512 research outputs found
The galaxy stellar mass-star formation rate relation: Evidence for an evolving stellar initial mass function?
The evolution of the galaxy stellar mass--star formation rate relationship
(M*-SFR) provides key constraints on the stellar mass assembly histories of
galaxies. For star-forming galaxies, M*-SFR is observed to be fairly tight with
a slope close to unity from z~0-2. Simulations of galaxy formation reproduce
these trends owing to the generic dominance of smooth and steady cold accretion
in these systems. In contrast, the amplitude of the M*-SFR relation evolves
markedly differently than in models. Stated in terms of a star formation
activity parameter alpha=(M*/SFR)/(t_H-1 Gyr), models predict a constant
alpha~1 out to redshifts z=4+, while the observed M*-SFR relation indicates
that alpha increases by X3 from z~2 until today. The low alpha at high-z not
only conflicts with models, but is also difficult to reconcile with other
observations of high-z galaxies. Systematic biases could significantly affect
measurements of M* and SFR, but detailed considerations suggest that none are
obvious candidates to reconcile the discrepancy. A speculative solution is
considered in which the stellar initial mass function (IMF) evolves towards
more high-mass star formation at earlier epochs. Following Larson, a model is
investigated in which the characteristic mass Mhat where the IMF turns over
increases with redshift. The observed and predicted M*-SFR evolution may be
brought into agreement if Mhat=0.5(1+z)^2 Mo out to z~2. Such evolution broadly
matches recent observations of cosmic stellar mass growth, and the resulting
z=0 cumulative IMF is similar to the paunchy IMF favored by Fardal et al to
reconcile the observed cosmic star formation history with present-day fossil
light measures. [abridged]Comment: 14 pages, MNRAS, accepted version. Significant expansion of
discussion; includes comparisons to new observation
Jamaan at the pass of Biârein. An Iron Age IIB-C Ammonite stronghold in central Jordan
In years 2015-2016 the Zarqa Directorate of the Department of Antiquities of the Hashemite Kingdom of Jordan carried out a rescue excavation at the site of Jamaan, an Iron Age IIB-C Ammonite stronghold 16 Km north of âAmman. The site survey and limited soundings allow to plot a plan of the structure, comprising an outer enclosure with a casemate wall, two cisterns, and a square podium tower, and to collect ceramic material dating from Iron Age IIB-C (c. 840-580 BC), as well as the head of a soft limestone statue, possibly depicting a local chief or an official. The latter adds to the relatively conspicuous number of statues from the Kingdom of Ammon, possibly illustrating the production of a non-royal commission
The Enrichment History of Baryons in the Universe
We present predictions for the cosmic metal budget in various phases of
baryons from redshift z=6-0, taken from a cosmological hydrodynamic simulation
that includes a well-constrained model for enriched galactic outflows. We find
that substantial amounts of metals are found in every baryonic phase at all
epochs, with diffuse intergalactic gas dominating the metal budget at early
epochs and stars and halo gas dominating at recent epochs. We provide a full
accounting of metals in the context of the missing metals problem at z~2.5,
showing that ~40% of the metals are in galaxies, and the remainder is divided
between diffuse IGM gas and shocked gas in halos and filamentary structures.
Comparisons with available observations of metallicity and metal mass fraction
evolution show broad agreement. We predict stars have a mean metallicity of
one-tenth solar already at z=6, which increases slowly to one-half solar today,
while stars just forming today have typically solar metallicity. Our HI column
density-weighted mean metallicity (comparable to Damped Ly-alpha system
metallicities) slowly increases from one-tenth to one-third solar from z=6-1,
then falls to one-quarter solar at z=0. The global mean metallicity of the
universe tracks ~50% higher than that of the diffuse phase down to z~1, and by
z=0 it has a value around one-tenth solar. Metals move towards higher densities
and temperatures with time, peaking around the mean cosmic density at z=2 and
an overdensity of 100 at z=0. We study how carbon and oxygen ions trace the
path of metals in phase space, and show that OIII-OVII lines provide the most
practical option for constraining intergalactic medium metals at z<2.Comment: 10 pages, MNRAS accepted. Minor changes, Figure 1c fixe
When Does the Intergalactic Medium Become Enriched?
We use cosmological hydrodynamic simulations including galactic feedback
based on observations of local starbursts to find a self-consistent
evolutionary model capable of fitting the observations of the intergalactic
metallicity history as traced by C IV between z=6.0->1.5. Our main finding is
that despite the relative invariance in the measurement of Omega(C IV) as well
as the column density and linewidth distributions over this range, continual
feedback from star formation-driven winds are able to reproduce the
observations, while an early enrichment scenario where a majority of the metals
are injected into the IGM at z>6 is disfavored. The constancy of the C IV
observations results from a rising IGM metallicity content balanced by a
declining C IV ionization fraction due to a 1) decreasing physical densities,
2) increasing ionization background strength, and 3) metals becoming more
shock-heated at lower redshift. Our models predict that ~20x more metals are
injected into the IGM between z=6->2 than at z>6. We show that the median C IV
absorber at z=2 traces metals injected 1 Gyr earlier indicating that the
typical metals traced by C IV are neither from very early times nor from very
recent feedback.Comment: 6 pages, 3 figures, to appear in the proceedings of "Chemodynamics:
from the First Stars to Local Galaxies", Lyon, France, July 10-14, 200
- âŠ