48 research outputs found
New insights into the interstellar medium of the dwarf galaxy IC 10 : connection between magnetic fields, the radio--infrared correlation and star formation
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. Available at doi: https://doi.org/10.1093/mnras/stx1567.We present the highest sensitivity and angular resolution study at 0.32 GHz of the dwarf irregular galaxy IC\,10, observed using the Giant Metrewave Radio Telescope, probing pc spatial scales. We find the galaxy-averaged radio continuum spectrum to be relatively flat, with a spectral index (), mainly due to a high contribution from free--free emission. At 0.32 GHz, some of the H{\sc ii} regions show evidence of free--free absorption as they become optically thick below GHz with corresponding free electron densities of . After removing the free--free emission, we studied the radio--infrared relations on 55, 110 and 165 pc spatial scales. We find that on all scales the non-thermal emission at 0.32 and 6.2 GHz correlates better with far-infrared (FIR) emission at m than mid-infrared emission at m. The dispersion of the radio--FIR relation arises due to variations in both magnetic field and dust temperature, and decreases systematically with increasing spatial scale. The effect of cosmic ray transport is negligible as cosmic ray electrons were only injected Myr ago. The average magnetic field strength () of G in the disc is comparable to that of large star-forming galaxies. The local magnetic field is strongly correlated with local star formation rate () as , indicating a star-burst driven fluctuation dynamo to be efficient ( per cent) in amplifying the field in IC\,10. The high spatial resolution observations presented here suggest that the high efficiency of magnetic field amplification and strong coupling with SFR likely sets up the radio--FIR correlation in cosmologically young galaxies.Peer reviewedFinal Accepted Versio
Azimuthal Anisotropy of Magnetic Fields in the Circumgalactic Medium Driven by Galactic Feedback Processes
We use the TNG50 cosmological magnetohydrodynamical simulation of the
IllustrisTNG project to show that magnetic fields in the circumgalactic medium
(CGM) have significant angular structure. This azimuthal anisotropy at fixed
distance is driven by galactic feedback processes that launch strong outflows
into the halo, preferentially along the minor axes of galaxies. These
feedback-driven outflows entrain strong magnetic fields from the interstellar
medium, dragging fields originally amplified by small-scale dynamos into the
CGM. At the virial radius, galaxies with M show the strongest anisotropy ( dex). This
signal weakens with decreasing impact parameter, and is also present but weaker
for lower mass as well as higher mass galaxies. Creating mock Faraday rotation
measure (RM) sightlines through the simulated volume, we find that the angular
RM trend is qualitatively consistent with recent observational measurements. We
show that rich structure is present in the circumgalactic magnetic fields of
galaxies. However, TNG50 predicts small RM amplitudes in the CGM that make
detection difficult as a result of other contributions along the line of sight.Comment: Submitted to MNRAS. Comments welcom
Cosmic-ray electron transport in the galaxy M 51
Context. Indirect observations of the cosmic-ray electron (CRE) distribution
via synchrotron emission is crucial for deepening the understanding of the CRE
transport in the interstellar medium, and in investigating the role of galactic
outflows.
Aims. In this paper, we quantify the contribution of diffusion- and
advection-dominated transport of CREs in the galaxy M51 considering relevant
energy loss processes.
Methods. We used recent measurement from M 51 that allow for the derivation
of the diffusion coefficient, the star formation rate, and the magnetic field
strength. With this input, we solved the 3D transport equation numerically
including the spatial dependence as provided by the measurements, using the
open-source transport framework CRPropa (v3.1). We included 3D transport
(diffusion and advection), and the relevant loss processes.
Results. We find that the data can be described well with the parameters from
recent measurements. For the best fit, it is required that the wind velocity,
following from the observed star formation rate, must be decreased by a factor
of 5. We find a model in which the inner galaxy is dominated by advective
escape and the outer galaxy is composed by both diffusion and advection.
Conclusions. Three-dimensional modelling of cosmic-ray transport in the
face-on galaxy M51 allows for conclusions about the strength of the outflow of
such galaxies by quantifying the need for a wind in the description of the
cosmic-ray signatures. This opens up the possibility of investigating galactic
winds in face-on galaxies in general.Comment: 6 pages, 7 figure
Radio constraints on dark matter annihilation in Canes Venatici I with LOFAR
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Dwarf galaxies are dark matter-dominated and therefore promising targets for the search for weakly interacting massive particles (WIMPs), which are well-known candidates for dark matter. Annihilation of WIMPs produce ultra-relativistic cosmic-ray electrons and positrons that emit synchrotron radiation in the presence of magnetic fields. For typical magnetic field strengths (few G) and (GeV-TeV) WIMP masses (and thus typical electron energies of the same order) this emission peaks at hundreds of MHz. Here, we use the non-detection of 150-MHz radio continuum emission from the dwarf spheroidal galaxy `Canes Venatici I' with the LOw-Frequency ARray (LOFAR) to derive constraints on the annihilation cross section of WIMPs into primary electron-positron and other fundamental particle-antiparticle pairs. In this first-of-its-kind LOFAR study, we obtain new constraints on annihilating WIMP dark matter (DM). Using conservative estimates for the magnetic field strengths and diffusion coefficients, we obtain limits that are comparable with those by the Fermi Large Area Telescope (Fermi-LAT) using gamma-ray observations. Assuming s-wave annihilation and WIMPs making up 100% of the DM density, our limits exclude several thermal WIMP realisations in the -GeV mass range. A more ambitious multi-wavelength and multi-target LOFAR study could improve these limits by a few orders of magnitude.Peer reviewe
The magnetic field structure in NGC 253 in presence of a galactic wind
We present radio continuum polarimetry observations of the nearby edge-on
galaxy NGC 253 which possesses a very bright radio halo. Using the vertical
synchrotron emission profiles and the lifetimes of cosmic-ray electrons, we
determined the cosmic-ray bulk speed as (300+/-30) km/s, indicating the
presence of a galactic wind in this galaxy. The large-scale magnetic field was
decomposed into a toroidal axisymmetric component in the disk and a poloidal
component in the halo. The poloidal component shows a prominent X-shaped
magnetic field structure centered on the nucleus, similar to the magnetic field
observed in other edge-on galaxies. Faraday rotation measures indicate that the
poloidal field has an odd parity (antisymmetric). NGC 253 offers the
possibility to compare the magnetic field structure with models of galactic
dynamos and/or galactic wind flows.Comment: 6 pages, 5 figures, in the proceedings of the IAU Symposium 259:
"Cosmic magnetic fields: from planets, to stars and galaxies", Teneriffe,
November 2008, in pres
The non-thermal superbubble in IC 10 : the generation of cosmic ray electrons caught in the act
Superbubbles are crucial for stellar feedback, with supposedly high (of the order of 10 per cent) thermalization rates. We combined multiband radio continuum observations from the Very Large Array (VLA) with Effelsberg data to study the non-thermal superbubble (NSB) in IC 10, a starburst dwarf irregular galaxy in the Local Group. Thermal emission was subtracted using a combination of Balmer Hα and VLA 32 GHz continuum maps. The bubble’s nonthermal spectrum between 1.5 and 8.8 GHz displays curvature and can be well fitted with a standard model of an ageing cosmic ray electron population. With a derived equipartition magnetic field strength of 44 ±8 μG, and measuring the radiation energy density from Spitzer MIPS maps as 5±1×10−11 erg cm−3, we determine, based on the spectral curvature, a spectral age of the bubble of 1.0 ± 0.3 Myr. Analysis of the LITTLE THINGS HI data cube shows an expanding HI hole with 100 pc diameter and a dynamical age of 3.8 ± 0.3 Myr, centred to within 16 pc on IC 10 X-1, a massive stellar mass black hole (M > 23 M⊙). The results are consistent with the expected evolution for a superbubble with a few massive stars, where a very energetic event like a Type Ic supernova/hypernova has taken place about 1 Myr ago. We discuss alternatives to this interpretationPeer reviewe
Magnetic Field Tomography in Nearby Galaxies with the Square Kilometre Array
Magnetic fields play an important role in shaping the structure and evolution
of the interstellar medium (ISM) of galaxies, but the details of this
relationship remain unclear. With SKA1, the 3D structure of galactic magnetic
fields and its connection to star formation will be revealed. A highly
sensitive probe of the internal structure of the magnetoionized ISM is the
partial depolarization of synchrotron radiation from inside the volume.
Different configurations of magnetic field and ionized gas within the
resolution element of the telescope lead to frequency-dependent changes in the
observed degree of polarization. The results of spectro-polarimetric
observations are tied to physical structure in the ISM through comparison with
detailed modeling, supplemented with the use of new analysis techniques that
are being actively developed and studied within the community such as Rotation
Measure Synthesis. The SKA will enable this field to come into its own and
begin the study of the detailed structure of the magnetized ISM in a sample of
nearby galaxies, thanks to its extraordinary wideband capabilities coupled with
the combination of excellent surface brightness sensitivity and angular
resolution.Comment: 11 pages, 1 figure; to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)10