3 research outputs found
Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CFI Bubble Chamber
Data from the operation of a bubble chamber filled with 3.5 kg of CFI
in a shallow underground site are reported. An analysis of ultrasound signals
accompanying bubble nucleations confirms that alpha decays generate a
significantly louder acoustic emission than single nuclear recoils, leading to
an efficient background discrimination. Three dark matter candidate events were
observed during an effective exposure of 28.1 kg-day, consistent with a neutron
background. This observation provides the strongest direct detection constraint
to date on WIMP-proton spin-dependent scattering for WIMP masses
GeV/c.Comment: 4 pages, 4 figures V2 submitted to match journal versio
Isolating Triggered Star Formation
Galaxy pairs provide a potentially powerful means of studying triggered star
formation from galaxy interactions. We use a large cosmological N-body
simulation coupled with a well-tested semi-analytic substructure model to
demonstrate that the majority of galaxies in close pairs reside within cluster
or group-size halos and therefore represent a biased population, poorly suited
for direct comparison to ``field'' galaxies. Thus, the frequent observation
that some types of galaxies in pairs have redder colors than ``field'' galaxies
is primarily a selection effect. We select galaxy pairs that are isolated in
their dark matter halos with respect to other massive subhalos (N=2 halos) and
a control sample of isolated galaxies (N=1 halos) for comparison. We then apply
these selection criteria to a volume-limited subset of the 2dF Galaxy Redshift
Survey with M_Bj <= -19 and obtain the first clean measure of the typical
fraction of galaxies affected by triggered star formation and the average
elevation in the star formation rate. We find that 24% (30.5%) of these L^\star
and sub-L^{\star} galaxies in isolated 50 (30) kpc/h pairs exhibit star
formation that is boosted by a factor of >~ 5 above their average past value,
while only 10% of isolated galaxies in the control sample show this level of
enhancement. Thus, 14% (20 %) of the galaxies in these close pairs show clear
triggered star formation. The isolation criteria we develop provide a means to
constrain star formation and feedback prescriptions in hydrodynamic simulations
and a very general method of understanding the importance of triggered star
formation in a cosmological context. (Abridged.)Comment: 12 pages, 10 figures, emulateapj format, accepted by Ap
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Fossils of reionization in the local group
We use a combination of high-resolution gas dynamics simulations of high-redshift dwarf galaxies and dissipationless simulations of a Milky Way sized halo to estimate the expected abundance and spatial distribution of the dwarf satellite galaxies that formed most of their stars around z {approx} 8 and evolved only little since then. Such galaxies can be considered as fossils of the reionization era, and studying their properties could provide a direct window into the early, pre-reionization stages of galaxy formation. We show that 5-15% of the objects existing at z {approx} 8 do indeed survive until the present in the MW like environment without significant evolution. This implies that it is plausible that the fossil dwarf galaxies do exist in the Local Group. Because such galaxies form their stellar systems early during the period of active merging and accretion, they should have spheroidal morphology regardless of their current distance from the host galaxy. We show that both the expected luminosity function and spatial distribution of dark matter halos which are likely to host fossil galaxies agree reasonably well with the observed distributions of the luminous (L{sub V} > 10{sup 6} Lsun) Local Group fossil candidates near the host galaxy (d<200 kpc). However, the predicted abundance is substantially larger (by a factor of 2-3) for fainter galaxies (L{sub V} < 10{sup 6} Lsun) at larger distances (d>300 kpc). We discuss several possible explanations for this discrepancy