J/A+A/636/A2 HII regions detected in absorption (Polderman+, 2020)
A more detailed look at Galactic magnetic field models:
using free-free absorption in HII regions.
Polderman I.M., Haverkorn M., Jaffe T.R.
<Astron. Astrophys., 636, A2 (2020)>
=2020A&A...636A...2P 2020A&A...636A...2P (SIMBAD/NED BibCode)
ADC_Keywords: H II regions ; Magnetic fields
Keywords: cosmic rays - ISM: magnetic fields - HII regions - Galaxy: structure -
radio continuum: ISM - catalogs
Abstract:
Cosmic rays (CRs) and the Galactic magnetic field (GMF) are
fundamental actors in many processes in the Milky Way. The observed
interaction product of these actors is Galactic synchrotron emission
integrated over the line of sight (LOS). A comparison to simulations
can be made with this tracer using existing GMF models and CR density
models. This probes the GMF strength and morphology and the CR
density.
Our aim is to provide insight into the Galactic CR density and the
distribution and morphology of the GMF strength by exploring and
explaining the differences between the simulations and observations of
synchrotron intensity.
At low radio frequencies HII regions become opaque due to free-free
absorption. Using these HII regions we can measure the synchrotron
intensity over a part of the LOS through the Galaxy. The measured
intensity per unit path length, that is, the emissivity, for HII
regions at different distances, allows us to probe the variation in
synchrotron emission not only across the sky but also in the third
dimension of distance. Performing these measurements on a large scale
is one of the new applications of the window opened by current
low-frequency arrays. Using a number of existing GMF models in
conjunction with the Galactic CR modeling code GALPROP, we can
simulate these synchrotron emissivities.
We present an updated catalog, compiled from the literature, of
low-frequency absorption measurements of HII regions, their distances,
and electron temperatures. We report a simulated emissivity that shows
a compatible trend for HII regions that are near the observer.
However, we observe a systematically increasing synchrotron emissivity
for HII regions that are far from the observer, which is not
compatible with the values simulated by the GMF models and GALPROP.
Current GMF models plus a GALPROP generated CR density model cannot
explain low-frequency absorption measurements. One possibility is that
distances to all HII regions catalogued at the kinematic "far"
distance are erroneously determined, although this is unlikely since
it ignores all evidence for far distances in the literature. However,
a detection bias due to the nature of this tracer requires us to keep
in mind that certain sources may be missed in an observation. The
other possibilities are an enhanced emissivity in the outer Galaxy or
a diminished emissivity in the inner Galaxy.
Description:
A significant part of the data in the Polderman et al.
(2019A&A...621A.127P 2019A&A...621A.127P, Cat. J/A+A/621/A127) catalog comes from the work
of Nord et al. (2006AJ....132..242N 2006AJ....132..242N, Cat. J/AJ/132/242; hereafter
N06). For some of these sources, newdistance information is now
available. Therefore, in our catalog we update both the number of
sources from N06 and their distances in three ways.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 82 144 Complete and updated catalog of HII regions
detected in absorption
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See also:
J/AJ/132/242 : H II absorption regions at 74MHz (Nord+, 2006)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 7 F7.3 deg GLON Galactic longitude
8- 14 F7.3 deg GLAT Galactic latitude
16- 20 F5.2 kpc Dist Distance from the Sun to the HII region
22 A1 --- n_Dist [5] Note on Dist (1)
24- 28 F5.3 kpc e_Dist rms uncertainty on Distance
29- 34 F6.2 10+3K Tobs Brightness temperature derived from the measured
intensity at the observing frequency
35 A1 --- n_Tobs [2] Note on Tobs (1)
37- 42 F6.3 10+3K e_Tobs rms uncertainty on Tobs
45- 49 F5.2 10+3K T Synchrotron brightness temperature of the column
in front (n_T=F) or behind (n_T=B)
the HII region
50 A1 --- f_T [a] Flag on T (1)
52- 55 F4.2 10+3K e_T rms uncertainty on T
56 A1 --- n_T [FB] front or behind for synchrotron brightness
temperature
58- 62 F5.2 10+3K Te Electron temperature of the HII region
64- 67 F4.2 10+3K e_Te rms uncertainty on Te
69 A1 --- n_Te [1,3] Note on e_Te (1)
71- 74 F4.2 K/pc eps Emissivity of the column in front (n_eps=F) or
behind (n_eps=B) the HII region
76- 79 F4.2 K/pc e_eps rms uncertainty on eps
80 A1 --- n_eps [FB] front or behind for emissivity
82 I1 --- Source Origin paper of the data (2)
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Note (1): Notes as follows:
1 = Electron temperatures updated with values from
Azcarate et al. (1990RMxAA..20...23A 1990RMxAA..20...23A)
2 = Observed values not given in original paper; values calculated in
Polderman et al. (2019A&A...621A.127P 2019A&A...621A.127P, Cat. J/A+A/621/A127)
3 = Uncertainty absent in paper, we adopt 1.0x103K
a = Not explained in the paper
Note (2): References as follows:
1 = Jones & Finlay (1974AuJPh..27..687J 1974AuJPh..27..687J)
2 = Roger et al. (1999A&AS..137....7R 1999A&AS..137....7R)
3 = Nord et al. (2006AJ....132..242N 2006AJ....132..242N, Cat. J/AJ/132/242)
4 = Hindson et al. (2016PASA...33...20H 2016PASA...33...20H, Cat. J/other/PASA/33.20)
5 = Su et al. (2017MNRAS.465.3163S 2017MNRAS.465.3163S)
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 19-Apr-2021