487 research outputs found
Detecting population III galaxies with HST and JWST
A small fraction of the atomic-cooling halos assembling at z<15 may form out
of minihalos that never experienced any prior star formation, and could in
principle host small galaxies of chemically unenriched stars. Since the
prospects of detecting isolated population III stars appear bleak even with the
upcoming James Webb Space Telescope (JWST), these population III galaxies may
offer one of the best probes of population III stars in the foreseeable future.
By projecting the results from population III galaxy simulations through
cluster magnification maps, we predict the fluxes and surface number densities
of pop III galaxy galaxies as a function of their typical star formation
efficiency. We argue that a small number of lensed population III galaxies in
principle could turn up at z=7-10 in the ongoing Hubble Space Telescope survey
CLASH, which covers a total of 25 low-redshift galaxy clusters.Comment: 4 pages, 2 figures, to appear in the proceedings of First Stars I
High-redshift microlensing and the spatial distribution of dark matter in the form of MACHOs
A substantial part of the dark matter of the Universe could be in the form of
compact objects (MACHOs), detectable through gravitational microlensing effects
as they pass through the line of sight to background light sources. So far,
most attempts to model the effects of high-redshift microlensing by a
cosmologically distributed population of MACHOs have assumed the compact
objects to be randomly and uniformly distributed along the line of sight. Here,
we present a more realistic model, in which the MACHOs are assumed to follow
the spatial clustering of cold dark matter. Because of sightline-to-sightline
variations in surface mass density, this scenario leads to substantial scatter
in MACHO optical depths, which we quantify as a function of source redshift. We
find that while optical depth estimates based on a uniform line-of-sight
distribution are reasonable for the highest-redshift light sources, such
estimates can be incorrect by a factor of ~2 for the nearby (z~0.25) Universe.
Hence, attempts to derive the cosmological density of MACHOs from microlensing
observations of only a few independent sightlines can be subject to substantial
uncertainties. We also apply this model to the prediction of
microlensing-induced variability in quasars not subject to macrolensing, and
demonstrate that relaxing the assumption of randomly and uniformly distributed
MACHOs only has a modest impact on the predicted light curve amplitudes. This
implies that the previously reported problems with microlensing as the dominant
mechanism for the observed long-term optical variability of quasars cannot be
solved by taking the large-scale clustering of dark matter into account.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Can the James Webb Space Telescope detect isolated population III stars?
Isolated population III stars are postulated to exist at approximately
z=10-30 and may attain masses up to a few hundred solar masses. The James Webb
Space telescope (JWST) is the next large space based infrared telescope and is
scheduled for launch in 2014. Using a 6.5 meter primary mirror, it will
probably be able to detect some of the first galaxies forming in the early
Universe. A natural question is whether it will also be able to see any
isolated population III stars. Here, we calculate the apparent broadband
AB-magnitudes for 300 solar masses population III stars in JWST filters at
z=10-20. Our calculations are based on realistic stellar atmospheres and take
into account the potential flux contribution from the surrounding HII region.
The gravitational magnification boost achieved when pointing JWST through a
foreground galaxy cluster is also considered. Using this machinery, we derive
the conditions required for JWST to be able to detect population III stars in
isolation. We find that a detection of individual population III stars with
JWST is unlikely at these redshifts. However, the main problem is not
necessarily that these stars are too faint, once gravitational lensing is taken
into account, but that their surface number densities are too low.Comment: 6 pages, 3 figures, proceedings of CRF2010, DESY Hamburg, Nov 9-12
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The dark matter halos of the bluest low surface brightness galaxies
We present BVI photometry and long-slit Halpha rotation curve data obtained
with ESO VLT/FORS2 for six low surface brightness galaxies with extremely blue
colours and very faint central regions. We find no evidence for a steep central
density cusp of the type predicted by many N-body simulations of cold dark
matter (CDM) halos. Our observations are instead consistent with dark matter
halos characterized by cores of roughly constant density, in agreement with
previous investigations. While unremarkable in terms of the central density
slope, these galaxies appear very challenging for existing CDM halo models in
terms of average central halo density, as measured by the Delta_(V/2)
parameter. Since most of our target galaxies are bulgeless disks, our
observations also disfavour a recently suggested mechanism for lowering the
central mass concentration of the halo by means of a fast collapse phase, as
this scenario predicts that the original CDM profile should still be detectable
in bulgeless galaxies. Other potential ways of reconciling the CDM predictions
with these observations are discussed.Comment: 14 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
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