J/A+A/619/A136 VLT/X-SHOOTER spectrum of COLA1 (Matthee+, 2018)
Confirmation of double peaked Lyα emission at z = 6.593.
Witnessing a galaxy directly contributing to the reionisation of the Universe.
Matthee J., Sobral D., Gronke M., Paulino-Afonso A., Stefanon M.,
Rottgering H.
<Astron. Astrophys. 619, A136 (2018)>
=2018A&A...619A.136M 2018A&A...619A.136M (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, spectra ; Spectra, ultraviolet ; Redshifts
Keywords: galaxies: high-redshift - galaxies: formation - dark ages -
reionization - first stars - techniques: spectroscopic -
intergalactic medium
Abstract:
Distant luminous Lyman-α emitters (LAEs) are excellent targets
for spectroscopic observations of galaxies in the epoch of
reionisation (EoR). We present deep high-resolution (R=5000)
VLT/X-shooter observations, along with an extensive collection of
photometric data of COLA1, a proposed double peaked LAE at z=6.6. We
rule out the possibility that COLA1's emission line is an [OII]
doublet at z=1.475 on the basis of i) the asymmetric red line-profile
and flux ratio of the peaks (blue/red=0.31±0.03) and ii) an
unphysical [OII]/Hα ratio ([OII]/Hα>22). We show that
COLA1's observed B-band flux is explained by a faint extended
foreground LAE, for which we detect Lyα and [OIII] at z=2.142.
We thus conclude that COLA1 is a real double-peaked LAE at z=6.593,
the first discovered at z>6. COLA1 is UV luminous
(M1500=-21.6±0.3), has a high equivalent width
(EW0,Lyα=120-40+50Å) and very compact Lyα emission
(r50,Lyα=0.33-0.04+0.07kpc). Relatively weak inferred
Hβ+[OIII] line-emission from Spitzer/IRAC indicates an extremely
low metallicity of Z<1/20Z☉ or reduced strength of nebular lines
due to high escape of ionising photons. The small Lyα peak
separation of 220±20km/s implies a low HI column density and an
ionising photon escape fraction of ∼15-30%, providing the first direct
evidence that such galaxies contribute actively to the reionisation of
the Universe at z>6. Based on simple estimates, we find that COLA1
could have provided just enough photons to reionise its own ∼0.3pMpc
(2.3cMpc) bubble, allowing the blue Lyα line to be observed.
However, we also discuss alternative scenarios explaining the detected
double peaked nature of COLA1. Our results show that future
high-resolution observations of statistical samples of double peaked
LAEs at z>5 are a promising probe of the occurrence of ionised
regions around galaxies in the EoR.
Description:
Reduced, flux- and wavelength-calibrated, 1-d spectrum of the double
peaked Lyman-alpha emitter 'COLA1' discovered by Hu et al.
(2016ApJ...825L...7H 2016ApJ...825L...7H) at z=6.593. Observations and data reduction are
described in the paper. The spectrum combines the three arms of
X-SHOOTER (UVB, VIS and NIR). The spectrum is in fits format, in
observed vacuum wavelengths, uncorrected for slit losses or
extinction. The wavelength is in Angstrom (Å) and the flux in
10-20erg/s/cm2/Å. The double peaked Lyman-alpha line is
observed at 9230Å.
Objects:
---------------------------------------------------------
RA (2000) DE Designation(s)
---------------------------------------------------------
10 02 35.40 +02 12 13.5 COLA1 = NAME COSMOS Lyalpha 1
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
spectrum.dat 45 20060 COLA 1 X-Shooter spectrum
xshoot_cola1.fits 2880 108 Fits spectrum
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Byte-by-byte Description of file: spectrum.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
2- 13 E12.6 0.1nm lambda Observed wavelength
16- 27 E12.6 10-22W/m2/nm Flux ?=- CGS flux density in
10-20erg/s/cm2/Å
31- 41 E11.6 10-22W/m2/nm e_Flux rms uncertainty on Flux in
10-20erg/s/cm2/Å
43- 45 A3 --- Arm [UVB VIS NIR] X-Shooter arm for
relevant spectral element
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Acknowledgements:
Jorryt Matthee, matthee(at)strw.leidenuniv.nl
(End) Jorryt Matthee [Univ. Leiden], Patricia Vannier [CDS] 07-Sep-2018