Time variations of narrow absorption lines in high resolution quasar spectra

Abstract

Aims. We have searched for temporal variations of narrow absorption lines in high resolution quasar spectra. A sample of five distant sources were assembled, for which two spectra are available, either VLT/UVES or Keck/HIRES, which were taken several years apart. Methods. We first investigate under which conditions variations in absorption line profiles can be detected reliably from high resolution spectra and discuss the implications of changes in terms of small-scale structure within the intervening gas or intrinsic origin. The targets selected allow us to investigate the time behaviour of a broad variety of absorption line systems by sampling diverse environments: the vicinity of active nuclei, galaxy halos, molecular-rich galaxy disks associated with damped Lyα systems, as well as neutral gas within our own Galaxy.Results. Intervening absorption lines from Mg ii, Fe ii, or proxy species with lines of lower opacity tracing the same kind of (moderately ionised) gas appear in general to be remarkably stable (1σ upper limits as low as 10% for some components on scales in the range 10-100 au), even for systems at z abs ≈ z e . Marginal variations are observed for Mg ii lines towards PKS 1229−021 at z abs = 0.83032; however, we detect no systems that display any change as large as those reported in low resolution SDSS spectra.The lack of clear variations for low β Mg ii systems does not support the existence of a specific population of absorbers made of sweptup gas towards blazars. In neutral or diffuse molecular media, clear changes are seen for Galactic Na i lines towards PKS 1229−02 (decrease in N by a factor of four for one of the five components over 9.7 yr), corresponding to structure on a scale of about 35 au, in good agreement with known properties of the Galactic interstellar medium. Tentative variations are detected for H 2 J = 3 lines towards FBQS J2340−0053 at z abs = 2.05454 ( 35% change in column density, N, over 0.7 yr in the rest frame), suggesting there is structure at the 10 au-scale for this warm molecular gas. A marginal change is also seen in C i from another velocity component of the same absorption system ( 70% variation in N(C i).

Extent and structure of intervening absorbers from absorption lines redshifted on quasar emission lines

“…In the interstellar medium of our own Galaxy, small-scale structure in the neutral medium (as traced by C i or Na i) is observed at all scales above about 10 au (Welty 2007;Watson & Meyer 1996). If structure over such small scales is also present in high-redshift galaxies, it should manifest itself through time changes in quasar absorption lines, as argued recently by Boissé et al (2015). Transverse peculiar velocities of a few 100 km s −1 are expected for the observer, quasars, and intervening galaxies.…”

“…The green curve for the C i, C i λ1560 absorptions corresponds to a fit with a spatial coverage factor C f = 0.85. At v helio = 0 km s −1 , the redshift is z = 2.054526. tive variation has been reported by Boissé et al (2015). The physical properties (density and temperature) of this multiple-component absorption system have been thoroughly investigated by Jorgenson et al (2010) using the C i finestructure lines and H 2 absorption.…”

“…This implies drifts of the line of sight through the absorber of tens to hundreds of au over a time interval of about 10 years. To date, only tentative variations (3σ significance level) have been observed for neutral (C i) and molecular (H 2 ) gas in a damped Lyα (DLA) absorber at z abs = 2.05454 towards FBQS J2340−0053 over a two-year time interval (Boissé et al 2015, and references therein for other approaches involving quasar pairs or lensed quasars). If internal structure were present in the intermediate 100 au -1pc range within distant neutral or molecular absorbers, this would potentially affect the behaviour of absorption lines that are detected on quasar emission features.…”

Nature of the DLA towards Q 0528−250: High pressure and strong UV field revealed by excitation of C i, H2, and Si ii

“…Studying the possible variation with time of the residual flux may provide clues to discriminate between these scenarios since each of them occur on different timescales. In the first scenario (geometric effects), flux variations are expected due to the relative projected motion of the absorber, quasaremitting region, and observer (Boissé et al 2015). The velocity of this motion can be estimated as ∼ 2000 km s −1 in the case of the DLA towards Q 0528−250 based on the velocity difference between QSO and DLA projected on the line-ofsight.…”

Improved Ni II oscillator strengths from quasar absorption systems

“…When using quasar absorption lines to derive relative fvalues, we can wonder which systems are best suited to provide the highest accuracy. Regarding resolved profiles, the reasoning presented in Boissé et al (2015) applies (see their relation 6), and we can easily show that the largest response to a small difference in f (i.e. the optimal sensitivity expressed in terms of δI n /(δ f / f ), where I n is the normalised intensity) is obtained for τ = 1 (or I n = 0.37) and remains good in a relatively broad opacity interval τ ≈ 0.6−1.5 (corresponding to I n ≈ 0.2−0.5).…”