396 research outputs found
Merging galaxies produce outliers from the Fundamental Metallicity Relation
From a large sample of local SDSS galaxies, we find that
the Fundamental Metallicity Relation (FMR) has an overabundance of outliers,
compared to what would be expected from a Gaussian distribution of residuals,
with significantly lower metallicities than predicted from their stellar mass
and star formation rate (SFR). This low-metallicity population has lower
stellar masses, bimodial specific SFRs with enhanced star formation within the
aperture and smaller half-light radii than the general sample, and is hence a
physically distinct population. We show that they are consistent with being
galaxies that are merging or have recently merged with a satellite galaxy. In
this scenario, low-metallicity gas flows in from large radii, diluting the
metallicity of star-forming regions and enhancing the specific SFR until the
inflowing gas is processed and the metallicity has recovered. We introduce a
simple model in which mergers with a mass ratio larger than a minimum dilute
the central galaxy's metallicity by an amount that is proportional to the
stellar mass ratio for a constant time, and show that it provides an excellent
fit to the distribution of FMR residuals. We find the dilution time-scale to be
Gyr, the average metallicity depression caused
by a 1:1 merger to be dex and the minimum
mass ratio merger that can be discerned from the intrinsic Gaussian scatter in
the FMR to be (these are
statistical errors only). From this we derive that the average metallicity
depression caused by a merger with mass ratio between 1:5 and 1:1 is 0.114 dex.Comment: 14 pages, 10 figures, published in MNRAS, updated to be essentially
identical to the published versio
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
Probing the Metal Enrichment of the Intergalactic Medium at Using the Hubble Space Telescope
We test the galactic outflow model by probing associated galaxies of four
strong intergalactic CIV absorbers at --6 using the Hubble Space Telescope
(HST) ACS ramp narrowband filters. The four strong CIV absorbers reside at
, , , and , with column densities ranging from
cm to cm. At , we
detect an i-dropout Ly emitter (LAE) candidate with a projected impact
parameter of 42 physical kpc from the CIV absorber. This LAE candidate has a
Ly-based star formation rate (SFR) of 2
yr and a UV-based SFR of 4 yr. Although we cannot
completely rule out that this -dropout emitter may be an [OII] interloper,
its measured properties are consistent with the CIV powering galaxy at
. For CIV absorbers at and , although we detect two
LAE candidates with impact parameters of 160 kpc and 200 kpc, such distances
are larger than that predicted from the simulations. Therefore we treat them as
non-detections. For the system at , we do not detect LAE candidates,
placing a 3- upper limit of SFR
yr. In summary, in these four cases, we only detect one plausible CIV
source at . Combining the modest SFR of the one detection and the three
non-detections, our HST observations strongly support that smaller galaxies
(SFR yr) are main sources of
intergalactic CIV absorbers, and such small galaxies play a major role in the
metal enrichment of the intergalactic medium at .Comment: Accepted for Publications in ApJ
Reionization in Technicolor
We present the Technicolor Dawn simulations, a suite of cosmological
radiation-hydrodynamic simulations of the first 1.2 billion years. By modeling
a spatially-inhomogeneous UVB on-the-fly with 24 frequencies and resolving dark
matter halos down to within 12 Mpc volumes, our
simulations unify observations of the intergalactic and circumgalactic media,
galaxies, and reionization into a common framework. The only empirically-tuned
parameter, the fraction of ionizing photons that
escape the interstellar medium, is adjusted to match observations of the
Lyman- forest and the cosmic microwave background. With this single
calibration, our simulations reproduce the history of reionization; the stellar
mass-star formation rate relation of galaxies; the number density and
metallicity of damped Lyman- absorbers (DLAs) at ; the
abundance of weak metal absorbers; the ultraviolet background (UVB) amplitude;
and the Lyman- flux power spectrum at . The galaxy stellar mass
and UV luminosity functions are underproduced by , suggesting an
overly vigorous feedback model. The mean transmission in the Lyman-
forest is underproduced at , indicating tension between measurements of
the UVB amplitude and Lyman- transmission. The observed SiIV column
density distribution is reasonably well-reproduced ( low). By
contrast, CIV remains significantly underproduced despite being boosted by an
intense Ryd UVB. Solving this problem by increasing metal yields would
overproduce both weak absorbers and DLA metallicities. Instead, the observed
strength of high-ionization emission from high-redshift galaxies and absorption
from their environments suggest that the ionizing flux from conventional
stellar population models is too soft.Comment: 24 pages, 17 figures, accepted to MNRA
The importance of nebular emission for SED modeling of distant star-forming galaxies
We highlight and discuss the importance of accounting for nebular emission in
the SEDs of high redshift galaxies, as lines and continuum emission can
contribute significantly or subtly to broad-band photometry. Physical
parameters such as the galaxy age, mass, star-formation rate, dust attenuation
and others inferred from SED fits can be affected to different extent by the
treatment of nebular emission.
We analyse a large sample of Lyman break galaxies from z~3-6, and show some
main results illustrating e.g. the importance of nebular emission for
determinations of the mass-SFR relation, attenuation and age. We suggest that a
fairly large scatter in such relations could be intrinsic. We find that the
majority of objects (~60-70%) is better fit with SEDs accounting for nebular
emission; the remaining galaxies are found to show relatively weak or no
emission lines. Our modeling, and supporting empirical evidence, suggests the
existence of two categories of galaxies, "starbursts" and "post-starbursts"
(lower SFR and older galaxies) among the LBG population, and relatively short
star-formation timescales.Comment: To appear in IAU Symp. 284, The Spectral Energy Distribution of
Galaxies, Preston (UK), September 2011, eds. R. J. Tuffs and C. C. Popesc
The Reionization of Carbon
Observations suggest that CII was more abundant than CIV in the intergalactic
medium towards the end of the hydrogen reionization epoch. This transition
provides a unique opportunity to study the enrichment history of intergalactic
gas and the growth of the ionizing background (UVB) at early times. We study
how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic
simulation that includes a self-consistent multifrequency UVB as well as a
well-constrained model for galactic outflows to disperse metals. Our predicted
UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement
given the uncertainties. Nonetheless, we use a calibration in post-processing
to account for Lyman-alpha forest measurements while preserving the predicted
spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way
that it always exceeds the volume average in regions where metals are found.
This implies both that a spatially-uniform UVB is a poor approximation and that
metal absorption is not sensitive to the epoch when HII regions overlap
globally even at column densites of 10^{12} cm^{-2}. We find, consistent with
observations, that the CII mass fraction drops to low redshift while CIV rises
owing the combined effects of a growing UVB and continued addition of carbon in
low-density regions. This is mimicked in absorption statistics, which broadly
agree with observations at z=6-3 while predicting that the absorber column
density distributions rise steeply to the lowest observable columns. Our model
reproduces the large observed scatter in the number of low-ionization absorbers
per sightline, implying that the scatter does not indicate a partially-neutral
Universe at z=6.Comment: 16 pages, 14 figures, accepted to MNRA
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