13 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
Dynamically driven inflow onto the galactic center and its effect upon molecular clouds
Funding: ERC via the ERC Synergy Grant “ECOGAL” (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% ± 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 ± 0.6 M⊙ yr−1. 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 (∼52% are strongly counter-rotating, and ∼7% 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
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
Kinematics of Galactic Centre clouds shaped by shear-seeded solenoidal turbulence
The Central Molecular Zone (CMZ; the central ~ 500 pc of the Galaxy) is a
kinematically unusual environment relative to the Galactic disc, with high
velocity dispersions and a steep size-linewidth relation of the molecular
clouds. In addition, the CMZ region has a significantly lower star formation
rate (SFR) than expected by its large amount of dense gas. An important factor
in explaining the low SFR is the turbulent state of the star-forming gas, which
seems to be dominated by rotational modes. However, the turbulence driving
mechanism remains unclear. In this work, we investigate how the Galactic
gravitational potential affects the turbulence in CMZ clouds. We focus on the
CMZ cloud G0.253+0.016 (`the Brick'), which is very quiescent and unlikely to
be kinematically dominated by stellar feedback. We demonstrate that several
kinematic properties of the Brick arise naturally in a cloud-scale
hydrodynamics simulation that takes into account the Galactic gravitational
potential. These properties include the line-of-sight velocity distribution,
the steepened size-linewidth relation, and the predominantly solenoidal nature
of the turbulence. Within the simulation, these properties result from the
Galactic shear in combination with the cloud's gravitational collapse. This is
a strong indication that the Galactic gravitational potential plays a crucial
role in shaping the CMZ gas kinematics, and is a major contributor to
suppressing the SFR by inducing predominantly solenoidal turbulent modes.Comment: 7 pages, 8 figures; accepted to MNRAS (July 24th 2023
Fuelling the nuclear ring of NGC 1097
Galactic bars can drive cold gas inflows towards the centres of galaxies. The
gas transport happens primarily through the so-called bar ``dust lanes'', which
connect the galactic disc at kpc scales to the nuclear rings at hundreds of pc
scales much like two gigantic galactic rivers. Once in the ring, the gas can
fuel star formation activity, galactic outflows, and central supermassive black
holes. Measuring the mass inflow rates is therefore important to understanding
the mass/energy budget and evolution of galactic nuclei. In this work, we use
CO datacubes from the PHANGS-ALMA survey and a simple geometrical method to
measure the bar-driven mass inflow rate onto the nuclear ring of the barred
galaxy NGC~1097. The method assumes that the gas velocity in the bar lanes is
parallel to the lanes in the frame co-rotating with the bar, and allows one to
derive the inflow rates from sufficiently sensitive and resolved
position-position-velocity diagrams if the bar pattern speed and galaxy
orientations are known. We find an inflow rate of averaged over a time span of 40 Myr, which varies by a
factor of a few over timescales of 10 Myr. Most of the inflow appears to
be consumed by star formation in the ring which is currently occurring at a
rate of -, suggesting that the
inflow is causally controlling the star formation rate in the ring as a
function of time.Comment: Accepted in MNRA
ISM dynamics in simulated galaxies: bridging the scales
The interstellar medium (ISM) and in particular giant molecular clouds (GMCs) are complex and dynamic entities, shaped by internal and external agents like stellar feedback and the galactic environment in which they reside. The aim of this thesis is to model the ISM to understand the connection of the smallest GMC scales to the large galactic scales and study the role of the environment in regulating their dynamics. We perform high resolution hydrodynamic simulations of the ISM in peculiar and rather extreme galactic configurations where we can stress test the ISM response to these environments. Our ISM model and resolution is fine-tuned to capture all important GMC physics while still retaining the large dynamic range in spatial scales necessary to follow them in the galactic environment.
In the first part of this thesis I focus on the gas dynamics of an M51-like galaxy encounter. I describe how the interaction affects the global ISM and star formation properties and I proceed with an analysis of the cloud population.
In the second part the focus falls on the central barred region of a Milky Way model. I describe the complex gas flows in this extreme environment and analyse the properties of the molecular ISM and the resulting star formation. These simulations are valuable tools to interpret observational data of the region