Faraday Rotation From Magnesium Ii Absorbers Toward Polarized Background Radio Sources

Abstract

Strong singly-ionized magnesium (MgII) absorption lines in quasar spectra typically serve as a proxy for intervening galaxies along the line of sight. Previous studies have found a correlation between the number of these MgII absorbers and the Faraday rotation measure (RM) at ≈ 5 GHz. We cross-match a sample of 35,752 optically-identified non-intrinsic MgII absorption systems with 25,649 polarized background radio sources for which we have measurements of both the spectral index and RM at 1.4 GHz. We use the spectral index to split the resulting sample of 599 sources into flat-spectrum and steep-spectrum subsamples. We find that our flat-spectrum sample shows significant (∼ 3.5σ) evidence for a correlation between MgII absorption and RM at 1.4 GHz, while our steep-spectrum sample shows no such correlation. We argue that such an effect cannot be explained by either luminosity or other observational effects, by evolution in another confounding variable, by wavelength-dependent polarization structure in an active galactic nucleus, by the Galactic foreground, by cosmological expansion, or by partial coverage models. We conclude that our data are most consistent with intervenors directly contributing to the Faraday rotation along the line of sight, and that the intervening systems must therefore have coherent magnetic fields of substantial strength (B = 1.8 ± 0.4 µG). Nevertheless, the weak nature of the correlation will require future high-resolution and broadband radio observations in order to place it on a much firmer statistical footing.

Using rotation measure grids to detect cosmological magnetic fields: A Bayesian approach

“…For more complex scenarios that include three components, we refer to Appendix C, where we introduce an additional constant and latitude-dependent term. Moreover, galaxies along the line of sight between a source and the observer can be responsible for high Faraday depth values (e.g., Kronberg & Perry 1982;Welter et al 1984;Kronberg et al 2008;Bernet et al 2012), indicating magnetic field strengths in these intervening galaxies of (1.8±0.4) µG (Farnes et al 2014a). The rotation of the polarization angle due to these sources adds to that associated with the large-scale structure and, therefore, should be taken into account in a proper modeling.…”

Detection of an ∼20 kpc coherent magnetic field in the outskirt of merging spirals: the Antennae galaxies

“…Implications on magnetic field measurements at high redshifts: Usually, one studies magnetic field properties in high redshift intervening objects by measuring the excess RM towards quasar absorption line systems: Mgii, damped Lyman-α (DLA), sub-DLA (see e.g., Oren & Wolfe 1995;Bernet et al 2008;Joshi & Chand 2013;Farnes et al 2014). It has been suggested that the sub-DLAs can originate from neutral gas that lies 20 kpc from the host galaxies and the absorbing gas is likely stripped via tidal interaction and/or ram pressure (Sembach et al 2001;Muzahid et al 2016).…”

“…The smallscale dynamo can efficiently amplify the magnetic fields on scales 1 kpc in ∼ 10 6 years (Subramanian 1999;Federrath et al 2011;Chamandy et al 2013;Schober et al 2013). On the other hand, the conventional α-Ω dynamo action requires ∼ 10 9 years to amplify the large-scale magnetic field in galaxies (Arshakian et al ⋆ E-mail: abasu@mpifr-bonn.mpg.de (AB); mao@mpifr-bonn.mpg.de (SAM) 2009; Pakmor et al 2014), which is too long to explain the detection of coherent fields in young systems (e.g., Bernet et al 2008;Farnes et al 2014). This suggests that there must be other magnetic field amplification processes at work.…”

Broadband Polarimetry with the Square Kilometre Array: A Unique Astrophysical Probe

“…Specifically, the detailed magneto-ionic properties of normal galaxies can be studied out to high redshift by determining RMs and polarised fractions as a function of redshift, and by cross-correlating this information with Mg II spectroscopy and deep optical imaging (Kronberg et al 2008;Bernet et al 2012Bernet et al , 2013Farnes et al 2014b). Through this technique, we can potentially measure the amplitude of turbulence in galactic disks and halos, the amplification time scales and coherence lengths of galactic dynamos, and the covering fraction and spatial extent of intervening systems, all as a function of redshift.…”

“…For the case of intervenors along the line of sight ( §3), we motivate our survey on the results of Farnes et al (2014b), who were able to show a difference in the radio polarisation properties of quasars with and without foreground Mg II absorbers at 3.5σ significance, using ∼140 quasars of which ∼40% have one or more Mg II absorbing systems along the line of sight. For SKA1, we aim to double this sample size in subcategories divided into 10 bins of redshift, 5 bins for absorber equivalent-width, and 5 bins of intervenor impact parameter, for a total of 50 000 quasar sightlines.…”