J/A+A/669/A8 Nearby galaxies in LoTSS-DR2. II. (Heesen+, 2023)
Nearby galaxies in the LOFAR Two-metre Sky Survey.
II. The magnetic field-gas relation.
Heesen V., Klocke T.-L., Brueggen M., Tabatabaei F.S., Basu A., Beck R.,
Drabent A., Nikiel-WroczyNski B., Paladino R., Schulz S., Stein M.
<Astron. Astrophys. 669, A8 (2023)>
=2023A&A...669A...8H 2023A&A...669A...8H (SIMBAD/NED BibCode)
ADC_Keywords: Surveys ; Radio continuum ; Galaxies, nearby ; Magnetic fields
Keywords: cosmic rays - galaxies: ISM - galaxies: magnetic fields -
galaxies: fundamental parameters - radio continuum: galaxies
Abstract:
Magnetic fields are key to understand galaxy evolution, regulating
stellar feedback and star formation in galaxies.
We probe the origin of magnetic fields in late-type galaxies,
measuring magnetic field strengths, exploring whether magnetic fields
are only passive constituents of the interstellar medium, or whether
they are active constituents being part of the local energy
equilibrium.
We measure equipartition magnetic field strengths in 39 galaxies from
LoTSS-DR2 using LOFAR observations at 144MHz with 6arcsec angular
resolution which (0.1-0.7kpc). For a subset of 9 galaxies, we
obtain atomic and molecular mass surface densities using HI and
CO(2-1) data, from the THINGS and HERACLES surveys, respectively.
These data are at 13 arcsec angular resolution, which corresponds to
0.3-1.2kpc at the distances of our galaxies. We measure kinetic
energy densities using HI and CO velocity dispersions.
We found a mean magnetic field strength of 3.6-12.5uG with a mean of
7.9±2.0G across the full sample. The magnetic field strength has the
tightest and steepest relation with the total gas surface density with
B∝Σ0.309±0.006HI+H2. The relation with the HI+H2
star-formation rate surface density and molecular gas surface density
has significantly flatter slopes. After accounting for the influence
of cosmic-ray transport, we found an even steeper relation of
B∝Σ0.393±0.009HI+H2.
These results suggest that the magnetic field is regulated by a
B-ρ relation, which has its origin in the saturation of the
small-scale dynamo. This is borne out by an agreement of kinetic and
magnetic energy densities although local deviations do exist in
particular in areas of high kinetic energy densities where the
magnetic field is sub-dominant.
Description:
The LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) Second
data release (LoTSS-DR2) contains 120-168MHz images covering 27% of
the northern sky. This work presents a sample of 39 nearby galaxies
from LoTSS-DR2 for which we have calculated equipartition magnetic
field strengths. For this we use the 6-arcsec maps from LoTSS-DR2 and
radio spectral index maps between 144 and approximately 1400MHz. If
they are not available, we use the integrated radio spectral index
within the same frequency range. These maps are presented as FITS
files.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 83 39 Magnetic field strengths and ancillary data in
the LoTSS-DR2 sample
list.dat 137 78 List of fits images
fits/* . 78 Individual fits images
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See also:
J/A+A/659/A1 : LOFAR Two-metre Sky Survey (LoTSS) DR2 (Shimwell+, 2022)
J/A+A/664/A83 : Nearby galaxies in LoTSS-DR2 (Heesen+, 2022)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Galaxy Name of the galaxy
10- 11 I2 deg incl Inclination angle with respect to
the line of sight
13- 17 F5.2 Mpc Dist Distance
19- 22 F4.1 kpc leff Path length
24- 28 F5.3 Jy S144MHz Integrated flux density at 144 MHz
30- 34 F5.2 --- alpha-int Integrated radio spectral index
36- 39 F4.2 --- e_alpha-int Integrated radio spectral index error
41- 44 F4.1 uG <Beq> Mean magnetic field strength
46- 48 F3.1 uG e_<Beq> Mean magnetic field strength error
50- 53 F4.1 uG Bmax-eq Maximum magnetic field strength
55- 58 F4.1 uG e_Bmax-eq Maximum magnetic field strength error
60- 65 F6.2 10+8Msun MHI ? HI mass from Walter et al.
(2008AJ....136.2563W 2008AJ....136.2563W, Cat. J/AJ/136/2563)
66 A1 --- n_MHI [a] Note on MHI (1)
68- 72 F5.2 10+8Msun MH2 ? H2 mass from Leroy et al.
(2008AJ....136.2782L 2008AJ....136.2782L, Cat. J/AJ/136/2782)
73 A1 --- n_MH2 [a] Note on MH2 (1)
75- 78 F4.1 km/s <sigmaHI> ? Mean turbulent velocity dispersion for HI
gas from Mogotsi et al.
(2016AJ....151...15M 2016AJ....151...15M)
79 A1 --- --- [/]
80- 83 F4.1 km/s <sigmaH2> ? Mean turbulent velocity dispersion for H2
gas from Mogotsi et al.
(2016AJ....151...15M 2016AJ....151...15M)
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Note (1): a: the CO detections of NGC 3077 and 4449 were marginal, so that these
galaxies were left out of the analysis of the ancillary data (Sect. 2.2);
for NGC 5474 we have only an HI map and so omitted this galaxy as well for
consistency.
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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- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 53 A26 "datime" Obs.date Observation date
55- 58 I4 Kibyte size Size of FITS file
60- 88 A29 --- FileName Name of FITS file, in subdirectory fits
90- 97 A8 --- Galaxy Name of the galaxy
99-137 A39 --- Title Title of the FITS file
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Acknowledgements:
Volker Heesen, volker.heesen(at)hs.uni-hamburg.de
References:
Heesen et al., Paper I 2022A&A...664A..83H 2022A&A...664A..83H, Cat. J/A+A/664/A83
(End) Patricia Vannier [CDS] 15-Dec-2022