129 research outputs found
The nature of damped HI absorbers probed by cosmological simulations: satellite accretion and outflows
We use state-of-the-art cosmological zoom simulations to explore the
distribution of neutral gas in and around galaxies that gives rise to high
column density \ion{H}{i} \mbox{Ly-} absorption (formally, sub-DLAs and
DLAs) in the spectra of background quasars. Previous cosmological hydrodynamic
simulations under-predict the mean projected separations of these
absorbers relative to the host, and invoke selection effects to bridge the gap
with observations. On the other hand, single lines of sight (LOS) in absorption
cannot uniquely constrain the galactic origin. Our simulations match all
observational data, with DLA and sub-DLA LOS existing over the entire probed
parameter space ([M/H], kpc) at all redshifts
(). We demonstrate how the existence of DLA LOS at kpc from a massive host galaxy require high numerical resolution, and
that these LOS are associated with dwarf satellites in the main halo, stripped
metal-rich gas and outflows. Separating the galaxy into interstellar
("\ion{H}{i} disc") and circumgalactic ("halo") components, we find that both
components significantly contribute to damped \ion{H}{i} absorption LOS. Above
the sub-DLA (DLA) limits, the disc and halo contribute with and
per cent, respectively. Our simulations confirm analytical
model-predictions of the DLA-distribution at . At high redshift
() sub-DLA and DLAs occupy similar spatial scales, but on average
separate by a factor of two by . On whether sub-DLA and DLA LOS
sample different stellar-mass galaxies, such a correlation can be driven by a
differential covering-fraction of sub-DLA to DLA LOS with stellar mass. This
preferentially selects sub-DLA LOS in more massive galaxies in the low-
universe.Comment: 12 pages, 5 figures, submitted to MNRAS 29/01/201
The Lyman-alpha glow of gas falling into the dark matter halo of a z=3 galaxy
Quasars are the visible signatures of super-massive black holes in the
centres of distant galaxies. It has been suggested that quasars are formed
during ``major merger events'' when two massive galaxies collide and merge,
leading to the prediction that quasars should be found in the centres of the
regions of largest overdensity in the early Universe. In dark matter
(DM)-dominated models of the early Universe, massive DM halos are predicted to
attract the surrounding gas, which falls towards its centre. The neutral gas is
not detectable in emission by itself, but gas falling into the ionizing cone of
such a quasar will glow in the Lyman-alpha line of hydrogen, effectively
imaging the DM halo. Here we present a Lyman-alpha image of a DM halo at
redshift 3, along with a two-dimensional spectrum of the gaseous halo. Our
observations are best understood in the context of the standard model for DM
halos; we infer a mass of (2-7) x 10^12 solar masses (Msun) for the halo.Comment: 4 pages, 4 figures. Published as a Letter to Nature in the August 26,
2004 issue; see accompanying News and Views article by Z. Haiman in the same
issu
All-Sky Near Infrared Space Astrometry
Gaia is currently revolutionizing modern astronomy. However, much of the
Galactic plane, center and the spiral arm regions are obscured by interstellar
extinction, rendering them inaccessible because Gaia is an optical instrument.
An all-sky near infrared (NIR) space observatory operating in the optical NIR,
separated in time from the original Gaia would provide microarcsecond NIR
astrometry and millimag photometry to penetrate obscured regions unraveling the
internal dynamics of the Galaxy.Comment: 7 page
HST Imaging of the Ionizing Radiation from a Star-forming Galaxy at z = 3.794
We report on the HST detection of the Lyman-continuum (LyC) radiation emitted
by a galaxy at redshift z=3.794, dubbed Ion1 (Vanzella et al. 2012). The LyC
from Ion1 is detected at rest-frame wavelength 820890 \AA with HST
WFC3/UVIS in the F410M band ( magnitude (AB), peak SNR =
4.17 in a circular aperture with radius r = 0.12'') and at 700830 \AA
with the VLT/VIMOS in the U-band ( magnitude (AB), peak SNR
= 6.7 with a r = 0.6'' aperture). A 20-hr VLT/VIMOS spectrum shows low- and
high-ionization interstellar metal absorption lines, the P-Cygni profile of CIV
and Ly in absorption. The latter spectral feature differs from what
observed in known LyC emitters, which show strong Ly emission. An HST
far-UV color map reveals that the LyC emission escapes from a region of the
galaxy that is bluer than the rest, presumably because of lower dust
obscuration. The F410M image shows that the centroid of the LyC emission is
offset from the centroid of the non-ionizing UV emission by 0.12''0.03'',
corresponding to 0.850.21 kpc (physical), and that its morphology is
likely moderately resolved. These morphological characteristics favor a
scenario where the LyC photons produced by massive stars escape from low HI
column-density "cavities" in the ISM, possibly carved by stellar winds and/or
supernova. We also collect the VIMOS U-band images of a sample of 107
Lyman-break galaxies with spectroscopic redshifts at , i.e.
sampling the LyC, and stack them with inverse-variance weights. No LyC emission
is detected in the stacked image, resulting in a 32.5 magnitude (AB) flux limit
(1) and an upper limit of absolute LyC escape fraction . LyC emitters like Ion1 are very likely at the bright-end of the LyC
luminosity function.Comment: 24 pages, 13 figures, accepted for publication in Ap
Science Education in a Nordic Welfare State
The problems related to science education in a small "Welfare State" such as Denmark are of a very different character and of a very different scope than those faced by many developing countries. In this paper I will try to describe the current situation for science education in Denmark based on my reading of different articles in educational magazines and material presented to the Danish parliament by the Danish Ministry of Education. Science education in Denmark suffers not from the lack of funding, but from the lack of students seeking technical and scientific studies
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