6 research outputs found
Lyman Alpha Blobs as an Observational Signature of Cold Accretion Streams into Galaxies
Recent hydrodynamic simulations of galaxy formation reveal streams of cold (T
~ 1e4 K) gas flowing into the centers of dark matter halos as massive as
1e12-1e13.5 M_sun at redshifts z~1-3. In this paper we show that if > 20% of
the gravitational binding energy of the gas is radiated away, then the
simulated cold flows are spatially extended Lyman Alpha (Lya) sources with
luminosities, Lya line widths, and number densities that are comparable to
those of observed Lya blobs. Furthermore, the filamentary structure of the cold
flows can explain the wide range of observed Lya blob morphologies. Since the
most massive halos form in dense environments, the association of Lya blobs
with overdense regions arise naturally. We argue that Lya blobs - even those
which are clearly associated with starburst galaxies or quasars - provide
direct observational support for the cold accretion mode of galaxies. We
discuss various testable predictions of this association.Comment: MNRAS in press. 13 pages, 6 figures. Discussion + references added.
Main conclusions unaffecte
The Late Reionization of Filaments
We study the topology of reionization using accurate three-dimensional
radiative transfer calculations post-processed on outputs from cosmological
hydrodynamic simulations. In our simulations, reionization begins in overdense
regions and then "leaks" directly into voids, with filaments reionizing last
owing to their combination of high recombination rate and low emissivity. This
result depends on the uniquely-biased emissivity field predicted by our
prescriptions for star formation and feedback, which have previously been shown
to account for a wide array of measurements of the post-reionization Universe.
It is qualitatively robust to our choice of simulation volume, ionizing escape
fraction, and spatial resolution (in fact it grows stronger at higher spatial
resolution) even though the exact overlap redshift is sensitive to each of
these. However, it weakens slightly as the escape fraction is increased owing
to the reduced density contrast at higher redshift. We also explore whether our
results are sensitive to commonly-employed approximations such as using
optically-thin Eddington tensors or substantially altering the speed of light.
Such approximations do not qualitatively change the topology of reionization.
However, they can systematically shift the overlap redshift by up to , indicating that accurate radiative transfer is essential for
computing reionization. Our model cannot simultaneously reproduce the observed
optical depth to Thomson scattering and ionization rate per hydrogen atom at
, which could owe to numerical effects and/or missing early sources of
ionization.Comment: 16 pages, 9 figures, accepted to MNRA
GeV Gamma-Ray Attenuation and the High-Redshift UV Background
We present new calculations of the evolving UV background out to the epoch of
cosmological reionization and make predictions for the amount of GeV gamma-ray
attenuation by electron-positron pair production. Our results are based on
recent semi-analytic models of galaxy formation, which provide predictions of
the dust-extinguished UV radiation field due to starlight, and empirical
estimates of the contribution due to quasars. We account for the reprocessing
of ionizing photons by the intergalactic medium. We test whether our models can
reproduce estimates of the ionizing background at high redshift from flux
decrement analysis and proximity effect measurements from quasar spectra, and
identify a range of models that can satisfy these constraints. Pair-production
against soft diffuse photons leads to a spectral cutoff feature for gamma rays
observed between 10 and 100 GeV. This cutoff varies with redshift and the
assumed star formation and quasar evolution models. We find only negligible
amounts of absorption for gamma rays observed below 10 GeV for any emission
redshift. With observations of high-redshift sources in sufficient numbers by
the Fermi Gamma-ray Space Telescope and new ground-based instruments it should
be possible to constrain the extragalactic background light in the UV and
optical portion of the spectrum.Comment: 19 pages, 12 figures, Accepted for publication in MNRAS, this version
includes minor correction
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Synthetic Gaia Surveys from the FIRE Cosmological Simulations of Milky Way-mass Galaxies
With Gaia Data Release 2, the astronomical community is entering a new era of multidimensional surveys of the Milky Way. This new phase-space view of our Galaxy demands new tools for comparing observations to simulations of Milky Way-mass galaxies in a cosmological context, to test the physics of both dark matter and galaxy formation. We present ananke, a framework for generating synthetic phase-space surveys from high-resolution baryonic simulations, and use it to generate a suite of synthetic surveys resembling Gaia DR2 in data structure, magnitude limits, and observational errors. We use three cosmological simulations of Milky Way-mass galaxies from the Latte suite of the Feedback In Realistic Environments project, which feature self-consistent clustering of star formation in dense molecular clouds and thin stellar/gaseous disks in live cosmological halos with satellite dwarf galaxies and stellar halos. We select three solar viewpoints from each simulation to generate nine synthetic Gaia-like surveys. We sample synthetic stars by assuming each star particle (of mass 7070 M o˙) represents a single stellar population. At each viewpoint, we compute dust extinction from the simulated gas metallicity distribution and apply a simple error model to produce a synthetic Gaia-like survey that includes both observational properties and a pointer to the generating star particle. We provide the complete simulation snapshot at z = 0 for each simulated galaxy. We describe data access points, the data model, and plans for future upgrades. These synthetic surveys provide a tool for the scientific community to test analysis methods and interpret Gaia data