1,148 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
ELVIS: Exploring the Local Volume in Simulations
We introduce a set of high-resolution dissipationless simulations that model
the Local Group (LG) in a cosmological context: Exploring the Local Volume in
Simulations (ELVIS). The suite contains 48 Galaxy-size halos, each within
high-resolution volumes that span 2-5 Mpc in size, and each resolving thousands
of systems with masses below the atomic cooling limit. Half of the ELVIS galaxy
halos are in paired configurations similar to the Milky Way (MW) and M31; the
other half are isolated, mass-matched analogs. We find no difference in the
abundance or kinematics of substructure within the virial radii of isolated
versus paired hosts. On Mpc scales, however, LG-like pairs average almost twice
as many companions and the velocity field is kinematically hotter and more
complex. We present a refined abundance matching relation between stellar mass
and halo mass that reproduces the observed satellite stellar mass functions of
the MW and M31 down to the regime where incompleteness is an issue, . Within a larger region spanning approximately 3
Mpc, the same relation predicts that there should be 1000 galaxies with
awaiting discovery. We show that up to 50% of halos
within 1 Mpc of the MW or M31 could be systems that have previously been within
the virial radius of either giant. By associating never-accreted halos with
gas-rich dwarfs, we show that there are plausibly 50 undiscovered dwarf
galaxies with HI masses within the Local Volume. The radial
velocity distribution of these predicted gas-rich dwarfs can be used to inform
follow-up searches based on ultra-compact high-velocity clouds found in the
ALFALFA survey.Comment: 22 pages, 19 figures, 3 tables; v2 -- accepted to MNRAS. Movies,
images, and data are available at http://localgroup.ps.uci.edu/elvi
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
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