drives an inflow from the Galactic disc (~kpc) down to the CMZ (~pc) of the order of
, consistent with observational determinations. (4)
Supernova feedback can drive an inflow from the CMZ inwards towards the
circumnuclear disc of the order of . (5) We
give a new interpretation for the 3D placement of the 20 and 50 km s
clouds, according to which they are close (~pc) to the Galactic
centre, but are also connected to the larger-scale streams at ~pc.Comment: Accepted for publication in MNRAS. Movies of the simulations can be
found at: https://www.youtube.com/channel/UCwnzfO-xLxzRDz9XsexfPo
Simulations of the star-forming molecular gas in an interacting M51-like galaxy: cloud population statistics
To investigate how molecular clouds react to different environmental
conditions at a galactic scale, we present a catalogue of giant molecular
clouds resolved down to masses of ~M from a simulation of
the entire disc of an interacting M51-like galaxy and a comparable isolated
galaxy. Our model includes time-dependent gas chemistry, sink particles for
star formation and supernova feedback, meaning we are not reliant on star
formation recipes based on threshold densities and can follow the physics of
the cold molecular phase. We extract giant molecular clouds at a given timestep
of the simulations and analyse their properties. In the disc of our simulated
galaxies, spiral arms seem to act merely as snowplows, gathering gas and clouds
without dramatically affecting their properties. In the centre of the galaxy,
on the other hand, environmental conditions lead to larger, more massive
clouds. While the galaxy interaction has little effect on cloud masses and
sizes, it does promote the formation of counter-rotating clouds. We find that
the identified clouds seem to be largely gravitationally unbound at first
glance, but a closer analysis of the hierarchical structure of the molecular
interstellar medium shows that there is a large range of virial parameters with
a smooth transition from unbound to mostly bound for the densest structures.
The common observation that clouds appear to be virialised entities may
therefore be due to CO bright emission highlighting a specific level in this
hierarchical binding sequence. The small fraction of gravitationally bound
structures found suggests that low galactic star formation efficiencies may be
set by the process of cloud formation and initial collapse.Comment: 22 pages, 26 figures, 2 tables. Properties of the clouds in the
catalog are provided as a supplementary fil
Simulations of the star-forming molecular gas in an interacting M51-like galaxy : cloud population statistics
Funding: They also acknowledge funding from the European Research Council in the ERC Synergy Grant āECOGAL ā Understanding our Galactic ecosystem: From the disk of the Milky Way to the formation sites of stars and planetsā (project ID 855130). RJS gratefully acknowledges an STFC Ernest Rutherford fellowship (grant ST/N00485X/1).To investigate how molecular clouds react to different environmental conditions at a galactic scale, we present a catalogue of giant molecular clouds (GMCs) resolved down to masses of ā¼10āMā from a simulation of the entire disc of an interacting M51-like galaxy and a comparable isolated galaxy. Our model includes time-dependent gas chemistry, sink particles for star formation, and supernova feedback, meaning we are not reliant on star formation recipes based on threshold densities and can follow the physics of the cold molecular phase. We extract GMCs from the simulations and analyse their properties. In the disc of our simulated galaxies, spiral arms seem to act merely as snowplows, gathering gas, and clouds without dramatically affecting their properties. In the centre of the galaxy, on the other hand, environmental conditions lead to larger, more massive clouds. While the galaxy interaction has little effect on cloud masses and sizes, it does promote the formation of counter-rotating clouds. We find that the identified clouds seem to be largely gravitationally unbound at first glance, but a closer analysis of the hierarchical structure of the molecular interstellar medium shows that there is a large range of virial parameters with a smooth transition from unbound to mostly bound for the densest structures. The common observation that clouds appear to be virialized entities may therefore be due to CO bright emission highlighting a specific level in this hierarchical binding sequence. The small fraction of gravitationally bound structures found suggests that low galactic star formation efficiencies may be set by the process of cloud formation and initial collapse.Peer reviewe
On the distribution of the CNM in spiral galaxies
The Cold Neutral Medium (CNM) is an important part of the galactic gas cycle
and a precondition for the formation of molecular and star forming gas, yet its
distribution is still not fully understood. In this work we present extremely
high resolution simulations of spiral galaxies with time-dependent chemistry
such that we can track the formation of the CNM, its distribution within the
galaxy, and its correlation with star formation. We find no strong radial
dependence between the CNM fraction and total HI due to the decreasing
interstellar radiation field counterbalancing the decreasing gas column density
at larger galactic radii.However, the CNM fraction does increase in spiral arms
where the CNM distribution is clumpy, rather than continuous, overlapping more
closely with H2. The CNM doesn't extend out radially as far as HI, and the
vertical scale height is smaller in the outer galaxy compared to HI with no
flaring. The CNM column density scales with total midplane pressure and
disappears from the gas phase below values of PT/kB =1000 K/cm3. We find that
the star formation rate density follows a similar scaling law with CNM column
density to the total gas Kennicutt-Schmidt law. In the outer galaxy we produce
realistic vertical velocity dispersions in the HI purely from galactic dynamics
but our models do not predict CNM at the extremely large radii observed in HI
absorption studies of the Milky Way. We suggest that extended spiral arms might
produce isolated clumps of CNM at these radii.Comment: 13 pages, 19 figures, submitted to MNRA
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