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EarlyMetalEnrichmentof
Gas-rich Galaxiesatz~5
SurajPoudel,Varsha Kulkarni(U.ofSouthCarolina),SeanMorrison(Lab.deAstrophys.deMarseille(LAM),formerlyU.ofSC),CélinePéroux,Debopam Som,SamuelQuiret (LAM),DonaldYork(U.ofChicago)
ABSTRACT:
Metal abundances in high-redshift quasar absorbers give constraints on the early
star formation and chemical enrichment history of galaxies. The first billion years
of the cosmic chemical enrichment history were an especially exciting epoch
influenced by the early generations of population II stars and the signatures of
population III stars. Nucleosynthesis by the early stars is expected to give rise to
rapid metallicity evolution and peculiar abundance patterns in galaxies at z >~ 5.
Unfortunately only a handful of robust measurements exist at these high redshifts
for elements that do not deplete much on dust grains. We report element
abundance measurements, including those for undepleted elements, in two
galaxies at redshifts z> 4.7. We compare these results with the lower-redshift
measurements to obtain improved constraints on the early chemical enrichment
history of galaxies. We also examine relative element abundances and their
variations along each sightline.
INTRODUCTION:
The damped Lyman-alpha (DLA) and sub-damped Lyman-alpha (sub-DLA)
absorbers are the most gas-rich of the quasar absorbers (log NHI >= 20.3 and 19.0
<= log NHI< 20.3, respectively), and dominate the neutral gas mass density in the
Universe (e.g., Prochaska & Wolfe 2009, Zafer et al. 2013, Popping et al. 2014).
Since DLAs are identified from the absorption line technique using background
quasars, DLA observations sample galaxies are independent of their brightness,
contrary to the flux limited imaging surveys which select brighter galaxies. Quasar
spectra also show absorption signatures from heavier elements in these systems.
Measurements of these metal lines provide a powerful tool to measure the
cosmic evolution of metals in the gas around galaxies. There have been claims of
a sudden drop in DLA metallicity at z > 4.7 (Rafelski et al. 2012, 2014; see Fig. 3).
Such a drop, if real, could signal a change in the chemical enrichment processes in
galaxies. However, such a drop would be steeper than predictions of models
excluding population III stars (Maio & Tescari 2015) or models including both
population II and III stars (Kulkarni et al. 2013). Furthermore, this claim for a
sudden drop was based primarily on the elements Si and Fe which are depleted
on dust grains. As found from the dust evolution models and from z ~ 5 sub-mm
galaxies, dust depletion can not be neglected even at z~5 (e.g., Walter et al. 2012,
Casey et al. 2014). Therefore absorption lines of essentially undepleted elements
like S or O are needed to find the intrinsic metallicity. We have obtained
observations of some high-z absorbers using VLT X-shooter. Here we report the
results for two absorbers at z~ 5 based on our VLT X-shooter observations and
Keck archival data.
RESULTS & DISCUSSION:
The H I column densities of the absorbers at z=4.8 and
z=5.3 are estimated to be log NHI = 20.90± 0.10 and
20.20± 0.15, respectively. Fig. 1 shows an example of the
Lyman series line fitting for one of the absorbers. Fig. 2
shows the metal absorption lines and their Voigt profile
fits. Table 1 lists the element abundances inferred for the
two absorbers. The abundances of O in the z=5.3
absorber appear to be lower than the z=4.8 absorber. For
the z=5.3 absorber, [O/H] seems to be lower than [Si/H],
which could indicate unusual nucleosynthetic signatures. Fig. 2. Velocity plots for metal lines for absorbers at (a) z=4.8 and (b) z=5.3. The vertical lines show
Furthermore, C/O and Si/O vary substantially between the centers of velocity components. The data are shown in black and the fitted profiles are shown
different velocity components, indicating variations in in green. Unrelated absorption features are shaded in grey.
dust depletion and/or early stellar nucleosynthesis.
Similar variations were found in our observations of a z=5
sub-DLA (Morrison et al. 2016).
Table 1. Element abundances.
Fig. 3. Metallicityredshift relation for
DLAs. Blue circles
show binned data
for DLAs at z<4.5
from undepleted
elements (Som et
al. 2015). The
Green squares and
magenta triangles
Fig. 1. Examples of Lyman series line fits for the absorber at
z=4.8. The data are shown in black and the fitted profiles show previous measurements for individual DLAs at z > 4.5 based on undepleted and depleted
are shown in green. The vertical dotted lines show the line measurements, respectively (Rafelski et al. 2012, 2014). Black circles show result from this work. Solid curve
shows mean gas metallicity from Maio & Tescari (2015). Dashed curve shows the prediction of the semicenters. The blue curves show the 1σ noise level.
analytic model of Kulkarni et al. (2013) including population II and III stars.
FUTURE WORK: Measurements of undepleted elements in many more high-z DLAs are
essential to understand how typical our findings are, and whether or not there is a
sudden drop in the DLA metallicity. We plan to increase the sample considerably with
our recent and ongoing observations of other high-z absorbers, in order to definitively
determine the evolution of metals and dust, and the correlation between metallicity
and velocity dispersion. The trends resulting from this work will be compared with
predictions of cosmic chemical evolution models.
Acknowledgements: SP, VK and SM acknowledge partial support from NSF
AST/1108830, NASA NNX14AG74G and HST-GO-12536.
References:
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