600 research outputs found
The Formation of a Disk Galaxy within a Growing Dark Halo
We present a dynamical model for the formation and evolution of a massive
disk galaxy, within a growing dark halo whose mass evolves according to
cosmological simulations of structure formation. The galactic evolution is
simulated with a new 3D chemo-dynamical code, including dark matter, stars and
a multi-phase ISM. The simulations start at redshift z=4.85 with a small dark
halo in a LCDM universe and we follow the evolution until the present epoch.
The energy release by massive stars and SNe prevents a rapid collapse of the
baryonic matter and delays the maximum star formation until z=1. The galaxy
forms radially from inside-out and vertically from halo to disk. The first
galactic component that forms is the halo, followed by the bulge, the disk-halo
transition region, and the disk. At z=1, a bar begins to form which later turns
into a triaxial bulge. There is a pronounced deficiency of low-metallicity disk
stars due to pre-enrichment of the disk ISM with metal-rich gas from the bulge
and inner disk (G-dwarf problem). The mean rotation and the distribution of
orbital eccentricities for all stars as a function of metallicity are not very
different from those observed in the solar neighbourhood, showing that
homogeneous collapse models are oversimplified. The approach presented here
provides a detailed description of the formation and evolution of an isolated
disk galaxy in a LCDM universe, yielding new information about the kinematical
and chemical history of the stars and the ISM, but also about the evolution of
the luminosity, the colours and the morphology of disk galaxies.Comment: 23 pages, LaTeX, 18 figures, A&A accepted, a high resolution version
of the paper can be found at http://www.astro.unibas.ch/leute/ms.shtm
The Structure of the Interstellar Medium of Star Forming Galaxies
We present numerical methods for including stellar feedback in galaxy-scale
simulations. We include heating by SNe (I & II), gas recycling and
shock-heating from O-star & AGB winds, HII photoionization, and radiation
pressure from stellar photons. The energetics and time-dependence are taken
directly from stellar evolution models. We implement these in simulations with
pc-scale resolution, modeling galaxies from SMC-like dwarfs and MW analogues to
massive z~2 starburst disks. Absent feedback, gas cools and collapses without
limit. With feedback, the ISM reaches a multi-phase steady state in which GMCs
continuously form, disperse, and re-form. Our primary results include: (1) Star
forming galaxies generically self-regulate at Toomre Q~1. Most of the volume is
in diffuse hot gas with most of the mass in dense GMC complexes. The phase
structure and gas mass at high densities are much more sensitive probes of
stellar feedback physics than integrated quantities (Toomre Q or gas velocity
dispersion). (2) Different feedback mechanisms act on different scales:
radiation & HII pressure are critical to prevent runaway collapse of dense gas
in GMCs. SNe and stellar winds dominate the dynamics of volume-filling hot gas;
however this primarily vents out of the disk. (3) The galaxy-averaged SFR is
determined by feedback. For given feedback efficiency, restricting star
formation to molecular gas or modifying the cooling function has little effect;
but changing feedback mechanisms directly translates to shifts off the
Kennicutt-Schmidt relation. (4) Self-gravity leads to marginally-bound GMCs
with an ~M^-2 mass function with a cutoff at the Jeans mass; they live a few
dynamical times before being disrupted by stellar feedback and turn ~1-10% of
their mass into stars (increasing from dwarfs through starburst galaxies).
Low-mass GMCs are preferentially unbound.Comment: 34 pages, 24 figures, accepted to MNRAS (matches accepted version).
Movies of the simulations are available at
https://www.cfa.harvard.edu/~phopkins/Site/Movies_sbw.htm
The Atomic to Molecular Transition and its Relation to the Scaling Properties of Galaxy Disks in the Local Universe
We extend existing semi-analytic models of galaxy formation to track atomic
and molecular gas in disk galaxies. Simple recipes for processes such as
cooling, star formation, supernova feedback, and chemical enrichment of the
stars and gas are grafted on to dark matter halo merger trees derived from the
Millennium Simulation. Each galactic disk is represented by a series of
concentric rings. We assume that surface density profile of infalling gas in a
dark matter halo is exponential, with scale radius r_d that is proportional to
the virial radius of the halo times its spin parameter . As the dark
matter haloes grow through mergers and accretion, disk galaxies assemble from
the inside out. We include two simple prescriptions for molecular gas formation
processes in our models: one is based on the analytic calculations by Krumholz,
McKee & Tumlinson (2008), and the other is a prescription where the H_2
fraction is determined by the kinematic pressure of the ISM. Motivated by the
observational results of Leroy et al. (2008), we adopt a star formation law in
which in the regime where the molecular gas
dominates the total gas surface density, and where atomic hydrogen dominates. We then fit these models to
the radial surface density profiles of stars, HI and H_2 drawn from recent high
resolution surveys of stars and gas in nearby galaxies. We explore how the
ratios of atomic gas, molecular gas and stellar mass vary as a function of
global galaxy scale parameters, including stellar mass, stellar surface
density, and gas surface density. We elucidate how the trends can be understood
in terms of three variables that determine the partition of baryons in disks:
the mass of the dark matter halo, the spin parameter of the halo, and the
amount of gas recently accreted from the external environment.Comment: Made some minor changes according to the reviewer's suggestion.
Accepted by MNRA
On the onset of galactic winds in quiescent star forming galaxies
We studied the effect of supernovae feedback on a disk galaxy, taking into
account the impact of infalling gas on both the star formation history and the
corresponding outflow structure, the apparition of a supernovae-driven wind
being highly sensitive to the halo mass, the galaxy spin and the star formation
efficiency. We model our galaxies as cooling and collapsing NFW spheres. The
dark matter component is modelled as a static external potential, while the
baryon component is described by the Euler equations using the AMR code RAMSES.
Metal-dependent cooling and supernovae-heating are also implemented using
state-of-the-art recipes coming from cosmological simulations. We allow for 3
parameters to vary: the halo circular velocity, the spin parameter and the star
formation efficiency. We found that the ram pressure of infalling material is
the key factor limiting the apparition of galactic winds. We obtain a very low
feedback efficiency, with supernovae to wind energy conversion factor around
one percent, so that only low cicrular velocity galaxies give rise to strong
winds. For massive galaxies, we obtain a galatic fountain, for which we discuss
the observational properties. We conclude that for quiescent isolated galaxies,
galactic winds appear only in very low mass systems. Although that can quite
efficiently enrich the IGM with metals, they don't carry away enough cold
material to solve the overcooling problem.Comment: 19 pages, 13 figures, 1 table, submited to A&
Returning home: heritage work among the Stl'atl'imx of the Lower Lillooet River Valley
This article focusses on heritage practices in the tensioned landscape of the Stl’atl’imx (pronounced Stat-lee-um) people of the Lower Lillooet River Valley, British Columbia, Canada. Displaced from their traditional territories and cultural traditions through the colonial encounter, they are enacting, challenging and remaking their heritage as part of their long term goal to reclaim their land and return ‘home’. I draw on three examples of their heritage work: graveyard cleaning, the shifting ‘official’/‘unofficial’ heritage of a wagon road, and marshalling of the mountain named Nsvq’ts (pronounced In-SHUCK-ch) in order to illustrate how the past is strategically mobilised in order to substantiate positions in the present. While this paper focusses on heritage in an Indigenous and postcolonial context, I contend that the dynamics of heritage practices outlined here are applicable to all heritage practices
Simulating Supersonic Turbulence in Galaxy Outflows
We present three-dimensional, adaptive mesh simulations of dwarf galaxy out-
flows driven by supersonic turbulence. Here we develop a subgrid model to track
not only the thermal and bulk velocities of the gas, but also its turbulent
velocities and length scales. This allows us to deposit energy from supernovae
directly into supersonic turbulence, which acts on scales much larger than a
particle mean free path, but much smaller than resolved large-scale flows.
Unlike previous approaches, we are able to simulate a starbursting galaxy
modeled after NGC 1569, with realistic radiative cooling throughout the
simulation. Pockets of hot, diffuse gas around individual OB associations sweep
up thick shells of material that persist for long times due to the cooling
instability. The overlapping of high-pressure, rarefied regions leads to a
collective central outflow that escapes the galaxy by eating away at the
exterior gas through turbulent mixing, rather than gathering it into a thin,
unstable shell. Supersonic, turbulent gas naturally avoids dense regions where
turbulence decays quickly and cooling times are short, and this further
enhances density contrasts throughout the galaxy- leading to a complex, chaotic
distribution of bubbles, loops and filaments as observed in NGC 1569 and other
outflowing starbursts.Comment: 22 pages, 13 figures, MNRAS, in pres
The Degeneracy of Galaxy Formation Models
We develop a new formalism for modeling the formation and evolution of
galaxies within a hierarchical universe. Similarly to standard semi-analytical
models we trace galaxies inside dark-matter merger-trees. The formalism
includes treatment of feedback, star-formation, cooling, smooth accretion, gas
stripping in satellite galaxies, and merger-induced star bursts. However,
unlike in other models, each process is assumed to have an efficiency which
depends only on the host halo mass and redshift. This allows us to describe the
various components of the model in a simple and transparent way. By allowing
the efficiencies to have any value for a given halo mass and redshift, we can
easily encompass a large range of scenarios. To demonstrate this point, we
examine several different galaxy formation models, which are all consistent
with the observational data. Each model is characterized by a different unique
feature: cold accretion in low mass haloes, zero feedback, stars formed only in
merger-induced bursts, and shutdown of star-formation after mergers. Using
these models we are able to examine the degeneracy inherent in galaxy formation
models, and look for observational data that will help to break this
degeneracy. We show that the full distribution of star-formation rates in a
given stellar mass bin is promising in constraining the models. We compare our
approach in detail to the semi-analytical model of De Lucia & Blaizot. It is
shown that our formalism is able to produce a very similar population of
galaxies once the same median efficiencies per halo mass and redshift are being
used. We provide a public version of the model galaxies on our web-page, along
with a tool for running models with user-defined parameters. Our model is able
to provide results for a 62.5 h^{-1} Mpc box within just a few seconds.Comment: Accepted for publication in MNRAS. Fig 6 & 7 corrected. For the
project page which allows running your own model, see
http://www.mpa-garching.mpg.de/galform/sesam
Gas Physics, Disk Fragmentation, and Bulge Formation in Young Galaxies
We investigate the evolution of star-forming gas-rich disks, using a 3D
chemodynamical model including a dark halo, stars, and a two-phase interstellar
medium with feedback processes from the stars. We show that galaxy evolution
proceeds along very different routes depending on whether it is the gas disk or
the stellar disk which first becomes unstable, as measured by the respective
Q-parameters. This in turn depends on the uncertain efficiency of energy
dissipation of the cold cloud component from which stars form. When the cold
gas cools efficiently and drives the instability, the galactic disk fragments
and forms a number of massive clumps of stars and gas. The clumps spiral to the
center of the galaxy in a few dynamical times and merge there to form a central
bulge component in a strong starburst. When the kinetic energy of the cold
clouds is dissipated at a lower rate, stars form from the gas in a more
quiescent mode, and an instability only sets in at later times, when the
surface density of the stellar disk has grown sufficiently high. The system
then forms a stellar bar, which channels gas into the center, evolves, and
forms a bulge whose stars are the result of a more extended star formation
history. We investigate the stability of the gas-stellar disks in both regimes,
as well as the star formation rates and element enrichment. We study the
morphology of the evolving disks, calculating spatially resolved colours from
the distribution of stars in age and metallicity, including dust absorption. We
then discuss morphological observations such as clumpy structures and chain
galaxies at high redshift as possible signatures of fragmenting, gas-rich
disks. Finally, we investigate abundance ratio distributions as a means to
distinguish the different scenarios for bulge formation.Comment: 16 pages, Latex, 14 figures, to appear in Astronomy and Astrophysics,
Version with high quality images available at
http://www.astro.unibas.ch/leute/ai.shtm
Galaxies in a Simulated CDM Universe II: Observable Properties and Constraints on Feedback
We compare the properties of galaxies that form in a cosmological simulation
without strong feedback to observations at z=0. We confirm previous findings
that models without strong feedback overproduce the observed galaxy baryonic
mass function, especially at the low and high mass extremes. Through
post-processing we investigate what kinds of feedback would be required to
reproduce observed galaxy masses and star formation rates. To mimic an extreme
form of "preventive" feedback (e.g., AGN radio mode) we remove all baryonic
mass that was originally accreted via "hot mode" from shock-heated gas. This
does not bring the high mass end of the galaxy mass function into agreement
with observations because much of the stellar mass in these systems formed at
high redshift from baryons that originally accreted via "cold mode" onto lower
mass progenitors. An efficient "ejective" feedback mechanism, such as supernova
driven winds, must reduce the masses of these progenitors. Feedback must also
reduce the masses of lower mass z=0 galaxies, which assemble at lower redshifts
and have much lower star formation rates. If we monotonically re-map galaxy
masses to reproduce the observed mass function, but retain the simulation's
predicted star formation rates, we obtain fairly good agreement with the
observed sequence of star-forming galaxies but fail to recover the observed
population of passive, low star formation rate galaxies. Suppressing all hot
mode accretion improves agreement for high mass galaxies but worsens the
agreement at intermediate masses. Reproducing these z=0 observations requires a
feedback mechanism that dramatically suppresses star formation in a fraction of
galaxies, increasing with mass, while leaving star formation rates of other
galaxies essentially unchanged.Comment: MNRAS in press. 15 pages, 5 figures, minimal changes from the first
versio
Feedback and Recycled Wind Accretion: Assembling the z=0 Galaxy Mass Function
We analyse cosmological hydrodynamic simulations that include
observationally-constrained prescriptions for galactic outflows. If these
simulated winds accurately represent winds in the real Universe, then material
previously ejected in winds provides the dominant source of gas infall for new
star formation at redshifts z<1. This recycled wind accretion, or wind mode,
provides a third physically distinct accretion channel in addition to the "hot"
and "cold" modes emphasised in recent theoretical studies. Because of the
interaction between outflows and gas in and around halos, the recycling
timescale of wind material (t_rec) is shorter in higher-mass systems, which
reside in denser gaseous environments. In these simulations, this differential
recycling plays a central role in shaping the present-day galaxy stellar mass
function (GSMF). If we remove all particles that were ever ejected in a wind,
then the predicted GSMFs are much steeper than observed; galaxy masses are
suppressed both by the direct removal of gas and by the hydrodynamic heating of
their surroundings, which reduces subsequent infall. With wind recycling
included, the simulation that incorporates our favoured momentum-driven wind
scalings reproduces the observed GSMF for stellar masses 10^9 < M < 5x10^10
Msolar. At higher masses, wind recycling leads to excessive galaxy masses and
excessive star formation rates relative to observations. In these massive
systems, some quenching mechanism must suppress the re-accretion of gas ejected
from star-forming galaxies. In short, as has long been anticipated, the form of
the GSMF is governed by outflows; the unexpected twist here for our simulated
winds is that it is not primarily the ejection of material but how the ejected
material is re-accreted that governs the GSMF.Comment: 16 pages, 7 figures, accepted by MNRA
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