641 research outputs found
On the interplay between star formation and feedback in galaxy formation simulations
We investigate the star formation-feedback cycle in cosmological galaxy
formation simulations, focusing on progenitors of Milky Way (MW)-sized
galaxies. We find that in order to reproduce key properties of the MW
progenitors, such as semi-empirically derived star formation histories and the
shape of rotation curves, our implementation of star formation and stellar
feedback requires 1) a combination of local early momentum feedback via
radiation pressure and stellar winds and subsequent efficient supernovae
feedback, and 2) efficacy of feedback that results in self-regulation of the
global star formation rate on kiloparsec scales. We show that such
feedback-driven self-regulation is achieved globally for a local star formation
efficiency per free fall time of . Although this
value is larger that the value usually inferred
from the Kennicutt-Schmidt (KS) relation, we show that it is consistent with
direct observational estimates of in molecular clouds.
Moreover, we show that simulations with local efficiency of reproduce the global observed KS relation. Such simulations
also reproduce the cosmic star formation history of the Milky Way sized
galaxies and satisfy a number of other observational constraints. Conversely,
we find that simulations that a priori assume an inefficient mode of star
formation, instead of achieving it via stellar feedback regulation, fail to
produce sufficiently vigorous outflows and do not reproduce observations. This
illustrates the importance of understanding the complex interplay between star
formation and feedback and the detailed processes that contribute to the
feedback-regulated formation of galaxies.Comment: 20 pages, 13 figures, accepted for publication in Ap
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
Observing the circumgalactic medium of simulated galaxies through synthetic absorption spectra
We explore the multiphase structure of the circumgalactic medium (CGM) probed
by synthetic spectra through a cosmological zoom-in galaxy formation
simulation. We employ a Bayesian method for modelling a combination of
absorption lines to derive physical properties of absorbers with a formal
treatment of detections, including saturated systems, and non-detections in a
uniform manner. We find that in the lines of sight passing through localized
density structures, absorption lines of low, intermediate and high ions are
present in the spectrum and overlap in velocity space. Low, intermediate and
high ions can be combined to derive the mass-weighted properties of a
density-varying peak, although the ions are not co-spatial within the
structure. By contrast, lines of sight that go through the hot halo only
exhibit detectable HI and high ions. In such lines of sight, the absorption
lines are typically broad due to the complex velocity fields across the entire
halo. We show that the derived gas density, temperature, and metallicity match
closely the corresponding HI mass-weighted averages along the LOS. We also show
that when the data quality allows, our Bayesian technique allows one to recover
the underlying physical properties of LOS by incorporating both detections and
non-detections. It is especially useful to include non-detections, of species
such as NV or NeVIII, when the number of detections of strong absorbers, such
as HI and OVI, is smaller than the number of model parameters (density,
temperature, and metallicity).Comment: Accepted for publication in MNRA
Characterizing gravitational instability in turbulent multi-component galactic discs
Gravitational instabilities play an important role in galaxy evolution and in
shaping the interstellar medium (ISM). The ISM is observed to be highly
turbulent, meaning that observables like the gas surface density and velocity
dispersion depend on the size of the region over which they are measured. In
this work we investigate, using simulations of Milky Way-like disc galaxies
with a resolution of pc, the nature of turbulence in the ISM and how
this affects the gravitational stability of galaxies. By accounting for the
measured average turbulent scalings of the density and velocity fields in the
stability analysis, we can more robustly characterize the average level of
stability of the galaxies as a function of scale, and in a straightforward
manner identify scales prone to fragmentation. Furthermore, we find that the
stability of a disc with feedback-driven turbulence can be well described by a
"Toomre-like" stability criterion on all scales, whereas the classical
can formally lose its meaning on small scales if violent disc instabilities
occur in models lacking pressure support from stellar feedback.Comment: 11 pages, 5 figures, submitted to MNRA
The stellar mass-halo mass relation of isolated field dwarfs: a critical test of CDM at the edge of galaxy formation
We fit the rotation curves of isolated dwarf galaxies to directly measure the
stellar mass-halo mass relation () over the mass range . By accounting for cusp-core
transformations due to stellar feedback, we find a monotonic relation with
little scatter. Such monotonicity implies that abundance matching should yield
a similar if the cosmological model is correct. Using the 'field
galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the
halo mass function from the Cold Dark Matter Bolshoi simulation, we
find remarkable agreement between the two. This holds down to M, and to M if we
assume a power law extrapolation of the SDSS stellar mass function below M.
However, if instead of SDSS we use the stellar mass function of nearby galaxy
groups, then the agreement is poor. This occurs because the group stellar mass
function is shallower than that of the field below M,
recovering the familiar 'missing satellites' and 'too big to fail' problems.
Our result demonstrates that both problems are confined to group environments
and must, therefore, owe to 'galaxy formation physics' rather than exotic
cosmology.
Finally, we repeat our analysis for a Warm Dark Matter cosmology,
finding that it fails at 68% confidence for a thermal relic mass of keV, and keV if we use the power law extrapolation
of SDSS. We conclude by making a number of predictions for future surveys based
on these results.Comment: 22 pages; 2 Tables; 10 Figures. This is the version accepted for
publication in MNRAS. Key changes: (i) added substantially more information
on the surveys used to measure the stellar mass functions; (ii) added tests
of the robustness of our results. Results and conclusions unchanged from
previously. Minor typos corrected from previous versio
Galaxies that Shine: radiation-hydrodynamical simulations of disk galaxies
Radiation feedback is typically implemented using subgrid recipes in
hydrodynamical simulations of galaxies. Very little work has so far been
performed using radiation-hydrodynamics (RHD), and there is no consensus on the
importance of radiation feedback in galaxy evolution. We present RHD
simulations of isolated galaxy disks of different masses with a resolution of
18 pc. Besides accounting for supernova feedback, our simulations are the first
galaxy-scale simulations to include RHD treatments of photo-ionisation heating
and radiation pressure, from both direct optical/UV radiation and
multi-scattered, re-processed infrared (IR) radiation. Photo-heating smooths
and thickens the disks and suppresses star formation about as much as the
inclusion of ("thermal dump") supernova feedback does. These effects decrease
with galaxy mass and are mainly due to the prevention of the formation of dense
clouds, as opposed to their destruction. Radiation pressure, whether from
direct or IR radiation, has little effect, but for the IR radiation we show
that its impact is limited by our inability to resolve the high optical depths
for which multi-scattering becomes important. While artificially boosting the
IR optical depths does reduce the star formation, it does so by smoothing the
gas rather than by generating stronger outflows. We conclude that although
higher-resolution simulations, and potentially also different supernova
implementations, are needed for confirmation, our findings suggest that
radiation feedback is more gentle and less effective than is often assumed in
subgrid prescriptions.Comment: 28 pages, 26 figures, accepted for publication in MNRAS. Revised to
match published versio
Reply to Melott's Comment on ``Discreteness Effects in Lambda Cold Dark Matter Simulations: A Wavelet-Statistical View'' by Romeo et al
Melott has made pioneering studies of the effects of particle discreteness in
N-body simulations, a fundamental point that needs careful thought and analysis
since all such simulations suffer from numerical noise arising from the use of
finite-mass particles. Melott (arXiv:0804.0589) claims that the conclusions of
our paper (arXiv:0804.0294) are essentially equivalent to those of his earlier
work. Melott is wrong: he has jumped onto one of our conclusions and
interpreted that in his own way. Here we point out the whys and the wherefores
Simulations of disk galaxies with cosmic ray driven galactic winds
We present results from high-resolution hydrodynamic simulations of isolated
SMC- and Milky Way-sized galaxies that include a model for feedback from
galactic cosmic rays (CRs). We find that CRs are naturally able to drive winds
with mass loading factors of up to ~10 in dwarf systems. The scaling of the
mass loading factor with circular velocity between the two simulated systems is
consistent with \propto v_c^{1-2} required to reproduce the faint end of the
galaxy luminosity function. In addition, simulations with CR feedback reproduce
both the normalization and the slope of the observed trend of wind velocity
with galaxy circular velocity. We find that winds in simulations with CR
feedback exhibit qualitatively different properties compared to SN driven
winds, where most of the acceleration happens violently in situ near star
forming sites. In contrast, the CR-driven winds are accelerated gently by the
large-scale pressure gradient established by CRs diffusing from the
star-forming galaxy disk out into the halo. The CR-driven winds also exhibit
much cooler temperatures and, in the SMC-sized system, warm (T~10^4 K) gas
dominates the outflow. The prevalence of warm gas in such outflows may provide
a clue as to the origin of ubiquitous warm gas in the gaseous halos of galaxies
detected via absorption lines in quasar spectra.Comment: ApJL accepted. Replaced with accepted version. Minor revision in
response to referee comment
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