52 research outputs found
Galaxy Size Problem at z=3: Simulated Galaxies Are Too Small
Using state-of-the-art adaptive mesh refinement cosmological hydrodynamic
simulations with a spatial resolution of proper 0.21kpc/h in refined subregions
embedded within a comoving cosmological volume (27.4Mpc/h)^3, we investigate
the sizes of galaxies at z=3 in the standard cold dark matter model where
reionization is assumed to complete at zri~6. Our simulated galaxies are found
to be significantly smaller than the observed ones: while more than one half of
the galaxies observed by HST and VLT ranging from rest-frame UV to optical
bands with stellar masses larger than 2E10 Msun have half-light radii larger
than 2kpc/h, none of the simulated massive galaxies in the same mass range have
half-light radii larger than 2kpc/h, after taking into account dust extinction.
Corroborative evidence is provided by the rotation curves of the simulated
galaxies with total masses of 1E11-1E12Msun, which display values 300-1000km/s
at small radii (0.5kpc/h) due to high stellar concentration in the central
regions, larger than those of any well observed galaxies. Possible physical
mechanisms to resolve this serious problem include: (1) an early reionization
at zri>>6 to suppress gas condensation hence star formation, (2) a strong,
internal energetic feedback from stars or central black holes to reduce the
overall star formation efficiency, or (3) a substantial small-scale cutoff in
the matter power spectrum.Comment: high resolution pdf file is available at
http://www.astro.princeton.edu/~cen/galaxysize.pdf 15 pages, 3 figures, in
press of ApJ Letter
Gas Accretion is Dominated by Warm Ionized Gas in Milky Way-Mass Galaxies at z ~ 0
We perform high-resolution hydrodynamic simulations of a Milky Way-mass
galaxy in a fully cosmological setting using the adaptive mesh refinement code,
Enzo, and study the kinematics of gas in the simulated galactic halo. We find
that the gas inflow occurs mostly along filamentary structures in the halo. The
warm-hot (10^5 K 10^6 K) ionized gases are found to
dominate the overall mass accretion in the system (with dM/dt = 3-5 M_solar/yr)
over a large range of distances, extending from the virial radius to the
vicinity of the disk. Most of the inflowing gas (by mass) does not cool, and
the small fraction that manages to cool does so primarily close to the galaxy
(R <~ 20 kpc), perhaps comprising the neutral gas that may be detectable as,
e.g., high-velocity clouds. The neutral clouds are embedded within larger,
accreting filamentary flows, and represent only a small fraction of the total
mass inflow rate. The inflowing gas has relatively low metallicity (Z/Z_solar <
0.2). The outer layers of the filamentary inflows are heated due to compression
as they approach the disk. In addition to the inflow, we find high-velocity,
metal-enriched outflows of hot gas driven by supernova feedback. Our results
are consistent with observations of halo gas at low z.Comment: 10 pages including 5 figures, submitted to Ap
Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium
We investigate models for the photoionization of the widespread diffuse
ionized gas in galaxies. In particular we address the long standing question of
the penetration of Lyman continuum photons from sources close to the galactic
midplane to large heights in the galactic halo. We find that recent
hydrodynamical simulations of a supernova-driven interstellar medium have low
density paths and voids that allow for ionizing photons from midplane OB stars
to reach and ionize gas many kiloparsecs above the midplane. We find ionizing
fluxes throughout our simulation grids are larger than predicted by one
dimensional slab models, thus allowing for photoionization by O stars of low
altitude neutral clouds in the Galaxy that are also detected in Halpha. In
previous studies of such clouds the photoionization scenario had been rejected
and the Halpha had been attributed to enhanced cosmic ray ionization or
scattered light from midplane H II regions. We do find that the emission
measure distributions in our simulations are wider than those derived from
Halpha observations in the Milky Way. In addition, the horizontally averaged
height dependence of the gas density in the hydrodynamical models is lower than
inferred in the Galaxy. These discrepancies are likely due to the absence of
magnetic fields in the hydrodynamic simulations and we discuss how
magnetohydrodynamic effects may reconcile models and observations.
Nevertheless, we anticipate that the inclusion of magnetic fields in the
dynamical simulations will not alter our primary finding that midplane OB stars
are capable of producing high altitude diffuse ionized gas in a realistic
three-dimensional interstellar medium.Comment: ApJ accepted. 17 pages, 7 figure
Vertical structure of a supernova-driven turbulent magnetized ISM
Stellar feedback drives the circulation of matter from the disk to the halo
of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a
vertical column of the interstellar medium with initial conditions typical of
the solar circle in which supernovae drive turbulence and determine the
vertical stratification of the medium. The simulations were run using a stable,
positivity-preserving scheme for ideal MHD implemented in the FLASH code. We
find that the majority (\approx 90 %) of the mass is contained in
thermally-stable temperature regimes of cold molecular and atomic gas at T <
200 K or warm atomic and ionized gas at 5000 K < T < 10^{4.2} K, with strong
peaks in probability distribution functions of temperature in both the cold and
warm regimes. The 200 - 10^{4.2} K gas fills 50-60 % of the volume near the
plane, with hotter gas associated with supernova remnants (30-40 %) and cold
clouds (< 10 %) embedded within. At |z| ~ 1-2 kpc, transition-temperature (10^5
K) gas accounts for most of the mass and volume, while hot gas dominates at |z|
> 3 kpc. The magnetic field in our models has no significant impact on the
scale heights of gas in each temperature regime; the magnetic tension force is
approximately equal to and opposite the magnetic pressure, so the addition of
the field does not significantly affect the vertical support of the gas. The
addition of a magnetic field does reduce the fraction of gas in the cold (< 200
K) regime with a corresponding increase in the fraction of warm (~ 10^4 K) gas.
However, our models lack rotational shear and thus have no large-scale dynamo,
which reduces the role of the field in the models compared to reality. The
supernovae drive oscillations in the vertical distribution of halo gas, with
the period of the oscillations ranging from ~ 30 Myr in the T < 200 K gas to ~
100 Myr in the 10^6 K gas, in line with predictions by Walters & Cox.Comment: Accepted for publication in ApJ. Replacement corrects an error in the
observed CNM pressure distribution in Figure 15 and associated discussio
The Origin and Distribution of Cold Gas in the Halo of a Milky Way-Mass Galaxy
We analyze an adaptive mesh refinement hydrodynamic cosmological simulation
of a Milky Way-sized galaxy to study the cold gas in the halo. HI observations
of the Milky Way and other nearby spirals have revealed the presence of such
gas in the form of clouds and other extended structures, which indicates
on-going accretion. We use a high-resolution simulation (136-272 pc throughout)
to study the distribution of cold gas in the halo, compare it with
observations, and examine its origin. The amount (10^8 Msun in HI), covering
fraction, and spatial distribution of the cold halo gas around the simulated
galaxy at z=0 are consistent with existing observations. At z=0 the HI mass
accretion rate onto the disk is 0.2 Msun/yr. We track the histories of the 20
satellites that are detected in HI in the redshift interval 0.5>z>0 and find
that most of them are losing gas, with a median mass loss rate per satellite of
3.1 x 10^{-3} Msun/yr. This stripped gas is a significant component of the HI
gas seen in the simulation. In addition, we see filamentary material coming
into the halo from the IGM at all redshifts. Most of this gas does not make it
directly to the disk, but part of the gas in these structures is able to cool
and form clouds. The metallicity of the gas allows us to distinguish between
filamentary flows and satellite gas. We find that the former accounts for at
least 25-75% of the cold gas in the halo seen at any redshift analyzed here.
Placing constraints on cloud formation mechanisms allows us to better
understand how galaxies accrete gas and fuel star formation at z=0.Comment: 13 pages, 8 figures. Accepted for publication in Ap
Gas Condensation in the Galactic Halo
Using adaptive mesh refinement (AMR) hydrodynamic simulations of vertically
stratified hot halo gas, we examine the conditions under which clouds can form
and condense out of the hot halo medium to potentially fuel star formation in
the gaseous disk. We find that halo clouds do not develop from linear isobaric
perturbations. This is a regime where the cooling time is longer than the
Brunt-Vaisala time, confirming previous linear analysis. We extend the analysis
into the nonlinear regime by considering mildly or strongly nonlinear
perturbations with overdensities up to 100, also varying the initial height,
the cloud size, and the metallicity of the gas. Here, the result depends on the
ratio of cooling time to the time required to accelerate the cloud to the sound
speed (similar to the dynamical time). If the ratio exceeds a critical value
near unity, the cloud is accelerated without further cooling and gets disrupted
by Kelvin-Helmholtz and/or Rayleigh-Taylor instabilities. If it is less than
the critical value, the cloud cools and condenses before disruption. Accreting
gas with overdensities of 10-20 is expected to be marginally unstable; the
cooling fraction will depend on the metallicity, the size of the incoming
cloud, and the distance to the galaxy. Locally enhanced overdensities within
cold streams have a higher likelihood of cooling out. Our results have
implications on the evolution of clouds seeded by cold accretion that are
barely resolved in current cosmological hydrodynamic simulations and absorption
line systems detected in galaxy halos.Comment: 13 pages, 8 figures, submitted to Ap
Simulated void galaxies in the standard cold dark matter model
We analyze a (120 h^{-1} Mpc)^3 adaptive mesh refinement hydrodynamic
simulation that contains a higher-resolution 31 x 31 x 35 h^{-3} Mpc subvolume
centered on a ~30 Mpc diameter void. Our detailed ~1 kpc resolution allows us
to identify 1300 galaxies within this void to a limiting halo mass of ~10^{10}
M_sun. Nearly 1000 galaxies are found to be in underdense regions, with 300
galaxies residing in regions less than half the mean density of the simulation
volume. We construct mock observations of the stellar and gas properties of
these systems, and reproduce the range of colors and luminosities observed in
the SDSS for nearby (z < 0.03) galaxies. We find no trends with density for the
most luminous (M_r
-16), though they are less reliably resolved, typically appear bluer, with
higher rates of star formation and specific star formation and lower mean
stellar ages than galaxies in average density environments. We find a
significant population of low luminosity (M_r ~ -14) dwarf galaxies that is
preferentially located in low density regions and specifically in the void
center. This population may help to reduce, but not remove, the discrepancy
between the predicted and observed number of void galaxies.Comment: 23 pages, 14 figures, submitted to Ap
Dependence of Interstellar Turbulent Pressure on Supernova Rate
Feedback from massive stars is one of the least understood aspects of galaxy
formation. We perform a suite of vertically stratified local interstellar
medium (ISM) simulations in which supernova rates and vertical gas column
densities are systematically varied based on the Schmidt-Kennicutt law. Our
simulations have a sufficiently high spatial resolution (1.95 pc) to follow the
hydrodynamic interactions among multiple supernovae that structure the ISM. At
a given supernova rate, we find that the mean mass-weighted sound speed and
velocity dispersion decrease as the inverse square root of gas density,
indicating that both thermal and turbulent pressures are nearly constant in the
midplane, so the effective equation of state is isobaric. In contrast, across
our four models having supernova rates that range from one to 512 times the
Galactic supernova rate, the mass-weighted velocity dispersion remains in the
range 4-6 km/s. Hence, gas averaged over ~100 pc regions follows an effective
equation of state that is close to isothermal. Simulated H I emission lines
have widths of 10-18 km/s, comparable to observed values. In our highest
supernova rate model, superbubble blow-outs occur, and the turbulent pressure
on large scales is >~4 times higher than the thermal pressure. We find a tight
correlation between the thermal and turbulent pressures averaged over ~100 pc
regions in the midplane of each model, as well as across the four ISM models.
We construct a subgrid model for turbulent pressure based on analytic arguments
and explicitly calibrate it against our stratified ISM simulations. The subgrid
model provides a simple yet physically motivated way to include supernova
feedback in cosmological simulations.Comment: 13 pages incl. 8 figures; accepted for publication in ApJ; contains a
new model of starburst galaxy showing superbubble blow-ou
Type-Ia Supernova-driven Galactic Bulge Wind
Stellar feedback in galactic bulges plays an essential role in shaping the
evolution of galaxies. To quantify this role and facilitate comparisons with
X-ray observations, we conduct 3D hydrodynamical simulations with the adaptive
mesh refinement code, FLASH, to investigate the physical properties of hot gas
inside a galactic bulge, similar to that of our Galaxy or M31. We assume that
the dynamical and thermal properties of the hot gas are dominated by mechanical
energy input from SNe, primarily Type Ia, and mass injection from evolved stars
as well as iron enrichment from SNe. We study the bulge-wide outflow as well as
the SN heating on scales down to ~4 pc. An embedding scheme that is devised to
plant individual SNR seeds, allows to examine, for the first time, the effect
of sporadic SNe on the density, temperature, and iron ejecta distribution of
the hot gas as well as the resultant X-ray morphology and spectrum. We find
that the SNe produce a bulge wind with highly filamentary density structures
and patchy ejecta. Compared with a 1D spherical wind model, the non-uniformity
of simulated gas density, temperature, and metallicity substantially alters the
spectral shape and increases the diffuse X-ray luminosity. The differential
emission measure as a function of temperature of the simulated gas exhibits a
log-normal distribution, with a peak value much lower than that of the
corresponding 1D model. The bulk of the X-ray emission comes from the
relatively low temperature and low abundance gas shells associated with SN
blastwaves. SN ejecta are not well mixed with the ambient medium, at least in
the bulge region. These results, at least partly, account for the apparent lack
of evidence for iron enrichment in the soft X-ray-emitting gas in galactic
bulges and intermediate-mass elliptical galaxies.[...]Comment: 37 pages, 19 figures, submitted to MNRAS; comments are welcom
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