Exploring magnetised galactic outflows in starburst dwarf galaxies NGC 3125 and IC 4662
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Exploring magnetised galactic outflows in starburst dwarf galaxies NGC
3125 and IC 4662
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Citation for the original published paper (version of record):
Taziaux, S., Müller, A., Adebahr, B. et al (2025). Exploring magnetised galactic outflows in starburst
dwarf galaxies NGC 3125 and IC 4662. Astronomy and Astrophysics, 696.
http://dx.doi.org/10.1051/0004-6361/202453311
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A&A, 696, A226 (2025)
https://doi.org/10.1051/0004-6361/202453311 Astronomy
c The Authors 2025 &
Astrophysics
Exploring magnetised galactic outflows in starburst dwarf galaxies
NGC 3125 and IC 4662
Sam Taziaux1,2,? , Ancla Müller1, Björn Adebahr1 , Aritra Basu3,4 , Christoph Pfrommer5 , Michael Stein1 ,
Krysztof T. Chyży6 , Dominik J. Bomans1,2 , Torsten Enßlin7 , Volker Heesen8 , Peter Kamphuis1 ,
Marian Soida6 , Marek Wezgowiec6 , Ralf-Jürgen Dettmar1,2 , Samata Das9 , and Julia Tjus2,9,10
1
Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Universitätsstraße 150,
44801 Bochum, Germany
2
Ruhr Astroparticle and Plasma Physics Center (RAPP Center), Ruhr-Universität Bochum, 44780 Bochum, Germany
3
Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
4
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
5
Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
6
Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
7
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
8
Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
9
Ruhr University Bochum, Faculty for Physics & Astronomy, Theoretical Physics IV: Plasma Astroparticle Physics,
44780 Bochum, Germany
10
Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden
Received 5 December 2024 / Accepted 17 March 2025
ABSTRACT
Context. The study of radio emission in starburst dwarf galaxies provides a unique opportunity to investigate the mechanisms respon-
sible for the amplification and transport of magnetic fields. Local dwarfs are often considered proxies for early Universe galaxies, so
this study may provide insights into the role of non-thermal components in the formation and evolution of larger galaxies.
Aims. By investigating the radio continuum spectra and maps of the starburst dwarf galaxies, we aim to draw conclusions on their
magnetic field strengths and configurations, as well as on the dynamics of cosmic ray (CR) transport.
Methods. We performed a radio continuum polarimetry study of two of the brightest starburst IRAS Revised Bright Galaxy Sam-
ple (RBGS) dwarf galaxies, NGC 3125 and IC 4662. By combining data of the Australian Telescope Compact Array (2.1 GHz) and
MeerKAT (1.28 GHz), we analysed the underlying emission mechanism and the CR transport in these systems.
Results. We find flat spectra in the dwarf galaxies over the entire investigated frequency range, which sharply contrasts with observa-
tions of massive spiral galaxies. Because the expected cooling time of CR electrons is much shorter than their escape time, we would
expect a steepened steady-state CR electron spectrum. The flat observed spectra suggest a substantial contribution from free-free
emission at high frequencies and absorption at low frequencies, which may solve this puzzle. For NGC 3125, we measured a degree
of polarisation between 0.75% and 2.6%, implying a turbulent field and supporting the picture of a comparably large thermal emission
component that could be sourced by stellar radiation feedback and supernovae.
Key words. galaxies: dwarf – galaxies: evolution – galaxies: individual: NGC 3125 – galaxies: individual: IC 4662 –
galaxies: magnetic fields
1. Introduction continuum observations provide insights into magnetic field
strength (Beck et al. 2019), with limitation of starburst galax-
Dwarf starburst galaxies serve as important laboratories for ies (e.g. Lacki 2013; Ruszkowski & Pfrommer 2023); field
analysing the effects of feedback processes, such as star forma- orientation through polarisation and rotation measure (RM)
tion, on galactic evolution. Their low escape velocities mean that synthesis (Brentjens & de Bruyn 2005; Pakmor et al. 2018;
even a modest starburst event can lead to winds (e.g. Chyży et al. Reissl et al. 2023); and the relation between ordered and total
2016) or outflows (e.g. Adebahr et al. 2013). Understanding the magnetic fields in galaxy disks and halos (Beck & Krause
complexities of starburst processes in dwarf galaxies is there- 2005). Furthermore, they enable the investigation of cos-
fore an important step in understanding the feedback period mic ray (CR) transport, using relativistic electrons as trac-
processes between star-formation and galactic evolution (e.g. ers (e.g. Lacki et al. 2010; Werhahn et al. 2021; Pfrommer et al.
Dekel & Silk 1986; Stevens et al. 2002; Chyży et al. 2016). 2022; Heesen et al. 2022). CRs propagate from their ori-
Two challenging questions in magnetic field research are gin in supernova remnants through the galactic disc into
understanding the role of magnetic fields in dwarf galaxies the halo via advection, diffusion, and streaming mechanisms
and how these fields formed and evolved in the early Uni- (e.g. Strong et al. 2007; Stein et al. 2019; Thomas et al. 2020;
verse (e.g. Pakmor et al. 2017, 2024; Liu et al. 2022). Radio Stein et al. 2023). CRs excite Alfvén and whistler waves
through resonant plasma instabilities, which in turn scatter CRs,
?
Corresponding author; sam.taziaux@rub.de thereby lowering their effective drift speed along the mean
A226, page 1 of 20
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0),
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Taziaux, S., et al.: A&A, 696, A226 (2025)
magnetic field (Kulsrud & Pearce 1969; Shalaby et al. 2021, 2000; Basu et al. 2017b). Taking the recent findings of the mag-
2023; Lemmerz et al. 2025). Hence, CRs primarily interact with netic field properties of dwarf galaxies into account, they are
the galactic magnetic field, transferring momentum to the ther- considered the best candidates for cosmic magnetisers in the
mal gas through resonant scattering at plasma waves. As a early Universe (e.g. Atek et al. 2024; Sanati et al. 2024), as they
result, CRs apply a pressure on the ambient plasma, which is resemble the primordial galaxies during the Epoch of Reionisa-
the basis of formulations of CR hydrodynamics (Zweibel 2013; tion (Sanati et al. 2024). The magnetisation of the intergalactic
Pfrommer et al. 2017a; Thomas & Pfrommer 2019). Galactic medium via galactic outflows can be important, as it can affect
wind models incorporating this hydrodynamic description indi- the formation and evolution of galaxies and help provide insight
cate that CR transport significantly influences mass-loss rates, into the structure and evolution of the Universe (e.g. Chyży et al.
gas distribution, and wind formation (e.g. Breitschwerdt et al. 2016; Kepley et al. 2010; Pakmor et al. 2020; Chyüy et al. 2000;
1991; Uhlig et al. 2012; Pakmor et al. 2016; Girichidis et al. Cannon et al. 2005; Klein et al. 1996).
2016, 2018; Recchia et al. 2017; Dashyan & Dubois 2020; In this paper, we present a radio continuum polarimetry
Thomas et al. 2023). Thomas et al. (2024) modelled CR trans- study of the two brightest IRAS Revised Bright Galaxy Sam-
port in a multiphase interstellar medium in a global Milky Way- ple (RBGS) dwarf galaxies, NGC 3125 and IC 4662, that allows
like galaxy, and they suggest that CRs have a significant impact us to determine the magnetic field strengths and the structure of
on the formation of galactic winds due to their long cooling these galaxies. These ATCA datasets represent the most sensitive
times and better plasma coupling compared to radiation. This set of radio continuum observations of NGC 3125 and IC 4662
makes galactic winds denser and increases the mass loading fac- to date, and the first observations of their polarised emission
tor in these CR-driven winds, thereby providing more effective at 16 cm. We describe our data set in Sect. 2 and present the
feedback on galaxies by moving this gas to larger heights in the basic properties of radio continuum of both dwarf galaxies in
halo and reducing the amount of gas available for future star Sect. 3. We place our observations in the context of what is
formation. Outflow describes any material moving away from a known of NGC 3125 and IC 4662 in Sect. 4. Finally, in Sect. 5,
central object, whereas wind specifically refers to gas ejected we present a summary of our work and our conclusions. In
from a galaxy, typically driven by mechanisms such as radi- Appendices A, B, and C, we provide the non-thermal, the ther-
ation pressure, and can encompass both diffuse flows and jets mal fraction, and non-thermal spectral index maps of NGC 3125
(Veilleux et al. 2005). and IC 4662, respectively. In Appendices D and E, we investigate
We focus on further exploring the properties of these winds, the properties of the magnetic field of NGC 3125 and IC 4662. In
including wind speed and magnetic field strength in the galac- Appendix F, we estimate the different timescale for CRE losses.
tic halo, driven by stellar feedback. Because of the very low Throughout the paper, we use the Gaussian cgs unit system.
radiative loss rates of CR protons (CRPs), they can only be
observed via the decay products in hadronic collisions with
the ambient gas, which scales with the gas density and thus 2. Observation and data reduction
mostly illuminates the dense gaseous phase (Pfrommer & Enßlin 2.1. Selection of targets
2004a; Pfrommer et al. 2017b; Werhahn et al. 2023). To study
the lower density outflow regions, we have to turn to the radia- NGC 3125 and IC 4662 are two of the brightest dwarf galax-
tive products of CR electrons (CREs), in particular the radio ies in the IRAS RBGS (Sanders et al. 2003), which is the most
synchrotron emission. In this context, CREs need to be mod- complete sample of optically bright star-forming galaxies in
elled alongside CRPs, which collectively form the entire CR the local Universe (see Table 1 for a list of the properties
population. The different loss timescales of (relativistic) elec- of these two dwarfs). NGC 3125, an irregular dwarf galaxy,
trons and protons cause a deviation of their energy spectra uniquely shows the presence of kiloparsec-scale super-bubbles
(Ruszkowski & Pfrommer 2023), and hence, it is critical to and filaments, which tend to be oriented along the galaxy minor
model their spectra separately in order to provide a realistic pre- axis out to several kiloparsec as traced by the Hα emission.
diction of the radio emission in galaxies (Chiu et al. 2024). Such a galaxy is expected to host a strong superwind capa-
In recent years, various studies have been conducted to bet- ble of escaping the galaxy and ejecting a significant fraction
ter understand the radio spectrum’s shape in dwarf galaxies of the newly synthesised heavy elements into the intragalac-
(e.g. Klein et al. 2018; Taziaux et al., in prep.). The few in- tic medium (Kronberg et al. 1999). Indeed, ground-based opti-
depth radio continuum and polarisation studies of dwarf galax- cal imaging has revealed a complex of Hα filaments and shells
ies so far have shown – (1) a mean spectral index (S ∝ (Marlowe et al. 1995) that is impressive even when compared to
ναnth ) of −0.6 that steepens at the galaxy outskirts to −1.1 famous starburst dwarfs such as NGC 1569, which has a sim-
(e.g. Chyüy et al. 2000; Kepley et al. 2010, 2011; Chyży et al. ilar mass and star formation rate (SFR) surface density. An
2016; Westcott et al. 2018); (2) a mean equipartition field of optical polarisation map of NGC 3125 shows that the com-
13.5 µG to 38 µG (e.g. Kepley et al. 2010, 2011; Chyży et al. plex of Hα filaments is in part a large-scale bipolar reflection
2016; Basu et al. 2017b); (3) dwarf galaxies having the ability nebula originating from the brightest inner feature of the cen-
to host a large-scale large-scale magnetised halo up to a few tral starburst region (Alton et al. 1994). It is assumed that the
kiloparsecs (e.g. Chyży et al. 2016; Chyüy et al. 2000) being dust in the nebula, which is concentrated along the major axis
partly polarised reaching an ordered field strength of about of the galaxy, originated in the central starburst regions and
6−8 µG and a degree of polarisation of 50%, providing impor- was driven out into the galaxy by starburst winds (Alton et al.
tant information about the structure and strength of the magnetic 1994). Stevens et al. (2002) has already conducted a resolved
field (Chyży et al. 2016); and (4) within the resolved star- observation of NGC 3125 at 4.80 and 8.64 GHz with the Aus-
forming knot, the spectral index can reach a flatter spectrum of tralian Telescope Compact Array (ATCA), which displayed dis-
−0.3 and a significantly lower degree of polarisation of about tinctive radio emission and multiple discrete emission regions
3% due to a combination of Faraday depolarisation caused by coinciding with massive star clusters. These emission regions
the high densities of ionised gas and magnetic fields in these consist of synchrotron radiation and optically thin Hii regions.
regions (e.g. Chyży et al. 2016; Kepley et al. 2010; Chyüy et al. The synchrotron emission typically exhibits a spectral index of
A226, page 2 of 20
Taziaux, S., et al.: A&A, 696, A226 (2025)
Table 1. Basic properties of NGC 3125 and IC 4662.
NGC 3125 IC 4662 Reference
RA (J2000) 10h 06m 33s.372 17h 47m 08s.86 LEDA database
Dec (J2000) −29d 56m 05s.50 −64d 38m 30s.3 LEDA database
Type BCD (WR) IBm (WR) Hadfield & Crowther (2006), Crowther & Bibby (2009)
Inclination/◦ 55.8 58 LEDA database, Van Eymeren et al. (2010)
D/Mpc 13.8 2.44 Marlowe et al. (1995), Karachentsev et al. (2006)
MB /mag −17.68 −16.27 LEDA database
log(MHI /M ) – 8.26 Zastrow et al. (2013), Koribalski et al. (2018)
12 + log(O/H) 8.3 8.1 Hadfield & Crowther (2006), Crowther & Bibby (2009)
log[SFR/(M yr−1 )] −0.15 −1.12 Marasco et al. (2023), Crowther & Bibby (2009)
log[Hα/(erg s−1 cm−2 )] −11.46 −11.14 Gil de Paz et al. (2003), Crowther & Bibby (2009)
FHI /(Jy km s−1 ) – 103.5 Koribalski et al. (2018)
S 12 µm /Jy 0.31 0.27 Condon et al. (2021)
S 25 µm /Jy 0.74 1.27 Condon et al. (2021)
S 60 µm /Jy 5.33 8.82 Condon et al. (2021)
S 100 µm /Jy 6.67 11.38 Condon et al. (2021)
(†)
S 0.2−2 keV /(10−13 erg s−1 cm−2 ) 1.902 0.70 SWIFT-XRT, Burrows et al. (2005)
(†)
S 2−12 keV /(10−13 erg s−1 cm−2 ) 54.89 33.67 SWIFT-XRT, Burrows et al. (2005)
(†)
S 20−40 keV /(10−13 erg s−1 cm−2 ) 31.72 32.64 Integral, Walter et al. (2003)
(†)
S 40−60 keV /(10−13 erg s−1 cm−2 ) 27.48 28.63 Integral, Walter et al. (2003)
(†)
S 60−100 keV /(10−13 erg s−1 cm−2 ) 28.42 29.88 Integral, Walter et al. (2003)
Notes. (†) The X-ray flux are taken from http://xmmuls.esac.esa.int/upperlimitserver/ and only represent an upper limit.
approximately −0.5, while optically thin Hii regions have a spec- abundances, and high extinctions of around 20–25 mag with
tral index of αtot = −0.1 (Stevens et al. 2002). NGC 3125 is dust well-mixed with the emitting gas (Gilbert & Vacca 2009).
highly X-ray luminous (LX ≈ 2 × 1039 erg s−1 in the 0.1−2.5 keV At high frequencies, the thermal emission from Hii regions
waveband; Stevens et al. 2002), primarily due to high-mass is expected to become optically thin, however; Johnson et al.
X-ray binaries associated with active star formation. There is no (2002) suggest this should be consistent with the data presented
evidence to suggest the presence of an active galactic nucleus in their paper as they note that the observed low-frequency
(AGN; Stevens et al. 2002). Although supernovae and supernova spectral index values indicate that the optically thick part of
remnants can be highly luminous in X-rays and radio waves (e.g. the emission at lower frequencies is contributing significantly
van Dyk et al. 1993; Immler et al. 2003), they have a relatively to the total observed flux density. Crowther & Bibby (2009)
short period. Consequently, it is probable that high-mass X-ray focused on spatially locating Wolf-Rayet stars through optical
binaries are the primary sources of the observed X-ray emission. and spectroscopic observations, revealing He ii excess and Hα
Zastrow et al. (2013) observed an optically thin ionisation cone emission regions associated with 13 WC and 28 WN stars. The
in NGC 3125, with a clear excess in [Siii]/[Sii], indicating that abundances of various chemical elements were determined in
the gas is highly ionised, implying a harder ionising radiation IC 4662, including hydrogen, helium, oxygen, nitrogen, neon,
field with more high-energy photons, likely from an AGN or argon, sulfur, and iron. The findings reveal a nearly constant
young-, hot stars and a deficiency of [Sii]/Hα, supporting the distribution of these elements throughout the face of the two
presence of strong ionising radiation, in the north-east side of massive star-forming regions in IC 4662 (Hidalgo-Gámez et al.
the cone. This behaviour is consistent with expectations for opti- 2001). The neutral and ionised gas kinematics are intricate to
cally thin gas (Pellegrini et al. 2012). compare due to the distorted velocity field of the Hi gas. Notably,
Our second target, IC 4662, a dwarf galaxy with a large Wolf- the Hii region in the southern part exhibits abnormal morphol-
Rayet star population indicating a high star formation rate sur- ogy. Analysis of Hi data suggests the gas in this dwarf galaxy is
face density of log[ΣSFR /(M yr−1 kpc−2 )] = −1.39 (Hunter et al. likely expanding (Van Eymeren et al. 2010).
2001), potentially serving as progenitors of Type Ic supernovae Thus, NGC 3125 and IC 4662 are excellent candidates to
gamma ray bursts (e.g. Hammer et al. 2006; Modjaz et al. 2008). study the suspected magnetised outflows from dwarf starburst
Johnson et al. (2002) discuss the optical and radio morphologies galaxies, which will contribute to our understanding of the influ-
of IC 4662 and note that the radio morphology is dominated by ence of CRs on galactic properties as well as on their ability to
two main regions of emission with thermal (inverted) spectra drive galactic winds. This understanding will clarify the evolu-
with αtot = +0.3 ± 0.2 and +0.4 ± 0.2 between 4.86 and 8.6 GHz, tionary paths of these galaxies and offer comprehensive insights
due to possible free–free absorption. The positive spectral into the feedback mechanisms that regulate the formation and
index is indicative of optically thick free–free thermal emission, evolution of starburst dwarf galaxies.
which could result from ionised free–free electrons produced by
O-type stars and/or supernovae (Johnson et al. 2002). The two
2.2. Observations
massive star-formation regions in IC 4662 consist of several star
clusters with ages of around 4 × 106 yr and masses of approxi- The two dwarf galaxies NGC 3125 and IC 4662 were observed
mately 3 × 105 M . The clusters have high excitation, sub-solar with the ATCA (project ID: C3531; PI: S. Taziaux). The
A226, page 3 of 20
Taziaux, S., et al.: A&A, 696, A226 (2025)
Table 2. Summary of the ATCA radio observations of NGC 3125.
Date 4. May 2023 6. May 2023 7. May 2023 4. June 2023 21. July 2023 8. August 2023
Observation time/min 720 210 720 720 90 90
Configuration 1.5A 1.5A 1.5A 6D 6D 6D
Central frequency/MHz 2100 2100 2100 2100 2100 2100
Bandwidth/MHz 2048 2048 2048 2048 2048 2048
Flux calibrator 1934-638 1934-638 0823-500 0823-500 1934-638 1934-638
Phase calibrator 1015-314 1015-314 1015-314 1015-314 1015-314 1015-314
Table 3. Summary of the ATCA radio observations of IC 4662.
Date 4. May 2023 5. May 2023 6. May 2023 7. May 2023 4. June 2023 28. January 2024
Observation time/min 330 420 420 330 330 420
Configuration 1.5A 1.5A 1.5A 1.5A 6D EW367
Central frequency/MHz 2100 2100 2100 2100 2100 2100
Bandwidth/MHz 2048 2048 2048 2048 2048 2048
Flux calibrator 1934-638 1934-638 1934-638 1934-638 1934-638 1934-638
Phase calibrator 1814-637 1814-637 1814-637 1814-637 1814-637 1814-637
observations were performed at 2.1 GHz (16 cm) between 4 May lines were used, simulations were conducted which revealed
2023 and 28 January 2024 with the array in the 1.5 km (1.5A), no issues.
6 km (6D) and (EW367) configurations. Observations of the tar- For polarisation imaging, we individually imaged the Stokes-
get galaxies and a phase calibrator were alternated through the Q and -U parameters. Each channel of 1 MHz was imaged
observing run, giving a total observing time of about 43 h on and cleaned (using the final mask from the total power imag-
NGC 3125 and 37.5 h on IC 4662. The phase calibrators used in ing) to mitigate bandwidth depolarisation effects. We then cor-
these observations were 1016-311 for NGC 3125 and 1814-637 rected each image for primary beam attenuation. To include
for IC 4662, along with the standard ATCA primary flux cal- also the lowest frequencies, in order to utilise the largest band-
ibrator, 1934-638, except for two measurement sets where we width possible, we convolved all our images to a circular beam
use the secondary flux calibrator 0823-500, which has been cal- of 1500 . This was performed for both galaxies. After imaging,
ibrated through the phase. A summary of the observations used we checked all individual Q- and U-images again and removed
in this paper is given in Tables 2 and 3. any image still showing artefacts to achieve optimal quality
for the following analysis steps. The final Stokes Q and U
images, detailed alongside the frequency range and the number
2.3. Data reduction
of images used in rotation measure synthesis in Table 5, served
We employed data reduction procedures based on the Multi- as input for the algorithm described by Burn (1966). Within
channel Image Reconstruction Imaging Analysis and Display this approach, the data is transformed from frequency space into
(miriad; Sault et al. 1995). The data reduction followed a stan- Faraday space via a fast Fourier transformation (FFT). This tech-
dard procedure. To mitigate the impact of radio frequency inter- nique is mathematically similar to aperture synthesis imaging
ference (RFI) during flux and phase calibration, we utilised the and known as rotation measure synthesis (Brentjens & de Bruyn
interactive flagging tool bflag. Additionally, automated flag- 2005). The instrumental parameters are determined by the instru-
ging routines were applied with the task pgflag to address ment’s frequency configuration, while the instrumental function
interference for the source. Any additional corrupted data iden- is described by the rotation measure transfer function (RMTF).
tified during calibration was manually flagged. Initially, after The resolution in Faraday space, the maximum observable scale
cross-calibration and excluding channels with Hi emission, and the maximum observable Faraday depth can be taken from
we employed iterative imaging and self-calibration selfcal the Table 5. The Q- and U-cubes have been sampled between
(phase-only, frequency-independent, with a solution interval −2048 rad m−2 and 2048 rad m−2 with a step size of 8 rad m−2 .
of 10 min down to 1 min) until image quality reached con- We then calculated a polarised intensity cube, which is the abso-
vergence. Multifrequency CLEANing mfclean was conducted lute value of the complex polarised intensity p = Q + iU, where
utilising interactive masks around visible sources to minimise Q and U are the fluxes in the Faraday Q- and U-cubes.
artefacts and flux scattering and applying detection thresholds to Due to the fact that the intensity can only be positive, the
retain genuine emission only. The continuum images for each polarised intensity
p can be described by the complex conjugated
galaxy were generated using Briggs weighting with robust = 0, P = pp∗ = Q2 + U 2 following a non-Gaussian statistic
resulting in resolutions, noise levels, and image sizes, as pro- (Wardle & Kronberg 1974), which is contingent upon the dis-
vided in Table 5. In the dataset observed on the 4-June-2023 tance from the pointing centre due to primary beam correction.
for IC 4662, data from antenna 6 has been excluded due to an When deriving a polarised intensity map from RM-Synthesis
error in correlation block 1-bcc16 on antenna 6, which could cubes, it is observed that the bias is elevated compared to the
not be fixed during the observation. In order to identify whether theoretical expectation. This is attributed to the calculation of
there is a missing zero spacing problem, as the long base- polarised intensity from the maximum value along the Faraday
A226, page 4 of 20
Taziaux, S., et al.: A&A, 696, A226 (2025)
optically thin regime. The non-thermal synchrotron component
has a spectral index (S ∝ ναnth ) which is known to dominate at
frequencies up to around 10 GHz, with a slope for SF regions of
around −0.7 ± 0.03 with little deviation (Resmi et al. 2021). By
separating the synchrotron emission from the free-free emission,
the total magnetic field strength can be estimated by assuming
equipartition (see Appendix D). Since free-free emission lacks
polarisation and synchrotron emission is polarised, any polarised
emission indicates synchrotron emission, providing insights into
the magnetic field component (see Sect. 3.5 and Appendix E).
3.1. Total intensity
Fig. 1. Total intensity emission at the central frequency of 1.28 GHz
with overlaid contours starting at 3σ and increasing by factor of two Radio continuum emission has been observed from NGC 3125 at
(σ = 20 µJy/beam). Left: Total intensity of NGC 3125. Right: Total a frequency of 2.1 GHz, displayed in the left panel of Fig. 2. The
intensity of IC 4662. The circular beam of 7.600 appears in the lower prominent radio emission is aligned with the optical main body
left corner. of the galaxy, with its peak coinciding with the location of the
giant Hii region. We detect both the bright central regions and
the extended diffuse component. There are two bright peaks of
axis. In case, noise dominates along this axis, there is a higher
emission in the central regions of the galaxy, which are the two
probability of encountering a higher value (Heald et al. 2009).
big SF knots. The brightest peak is on the northeastern side of the
The extent of this deviation hinges on the length of the sampled
disk, and the fainter, slightly elongated peak is on the southwest-
Faraday axis and the Faraday resolution of the observation. Con-
ern. We observe extended emission of NGC 3125 perpendicular
sequently, an approximate method was adopted to mitigate this
to the disk, resulting in a symmetric envelope of radio emission.
bias (Adebahr et al. 2017). To address this, a higher-order poly-
Using the same nomenclature as Johnson et al. (2002) and
nomial with a parabolic shape was fitted to the distance of each
Crowther & Bibby (2009) for IC 4662, the two central regions,
pixel from the pointing centre against the average value along the
A1 and A2, exhibit intense radio emission attributed to active
polarised intensity axis in the Faraday polarised intensity cube.
star formation. The right panel of Fig. 2 shows the total inten-
This was performed solely in emission-free regions, termed the
sity of IC 4662 and displays elongated structures, or filaments,
background. The background image was then extended along the
extending from the central region to regions B, C, N and E.
third axis and subtracted from the polarised intensity cube. The
Additionally, several smaller, isolated regions of emission are
polarised intensity map and the RM map were derived by fitting
scattered around the central area, with the most significant radio
a parabola along the Faraday axis of the polarised intensity cube
emission observed in the south-east (region D). This raises the
to values exceeding 5σ (Heald et al. 2009). The peak value of
question whether region D is an isolated Hii region, a companion
this parabola yields the polarised flux density, while the position
galaxy, or an extension of the main galaxy, which is discussed
of the peak in the Faraday spectrum determines the RM value.
further in Sect. 4.2.2.
RM is an observable for the frequency of rotation of the electric
field vector along the line-of-sight due to Faraday rotation. Pos-
itive values represent a magnetic field that is pointing towards 3.2. Estimating the free–free emission
us while negative ones describe the opposite. The orientation
of the electric field vectors was derived using the polarisation A separation of the non-thermal and thermal components of
angle χ, calculated as χ = 21 tan−1 UQ . Assuming that the RM the radio emission is essential for the analysis of the spec-
tral index (Sect. 3.4) and estimating the magnetic field strength
and polarised intensity originate from the same region, the elec- (Appendix D). To obtain the thermal component, we utilise the
tric field vectors E were derotated back to the intrinsic magnetic free–free emission density function in relation to the Hα flux
field using χ0 = χ − RM λ2 + π2 . (Deeg et al. 1997, equation 3). For this purpose, we specifically
use the Hα maps from Gil de Paz et al. (2003) for NGC 3125
2.4. MeerKAT archive data and Hunter & Elmegreen (2004) for IC 4662 (see Fig. 3). It is
important to note that these maps have not been adjusted to
The MeerKAT radio telescope consists of 64 dish antennas and account for potential contamination from the [Nii] doublet emis-
observes in a frequency range of 856−1712 MHz. Its data is col- sion lines or corrected for dust extinction, which could introduce
lected across 4096 spectral channels. The MeerKAT maps, taken systematic uncertainties into the thermal flux estimates. How-
from Condon et al. (2021) and shown in Fig. 1, are 15 min snap- ever, Gil de Paz et al. (2003) and Hunter & Elmegreen (2004)
shot observations at a central frequency of 1.28 GHz. The data corrected the Hα flux of both galaxies, NGC 3125 and IC 4662,
reduction, including amplitude, phase and polarisation calibra- for both internal and galactic extinction.
tion, imaging processing of the Stokes parameters and phase- In the later stages of the analysis, we encountered some
only self calibration are done with Obit (Cotton 2008) at a robust uncertainties regarding the reliability of the thermal–non-
briggs weighting = −1.5. For more details about the data reduc- thermal separation. Consequently, the thermal and non-thermal
tion see Condon et al. (2021). fractions have been included in the Appendices A and B. In the
left panels of Fig. A.1 in Appendix A, the non-thermal emission
3. Properties of radio continuum polarimetry of NGC 3125 extends beyond the core, forming a synchrotron
halo around the galaxy. The radio envelope extends along the
The radio spectrum of galaxies is a combination of free-free minor axis of the galaxy. The thermal contribution reveals a
emission and synchrotron radiation (e.g. Klein et al. 2018), peak of 17.5% to 22.5% at 2.1 GHz in the galaxy centre, with
which can be written as a combination of two power-laws in the a rapid decline further out, shown in the left panels of Fig. B.1
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Taziaux, S., et al.: A&A, 696, A226 (2025)
Fig. 2. Total intensity emission at the central frequency of 2.1 GHz with overlaid contours starting at 3σ and increasing by factor of two (σ =
11 µJy/beam). Left: Total intensity of NGC 3125. Right: Total intensity of IC 4662. The beam of 7.300 × 3.500 for NGC 3125 and 7.400 × 2.900 for
IC 4662 appears in the lower left corner and can also be taken from Table 5.
Fig. 3. Left: Hα map of NGC 3125 from Gil de Paz et al. (2003). Right: Hα map of IC 4662 from Hunter & Elmegreen (2004). The different
regions are marked in orange.
in Appendix B. The mean thermal fraction of NGC 3125 is esti- Note that this does not include systematic uncertainties and may
mated to be 11.08% at 1.28 GHz and 14.87% at 2.1 GHz. be a gross underestimate.
In the right panels of Fig. A.1 in Appendix A, we show the
non-thermal emission from IC 4662, which exhibits pronounced
3.3. Box-integrated intensity profiles
peaks corresponding to the bright star-forming regions A1 and
A2. Additionally, there is a diffuse emission extending to the Spectral index derived from observed total intensity radio con-
north toward region N and to the south, linking to the galaxy’s tinuum emissions serve as crucial analysis tools for explaining
tail (regions B and C). The overall emission from this dwarf the underlying emission mechanisms, and energy loss/gain pro-
galaxy is predominantly thermal, as evidenced by the thermal cesses of the synchrotron emitting CREs. Furthermore, analysis
fraction shown in the right panels of Fig. B.1 in Appendix B. of observed spectral indices provide insights into whether ther-
The mean thermal fraction of the main body of IC 4662 is esti- mal or synchrotron emissions dominate within a specific region.
mated to be 36.06% at 1.28 GHz and 46.47% at 2.1 GHz. The However, it is important to recognise that spectral index
thermal fraction reaches its maximum value of approximately maps are susceptible to significant distortion due to minor sys-
70% at 2.1 GHz in both the main body and the southern region, tematic fluctuations in flux density. For instance, in the case
which corresponds to Hii region D. In this region D, the mean of a frequency range between 1.28 GHz and 2.1 GHz, even a
thermal fraction is 48.52% at 1.28 GHz and 45.57% at 2.1 GHz. marginal 5% fluctuation in flux density can cause the spectral
To estimate the uncertainties in the non-thermal emission, index to change by as much as ±0.104, depending on whether
we start with a 10% uncertainty in the total flux density, com- the flux density increases or decreases.
ing from the telescopes. Additional errors arise from the thermal To assess the validity of the thermal–non-thermal separation
correction, due to electron temperature variations T e = 10 000 ± and extend the study of radio continuum emission into the galac-
3000 K and a 10% uncertainty in the treatment of the [Nii] line tic halo, we utilise a box integration method to average data over
flux relative to Hα measurements (Vargas et al. 2018). Combin- large regions of NGC 3125 and IC 4662. We use one strip con-
ing these factors, we estimate a total uncertainty of approxi- taining 14 boxes with the FWHM of our synthesised beam (7.600 )
mately 20% in the final non-thermal flux density measurement. as box height for each galaxy, determined by the spatial extent
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Taziaux, S., et al.: A&A, 696, A226 (2025)
st
We
t
rth Ea s
No
uth
So
Fig. 4. Left: Thermal emission map of NGC 3125 with overlaid boxes extending from the south to the north. Right: Thermal emission map of
IC 4662 with overlaid boxes extending from the west to the east. The circular beam of 7.600 is shown in the left corner.
100
North Box Profile 100
South Box Profile
ATCA Total Emission ATCA Total Emission
ATCA Thermal Emission ATCA Thermal Emission
MeerKAT Total Emission MeerKAT Total Emission
MeerKAT Thermal Emission MeerKAT Thermal Emission
10 1 10 1
Intensity / Jy/beam
10 2 10 2
10 3 10 3
0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Distance to galaxy center / kpc Distance to galaxy center / kpc
Thermal Fraction North Thermal Fraction South
30 30
ATCA ATCA
25 MeerKAT 25 MeerKAT
Thermal Fraction / %
20 20
15 15
10 10
5 5
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Distance to galaxy center / kpc Distance to galaxy center / kpc
Spectral Index North Spectral Index South
1 1
0 0
1 1
Spectral Index
2 2
3 3
4 Total Spectral Index 4 Total Spectral Index
Non-thermal Spectral Index Non-thermal Spectral Index
5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 5 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Distance to galaxy center / kpc Distance to galaxy center / kpc
Fig. 5. Upper panel: Box-integrated radio continuum profiles of NGC 3125, separated into northern (left) and southern (right) sections, showing the
total and thermal emission at 1.28 GHz (MeerKAT) and 2.1 GHz (ATCA). Middle panel: Thermal fraction profiles of NGC 3125 for the northern
(left) and southern (right) sections. Bottom panel: Comparison of non-thermal and total spectral indices for the northern (left) and southern (right)
sections of NGC 3125. Spectral index data points on the x-axis are slightly offset to improve differentiation between close values.
necessary to include the halo region fully. This approach allows For NGC 3125, we separate the northern and southern sections,
us to differentiate between the central star-forming regions, the whereas for IC 4662, we distinguish the western and eastern
outskirts, and the more distant outer regions. The total absolute sections, using the optical major axis as the dividing line (see
flux density error arises from multiple sources, including back- Fig. 4).
ground noise level and calibration uncertainties. Background Figure 5 (upper panel) shows the integrated total and ther-
noise increases with the number of beams within the integration mal radio emissions for various regions of NGC 3125, with sep-
area, contributing to the overall uncertainty (Klein & Emerson arate analyses for the northern and southern sections. NGC 3125
1981). However, in our analysis, the dominant source of error shows extended radio continuum emission up to a distance of
is the calibration uncertainty, which is estimated to be 10%. 2 kpc from the galaxy’s centre. In the northern section, it is
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Taziaux, S., et al.: A&A, 696, A226 (2025)
100
East Box Profile 100
West Box Profile
ATCA Total Emission ATCA Total Emission
ATCA Thermal Emission ATCA Thermal Emission
MeerKAT Total Emission MeerKAT Total Emission
MeerKAT Thermal Emission MeerKAT Thermal Emission
10 1 10 1
Intensity / Jy/beam
10 2 10 2
10 3 10 3
0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.2 0.3 0.4 0.5 0.6
Distance to galaxy center / kpc Distance to galaxy center / kpc
Thermal Fraction East Thermal Fraction West
60 60
ATCA ATCA
50 MeerKAT 50 MeerKAT
Thermal Fraction / %
40 40
30 30
20 20
10 10
0 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5 0.6
Distance to galaxy center / kpc Distance to galaxy center / kpc
Spectral Index East Spectral Index West
2 Total Spectral Index 2
Non-thermal Spectral Index
1 1
Spectral Index
0 0
1 1
2 2
Total Spectral Index
3 3 Non-thermal Spectral Index
0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.2 0.3 0.4 0.5 0.6
Distance to galaxy center / kpc Distance to galaxy center / kpc
Fig. 6. Upper panel: Box-integrated radio continuum profiles of IC 4662, separated into eastern (left) and western (right) sections, showing the
total and thermal emission at 1.28 GHz (MeerKAT) and 2.1 GHz (ATCA). Middle panel: Thermal fraction profiles of IC 4662 for the eastern (left)
and western (right) sections. Bottom panel: Comparison of non-thermal and total spectral indices for the eastern (left) and western (right) sections
of IC 4662. Spectral index data points on the x-axis are slightly offset to improve differentiation between close values.
noteworthy that there is a lack of detectable radio emission at in star formation activity, magnetic field configurations, or dif-
2.1 GHz, while total emission is still discernible at a lower fre- ferent contributions from various emission processes across the
quency of 1.28 GHz. We further extend the profile up to 3.5 kpc, galaxy.
where thermal emission remains detectable (see left panel of In Fig. 6, we observe the significant part of thermal emission
Fig. 4). This extended range indicates that thermal processes in the central regions of IC 4662, with thermal fractions reach-
are still active, even as radio continuum emission diminishes. ing values up to 40%. The total and non-thermal spectral indices
Within this northern region, the morphology of the feature might in this galaxy show minimal distinction, resulting in a generally
indicate an outflow, prominently visible in the thermal emission flat spectral profile across the entire galaxy. Notably, the western
maps. This outflow is less apparent in the total intensity map, section, which encompasses regions A1, B, C, and part of region
where it falls below the 3σ sensitivity threshold, suggesting that N, exhibits a markedly flatter spectrum compared to the eastern
the feature might become more distinguishable with increased region, which includes areas A2, E, and the other half of region
observational sensitivity. At 2.1 GHz, we observe an increase N. The pronounced flatness of the spectrum in IC 4662, as con-
in the thermal fraction at 2 kpc, which can be attributed to the firmed by multiple analyses through this study, may be attributed
absence of detectable radio continuum emission in this region, to the combined effects of free–free emission and an extreme
despite ongoing thermal emission. Furthermore, the total and degree of low-frequency absorption (Werhahn et al. 2021). This
non-thermal spectral indices are found to be closely aligned, observation suggests that the traditional methodologies for esti-
indicating similar contributions from different emission mecha- mating free-free emission, commonly derived from Hα, may not
nisms. Beyond the 2 kpc threshold, an increase in the spectral be applicable or effective in this particular dwarf galaxy.
index is detected, which may serve as evidence for the weak
total radio continuum emission associated with the outflow arm
3.4. Integrated spectral index
in the northern section of the galaxy. This suggests a poten-
tial link between the outflow dynamics and the radio emissions. Since we observe no significant difference between the total and
In addition, the southern part of NGC 3125 exhibits a flatter non-thermal spectral index in the box integrated profiles, and to
spectrum compared to the northern section, which can also be minimise systematic errors, we present the spectral index analy-
observed in the left panel of Fig. 8, highlighting distinct differ- sis based on the total uncorrected flux densities. A corresponding
ences in the physical conditions and processes occurring in these analysis was also performed using non-thermal emission maps,
two regions of the galaxy. This disparity may reflect variations with the results provided in Appendix C.
A226, page 8 of 20
Taziaux, S., et al.: A&A, 696, A226 (2025)
NGC 3125 IC 4662
Fig. 7. Spectral energy distribution of NGC 3125 (left panel) and IC 4662 (right panel). The plot displays total intensity data with different
observations: GLEAM in blue, RACS in orange, MeerKAT in green, ATCA data from this study in red, and high-frequency data from ATCA for
NGC 3125 (Stevens et al. 2002) and for IC 4662 (Johnson et al. 2002) in violet. As GLEAM and RACS are survey data, we only use lower limits
as we expect more flux to be there. Additionally, for NGC 3125 there may be some flux loss at 2740 MHz. Therefore, the data point marked in dark
red and shown as a lower limit, does not contribute to the fit. Thermally corrected emissions are shown in gray. Additionally, the polynomial fits
from Perley & Butler (2017) are provided, indicating the spectral index α0 (SI) and spectral curvature β (SC) of the source. A standard synchrotron
spectrum fit with a spectral index of α = −0.7 is shown in blue. For IC 4662, we additionally present a thermal radio corrected emission in black
by assuming only thermal emission above 4.86 GHz and using the black dashed line as new thermal emission model. The 4.8 and 8.64 GHz data
points for IC 4662 are excluded from the fit.
Fig. 8. Left: Fitted in-band total spectral index and its error of NGC 3125 between the frequency ranges 1460 MHz and 2740 MHz. Right: Fitted in-
band total spectral curvature and its error of NGC 3125 between the frequency ranges 1460 MHz and 2740 MHz. The contours show the 2228 MHz
total radio emission at (5, 10, 20, 40, 80, 160, 320) × 37.5 µJy/beam. The circular beam of 12.500 is shown in the left corner.
In order to estimate the accuracy of the spectral index, the In the left panel of Fig. 7, we present the integrated spec-
data set was divided into eight segments, each with a bandwidth tral energy distribution of NGC 3125, as derived from the data
of 256 MHz. Due to high noise levels, we excluded the two edge in Table 4. At frequencies above 4.86 GHz, we have used
bins at 1205 MHz and 2996 MHz. We then fitted a model spec- the integrated values from all star-forming regions reported by
trum based on a polynomial equation (Perley & Butler 2017): Stevens et al. (2002) as lower limits. The observed total spectral
!#2 index is −0.48 ± 0.03, with a spectral curvature of −0.35 ± 0.04.
ν ν
! "
The non-thermal spectral index, after accounting for Hα emis-
log S ν = log S 0 + α0 log + β log , (1) sion, is −0.63 ± 0.05, while the corresponding spectral curvature
ν0 ν0
is −0.48±0.05. However, for NGC 3125, there may be some flux
where S ν is the flux density at a given frequency ν (in GHz), loss at 2740 MHz (the final data point of the splitted data) due to
S 0 is the flux density at the reference frequency ν0 = 2.1 GHz, the use of only long baselines. This effect is noticeable, so the
α0 represents the polynomial spectral index at the reference 2740 MHz point should be treated as a lower limit.
frequency ν0 , and β is the spectral curvature. We begin with We perform model comparisons using the Akaike Informa-
this purely phenomenological model to investigate the general tion Criterion (AIC) and the Residual Sum of Squares (RSS)
shape of the spectrum without bias towards any specific physi- to assess whether including curvature improves model perfor-
cal mechanism. For the integrated spectra, we incorporate addi- mance. To do so, we fitted the model both with and without the
tional data from several sources: GLEAM (Wayth et al. 2015), curvature term. Summarised, AIC helps select the best model by
RACS (McConnell et al. 2020), MeerKAT (Condon et al. 2021), penalizing complexity and avoiding overfitting and RSS mea-
and high-frequency observations from ATCA for NGC 3125 sures how well the model fits the data, for both the lower, the
(Stevens et al. 2002) and for IC 4662 (Johnson et al. 2002). better. Here, we consider the full spectrum in one case and only
These data are also detailed in Table 4. the ATCA data (excluding 2.7 GHz) along with MeerKAT in the
A226, page 9 of 20Taziaux, S., et al.: A&A, 696, A226 (2025)
Table 4. Summary of the integrated total radio continuum flux density values and its error for NGC 3125 and IC 4662.
ν/MHz S tot,NGC 3125 /mJy S tot,IC 4662 /mJy Reference
87.7 18 ± 5.3 – Wayth et al. (2015)
118.4 21 ± 6.8 – Wayth et al. (2015)
154.2 25 ± 5.5 – Wayth et al. (2015)
887 22.98 ± 2.31 29.57 ± 2.96 McConnell et al. (2020)
1280 25.0 ± 2.50 34.06 ± 3.4 Condon et al. (2021)
1460 22.88 ± 2.28 31.86 ± 3.18 This work
1716 21.35 ± 2.13 31.87 ± 3.18 This work
1972 20.13 ± 2.01 30.69 ± 3.07 This work
2228 17.96 ± 1.79 28.99 ± 2.89 This work
2484 17.08 ± 1.70 26.93 ± 2.69 This work
(†)
2740 14.64 ± 1.46 26.92 ± 2.69 This work
4800 10.2 ± 1.02 8.2 ± 0.93 Stevens et al. (2002), Johnson et al. (2002)
8640 7.5 ± 0.75 9.7 ± 1.11 Stevens et al. (2002), Johnson et al. (2002)
Notes. (†) There may be some flux loss due to the use of only long baselines.
Table 5. Overview of radio continuum and polarisation properties for NGC 3125 and IC 4662.
NGC 3125 IC 4662
Radio continuum properties
Angular resolution/00 7.3 × 3.5 7.4 × 2.9
Noise level/µJy/beam 11 11
Path length/kpc 2.1 ± 0.2 0.9 ± 0.1
S tot,1.28 GHz /mJy 24.792 ± 2.479 28.055 ± 2.806
S tot,2.1 GHz /mJy 17.306 ± 1.731 25.443 ± 2.544
αtot,1.28−2.1 GHz −0.726 ± 0.286 −0.197 ± 0.286
S nth,1.28 GHz /mJy 21.799 ± 4.360 17.708 ± 3.541
S nth,2.1 GHz /mJy 14.247 ± 2.850 15.784 ± 3.156
αnth,1.28−2.1 GHz −0.859 ± 0.554 −0.232 ± 0.596
hBeq,2.1 GHz i/µG 14.70+0.76
−0.69 15.48+1.04
−0.79
(†)
Polarisation properties
Frequency range/MHz [1323, 2869] [1489, 2869]
nimages 1450 1311
Resolution/rad m−2 83.290 90.206
Scale/rad m−2 319.437 319.436
Φ/rad m−2 106453.903 112381.847
λ2 /rad m−2 0.0217 0.0202
PI/µJy 34.2 ± 3.5 121.6 ± 27.5
Notes. (†) Equipartition magnetic field strength assuming a non-thermal spectral index of −0.7 This overview includes the beam resolution, noise
levels of the ATCA total radio continuum maps, and the estimated path length for both dwarf galaxies. The total and non-thermal flux densities
from 1.28 GHz (MeerKAT) and 2.1 GHz (ATCA) data are provided, along with the calculated total and non-thermal spectral indices between these
frequencies. Additionally, the equipartition magnetic field strength is reported. For the polarisation properties, we present the frequency range and
number of images used for RM synthesis, the resolution in Faraday depth space, the maximum observable scale, the maximum observed Faraday
depth, the observing wavelengths, and the polarised intensity.
other. For the full spectrum, the RSS is significantly reduced and ATCA data, extrapolated to 87 MHz (GLEAM), predicts
when including curvature (1.71 × 10−5 vs. 7.42 × 10−4 ), and the an integrated flux density of 32 mJy, whereas the observed
AIC confirms a better fit with curvature (−6.95 vs. 25.03). How- value is 18 mJy, approximately 50% lower. Additionally, within
ever, when considering only the ATCA and MeerKAT data, the the GLEAM survey, the integrated flux density decreases with
RSS is slightly lower with curvature (4.70×10−7 vs. 5.51×10−7 ), decreasing frequency, which cannot be attributed to missing cov-
but the AIC increases (−17.95 vs. −18.75), indicating that the erage. This suggests that detecting spectral curvature requires
additional curvature parameter is not justified in this limited fre- including low-frequency data, also shown in Stein et al. (2023),
quency range. Overall, while curvature improves the fit across Gajović et al. (2024) and Gajović et al. (2025).
the entire spectrum, a straight-line model is sufficient for the To compute an in-band spatial distributed spectral index, we
ATCA and MeerKAT data alone. Furthermore GLEAM and split our ATCA dataset as described above, convolved it to the
RACS, as survey data, are expected to show slightly higher inte- same resolution of a 12.500 circular beam, clipped each at 5 times
grated flux densities. A linear regression based on MeerKAT the rms noise level, and plotted the fitted total spectral index
A226, page 10 of 20Taziaux, S., et al.: A&A, 696, A226 (2025)
Fig. 9. Left: Fitted in-band total spectral index and its error of IC 4662 between the frequency ranges 1460 MHz and 2740 MHz. Right: Fitted in-
band total spectral curvature and its error of IC 4662 between the frequency ranges 1460 MHz and 2740 MHz. The contours show the 2228 MHz
total radio emission at (5, 10, 20, 40, 80, 160, 320) × 38 µJy/beam. The circular beam of 1500 is shown in the left corner.
and spectral curvature using the Perley & Butler (2017) polyno- of the galaxy, the spectral curvature shows a variation between
mial equation, as shown in the left and right panel of Fig. 8. −1.5 and 1.5. The positive spectral indices observed in north
We find a total spectral index of −0.64 ± 0.03 in the galactic region indicate a dominance of optically thick thermal emission,
disk and decreasing to −1.22 ± 0.19 towards the edges of the further discussed in Sect. 4.1. In the top right panel of Fig. C.1 in
galaxy with a spectral curvature ranging from −2.44 ± 0.34 to Appendix C, we present the fitted non-thermal spectral index. In
−3.35 ± 0.62. the same regions, the non-thermal spectral index is found to be
Note that, the polynomial spectral index α0 and 2-point spec- positive, which is impossible to reconcile with CREs that have
tral index α are not the same. From Eq. (1), we get that α0 = been accelerated via diffusive shock acceleration at supernova
α−2β[log(ν/ν0 )]. In order to compare α0 and α between 1.28 and remnants (Marcowith et al. 2016). Hence, this suggests that the
2.1 GHz, seen in Figs. 5 and 6, the reference frequency ν0 should Hα-based thermal correction is insufficient and not applicable
be chosen somewhere between 1.28 and 2.1 GHz, depending on for this type of dwarf galaxy. Similar flat spectra, with even pos-
the spectral curvature β. itive total spectral indices for IC 4662, have also been observed
In right panel of Fig. 7, we show the integrated spectral at higher frequencies by Johnson et al. (2002).
energy distribution of IC 4662 from Table 4. At frequencies
above 4.86 GHz, we have used the integrated values of all the 3.5. Polarised emission
regions provided by Johnson et al. (2002) in their study, shown
as lower limits but excluded from the fit. First, we observe a The left panel of Fig. 10 shows the distribution of polarised
total spectral index of −0.39 ± 0.06 with a spectral curvature emission of NGC 3125 across two distinct regions. Two regions
of −1.07 ± 0.19. To estimate the non-thermal spectral index, were identified: the first is situated in the south of the two star-
the contribution from the thermal emission is subtracted using forming knots, while the second is located to the north-west of
the integrated value of Hα from Table 1 (Deeg et al. 1997, the central area. In the southern region, the magnetic field orien-
Eq. (3)). The non-thermal spectral index and spectral curvature tations are oriented nearly perpendicular to the optical major axis
are −0.50 ± 0.07 and −1.45 ± 0.26, respectively. Additionally, of the galaxy, while moving towards the north-west, the mag-
assuming that only free-free emission dominates at higher fre- netic field orientations progressively align more closely with the
quencies, we modelled the thermal emission using data at 4.8 major axis of the galaxy. The integrated polarised emission of
and 8.64 GHz. By subtracting this thermal contribution from the galaxy is approximately 34.2 ± 3.5 µJy. In the right panel of
the total intensity, we derived a non-thermal spectral index of Fig. 10, the Hα emission is overlaid with magnetic field orienta-
−0.59 ± 0.07 and a spectral curvature of −1.41 ± 0.23. For these tions. The filaments, as traced by the Hα emission, do not show
latter two models, which include the Hα corrected emission and any detectable polarised emission. While filaments observed in
the radio corrected emission, we did not incorporate the values Hα can sometimes be associated with polarised emission (e.g.
provided by Johnson et al. (2002) when calculating the spectral Kepley et al. 2010; Chyży et al. 2016) due to aligned magnetic
index and its spectral curvature. fields, in this case, no significant polarisation is observed within
Comparing the total and non-thermal spectral indices from them. We observe a degree of polarisation in these regions of
the integrated spectrum with the two-point spectral index pre- 0.1% up to 1.8%, seen in the middle panel of Fig. 10. The leak-
sented in Table 5, a discrepancy is observed. This arises because age of polarisation of the standard calibrator 1934-638 is 0.08%
the polynomial spectral index and the two-point spectral index (Schnitzeler et al. 2011).
are not directly comparable, as spectral curvature must be con- For IC 4662, we do not detect any polarised emission (see
sidered. Additionally, the two-point spectral index between the Fig. 11), likely due to turbulence obscuring the presence of
central frequencies of 1.28 and 2.1 GHz represents only a narrow ordered and/or anisotropic random magnetic fields, potentially
portion of the overall spectrum. resulting in depolarisation. The several processes of depolar-
To compute the distribution in-band spectral index, we split isation, possibly happening this galaxy are discussed in the
our ATCA dataset as described, as shown in the middle and right Sect. 4.3. A clear detection of polarised emission at a mean
panel of Fig. 9. The central region remains flat or shows a pos- weighted λ2 of 0.0202 rad m−2 , with an integrated value of
itive spectrum, indicating a dominance of thermal emission and 121.6 ± 27.5 µJy, is observed between the southern Hii region
a reduced contribution from synchrotron radiation. In the core D and the main body of IC 4662.
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