J/A+A/653/A155 Lockman Hole Polarised Sources at 1.4GHz (Berger+, 2021)
Faint polarised sources in the Lockman Hole field at 1.4 GHz.
Berger A., Adebahr B., Herrera Ruiz N., Wright A.H., Prandoni I.,
Dettmar R.-J.
<Astron. Astrophys. 653, A155 (2021)>
=2021A&A...653A.155B 2021A&A...653A.155B (SIMBAD/NED BibCode)
ADC_Keywords: Magnetic fields ; Polarization ; Radio continuum
Keywords: magnetic fields - polarization - cosmology: observations -
galaxies: active - radio continuum: general
Abstract:
In the context of structure formation and galaxy evolution, the
contribution of magnetic fields is not well understood.
Feedback processes originating from active galactic nucleus (AGN)
activity and star formation can be actively influenced by magnetic
fields, depending on their strength and morphology. One of the best
tracers of magnetic fields is polarised radio emission. Tracing this
emission over a broad redshift range therefore allows an investigation
of these fields and their evolution.
We aim to study the nature of the faint, polarised radio source
population whose source composition and redshift dependence contain
information about the strength, morphology, and evolution of magnetic
fields over cosmic timescales.
We use a 15-pointing radio continuum L-band mosaic of the Lockman
Hole, observed in full polarisation, generated from archival data of
the Westerbork Synthesis Radio Telescope (WSRT). The data were
analysed using the rotation measure synthesis technique. We achieved a
noise of 7uJy/beam in polarised intensity, with a resolution of 15".
Using infrared and optical images and source catalogues, we were
able to cross-identify and determine redshifts for one-third of our
detected polarised sources. Results. We detected 150 polarised
sources, most of which are weakly polarised with a mean fractional
polarisation of 5.4%. No source was found with a fractional
polarisation higher than 21%. With a total area of 6.5deg2 and a
detection threshold of 6.25σ, we find 23 polarised sources per
deg2. Based on our multi-wavelength analysis, we find that our
sample consists of AGN only. We find a discrepancy between archival
number counts and those present in our data, which we attribute to
sample variance (i.e. large-scale structures). Considering the
absolute radio luminosity, we find a general trend of increased
probability of detecting weak sources at low redshift and strong
sources at high redshift. We attribute this trend to a selection bias.
Further, we find an anti-correlation between fractional polarisation
and redshift for our strong-source sample at z≥0.6.
A decrease in the fractional polarisation of strong sources with
increasing redshift cannot be explained by a constant magnetic field
and electron density over cosmic scales; however, the changing
properties of cluster environments over cosmic time may play an
important role. Disentangling these two effects requires deeper and
wider polarisation observations as well as better models of the
morphology and strength of cosmic magnetic fields.
Description:
This study is based on the WSRT observations of the Lockman Hole field
at 1.4GHz (Prandoni et al., 2018MNRAS.481.4548P 2018MNRAS.481.4548P). The data consist of
16 individual pointings, each observed for a full synthesis of 12-hrs
between December 2006 and January 2007. The centre of the mosaic was
chosen to be at RA=10:53:16.6; DE=+58:01:15 (J2000).
We present a new deep-field analysis of polarised sources in the
Lockman Hole at 1.4GHz, using a bespoke polarised mosaic with a
central RMS of 7uJy/beam. We find 150 polarised sources in an area of
6.5deg2 out of 1708 total-intensity sources in this field (8.8%).
This equates to a polarised source density of 23deg-2.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
catalog.dat 163 190 Full source catalogue
list.dat 88 1 Information on fits image
fits/* . 1 fits image
--------------------------------------------------------------------------------
See also:
J/PASJ/53/445 : ASCA Deep survey in Lockman Hole Field (Ishisaki+, 2001)
J/ApJS/185/433 : SWIRE/Chandra survey in Lockman Hole Field (Wilkes+, 2009)
J/ApJS/198/1 : Photometry catalogs for the Lockman Hole (Fotopoulou+, 2012)
Byte-by-byte Description of file: catalog.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 4 A4 --- IslIdPI Source ID, NNN or NNNA
6- 15 F10.6 deg RAdeg Right ascension (J2000)
17- 24 F8.6 deg e_RAdeg ? Right ascension uncertainty (J2000)
26- 34 F9.6 deg DEdeg Declination (J2000)
36- 43 F8.6 deg e_DEdeg ? Declination uncertainty (J2000)
45- 51 F7.4 mJy PI Polarised intensity
53- 58 F6.4 mJy e_PI Polarised intensity uncertainty
60- 67 F8.4 mJy I ? Total intensity
69- 74 F6.4 mJy e_I ? Total intensity uncertainty
76- 82 F7.4 % fracpol ? Fractional polarisation
84- 89 F6.4 % e_fracpol ? Fractional polarisation uncertainty
91- 97 A7 --- SCode S_code; see paper appendix for details
99-107 A9 --- Flag Flag on intensity (1)
109-112 F4.1 --- SpIndex ? Spectral index to 150MHz (2)
114-117 F4.2 --- zB ? Photometric redshift (3)
119-123 F5.3 --- b_zB ? Photometric redshift minimum (3)
125-129 F5.3 --- B_zB ? Photometric redshift maximum (3)
131-136 F6.2 mag BMAG ? Absolute B magnitude (3)
138-163 A26 --- SWIRE ID of crossmatches SWIRE source (4)
--------------------------------------------------------------------------------
Note (1): Flag indicating if the source is not found in the total intensity
image and thus taken from Prandoni et al. (2018MNRAS.481.4548P 2018MNRAS.481.4548P) with
their source ID.
Note (2): Taken from Mahony et al. (2016MNRAS.463.2997M 2016MNRAS.463.2997M, Cat. J/MNRAS/463/2997)
Note (3): Taken from Tudorica et al. (2017A&A...608A.141T 2017A&A...608A.141T)
Note (4): From Lonsdale et al. (2003PASP..115..897L 2003PASP..115..897L)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 50 A21 "datime" Obs.date Observation date
52- 56 I5 Kibyte size Size of FITS file
58- 64 A7 --- FileName Name of FITS file, in subdirectory fits
66- 88 A23 --- Title Title of the FITS file
--------------------------------------------------------------------------------
Acknowledgements:
Anna Berger, berger(at)astro.rub.de
(End) Patricia Vannier [CDS] 27-Jul-2021