Radio continuum observations of external galaxies provide us with an excellent outside view on the distribution of cosmic-ray electrons in the disc and halo. In this review, we summarise the current ...state of what we have learned from modelling such observations with cosmic-ray transport, paying particular attention to the question to what extent we can exploit radio haloes when studying galactic winds. We have developed the user-friendly framework
spinnaker
to model radio haloes with either pure advection or diffusion, allowing us to study both diffusion coefficients and advection speeds in nearby galaxies. Using these models, we show that we can identify galaxies with winds using both morphology and radio spectral indices of radio haloes. Advective radio haloes are ubiquitous, indicating that already fairly low values of the star formation rate (SFR) surface density (
Σ
SFR
) can trigger galactic winds. The advection speeds scale with SFR,
Σ
SFR
, and rotation speed as expected for stellar feedback-driven winds. Accelerating winds are in agreement with our radio spectral index data, but this is sensitive to the magnetic field parametrisation, so that constant wind speeds cannot be ruled out either. The question to what extent cosmic rays can be a driving force behind winds is still an open issue and we discuss only in passing how a simple iso-thermal wind model could fit our data. Nevertheless, the comparison with inferences from observations and theory looks promising with radio continuum offering a complementary view on galactic winds. We finish with a perspective on future observations and challenges lying ahead.
We present a study of the spatially resolved radio continuum-star formation rate (RC-SFR) relation using state-of-the-art star formation tracers in a sample of 17 THINGS galaxies. We use SFR surface ...density ( capital sigma sub(sfr)) maps created by a linear combination of GALEX far-UV (FUV) and Spitzer 24 mu m maps. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based capital sigma sub(sfr) for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based capital sigma sub(sfr) agrees with the integrated ratio and has only quasi-random fluctuations with galactocentric radius that are relatively small. We studied the ratio R of radio to FUV/MIR-based integrated SFR as a function of global galaxy parameters and found no clear correlation. We can reconcile our finding of an almost linear RC-S FR relation and sub-linear resolved RC- capital sigma sub(sfr) relation by proposing a non-linear magnetic field-SFR relation, B is proportional to SFR super(0.30 + or - 0.02) sub(hyb), which holds both globally and locally.
Abstract We have carried out spatially resolved thermal–nonthermal separation on two edge-on galaxies, NGC 3044 and NGC 5775, using only radio data. Narrowband imaging within a frequency band that is ...almost contiguous from 1.25 to 7.02 GHz ( L band, S band, and C band) has allowed us to fit spectra and construct thermal, nonthermal, and nonthermal spectral index maps. This method does not require any ancillary H α and IR data or rely on dust corrections that are challenging in edge-on galaxies. For NGC 3044, at 15″ resolution, we find a median thermal fraction of ∼13% with an estimated uncertainty in this fraction of ∼50% at 4.13 GHz. This compares well with the H α mixture method results. We uncovered evidence for a vertical outflow feature reaching at least z ∼ 3.5 kpc in projection above the plane, reminiscent of M82's starburst wind. For the higher star formation rate galaxy, NGC 5775, at 12″ resolution, we find a median thermal fraction of 44% at 4.13 GHz with an estimated error on this fraction of 17%. Both galaxies show a change of slope (flattening) in L band. These results suggest that a radio-only method for separating thermal from nonthermal emission is not only feasible, but able to reveal new features that might otherwise be obscured in edge-on disks.
In this paper, we examine to what extent the radio continuum can be used as an extinction-free probe of star formation in dwarf galaxies. To that aim, we observe 40 nearby dwarf galaxies with the ...Very Large Array at 6 cm (4-8 GHz) in C-configuration. We obtained images with 3″-8″ resolution and noise levels of 3-15 Jy beam−1. We detected emission associated with 22 of the 40 dwarf galaxies, eight of which are new detections. The general picture is that of an interstellar medium largely devoid of radio continuum emission, interspersed by isolated pockets of emission associated with star formation. We find an average thermal fraction of ∼50%-70% and an average magnetic field strength of ∼5-8 G, only slightly lower than that found in larger, spiral galaxies. At 100 pc scales, we find surprisingly high values for the average magnetic field strength of up to 50 G. We find that dwarf galaxies follow the theoretical predictions of the radio continuum-star formation rate relation within regions of significant radio continuum emission but that the nonthermal radio continuum is suppressed relative to the star formation rate when considering the entire optical disk. We examine the far-infrared-star formation rate relation for our sample and find that the far-infrared is suppressed compared to the expected star formation rate. We discuss explanations for these observed relations and the impact of our findings on the radio continuum-far-infrared relation. We conclude that radio continuum emission at centimeter wavelengths has the promise of being a largely extinction-free star formation rate indicator. We find that star formation rates of gas-rich, low-mass galaxies can be estimated with an uncertainty of 0.2 dex between the values of 2 × 10−4 and 0.1 M yr−1.
ABSTRACT We present far-infrared (FIR) spectral line observations of five galaxies from the Little Things sample: DDO 69, DDO 70, DDO 75, DDO 155, and WLM. While most studies of dwarfs focus on ...bright systems or starbursts due to observational constraints, our data extend the observed parameter space into the regime of low surface brightness dwarf galaxies with low metallicities and moderate star formation rates. Our targets were observed with Herschel at the C ii 158 m, O i 63 m, O iii 88 m, and N ii 122 m emission lines using the PACS Spectrometer. These high-resolution maps allow us for the first time to study the FIR properties of these systems on the scales of larger star-forming complexes. The spatial resolution in our maps, in combination with star formation tracers, allows us to identify separate photodissociation regions (PDRs) in some of the regions we observed. Our systems have widespread C ii emission that is bright relative to continuum, averaging near 0.5% of the total infrared (TIR) budget-higher than in solar-metallicity galaxies of other types. N ii is weak, suggesting that the C ii emission in our galaxies comes mostly from PDRs instead of the diffuse ionized interstellar medium (ISM). These systems exhibit efficient cooling at low dust temperatures, as shown by (O i+C ii)/TIR in relation to 60 m/100 m, and low O i/C ii ratios which indicate that C ii is the dominant coolant of the ISM. We observe O iii/C ii ratios in our galaxies that are lower than those published for other dwarfs, but similar to levels noted in spirals.
ABSTRACT We aim to explore whether strong magnetic fields can be effectively generated in low-mass dwarf galaxies and, if so, whether such fields can be affected by galactic outflows and spread out ...into the intergalactic medium (IGM). We performed a radio continuum polarimetry study of IC 10, the nearest starbursting dwarf galaxy, using a combination of multifrequency interferometric (VLA) and single-dish (Effelsberg) observations. VLA observations at 1.43 GHz reveal an extensive and almost spherical radio halo of IC 10 in total intensity, extending twice more than the infrared-emitting galactic disk. The halo is magnetized with a magnetic field strength of 7 G in the outermost parts. Locally, the magnetic field reaches about in H ii complexes, becomes more ordered, and weakens to in the synchrotron superbubble and to 7-10 G within H i holes. At the higher frequency of 4.86 GHz, we found a large-scale magnetic field structure of X-shaped morphology, similar to that observed in several edge-on spiral galaxies. The X-shaped magnetic structure can be caused by the galactic wind, advecting magnetic fields injected into the interstellar medium by stellar winds and supernova explosions. The radio continuum scale heights at 1.43 GHz indicate the bulk speed of cosmic-ray electrons outflowing from H ii complexes of about 60 km s−1, exceeding the escape velocity of 40 km s−1. Hence, the magnetized galactic wind in IC 10 inflates the extensive radio halo visible at 1.43 GHz and can seed the IGM with both random and ordered magnetic fields. These are signatures of intense material feedback onto the IGM, expected to be prevalent in the protogalaxies of the early universe.
CHANG-ES (Continuum Halos in Nearby Galaxies—an EVLA Survey) is an ambitious project to target 35 nearby disk galaxies that are edge-on to the line of sight. The orientation permits both the disk and ...halo regions to be studied. The observations were initially at 1.5 GHz (L-band) and 6.0 GHz (C-band) in a variety of VLA array configurations, and in all four Stokes parameters, which allowed for spatially resolved images in total intensity plus polarization. The inclusion of polarization is unique to an edge-on galaxy survey and reveals the galaxies’ halo magnetic fields. This paper will summarize the results to date, some of which are new phenomena, never seen prior to CHANG-ES. For example, we see that ‘X-type’ fields, as well as rotation measure reversals, are common features of spiral galaxies. Further observations at 3.0 GHz (S-band) as well as future scientific opportunities will also be described.
Aim. The vertical halo scale height is a crucial parameter to understand the transport of cosmic-ray electrons (CRE) and their energy loss mechanisms in spiral galaxies. Until now, the radio scale ...height could only be determined for a few edge-on galaxies because of missing sensitivity at high resolution. Methods. We developed a sophisticated method for the scale height determination of edge-on galaxies. With this we determined the scale heights and radial scale lengths for a sample of 13 galaxies from the CHANG-ES radio continuum survey in two frequency bands. Results. The sample average values for the radio scale heights of the halo are 1.1 ± 0.3 kpc in C-band and 1.4 ± 0.7 kpc in L-band. From the frequency dependence analysis of the halo scale heights we found that the wind velocities (estimated using the adiabatic loss time) are above the escape velocity. We found that the halo scale heights increase linearly with the radio diameters. In order to exclude the diameter dependence, we defined a normalized scale height h˜ which is quite similar for all sample galaxies at both frequency bands and does not depend on the star formation rate or the magnetic field strength. However, h˜ shows a tight anticorrelation with the mass surface density. Conclusions. The sample galaxies with smaller scale lengths are more spherical in the radio emission, while those with larger scale lengths are flatter. The radio scale height depends mainly on the radio diameter of the galaxy. The sample galaxies are consistent with an escape-dominated radio halo with convective cosmic ray propagation, indicating that galactic winds are a widespread phenomenon in spiral galaxies. While a higher star formation rate or star formation surface density does not lead to a higher wind velocity, we found for the first time observational evidence of a gravitational deceleration of CRE outflow, e.g. a lowering of the wind velocity from the galactic disk.
ABSTRACT
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, z = 0 galaxies with M⋆ ∼ $10^{10}\, \rm {M_\odot }$ show the strongest anisotropy (∼0.35 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.
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.
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