811 research outputs found
The Small Covering Factor of Cold Accretion Streams
Theoretical models of galaxy formation predict that galaxies acquire most of
their baryons via cold mode accretion. Observations of high-redshift galaxies,
while showing ubiquitous outflows, have so far not revealed convincing traces
of the predicted cold streams, which has been interpreted as a challenge for
the current models. Using high-resolution, zoom-in smooth particle
hydrodynamics simulations of Lyman break galaxy (LBG) halos combined with
ionizing radiative transfer, we quantify the covering factor of the cold
streams at z=2-4. We focus specifically on Lyman limit systems (LLSs) and
damped Ly-alpha absorbers (DLAs), which can be probed by absorption
spectroscopy using a background galaxy or quasar sightline, and which are
closely related to low-ionization metal absorbers. We show that the covering
factor of these systems is relatively small and decreases with time. At z=2,
the covering factor of DLAs within the virial radius of the simulated galaxies
is ~3% (~1% within twice this projected distance), and arises principally from
the galaxy itself. The corresponding values for LLSs are ~10% and ~4%. Because
of their small covering factor compared to the order unity covering fraction
expected for galactic winds, the cold streams are naturally dominated by
outflows in stacked spectra. We conclude that the existing observations are
consistent with the predictions of cold mode accretion, and outline promising
kinematic and chemical diagnostics to separate out the signatures of galactic
accretion and feedback.Comment: 5 pages, 2 figures. MNRAS, in pres
Stellar feedback and bulge formation in clumpy discs
We use numerical simulations of isolated galaxies to study the effects of stellar feedback on the formation and evolution of giant star-forming gas ‘clumps’ in high-redshift, gas-rich galaxies. Such galactic discs are unstable to the formation of bound gas-rich clumps whose properties initially depend only on global disc properties, not the microphysics of feedback. In simulations without stellar feedback, clumps turn an order-unity fraction of their mass into stars and sink to the centre, forming a large bulge and kicking most of the stars out into a much more extended stellar envelope. By contrast, strong radiative stellar feedback disrupts even the most massive clumps after they turn ∼10–20 per cent of their mass into stars, in a time-scale of ∼10–100 Myr, ejecting some material into a superwind and recycling the rest of the gas into the diffuse interstellar medium (ISM). This suppresses the bulge formation rate by direct ‘clump coalescence’ by a factor of several. However, the galactic discs do undergo significant internal evolution in the absence of mergers: clumps form and disrupt continuously and torque gas to the galactic centre. The resulting evolution is qualitatively similar to bar/spiral evolution in simulations with a more homogeneous ISM
Intergalactic Dust Extinction in Hydrodynamic Cosmological Simulations
Recently Menard et al. detected a subtle but systematic change in the mean
color of quasars as a function of their projected separation from foreground
galaxies, extending to comoving separations of ~10Mpc/h, which they interpret
as a signature of reddening by intergalactic dust. We present theoretical
models of this remarkable observation, using SPH cosmological simulations of a
(50Mpc/h)^3 volume. Our primary model uses a simulation with galactic winds and
assumes that dust traces the intergalactic metals. The predicted galaxy-dust
correlation function is similar in form to the galaxy-mass correlation
function, and reproducing the MSFR data requires a dust-to-metal mass ratio of
0.24, about half the value in the Galactic ISM. Roughly half of the reddening
arises in dust that is more than 100Kpc/h from the nearest massive galaxy. We
also examine a simulation with no galactic winds, which predicts a much smaller
fraction of intergalactic metals (3% vs. 35%) and therefore requires an
unphysical dust-to-metal ratio of 2.18 to reproduce the MSFR data. In both
models, the signal is dominated by sightlines with E(g-i)=0.001-0.1. The
no-wind simulation can be reconciled with the data if we also allow reddening
to arise in galaxies up to several x 10^10 Msun. The wind model predicts a mean
visual extinction of A_V ~0.0133 mag out to z=0.5, with a
sightline-to-sightline dispersion similar to the mean, which could be
significant for future supernova cosmology studies. Reproducing the MSFR
results in these simulations requires that a large fraction of ISM dust survive
its expulsion from galaxies and its residence in the intergalactic medium.
Future observational studies that provide higher precision and measure the
dependence on galaxy type and environment will allow detailed tests for models
of enriched galactic outflows and the survival of IG dust.Comment: Matches version accepted by MNRA
Galactic outflows and the kinematics of damped Lyman alpha absorbers
The kinematics of damped Lyman alpha absorbers (DLAs) are difficult to
reproduce in hierarchical galaxy formation models, particularly the
preponderance of wide systems. We investigate DLA kinematics at z=3 using
high-resolution cosmological hydrodynamical simulations that include a
heuristic model for galactic outflows. Without outflows, our simulations fail
to yield enough wide DLAs, as in previous studies. With outflows, predicted DLA
kinematics are in much better agreement with observations. Comparing two
outflow models, we find that a model based on momentum-driven wind scalings
provides the best match to the observed DLA kinematic statistics of Prochaska &
Wolfe. In this model, DLAs typically arise a few kpc away from galaxies that
would be identified in emission. Narrow DLAs can arise from any halo and galaxy
mass, but wide ones only arise in halos with mass >10^11 Mo, from either large
central or small satellite galaxies. This implies that the success of this
outflow model originates from being most efficient at pushing gas out from
small satellite galaxies living in larger halos. This increases the
cross-section for large halos relative to smaller ones, thereby yielding wider
kinematics. Our simulations do not include radiative transfer effects or
detailed metal tracking, and outflows are modeled heuristically, but they
strongly suggest that galactic outflows are central to understanding DLA
kinematics. An interesting consequence is that DLA kinematics may place
constraints on the nature and efficiency of gas ejection from high-z galaxies.Comment: submitted to MNRA
A Direct Detection of Gas Accretion: The Lyman Limit System in 3C 232
The gas added and removed from galaxies over cosmic time greatly affects
their stellar populations and star formation rates. QSO absorption studies in
close QSO/galaxy pairs create a unique opportunity to study the physical
conditions and kinematics of this gas. Here we present new Hubble Space
Telescope (HST) images of the QSO/galaxy pair 3C 232/NGC 3067. The quasar
spectrum contains a Lyman-limit absorption system (LLS) due to NGC 3067 at cz =
1421 km/s that is associated with the nearby SAB galaxy NGC 3067. Previous work
identifies this absorber as a high-velocity cloud (HVC) in NGC 3067 but the
kinematics of the absorbing gas, infalling or outflowing, were uncertain. The
HST images presented here establish the orientation of NGC 3067 and so
establish that the LLS/HVC is infalling. Using this system as a prototype, we
extend these results to higher-z Mg II/LLS to suggest that Mg II/LLSs are a
sight line sampling of the so-called "cold mode accretion" (CMA) infalling onto
luminous galaxies. But to match the observed Mg II absorber statistics, the CMA
must be more highly ionized at higher redshifts. The key observations needed to
further the study of low-z LLSs is HST/UV spectroscopy, for which a new
instrument, the Cosmic Origins Spectrograph, has just been installed greatly
enhancing our observational capabilities.Comment: 9 pages, 5 figures, accepted by PAS
When Feedback Fails: The Scaling and Saturation of Star Formation Efficiency
We present a suite of 3D multi-physics MHD simulations following star
formation in isolated turbulent molecular gas disks ranging from 5 to 500
parsecs in radius. These simulations are designed to survey the range of
surface densities between those typical of Milky Way GMCs (\sim 10^2
M_\odot\,pc^{-2}}) and extreme ULIRG environments (\sim 10^2
M_\odot\,pc^{-2}}) so as to map out the scaling of the cloud-scale star
formation efficiency (SFE) between these two regimes. The simulations include
prescriptions for supernova, stellar wind, and radiative feedback, which we
find to be essential in determining both the instantaneous per-freefall
() and integrated () star formation
efficiencies. In all simulations, the gas disks form stars until a critical
stellar surface density has been reached and the remaining gas is blown out by
stellar feedback. We find that surface density is a good predictor of
, as suggested by analytic force balance arguments from
previous works. SFE eventually saturates to at high surface density.
We also find a proportional relationship between and
, implying that star formation is feedback-moderated even over
very short time-scales in isolated clouds. These results have implications for
star formation in galactic disks, the nature and fate of nuclear starbursts,
and the formation of bound star clusters. The scaling of with
surface density is not consistent with the notion that is
always on the scale of GMCs, but our predictions recover the value for GMC parameters similar to those found in sprial galaxies,
including our own.Comment: 21 pages, 7 figures. Accepted to MNRA
Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution and implications for the IMF
High-redshift submillimetre galaxies (SMGs) are some of the most rapidly star-forming galaxies in the Universe. Historically, galaxy formation models have had difficulty explaining the observed number counts of SMGs. We combine a semi-empirical model with 3D hydrodynamical simulations and 3D dust radiative transfer to predict the number counts of unlensed SMGs. Because the stellar mass functions, gas and dust masses, and sizes of our galaxies are constrained to match observations, we can isolate uncertainties related to the dynamical evolution of galaxy mergers and the dust radiative transfer. The number counts and redshift distributions predicted by our model agree well with observations. Isolated disc galaxies dominate the faint (S_(1.1) ≲ 1 or S_(850) ≲ 2 mJy) population. The brighter sources are a mix of merger-induced starbursts and galaxy-pair SMGs; the latter subpopulation accounts for ∼30–50 per cent of all SMGs at all S_(1.1) ≳ 0.5 mJy (S_(850) ≳ 1 mJy). The mean redshifts are ∼3.0–3.5, depending on the flux cut, and the brightest sources tend to be at higher redshifts. Because the galaxy-pair SMGs will be resolved into multiple fainter sources by the Atacama Large Millimeter/submillimeter Array (ALMA), the bright ALMA counts should be as much as two times less than those observed using single-dish telescopes. The agreement between our model, which uses a Kroupa initial mass function (IMF), and observations suggests that the IMF in high-redshift starbursts need not be top heavy; if the IMF were top heavy, our model would overpredict the number counts. We conclude that the difficulty some models have reproducing the observed SMG counts is likely indicative of more general problems – such as an underprediction of the abundance of massive galaxies or a star formation rate and stellar mass relation normalization lower than that observed – rather than a problem specific to the SMG population
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