Partial covering of emission regions of Q 0528-250 by intervening H${}_2$ clouds

Abstract

We present an analysis of the molecular hydrogen absorption system at z${}_{\mathrm{abs}}$ = 2.811 in the spectrum of the blazar Q0528-250. We demonstrate that the molecular cloud does not cover the background source completely. The partial coverage reveals itself as a residual flux in the bottom of saturated H_2 absorption lines. This amounts to about (2.22$\pm$0.54)% of the continuum and does not depend on the wavelength. This value is small and it explains why this effect has not been detected in previous studies of this quasar spectrum. However, it is robustly detected and significantly higher than the zero flux level in the bottom of saturated lines of the Ly-alpha forest, (-0.21$\pm$0.22)%. The presence of the residual flux could be caused by unresolved quasar multicomponents, by light scattered by dust, and/or by jet-cloud interaction. The H${}_2$ absorption system is very well described by a two-component model without inclusion of additional components when we take partial coverage into account. The derived total column densities in the H${}_2$ absorption components A and B are logN(H${}_2$)[cm${}^{-2}$] = 18.10$\pm$0.02 and 17.82$\pm$0.02, respectively. HD molecules are present only in component B. Given the column density, logN(HD)= 13.33$\pm$0.02, we find N(HD)/2N(H${}_2$)=(1.48$\pm$0.10)x10${}^{-5}$, significantly lower than previous estimations. We argue that it is crucial to take into account partial coverage effects for any analysis of H${}_2$ bearing absorption systems, in particular when studying the physical state of high-redshift interstellar medium.

Accepted for MNRAS

References in corpus (7)

• A change in the optical polarization associated with a gamma-ray flare in the blazar 3C 279
• Sizes and Temperature Profiles of Quasar Accretion Disks from Chromatic Microlensing
• Molecular hydrogen in high-redshift Damped Lyman-alpha systems: The VLT/UVES database
• Physical conditions in the neutral interstellar medium at z=2.43 toward Q2348-011
• HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium
• HD molecules at high redshift: The absorption system at z=2.3377 towards Q 1232+082
• HST/COS observations of a new population of associated QSO absorbers

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• Physical conditions in diffuse interstellar medium of local and high redshift galaxies: measurements based on excitation of H${}_2$ rotational and CI fine-structure levels
• HD molecules at high redshift: cosmic-ray ionization rate in the diffuse interstellar medium
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• Nature of the DLA towards Q0528-250: High pressure and strong UV field revealed by excitation of CI, H2 and SiII
• Extent and structure of intervening absorbers from absorption lines redshifted on quasar emission lines
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