5,285 research outputs found
Near-Field Limits on the Role of Faint Galaxies in Cosmic Reionization
Reionizing the Universe with galaxies appears to require significant star
formation in low-mass halos at early times, while local dwarf galaxy counts
tell us that star formation has been minimal in small halos around us today.
Using simple models and the ELVIS simulation suite, we show that reionization
scenarios requiring appreciable star formation in halos with at are in serious tension with galaxy counts
in the Local Group. This tension originates from the seemingly inescapable
conclusion that 30 - 60 halos with at
will survive to be distinct bound satellites of the Milky Way at .
Reionization models requiring star formation in such halos will produce dozens
of bound galaxies in the Milky Way's virial volume today (and 100 - 200
throughout the Local Group), each with of old stars
( Gyr). This exceeds the stellar mass function of classical Milky
Way satellites today, even without allowing for the (significant)
post-reionization star formation observed in these galaxies. One possible
implication of these findings is that star formation became sharply inefficient
in halos smaller than at early times, implying that the
high- luminosity function must break at magnitudes brighter than is often
assumed (at ). Our results suggest that JWST (and
possibly even HST with the Frontier Fields) may realistically detect the
faintest galaxies that drive reionization. It remains to be seen how these
results can be reconciled with the most sophisticated simulations of early
galaxy formation at present, which predict substantial star formation in
halos during the epoch of reionization.Comment: 6 pages, 4 figures; minor updates. Published in MNRAS Letter
A Study of Music in the Elementary School as Taught by Graduates of Central Washington College Who are Non-Music Majors and Minors, Years 1949-1953
The majority of music supervisors in the public schools examined in the study said that the classroom teacher should teach music in her classroom from the grade level of kindergarten through fourth grade. These supervisors felt that the classroom teacher was better qualified to correlate and enrich the curriculum for her children than the special music teacher
Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxies
We use Local Group galaxy counts together with the ELVIS N-body simulations
to explore the relationship between the scatter and slope in the stellar mass
vs. halo mass relation at low masses, .
Assuming models with log-normal scatter about a median relation of the form
, the preferred log-slope steepens from
in the limit of zero scatter to in the
case of dex of scatter in at fixed halo mass. We provide fitting
functions for the best-fit relations as a function of scatter, including cases
where the relation becomes increasingly stochastic with decreasing mass. We
show that if the scatter at fixed halo mass is large enough ( dex)
and if the median relation is steep enough (), then the
"too-big-to-fail" problem seen in the Local Group can be self-consistently
eliminated in about of realizations. This scenario requires that
the most massive subhalos host unobservable ultra-faint dwarfs fairly often; we
discuss potentially observable signatures of these systems. Finally, we compare
our derived constraints to recent high-resolution simulations of dwarf galaxy
formation in the literature. Though simulation-to-simulation scatter in
at fixed is large among separate authors (
dex), individual codes produce relations with much less scatter and usually
give relations that would over-produce local galaxy counts.Comment: 15 pages, 6 figures, 1 table. Accepted for publication into MNRA
On the stark difference in satellite distributions around the Milky Way and Andromeda
We compare spherically-averaged radial number counts of bright (> 10^5 Lsun)
dwarf satellite galaxies within 400 kpc of the Milky Way (MW) and M31 and find
that the MW satellites are much more centrally concentrated. Remarkably, the
two satellite systems are almost identical within the central 100 kpc, while
M31 satellites outnumber MW satellites by about a factor of four at deprojected
distances spanning 100 - 400 kpc. We compare the observed distributions to
those predicted for LCDM suhbalos using a suite of 44 high-resolution ~10^12
halo zoom simulations, 22 of which are in pairs like the MW and M31. We find
that the radial distribution of satellites around M31 is fairly typical of
those predicted for subhalos, while the Milky Way's distribution is more
centrally concentrated that any of our simulated LCDM halos. One possible
explanation is that our census is bright (> 10^5 Lsun) MW dwarf galaxies is
significantly incomplete beyond ~ 100 kpc of the Sun. If there were ~8 - 20
more bright dwarfs orbiting undetected at 100 - 400 kpc, then the Milky Way's
radial distribution would fall within the range expected from subhalo
distributions and alos look very much like the known M31 system. We use our
simulations to demonstrate that there is enough area left unexplored by the
Sloan Digital Sky Survey and its extensions that the discovery of ~10 new
bright dwarfs is not implausible given the expected range of angular anisotropy
of subhalos in the sky.Comment: 10 pages, 8 figures, submitted to MNRA
Too Big to Fail in the Local Group
We compare the dynamical masses of dwarf galaxies in the Local Group (LG) to
the predicted masses of halos in the ELVIS suite of CDM simulations, a
sample of 48 Galaxy-size hosts, 24 of which are in paired configuration similar
to the LG. We enumerate unaccounted-for dense halos ( km s) in these volumes that at some point in their histories were
massive enough to have formed stars in the presence of an ionizing background
( km s). Within 300 kpc of the Milky Way, the
number of unaccounted-for massive halos ranges from 2 - 25 over our full
sample. Moreover, this "too big to fail" count grows as we extend our
comparison to the outer regions of the Local Group: within 1.2 Mpc of either
giant we find that there are 12-40 unaccounted-for massive halos. This count
excludes volumes within 300 kpc of both the MW and M31, and thus should be
largely unaffected by any baryonically-induced environmental processes.
According to abundance matching -- specifically abundance matching that
reproduces the Local Group stellar mass function -- all of these missing
massive systems should have been quite bright, with .
Finally, we use the predicted density structure of outer LG dark matter halos
together with observed dwarf galaxy masses to derive an
relation for LG galaxies that are outside the virial
regions of either giant. We find that there is no obvious trend in the relation
over three orders of magnitude in stellar mass (a "common mass" relation), from
, in drastic conflict with the tight relation
expected for halos that are unaffected by reionization. Solutions to the too
big to fail problem that rely on ram pressure stripping, tidal effects, or
statistical flukes appear less likely in the face of these results.Comment: 16 pages, 14 figures, 2 tables, submitted to MNRA
Running with BICEP2: Implications for Small-Scale Problems in CDM
The BICEP2 results, when interpreted as a gravitational wave signal and
combined with other CMB data, suggest a roll-off in power towards small scales
in the primordial matter power spectrum. Among the simplest possibilities is a
running of the spectral index. Here we show that the preferred level of running
alleviates small-scale issues within the CDM model, more so even than
viable WDM models. We use cosmological zoom-in simulations of a Milky Way-size
halo along with full-box simulations to compare predictions among four separate
cosmologies: a BICEP2-inspired running index model ( = -0.024), two
fixed-tilt CDM models motivated by Planck, and a 2.6 keV thermal WDM
model. We find that the running BICEP2 model reduces the central densities of
large dwarf-size halos ( ~ 30 - 80 km s) and alleviates
the too-big-to-fail problem significantly compared to our adopted Planck and
WDM cases. Further, the BICEP2 model suppresses the count of small subhalos by
~50% relative to Planck models, and yields a significantly lower "boost" factor
for dark matter annihilation signals. Our findings highlight the need to
understand the shape of the primordial power spectrum in order to correctly
interpret small-scale data.Comment: 10 pages, 8 figures, 2 tables, published in MNRA
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