732 research outputs found
The origin of ultra diffuse galaxies: stellar feedback and quenching
We test if the cosmological zoom-in simulations of isolated galaxies from the
FIRE project reproduce the properties of ultra diffuse galaxies. We show that
stellar feedback-generated outflows that dynamically heat galactic stars,
together with a passively aging stellar population after imposed quenching
(from e.g. infall into a galaxy cluster), naturally reproduce the observed
population of red UDGs, without the need for high spin halos or dynamical
influence from their host cluster. We reproduce the range of surface
brightness, radius and absolute magnitude of the observed z=0 red UDGs by
quenching simulated galaxies at a range of different times. They represent a
mostly uniform population of dark matter-dominated galaxies with M_star ~1e8
Msun, low metallicity and a broad range of ages. The most massive simulated
UDGs require earliest quenching and are therefore the oldest. Our simulations
provide a good match to the central enclosed masses and the velocity
dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the
simulated UDGs remain largely fixed across a broad range of quenching times
because the central regions of their dark matter halos complete their growth
early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The
most massive red UDG in our sample requires quenching at z~3 when its halo
reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0
its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If
our simulated dwarfs are not quenched, they evolve into bluer low-surface
brightness galaxies with mass-to-light ratios similar to observed field dwarfs.
While our simulation sample covers a limited range of formation histories and
halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy
type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio
CHIANTI - an Atomic Database for Emission Lines. Paper VI: Proton Rates and Other Improvements
The CHIANTI atomic database contains atomic energy levels, wavelengths,
radiative transition probabilities and electron excitation data for a large
number of ions of astrophysical interest. Version 4 has been released, and
proton excitation data is now included, principally for ground configuration
levels that are close in energy. The fitting procedure for excitation data,
both electrons and protons, has been extended to allow 9 point spline fits in
addition to the previous 5 point spline fits. This allows higher quality fits
to data from close-coupling calculations where resonances can lead to
significant structure in the Maxwellian-averaged collision strengths. The
effects of photoexcitation and stimulated emission by a blackbody radiation
field in a spherical geometry on the level balance equations of the CHIANTI
ions can now be studied following modifications to the CHIANTI software. With
the addition of H I, He I and N I, the first neutral species have been added to
CHIANTI. Many updates to existing ion data-sets are described, while several
new ions have been added to the database, including Ar IV, Fe VI and Ni XXI.
The two-photon continuum is now included in the spectral synthesis routines,
and a new code for calculating the relativistic free-free continuum has been
added. The treatment of the free-bound continuum has also been updated.Comment: CHIANTI is available at http://wwwsolar.nrl.navy.mil/chianti.htm
The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman--Break Galaxies at 3.4<z<4.5
We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1
narrow-band imaging in the GOODS Southern field to derive limits on the
distribution of the escape fraction (f_esc) of ionizing radiation for L >~
L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at
redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the
highest redshift galaxy currently known with a direct detection. Its
ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of
seven AGN are also detected in their LyC, including one at redshift z=3.951 and
z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average
f_esc in the range 5%--20%, depending on the how the data are selected (e.g.,
by magnitude and/or redshift). We undertake extensive Monte Carlo simulations
that take into account intergalactic attenuation, stellar population synthesis
models, dust extinction and photometric noise in order to explore the moments
of the distribution of the escaping radiation. Various distributions
(exponential, log-normal and Gaussian) are explored. We find that the median
f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If
this result remains valid for fainter LBGs down to current observational
limits, then the LBG population might be not sufficient to account for the
entire photoionization budget at the redshifts considered here, with the exact
details dependent upon the assumed ionizing background and QSO contribution
thereto. It is possible that f_esc depends on the UV luminosity of the
galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc
from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative
agreement with such a scenario.Comment: 58 pages, 23 figures; submitted to ApJ, revised version in response
to referee's comment
SOI-based micro-mechanical terahertz detector operating at room-temperature
We present a micro-mechanical terahertz (THz) detector fabricated on a
silicon on insulator (SOI) substrate and operating at room-temperature. The
device is based on a U-shaped cantilever of micrometric size, on top of which
two aluminum half-wave dipole antennas are deposited. This produces an
absorption extending over the THz frequency range. Due to the
different thermal expansion coefficients of silicon and aluminum, the absorbed
radiation induces a deformation of the cantilever, which is read out optically
using a m laser diode. By illuminating the detector with an amplitude
modulated, 2.5 THz quantum cascade laser, we obtain, at room-temperature and
atmospheric pressure, a responsivity of pm/W for the
fundamental mechanical bending mode of the cantilever. This yields an
noise-equivalent-power of 20 nW/Hz at 2.5THz. Finally, the low
mechanical quality factor of the mode grants a broad frequency response of
approximately 150kHz bandwidth, with a response time of s
Observational Diagnostics of Gas Flows: Insights from Cosmological Simulations
Galactic accretion interacts in complex ways with gaseous halos, including
galactic winds. As a result, observational diagnostics typically probe a range
of intertwined physical phenomena. Because of this complexity, cosmological
hydrodynamic simulations have played a key role in developing observational
diagnostics of galactic accretion. In this chapter, we review the status of
different observational diagnostics of circumgalactic gas flows, in both
absorption (galaxy pair and down-the-barrel observations in neutral hydrogen
and metals; kinematic and azimuthal angle diagnostics; the cosmological column
density distribution; and metallicity) and emission (Lya; UV metal lines; and
diffuse X-rays). We conclude that there is no simple and robust way to identify
galactic accretion in individual measurements. Rather, progress in testing
galactic accretion models is likely to come from systematic, statistical
comparisons of simulation predictions with observations. We discuss specific
areas where progress is likely to be particularly fruitful over the next few
years.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dave, to be published by
Springer. Typos correcte
A low escape fraction of ionizing photons of L>L* Lyman break galaxies at z=3.3
We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of
ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs)
with deep imaging in the U band obtained with the Large Binocular Camera,
mounted on the prime focus of the Large Binocular Telescope. We selected 11
LBGs with secure redshift in the range 3.27<z<3.35, from 3 independent fields.
We stacked the images of our sources in the R and U band, which correspond to
an effective rest-frame wavelength of 1500\AA and 900\AA respectively,
obtaining a limit in the U band image of >=30.7(AB)mag at 1 sigma. We derive a
1 sigma upper limit of f_{esc}^{rel}~5%, which is one of the lowest values
found in the literature so far at z~3.3. Assuming that the upper limit for the
escape fraction that we derived from our sample holds for all galaxies at this
redshift, the hydrogen ionization rate that we obtain (Gamma_{-12}<0.3 s^{-1})
is not enough to keep the IGM ionized and a substantial contribution to the UV
background by faint AGNs is required. Since our sample is clearly still limited
in size, larger z~3 LBG samples, at similar or even greater depths are
necessary to confirm these results on a more firm statistical basis.Comment: 15 pages, 2 figures, 1 table, accepted for publication in Ap
On Pulsar Distance Measurements and their Uncertainties
Accurate distances to pulsars can be used for a variety of studies of the
Galaxy and its electron content. However, most distance measures to pulsars
have been derived from the absorption (or lack thereof) of pulsar emission by
Galactic HI gas, which typically implies that only upper or lower limits on the
pulsar distance are available. We present a critical analysis of all measured
HI distance limits to pulsars and other neutron stars, and translate these
limits into actual distance estimates through a likelihood analysis that
simultaneously corrects for statistical biases. We also apply this analysis to
parallax measurements of pulsars in order to obtain accurate distance estimates
and find that the parallax and HI distance measurements are biased in different
ways, because of differences in the sampled populations. Parallax measurements
typically underestimate a pulsar's distance because of the limited distance to
which this technique works and the consequential strong effect of the Galactic
pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance
limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading
to overestimated distances because the bright pulsars on which this technique
is applicable are more likely to be nearby given their brightness.Comment: 32 pages, 1 figure, 2 tables; Accepted for publication in the
Astrophysical Journa
Sinking of a magnetically confined mountain on an accreting neutron star
We perform ideal-magnetohydrodynamic axisymmetric simulations of magnetically
confined mountains on an accreting neutron star, with masses less than ~0.12
solar masses. We consider two scenarios, in which the mountain sits atop a hard
surface or sinks into a soft, fluid base. We find that the ellipticity of the
star, due to a mountain grown on a hard surface, approaches ~2e-4 for accreted
masses greater than ~1.2e-3 solar masses, and that sinking reduces the
ellipticity by between 25% and 60%. The consequences for gravitational
radiation from low-mass x-ray binaries are discussed.Comment: 13 pages, 12 figures, and 3 tables; accepted for publication in MNRA
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