967 research outputs found
The origin of the Milky Way globular clusters
We present a cosmological zoom-in simulation of a Milky Way-like galaxy used
to explore the formation and evolution of star clusters. We investigate in
particular the origin of the bimodality observed in the colour and metallicity
of globular clusters, and the environmental evolution through cosmic times in
the form of tidal tensors. Our results self-consistently confirm previous
findings that the blue, metal-poor clusters form in satellite galaxies which
are accreted onto the Milky Way, while the red, metal-rich clusters form mostly
in situ or, to a lower extent in massive, self-enriched galaxies merging with
the Milky Way. By monitoring the tidal fields these populations experience, we
find that clusters formed in situ (generally centrally concentrated) feel
significantly stronger tides than the accreted ones, both in the present-day,
and when averaged over their entire life. Furthermore, we note that the tidal
field experienced by Milky Way clusters is significantly weaker in the past
than at present-day, confirming that it is unlikely that a power-law cluster
initial mass function like that of young massive clusters, is transformed into
the observed peaked distribution in the Milky Way with relaxation-driven
evaporation in a tidal field.Comment: MNRAS accepte
Supernovae feedback propagation: the role of turbulence
Modelling the propagation of supernova (SN) bubbles, in terms of energy,
momentum and spatial extent, is critical for simulations of galaxy evolution
which do not capture these scales. To date, small scale models of SN feedback
predict that the evolution of above-mentioned quantities can be solely
parameterised by average quantities of the surrounding gas, such as density.
However, most of these studies neglect the turbulent motions of this medium. In
this paper, we study the propagation and evolution of SNe in turbulent
environments. We confirm that the time evolution of injected energy and
momentum can be characterised by the average density. However, the details of
the density structure of the interstellar medium play a crucial role in the
spatial extent of the bubble, even at a given average density. We demonstrate
that spherically symmetric models of SN bubbles do not model well their spatial
extent, and therefore cannot not be used to design sub-grid models of SNe
feedback at galactic and cosmological scales.Comment: Accepted by MNRA
The impact of stellar feedback on the density and velocity structure of the interstellar medium
We study the impact of stellar feedback in shaping the density and velocity
structure of neutral hydrogen (HI) in disc galaxies. For our analysis, we carry
out pc resolution -body+adaptive mesh refinement (AMR)
hydrodynamic simulations of isolated galaxies, set up to mimic a Milky Way
(MW), and a Large and Small Magellanic Cloud (LMC, SMC). We quantify the
density and velocity structure of the interstellar medium using power spectra
and compare the simulated galaxies to observed HI in local spiral galaxies from
THINGS (The HI Nearby Galaxy Survey). Our models with stellar feedback give an
excellent match to the observed THINGS HI density power spectra. We find that
kinetic energy power spectra in feedback regulated galaxies, regardless of
galaxy mass and size, show scalings in excellent agreement with super-sonic
turbulence () on scales below the thickness of the HI
layer. We show that feedback influences the gas density field, and drives gas
turbulence, up to large (kpc) scales. This is in stark contrast to density
fields generated by large scale gravity-only driven turbulence. We conclude
that the neutral gas content of galaxies carries signatures of stellar feedback
on all scales.Comment: 19 pages, 13 figures, 2 tables, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
From giant clumps to clouds - I. The impact of gas fraction evolution on the stability of galactic discs
The morphology of gas-rich disc galaxies at redshift is dominated by a few massive clumps. The process of formation or assembly of these clumps and their relation to molecular clouds in contemporary spiral galaxies are still unknown. Using simulations of isolated disc galaxies, we study how the structure of the interstellar medium and the stability regime of the discs change when varying the gas fraction. In all galaxies, the stellar component is the main driver of instabilities. However, the molecular gas plays a non-negligible role in the interclump medium of gas-rich cases, and thus in the assembly of the massive clumps. At scales smaller than a few 100 pc, the Toomre-like disc instabilities are replaced by another regime, especially in the gas-rich galaxies. We find that galaxies at low gas fraction (10 percent) stand apart from discs with more gas, which all share similar properties in virtually all aspects we explore. For gas fractions below , the clump-scale regime of instabilities disappears, leaving only the large-scale disc-driven regime. Associating the change of gas fraction to the cosmic evolution of galaxies, this transition marks the end of the clumpy phase of disc galaxies, and allows for the onset of spiral structures, as commonly found in the local Universe
The nature of damped HI absorbers probed by cosmological simulations: satellite accretion and outflows
We use state-of-the-art cosmological zoom simulations to explore the
distribution of neutral gas in and around galaxies that gives rise to high
column density \ion{H}{i} \mbox{Ly-} absorption (formally, sub-DLAs and
DLAs) in the spectra of background quasars. Previous cosmological hydrodynamic
simulations under-predict the mean projected separations of these
absorbers relative to the host, and invoke selection effects to bridge the gap
with observations. On the other hand, single lines of sight (LOS) in absorption
cannot uniquely constrain the galactic origin. Our simulations match all
observational data, with DLA and sub-DLA LOS existing over the entire probed
parameter space ([M/H], kpc) at all redshifts
(). We demonstrate how the existence of DLA LOS at kpc from a massive host galaxy require high numerical resolution, and
that these LOS are associated with dwarf satellites in the main halo, stripped
metal-rich gas and outflows. Separating the galaxy into interstellar
("\ion{H}{i} disc") and circumgalactic ("halo") components, we find that both
components significantly contribute to damped \ion{H}{i} absorption LOS. Above
the sub-DLA (DLA) limits, the disc and halo contribute with and
per cent, respectively. Our simulations confirm analytical
model-predictions of the DLA-distribution at . At high redshift
() sub-DLA and DLAs occupy similar spatial scales, but on average
separate by a factor of two by . On whether sub-DLA and DLA LOS
sample different stellar-mass galaxies, such a correlation can be driven by a
differential covering-fraction of sub-DLA to DLA LOS with stellar mass. This
preferentially selects sub-DLA LOS in more massive galaxies in the low-
universe.Comment: 12 pages, 5 figures, submitted to MNRAS 29/01/201
The merger-starburst connection across cosmic times
The correspondence between galaxy major mergers and starburst activity is
well-established observationally and in simulations of low redshift galaxies.
However, the evolution of the properties of interactions and of the galaxies
involved suggests that the starburst response of galaxies to merger events
could vary across cosmic time. Using the VINTERGATAN cosmological zoom-in
simulation of a Milky Way-like galaxy, we show here that starbursts, i.e.
episodes of fast star formation, are connected with the onset of tidal
compression, itself induced by mergers. However, this compression becomes
strong enough to trigger starbursts only after the formation of the galactic
disc. As a consequence, starburst episodes are only found during a precise
phase of galaxy evolution, after the formation of the disc and until the last
major merger. As the depletion time quantifies the instantaneous star formation
activity, while the specific star formation rate involves the integrated result
of the past activity (via the stellar mass), starburst episodes do not
necessarily coincide with elevated specific star formation rate. This suggests
that not all starburst galaxies are outliers above the main sequence of galaxy
formation.Comment: MNRAS accepted, 10 page
From lenticulars to blue compact dwarfs: the stellar mass fraction is regulated by disc gravitational instability
The stellar-to-halo mass relation (SHMR) is one of the main sources of
information we have on the connection between galaxies and their dark matter
haloes. Here we analyse in detail two popular forms of the SHMR, M*/Mh vs Mh
and M*/Mh vs M*, and compare them with another physically motivated scaling
relation, M*/Mh vs GMh/j*sigmahat*. Although this relation cannot predict the
halo mass explicitely, it connects the stellar mass fraction to fundamental
galaxy properties such as specific angular momentum (j*) and velocity
dispersion (sigmahat*) via disc gravitational instability. Our detailed
comparative analysis is based on one of the largest sample of galaxies with
both high-quality rotation curves and near-infrared surface photometry, and
leads to the following results: (i) M*/Mh vs Mh and M*/Mh vs M* are not just
two alternative parametrizations of the same relation, but two significantly
different relations; (ii) M*/Mh vs GMh/j*sigmahat* outperforms the two popular
relations in terms of tightness, correlation strength and significance; (iii)
j* and sigmahat* play an equally important role in our scaling relation, and it
is their interplay that constrains M*/Mh so tightly; (iv) the evolution of
M*/Mh, j* and sigmahat* is regulated by disc gravitational instability: when
M*/Mh varies, j* and sigmahat* also vary as predicted by our scaling relation,
thus erasing the memory of such evolution. This implies that the process of
disc gravitational instability is intriguingly uniform across disc galaxies of
all morphological types: from lenticulars to blue compact dwarfs. In
particular, the cosmic variance of Toomre's Q is 0.2 dex, a universal value for
both stars and atomic gas.Comment: Submitted to MNRA
Runaway stars masquerading as star formation in galactic outskirts
In the outskirts of nearby spiral galaxies, star formation is observed in
extremely low gas surface densities. Star formation in these regions, where the
interstellar medium is dominated by diffuse atomic hydrogen, is difficult to
explain with classic star formation theories. In this work, we introduce
runaway stars as an explanation to this observation. Runaway stars, produced by
collisional dynamics in young stellar clusters, can travel kilo-parsecs during
their main sequence life time. Using galactic-scale hydrodynamic simulations
including a treatment of individual stars, we demonstrate that this mechanism
enables the ejection of young massive stars into environments where the gas is
not dense enough to trigger star formation. This results in the appearance of
star formation in regions where it ought to be impossible. We conclude that
runaway stars are a contributing, if not dominant, factor to the observations
of star formation in the outskirts of spiral galaxies.Comment: Submitted to MNRAS Letters, comments welcom
The specific angular momentum of disc galaxies and its connection with galaxy morphology, bar structure and disc gravitational instability
The specific angular momenta () of stars (), gas
(), baryons as a whole () and dark matter
haloes () contain clues of vital importance about how galaxies
form and evolve. Using one of the largest samples of disc galaxies (S0-BCD)
with high-quality rotation curves and near-infrared surface photometry, we
perform a detailed comparative analysis of that stretches across a variety
of galaxy properties. Our analysis imposes tight constraints on the "retained"
fractions of specific angular momentum (,
and ), as well
as on their systematic trends with mass fraction and galaxy morphology, thus on
how well specific angular momentum is conserved in the process of disc galaxy
formation and evolution. Besides, our analysis demonstrates how challenging it
is to characterize barred galaxies from a gravitational instability point of
view. This is true not only for the popular Efstathiou, Lake & Negroponte
(1982) bar instability criterion, which fails to separate barred from
non-barred galaxies in about 55% of the cases, but also for the mass-weighted
Toomre (1964) parameter of atomic gas, , which
succeeds in separating barred from non-barred galaxies, but only in a
statistical sense.Comment: Submitted to MNRA
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