Exploring the origins of a new, apparently metal-free gas cloud at z = 4.4. (arXiv:1812.05098v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Robert_P/0/1/0/all/0/1">P. Frédéric Robert</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Murphy_M/0/1/0/all/0/1">Michael T. Murphy</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+OMeara_J/0/1/0/all/0/1">John M. O'Meara</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Crighton_N/0/1/0/all/0/1">Neil H. M. Crighton</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Fumagalli_M/0/1/0/all/0/1">Michele Fumagalli</a> (3) ((1) Centre for Astrophysics and Supercomputing, Swinburne University of Technology, (2) Department of Chemistry & Physics, Saint Michael's College, (3) Institute for Computational Cosmology and Centre for Extragalactic Astronomy, Durham University)
We report the discovery and analysis of only the third Lyman-limit system in
which a high-quality resolution, echelle spectrum reveals no metal absorption
lines, implying a metallicity $lesssim$1/10000 solar. Our HIRES spectrum of
the background quasar, PSS1723$+$2243, provides a neutral hydrogen column
density range for LLS1723 of $N_textrm{HI}=10^{text{17.9–18.3}}$ cm$^{-2}$
at redshift $z_textrm{abs}approx4.391$. The lower bound on this range, and
the lack of detectable absorption from the strongest low-ionisation metal
lines, are combined in photoionisation models to infer a robust, conservative
upper limit on the metallicity: $log(Z/Z_odot)<-4.14$ at 95% confidence. Such
a low metallicity raises the question of LLS1723's origin and enrichment
history. Previous simulations of the circumgalactic medium imply that LLS1723
is a natural candidate for a cold gas stream accreting towards a galaxy.
Alternatively, LLS1723 may represent a high-density portion of the
intergalactic medium containing either pristine gas -- unpolluted by stellar
debris for 1.4 Gyr after the Big Bang -- or the remnants of low-energy
supernovae from (likely low-mass) Population III stars. Evidence for the
circumgalactic scenario could be obtained by mapping the environment around
LLS1723 with optical integral-field spectroscopy. The intergalactic
possibilities highlight the need for -- and opportunity to test -- simulations
of the frequency with which such high-density, very low-metallicity systems
arise in the intergalactic medium.
We report the discovery and analysis of only the third Lyman-limit system in
which a high-quality resolution, echelle spectrum reveals no metal absorption
lines, implying a metallicity $lesssim$1/10000 solar. Our HIRES spectrum of
the background quasar, PSS1723$+$2243, provides a neutral hydrogen column
density range for LLS1723 of $N_textrm{HI}=10^{text{17.9–18.3}}$ cm$^{-2}$
at redshift $z_textrm{abs}approx4.391$. The lower bound on this range, and
the lack of detectable absorption from the strongest low-ionisation metal
lines, are combined in photoionisation models to infer a robust, conservative
upper limit on the metallicity: $log(Z/Z_odot)<-4.14$ at 95% confidence. Such
a low metallicity raises the question of LLS1723’s origin and enrichment
history. Previous simulations of the circumgalactic medium imply that LLS1723
is a natural candidate for a cold gas stream accreting towards a galaxy.
Alternatively, LLS1723 may represent a high-density portion of the
intergalactic medium containing either pristine gas — unpolluted by stellar
debris for 1.4 Gyr after the Big Bang — or the remnants of low-energy
supernovae from (likely low-mass) Population III stars. Evidence for the
circumgalactic scenario could be obtained by mapping the environment around
LLS1723 with optical integral-field spectroscopy. The intergalactic
possibilities highlight the need for — and opportunity to test — simulations
of the frequency with which such high-density, very low-metallicity systems
arise in the intergalactic medium.
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