J/A+A/621/A127 HII regions synchrotron radiation (Polderman+, 2019)
Low-frequency measurements of synchrotron absorbing HII regions and modeling
of observed synchrotron emissivity.
Polderman I.M., Haverkorn M., Jaffe T.R., Alves M.I.R.
<Astron. Astrophys., 621A, A127 (2019)>
=2019A&A...621A.127P 2019A&A...621A.127P (SIMBAD/NED BibCode)
ADC_Keywords: H II regions ; Radio continuum ; Effective temperatures
Keywords: cosmic rays - ISM: magnetic fields - HII regions - Galaxy: structure -
radio continuum: ISM - catalogs
Abstract:
Cosmic rays (CRs) and magnetic fields are dynamically important
components in the Galaxy, and their energy densities are comparable to
that of the turbulent interstellar gas. The interaction of CRs and
Galactic magnetic fields (GMF) produces synchrotron radiation clearly
visible in the radio regime. Detailed measurements of synchrotron
radiation averaged over the line-of-sight (LOS), so-called synchrotron
emissivities, can be used as a tracer of the CR density and GMF
strength.
Our aim is to model the synchrotron emissivity in the Milky Way using
a three-dimensional dataset instead of LOS-integrated intensity maps
on the sky.
Using absorbed HII regions, we measured the synchrotron emissivity
over a part of the LOS through the Galaxy, changing from a
two-dimensional to a three-dimensional view. Performing these
measurements on a large scale is one of the new applications of the
window opened by current low-frequency arrays. Using various simple
axisymmetric emissivity models and a number of GMF-based emissivity
models, we were able to simulate the synchrotron emissivities and
compare them to the observed values in the catalog.
We present a catalog of low-frequency absorption measurements of HII
regions, their distances and electron temperatures, compiled from
literature. These data show that the axisymmetric emissivity models
are not complex enough, but the GMF-based emissivity models deliver a
reasonable fit. These models suggest that the fit can be improved by
either an enhanced synchrotron emissivity in the outer reaches of the
Milky Way or an emissivity drop near the Galactic center.
Current GMF models plus a constant CR density model cannot explain
low-frequency absorption measurements, but the fits improved with
slight (ad hoc) adaptations. It is clear that more detailed models are
needed, but the current results are very promising.
Description:
In this work we have combined existing low-frequency observations of
HII regions, from five papers, with known distances in the Milky Way,
into one catalog.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
tablea1.dat 81 123 Complete catalog of HII regions detected in
absorption
--------------------------------------------------------------------------------
See also:
J/AJ/132/242 : H II absorption regions at 74MHz (Nord+, 2006)
J/other/PASA/33.20 : MWA Survey of Galactic HII regions (Hindson+, 2016)
Byte-by-byte Description of file: tablea1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 F7.3 deg GLON Galactic longitude
9- 15 F7.3 deg GLAT Galactic latitude
17- 21 F5.2 kpc Dist Distance from the Sun to the HII region
22 A1 --- n_Dist [ef] Note on Dist (1)
24- 28 F5.3 kpc e_Dist rms uncertainty on Dist
30- 35 F6.2 10+3K Tobs Sky brightness temperature derived from the
measured intensity at the observing frequency
36 A1 --- n_Tobs [b] Note on Tobs (1)
38- 43 F6.3 10+3K e_Tobs rms uncertainty on Tobs
44 A1 --- neTobs [a] Note on e_Tobs (1)
46- 50 F5.2 10+3K TF Synchrotron brightness temperature of the
column
52- 55 F4.2 10+3K e_TF rms uncertainty on TF
56 A1 --- n_TF [*] TF in front of the HII region (if *),
else behind the HII region
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
68 A1 --- n_Te [cd] Note on Te (1)
70- 73 F4.2 K/pc epsF Emissivity of the column
75- 78 F4.2 K/pc e_epsF rms uncertainty on epsF
79 A1 --- n_epsF [*] in front of the HII regions (if *),
else behind the HII region
81 I1 --- Ref Reference (2)
--------------------------------------------------------------------------------
Note (1): Notes as follows:
a = The HII region was observed in different fields. The weighted mean for
this HII region is calculated from the different measurements and is
given in this table. This weighted mean was also used in our modeling
effort.
b = Observed values not given in original paper; values calculated here.
c = Uncertainty absent in paper, as discussed in Sect. 4.1;
we adopted 1.0x103K.
d = Electron temperatures updated with values from Azcarate et al.
(1990RMxAA..20...23A 1990RMxAA..20...23A).
e = No error on the distance was provided by Paper 2 or references therein.
A 10% uncertainty was assumed.
f = Distance as calculated from the radial velocities in Paladini et al.
(2003, Cat. J/A+A/397/213) different from cited distance in Paper 3
(Nord et al., 2006, Cat. J/AJ/132/242).
The new calculated value was adopted.
Note (2): References as follows:
1 = Jones & Finlay (1974AuJPh..27..687J 1974AuJPh..27..687J)
2 = Roger et al. (1999A&A....19....7R 1999A&A....19....7R)
3 = Nord et al. (2006, Cat. J/AJ/132/242)
4 = Hindson et al. (2016, Cat J/other/PASA/33.20)
5 = Su et al. (2017MNRAS.465.3163S 2017MNRAS.465.3163S)
--------------------------------------------------------------------------------
History:
From electronic version of the journal
References:
Jones & Finlay, Paper I 1974AuJPh..27..687J 1974AuJPh..27..687J
Roger et al., Paper II 1999A&A....19....7R 1999A&A....19....7R
Nord et al., Paper III 2006AJ....132..242N 2006AJ....132..242N, Cat. J/AJ/132/242
Hindson et al., Paper IV 2016PASA...33...20H 2016PASA...33...20H, Cat J/other/PASA/33.20H
Su et al., Paper V 2017MNRAS.465.3163S 2017MNRAS.465.3163S
(End) Patricia Vannier [CDS] 15-Mar-2019