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The Cosmic Neutral Hydrogen ! Mass Density at z=5

arXiv: 1506.02037
Neil Crighton Swinburne University ! J. Xavier Prochaska UCSC Michael Murphy Swinburne Gabor Worseck MPIA Marc Rafelski GSFC !


Cosmological mass density of neutral hydrogen
It is required to measure cosmological parameters via HI intensity mapping (e.g. Wyithe & Loeb 2008). It provides a useful constraint for theoretical models of galaxy formation (Lagos+ 2011, Dave+ 2013, Popping+ 2014, Rahmati+ 2015). The dense, mostly neutral clouds which dominate the mass density are expected to be closely linked to star formation. We extend this measurement to z ~ 5: approaching reionization epoch,.


HI cosmic mass density from absorption
Incidence rate of DLAs N(HI) physical area! of a DLA

Mass per DLA =

comoving DLA ! number density

critical cosmological density at z=0

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z=3

z=0
Noterdame+ 2012

Gas below the DLA threshold contributes < 20% of HI mass. (O'Meara+ 07, Zafar+ 13)
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High-redshift QSO Sample
162 QSOs at 4.4 < z < 5.4 with 69 DLAs (The Giant Gemini GMOS survey, Worseck+ 2014)! QSOs colour-selected from SDSS (unbiased with respect to the presence of a DLA)! Low resolution spectra (~300 km/s FWHM) covering rest frame 912-1300 A with SNR ~20.
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z=0.2

z=1.3

Finding DLAs becomes difficult at high redshift

z=2.4

z=3.6

z=4.8

May bias f(N,X) and N(HI)
z=6.3
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from Bob Carswell


Dealing with systematics

· Use mock spectra.! · Confirm DLAs with higher resolution spectra. ! · Perform independent analyses.

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Systematics:! correction factors
We do all of this twice, for two authors: NHMC and JXP.! None of the systemics are as large as the statistical uncertainty, estimated using bootstrapping.! There is an increase of 30 ± 20% in the DLA incidence towards bright QSOs (z mag <19.2) compared to fainter QSOs (see also Prochaska+ 2005). This is probably a statistical fluctuation (Menard+ 2008).

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Results

See also Zafar+ 2013, Prochaska+ 2009

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Results

See also Zafar+ 2013, Prochaska+ 2009

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Comparison to theory

See also Duffy+ 2012, Popping+ 2014, Rahmati+ 2015

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Comparison to theory

21cm

See also Duffy+ 2012, Popping+ 2014, Rahmati+ 2015

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Comparison to theory

Lensing

See also Duffy+ 2012, Popping+ 2014, Rahmati+ 2015

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Comparison to theory

Dust

See also Duffy+ 2012, Popping+ 2014, Rahmati+ 2015

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Results

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Madau+ 2014

HI is a short-lived buffer, not a large reservoir. Is the IGM driving star formation?
See also Peroux+ 2003, Prochaska+ 2005, ! Dave+ 2013, Zafar+ 2013

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Madau+ 2014

HI is a short-lived buffer, not a large reservoir. Is the IGM driving star formation?!
See also Peroux+ 2003, Prochaska+ 2005, ! Dave+ 2013, Zafar+ 2013

YES!
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Summary
We assemble the largest survey of DLAs at z~5, and measure the cosmological HI mass density.! Systematic uncertainties do not significantly affect our results.! Theoretical models do not currently match the observations.! The HI mass density consistent with a gradual decrease from z=5.5 to z=0, and we interpret the HI phase as a short-lived `buffer '.
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