C.D.S. - SIMBAD4 rel 1.7 - 2020.03.31CEST00:55:15

2019A&A...623A.149D - Astronomy and Astrophysics, volume 623A, 149-149 (2019/3-1)

GALEX colours of quasars and intergalactic medium opacity at low redshift.


Abstract (from CDS):

Aims. The distribution of neutral hydrogen in the intergalactic medium (IGM) is currently explored at low redshift by means of UV spectroscopy of quasars. We propose here an alternative approach based on UV colours of quasars as observed from GALEX surveys. We built a NUV-selected sample of 9033 quasars with (FUV-NUV) colours. The imprint of HI absorption in the observed colours is suggested qualitatively by their distribution as a function of quasar redshift.
Methods. Because broad band fluxes lack spectral resolution and are sensitive to a large range of HI column densities a Monte Carlo simulation of IGM opacity is required for quantitative analysis. It was performed with absorbers randomly distributed along redshift and column density distributions. The column density distribution was assumed to be a broken power law with index β1 (1015cm–2<NHI<1017.2cm–2) and β2 (1017.2cm–2<NHI<1019cm–2). For convenience the redshift distribution is taken proportional to the redshift evolution law of the number density of Lyman limit systems (LLS) per unit redshift as determined by existing spectroscopic surveys. The simulation is run with different assumptions on the spectral index αν of the quasar ionising flux.
Results. The fits between the simulated and observed distribution of colours require an LLS redshift density larger than that derived from spectroscopic counting. This result is robust in spite of difficulties in determining the colour dispersion other than that due to neutral hydrogen absorption. This difference decreases with decreasing αν (softer ionising quasar spectrum) and would vanish only with values of αν which are not supported by existing observations.
Conclusions. We provide arguments to retain αν=-2, a value already extreme with respect to those measured with HST/COS. Further fitting of power law index β1 and β2 leads to a higher density by a factor of 1.7 (β1=-1.7, β2=-1.5), possibly 1.5 (β1=-1.7, β2=-1.7). Beyond the result in terms of density the analysis of UV colours of quasars reveals a tension between the current description of IGM opacity at low z and the published average ionising spectrum of quasars.

Abstract Copyright: © J.-M. Deharveng et al. 2019

Journal keyword(s): intergalactic medium - ultraviolet: general - quasars: general

Status at CDS:  

Simbad objects: 3

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Number of rows : 3

N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2020
1 SDSS J141801.50+525200.7 QSO 14 18 01.509 +52 52 00.76   20.45   20.18 20.13 ~ 9 0
2 GALEX J141824.9+522329 AGN 14 18 24.945 +52 23 30.71   24.638 23.993 22.98 22.112 ~ 7 0
3 SDSS J171231.51+600719.8 QSO 17 12 31.5260521610 +60 07 19.797213865   21.05 20.58     ~ 6 0

    Equat.    Gal    SGal    Ecl

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