J/ApJ/795/63 Faraday rotation from magnesium II absorbers (Farnes+, 2014)
Faraday rotation from magnesium II absorbers toward polarized background
radio sources.
Farnes J.S., O'Sullivan S.P., Corrigan M.E., Gaensler B.M.
<Astrophys. J., 795, 63 (2014)>
=2014ApJ...795...63F 2014ApJ...795...63F (SIMBAD/NED BibCode)
ADC_Keywords: QSOs ; Galaxies, rotation ; Galaxies, radio ; Magnetic fields ;
Polarization ; Redshifts
Keywords: galaxies: magnetic fields - magnetic fields - polarization -
quasars: absorption lines - radio continuum: galaxies
Abstract:
Strong singly ionized magnesium (Mg II) 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 Mg II absorbers and the Faraday rotation measure (RM)
at ∼5 GHz. We cross-match a sample of 35752 optically identified
non-intrinsic Mg II absorption systems with 25649 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 Mg II 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{bar}=1.8±0.4µG). 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.
Description:
We use the broadband radio polarization catalog of Farnes et al.
(2014, J/ApJS/212/15) as our primary data source. We cross-match the
data from Farnes et al. (2014, J/ApJS/212/15) with the catalog of
Zhu & Menard (2013ApJ...770..130Z 2013ApJ...770..130Z), which presents a sample of 84534
quasars with a total of 35752 non-intrinsic Mg II absorption systems
along their lines of sight, as derived from SDSS spectra. To combine
the Zhu & Menard (2013ApJ...770..130Z 2013ApJ...770..130Z) data with the Farnes et al.
(2014, J/ApJS/212/15) catalog, we therefore use the redshift of the
background quasar, z, in the cross-matching criteria. Cross-matching
was carried out relative to the radio source positions provided by
Taylor et al. (2009, J/ApJ/702/1230), each of which has an associated
RM measurement at 1.4 GHz.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 104 599 Details of the Main Sample, Listed in Order
of NVSS Right Ascension
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See also:
VIII/65 : 1.4GHz NRAO VLA Sky Survey (NVSS) (Condon+ 1998)
J/ApJS/45/97 : Extragal. Radio Sources Faraday Rotation
(Simard-Normandin+ 1981)
J/MNRAS/376/371 : Polarisation of flat-spectrum radio sources (Jackson+, 2007)
J/ApJ/702/1230 : Rotation measure image of the sky (Taylor+, 2009)
J/MNRAS/409/821 : Radio sources with ultrahigh polarization (Shi+, 2010)
J/ApJS/212/15 : Polarized NVSS sources SEDs (Farnes+, 2014)
J/MNRAS/442/3329 : Rotation measures of 2642 quasars (Xu+, 2014)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Number [1/599] Source number
5- 6 I2 h RAh Hour of Right Ascension (J2000)
8- 9 I2 min RAm Minute of Right Ascension (J2000)
11- 15 F5.2 s RAs Second of Right Ascension (J2000)
17 A1 --- DE- Sign of the Declination (J2000)
18- 19 I2 deg DEd Degree of Declination (J2000)
21- 22 I2 arcmin DEm Arcminute of Declination (J2000)
24- 27 F4.1 arcsec DEs Arcsecond of Declination (J2000)
29- 35 F7.4 --- alpha ? Total intensity spectral index
37- 42 F6.4 --- Dalpha ? Total intensity spectral index difference
44- 48 F5.3 --- Chi2 ? Χ2 value
50 A1 --- Sample [FS-] Source subsample (1)
52 I1 --- NMgII [0/5] Number of Mg II absorbers
54- 58 F5.1 rad/m2 RM Faraday rotation measure
60- 63 F4.1 rad/m2 DRM Faraday rotation measure difference
65- 69 F5.2 % Pi Fractional polarization
71- 74 F4.2 % DPi Fractional polarization difference
76- 81 F6.3 --- beta ? Polarization spectral index
83- 88 F6.4 --- Dbeta ? Polarization spectral index difference
90- 96 F7.5 --- z Redshift for the polarized background
radio sources
98-104 E7.1 --- Dz Redshift difference
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Note (1): Subsample as follows:
F = Source that is used in the "flat" spectrum subsample;
S = Source that is used in the "steep" spectrum subsample.
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History:
From electronic version of the journal
(End) Prepared by Tiphaine Pouvreau [CDS] 19-May-2017