Nonthermal Radio Continuum Research Articles

A spatially resolved radio spectral study of the galaxy M 51

Context. Radio continuum emission from galaxies at gigahertz frequencies can be used as an extinction-free tracer of star formation. However, at frequencies of a few hundred megahertz, there is evidence for low-frequency spectral flattening. Aims. We wish to understand the origin of this low-frequency flattening better, and to this end, we performed a spatially resolved study of the nearby spiral galaxy M51. We explored the different effects that can cause a flattening of the spectrum towards lower frequencies, such as free–free absorption and cosmic-ray ionisation losses. Methods. We used radio continuum intensity maps between 54 and 8350 MHz at eight different frequencies, with observations at 240 MHz from the Giant Metrewave Radio Telescope presented for the first time. We corrected for the contribution from thermal free–free emission using an H α map that was corrected for extinction with 24 μm data. We fitted free–free absorption models to the radio spectra to determine the emission measure (EM) as well as polynomial functions to measure the non-thermal spectral curvature. We also obtained a new extinction-corrected H α intensity map from the Metal-THINGS survey using integral field unit spectroscopy. Results. The non-thermal low-frequency radio continuum spectrum between 54 and 144 MHz is very flat and even partially inverted, particularly in the spiral arms; in contrast, the spectrum at higher frequencies is typical for a non-thermal radio continuum spectrum. However, we did not find any correlation between the EMs calculated from radio and from H α observations; instead, the non-thermal spectral curvature weakly correlates with the H I gas-mass surface density. This suggests that cosmic-ray ionisation losses play an important role in the low-frequency spectral flattening. Conclusions. The observed spectral flattening towards low frequencies in M51 is caused by a combination of ionisation losses and free–free absorption. The reasons for this flattening need to be understood in order to use sub-gigahertz frequencies as a tracer of star formation.

Synchrotron signatures of cosmic ray transport physics in galaxies

ABSTRACT Cosmic rays (CRs) may drive outflows and alter the phase structure of the circumgalactic medium, with potentially important implications on galaxy formation. However, these effects ultimately depend on the dominant mode of transport of CRs within and around galaxies, which remains highly uncertain. To explore potential observable constraints on CR transport, we investigate a set of cosmological fire-2 CR-magnetohydrodynamic simulations of L* galaxies which evolve CRs with transport models motivated by self-confinement (SC) and extrinsic turbulence (ET) paradigms. To first order, the synchrotron properties diverge between SC and ET models due to a CR physics-driven hysteresis. SC models show a higher tendency to undergo ‘ejective’ feedback events due to a runaway buildup of CR pressure in dense gas due to the behaviour of SC transport scalings at extremal CR energy densities. The corresponding CR wind-driven hysteresis results in brighter, smoother, and more extended synchrotron emission in SC runs relative to ET and constant diffusion runs. The differences in synchrotron arise from different morphology, interstellar medium gas, and B properties, potentially ruling out SC as the dominant mode of CR transport in typical star-forming L* galaxies, and indicating the prospect for non-thermal radio continuum observations to constrain CR transport physics.

Deciphering the radio–star formation correlation on kpc scales

The relation between the resolved star formation rate (SFR) per unit area and the nonthermal radio continuum emission is studied in 21 Virgo cluster galaxies and the two nearby spiral galaxies, NGC 6946 and M 51. For the interpretation and understanding of our results, we used a 3D model where star formation, 2D cosmic-ray (CR) propagation, and the physics of synchrotron emission are included. Based on the linear correlation between the SFR per unit area and the synchrotron emission and its scatter, radio-bright and radio-dim regions can be robustly defined for our sample of spiral galaxies. We identified CR diffusion or streaming as the physical causes of radio-bright regions of unperturbed symmetric spiral galaxies as NGC 6946. The enhanced magnetic field in the region of interstellar medium (ISM) compression via ram pressure is responsible for the southwestern radio-bright region in NGC 4501. We identified the probable causes of radio-bright regions in several galaxies as CR transport, via either gravitational tides (M 51) or galactic winds (NGC 4532) or ram pressure stripping (NGC 4330 and NGC 4522). Three galaxies are overall radio dim: NGC 4298, NGC 4535, and NGC 4567. Based on our model of synchrotron-emitting disks, we suggest that the overall radio-dim galaxies have a significantly lower magnetic field than expected by equipartition between the magnetic and turbulent energy densities. We suggest that this is linked to the difference between the timescales of the variation in the SFR and the small-scale dynamo. In NGC 4535, shear motions increase the total magnetic field strength via the induction equation, which leads to enhanced synchrotron emission with respect to the SFR in an otherwise radio-dim galactic disk. Radio-bright regions frequently coincide with asymmetric ridges of polarized radio continuum emission, and we found a clear albeit moderate correlation between the polarized radio continuum emission and the radio/SFR ratio. When compression or shear motions of the ISM are present in the galactic disk, the radio-bright regions are linked to the commonly observed asymmetric ridges of polarized radio continuum emission and represent a useful tool for the interaction diagnostics. The magnetic field is enhanced (as observed in NGC 4535 and NGC 4501) and ordered by these ISM compression and shear motions. Whereas the enhancement of the magnetic field is rather modest and does not significantly influence the radio-SFR correlation, the main effect of ISM compression and shear motions is the ordering of the magnetic field, which significantly affects the CR transport. Cosmic-ray energy losses and transport also affect the spectral index, which we measured between 4.85 and 1.4 GHz. The influence of CR losses and transport on the spectral index distribution with respect to the synchrotron/SFR ratio is discussed with the help of model calculations. Based on our results, we propose a scenario for the interplay between star formation, CR electrons, and magnetic fields in spiral galaxies.

Evolution of thermal and non-thermal radio continuum emission on kpc scales – predictions for SKA

ABSTRACT Resolved maps of the thermal and non-thermal radio continuum (RC) emission of distant galaxies are a powerful tool for understanding the role of the interstellar medium (ISM) in the evolution of galaxies. We simulate the RC surface brightness of present-day star-forming galaxies in the past at 0.15 < z < 3, considering two cases of radio size evolution: (1) no evolution and (2) same evolution as in the optical. We aim to investigate the (a) structure of the thermal and non-thermal emission on kpc scales, (b) evolution of the thermal fraction and synchrotron spectrum at mid-radio frequencies (≃1–10 GHz), and (c) capability of the proposed SKA phase 1 MID-frequency (SKA1-MID) reference surveys in detecting the RC emitting structures. The synchrotron spectrum flattens with z causing curvature in the observed mid-radio SEDs of galaxies at higher z. The spectral index reported in recent observational studies agrees better with the no size evolution scenario. In this case, the mean thermal fraction observed at 1.4 GHz increases with redshift by more than 30 per cent from z = 0.15 to z = 2 because of the drop of the synchrotron emission at higher rest-frame frequencies. More massive galaxies have lower thermal fractions and experience a faster flattening of the non-thermal spectrum. The proposed SKA1-MID band 2 reference survey, unveils the ISM in M51- and NGC 6946-like galaxies (with ${\rm M_{\star }}\simeq 10^{10}\, {\rm M}_{\odot }$) up to z = 3. This survey detects lower-mass galaxies like M33 (${\rm M_{\star }}\simeq 10^{9}\, {\rm M}_{\odot }$) only at low redshifts z ≲ 0.5. For a proper separation of the RC emitting processes at the peak of star formation, it is vital to include band 1 into the SKA1-MID reference surveys.

A Novel Method for Estimating the Ambient Medium Density Around Distant Radio Sources from Their Observed Radio Spectra

Abstract The dynamical evolution and radiative properties of luminous radio galaxies and quasars of the FR II type, are well understood. As a result, through the use of detailed modeling of the observed radio emission of such sources, one can estimate various physical parameters of the systems, including the density of the ambient medium into which the radio structure evolves. This, however, requires rather comprehensive observational information, i.e., sampling the broadband radio continua of the targets at several frequencies, and imaging their radio structures with high resolution. Such observations are, on the other hand, not always available, especially for high-redshift objects. Here, we analyze the best-fit values of the source physical parameters, derived from extensive modeling of the largest currently available sample of FR II radio sources, for which good-quality multiwavelength radio flux measurements could be collected. In the analyzed data set, we notice a significant and nonobvious correlation between the spectral index of the nonthermal radio emission continuum, and density of the ambient medium. We derive the corresponding correlation parameters, and quantify the intrinsic scatter by means of Bayesian analysis. We propose that the discovered correlation could be used as a cosmological tool to estimate the density of ambient medium for large samples of distant radio galaxies. Our method does not require any detailed modeling of individual sources, and relies on limited observational information, namely, the slope of the radio continuum between the rest-frame frequencies 0.4 and 5 GHz, possibly combined with the total linear size of the radio structure.

Radio emission during the formation of stellar clusters in M 33

Aims. We investigate thermal and nonthermal radio emission associated with the early formation and evolution phases of young stellar clusters (YSCs) selected by their mid-infrared (MIR) emission at 24 μm in M 33. We consider regions in their early formation period, which are compact and totally embedded in the molecular cloud, and in the more evolved and exposed phase. Methods. Thanks to recent radio continuum surveys between 1.4 and 6.3 GHz we are able to find radio source counterparts to more than 300 star forming regions of M 33. We identify the thermal free–free component for YSCs and their associated molecular complexes using the Hα line emission. Results. A cross-correlation of MIR and radio continuum is established from bright to very faint sources, with the MIR-to-radio emission ratio that shows a slow radial decline throughout the M 33 disk. We confirm the nature of candidate embedded sources by recovering the associated faint radio continuum luminosities. By selecting exposed YSCs with reliable Hα flux, we establish and discuss the tight relation between Hα and the total radio continuum at 5 GHz over four orders of magnitude. This holds for individual YSCs as well as for the giant molecular clouds hosting them, and allows us to calibrate the radio continuum–star formation rate relation at small scales. On average, about half of the radio emission at 5 GHz in YSCs is nonthermal with large scatter. For exposed but compact YSCs and their molecular clouds, the nonthermal radio continuum fraction increases with source brightness, while for large HII regions the nonthermal fraction is lower and shows no clear trend. This has been found for YSCs with and without identified supernova remnants and underlines the possible role of massive stars in triggering particle acceleration through winds and shocks: these particles diffuse throughout the native molecular cloud prior to cloud dispersal.

Warped diffusive radio halo around the quiescent spiral edge-on galaxy NGC 4565

Context. Cosmic rays play a pivotal role in launching galactic winds, particularly in quiescently star-forming galaxies where the hot gas alone is not sufficient to drive a wind. Except for the Milky Way, not much is known about the transport of cosmic rays in galaxies. Aims. In this Letter, we present low-frequency observations of the nearby edge-on spiral galaxy NGC 4565 using the LOw-Frequency ARray (LOFAR). With our deep 144 MHz observations, we obtain a clean estimate of the emission originating from old cosmic-ray electrons (CRe), which is almost free from contamination by thermal emission. Methods. We measured vertical profiles of the non-thermal radio continuum emission that we fitted with Gaussian and exponential functions. The different profile shapes correspond to 1D cosmic-ray transport models of pure diffusion and advection, respectively. Results. We detect a warp in the radio continuum that is reminiscent of the previously known H I warp. Because the warp is not seen at GHz-frequencies in the radio continuum, its minimum age must be about 100 Myr. The warp also explains the slight flaring of the thick radio disc that can otherwise be well described by a Gaussian profile with an FWHM of 65 arcsec (3.7 kpc). Conclusions. The diffusive radio halo together with the extra-planar X-ray emission may be remnants of enhanced star-forming activity in the past where the galaxy had a galactic wind, as GHz-observations indicate only a weak outflow in the last 40 Myr. NGC 4565 could be in transition from an outflow- to an inflow-dominated phase.

ALMA, ATCA, and Spitzer Observations of the Luminous Extragalactic Supernova SN 1978K

Abstract Only three extragalactic supernovae have been detected at late times at millimeter wavelengths: SN 1987A, SN 1978K, and SN 1996cr. SN 1978K is a remarkably luminous Type IIn supernova that remains bright at all wavelengths 40 years after its explosion. Here, we present Atacama Large Millimeter/submillimeter Array (ALMA) observations taken in 2016 using Bands 3, 4, 6, and 7 that show a steepening in the spectrum. An absorbed single power-law model broadly fits all of the radio and millimeter observations, but would require significant chromatic variability. Alternatively, a broken power law fits the radio-millimeter spectrum; this can be explained using an ultra-relativistic spherical blast wave in a wind scaling with a cooling break, as in a gamma-ray burst afterglow. Using updated Australia Telescope Compact Array light curves, we show that the non-thermal radio continuum continues to decay as t −1.53; in the fireball model, this independently defines the power-law indices found in the radio-millimeter spectrum. Supernovae such as SN 1978K might be important contributors to the universal dust budget: only SN 1978K was detected in a search for warm dust in supernovae in the transitional phase (age 10–100 yr). Using Spitzer Space Telescope observations, we show that at least some of this dust emission has been decaying rapidly as t −2.45 over the past decade, suggesting it is being destroyed. Depending on the modeling of the synchrotron emission, the ALMA observations suggest there may be emission from a cold dust component.

Global correlations between the radio continuum, infrared, and CO emissions in dwarf galaxies

Correlations between the radio continuum, infrared and CO emission are known to exist for several types of galaxies and across several orders of magnitude. However, the low-mass, low-luminosity and low-metallicity regime of these correlations is not well known. A sample of metal-rich and metal-poor dwarf galaxies from the literature has been assembled to explore this extreme regime. The results demonstrate that the properties of dwarf galaxies are not simple extensions of those of more massive galaxies; the different correlations reflect different star-forming conditions and different coupling between the star formation and the various quantities. It is found that dwarfs show increasingly weaker CO and infrared emission for their luminosity, as expected for galaxies with a low dust content, slower reaction rates, and a hard ionizing radiation field. In the higher-luminosity dwarf regime (L_1.4GHz > 10^27 W, where L_1.4GHz ~ 10^29 W for a Milky Way star formation rate of ~1 M_sun yr^-1), the total and non-thermal radio continuum emission appear to adequately trace the star formation rate. A breakdown of the dependence of the (Halpha-based) thermal, non-thermal, and, hence, total radio continuum emission on star formation rate occurs below L_1.4GHz ~ 10^27 W, resulting in a steepening or downturn of the relations at extreme low luminosity. Below L_FIR ~ 10^36 W ~ 3 x 10^9 L_sun, the infrared emission ceases to adequately trace the star formation rate. A lack of a correlation between the magnetic field strength and the star formation rate in low star formation rate dwarfs suggests a breakdown of the equipartition assumption. As extremely metal-poor dwarfs mostly populate the low star formation rate and low luminosity regime, they stand out in their infrared, radio continuum and CO properties.

The Extraordinary Outburst in the Massive Protostellar System NGC 6334I-MM1: Emergence of Strong 6.7 GHz Methanol Masers

Abstract We report the first sub-arcsecond VLA imaging of 6 GHz continuum, methanol maser, and excited-state hydroxyl maser emission toward the massive protostellar cluster NGC 6334I following the recent 2015 outburst in (sub)millimeter continuum toward MM1, the strongest (sub)millimeter source in the protocluster. In addition to detections toward the previously known 6.7 GHz Class II methanol maser sites in the hot core MM2 and the UCHII region MM3 (NGC 6334F), we find new maser features toward several components of MM1, along with weaker features ∼1″ north, west, and southwest of MM1, and toward the nonthermal radio continuum source CM2. None of these areas have heretofore exhibited Class II methanol maser emission in three decades of observations. The strongest MM1 masers trace a dust cavity, while no masers are seen toward the strongest dust sources MM1A, 1B, and 1D. The locations of the masers are consistent with a combination of increased radiative pumping due to elevated dust grain temperature following the outburst, the presence of infrared photon propagation cavities, and the presence of high methanol column densities as indicated by ALMA images of thermal transitions. The nonthermal radio emission source CM2 (2″ north of MM1) also exhibits new maser emission from the excited 6.035 and 6.030 GHz OH lines. Using the Zeeman effect, we measure a line-of-sight magnetic field of +0.5 to +3.7 mG toward CM2. In agreement with previous studies, we also detect numerous methanol and excited OH maser spots toward the UCHII region MM3, with predominantly negative line-of-sight magnetic field strengths of −2 to −5 mG and an intriguing south–north field reversal.

Exploring the making of a galactic wind in the starbursting dwarf irregular galaxy IC 10 with LOFAR

Low-mass galaxies are subject to strong galactic outflows, in which cosmic rays may play an important role, they can be best traced with low-frequency radio continuum observations, which are less affected by spectral ageing. We present a study of the nearby star burst dwarf irregular galaxy IC 10 using observations at 140 MHz with the LOw-Frequency ARray (LOFAR), at 1580 MHz with the Very Large Array (VLA) and at 6200 MHz with the VLA and the 100-m Effelsberg telescope. We find that IC 10 has a low-frequency radio halo, which manifests itself as a second component (thick disc) in the minor axis profiles of the non-thermal radio continuum emission at 140 and 1580 MHz. These profiles are then fitted with 1D cosmic-ray transport models for pure diffusion and advection. We find that a diffusion model fits best, with a diffusion coefficient of $D=(0.4$-$0.8) \times 10^{26}(E/{\rm GeV})^{0.5}~{\rm cm^2\,s^{-1}}$, which is at least an order of magnitude smaller than estimates both from anisotropic diffusion and the diffusion length. In contrast, advection models, which cannot be ruled out due to the mild inclination, while providing poorer fits, result in advection speeds close to the escape velocity of $\approx$$50~\rm km\,s^{-1}$, as expected for a cosmic-ray driven wind. Our favoured model with an accelerating wind provides a self-consistent solution, where the magnetic field is in energy equipartition with both the warm neutral and warm ionized medium with an important contribution from cosmic rays. Consequently, cosmic rays can play a vital role for the launching of galactic winds in the disc--halo interface.

A Radio Continuum Study of Dwarf Galaxies: 6 cm Imaging of LITTLE THINGS

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 $3$-$8^{\prime\prime}$ resolution and noise levels of $3$-$15{\rm \,\mu Jy\,beam^{-1}}$. We detected emission associated with $22$ of the $40$ dwarf galaxies, $8$ 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 $\sim 50$-$70$% and an average magnetic field strength of $\sim 5$-$8\,{\rm \mu G}$, 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$\,{\rm \mu G}$. 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 non-thermal 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 centimetre 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 $\pm 0.2$ dex between the values of $2 \times 10^{-4}$ and $0.1 {\rm M_\odot\,yr^{-1}}$.

THE MASSIVE PROTOSTELLAR CLUSTER NGC 6334I AT 220 au RESOLUTION: DISCOVERY OF FURTHER MULTIPLICITY, DIVERSITY, AND A HOT MULTI-CORE

ABSTRACT We present Very Large Array and Atacama Large Millimeter/submillimeter Array imaging of the deeply embedded protostellar cluster NGC 6334I from 5 cm to 1.3 mm at angular resolutions as fine as 0.″17 (220 au). The dominant hot core MM1 is resolved into seven components at 1.3 mm, clustered within a radius of 1000 au. Four of the components have brightness temperatures >200 K, radii ∼300 au, minimum luminosities ∼104 L ⊙, and must be centrally heated. We term this new phenomenon a “hot multi-core.” Two of these objects also exhibit compact free–free emission at longer wavelengths, consistent with a hypercompact H ii region (MM1B) and a jet (MM1D). The spatial kinematics of the water maser emission centered on MM1D are consistent with it being the origin of the high-velocity bipolar molecular outflow seen in CO. The close proximity of MM1B and MM1D (440 au) suggests a proto-binary or a transient bound system. Several components of MM1 exhibit steep millimeter spectral energy distributions indicative of either unusual dust spectral properties or time variability. In addition to resolving MM1 and the other hot core (MM2) into multiple components, we detect five new millimeter and two new centimeter sources. Water masers are detected for the first time toward MM4A, confirming its membership in the protocluster. With a 1.3 mm brightness temperature of 97 K coupled with a lack of thermal molecular line emission, MM4A appears to be a highly optically thick 240 L ⊙ dust core, possibly tracing a transient stage of massive protostellar evolution. The nature of the strongest water maser source CM2 remains unclear due to its combination of non-thermal radio continuum and lack of dust emission.

G192.8-1.1: A CANDIDATE OF AN EVOLVED THERMAL COMPOSITE SUPERNOVA REMNANT REIGNITED BY NEARBY MASSIVE STARS

G192.8-1.1 has been known as one of the faintest supernova remnants (SNRs) in the Galax until the radio continuum of G192.8-1.1 is proved to be thermal by Gao et al. (2011). Yet, the nature of G192.8-1.1 has not been fully investigated. Here, we report the possible discovery of faint non-thermal radio continuum components with a spectral index <TEX>${\alpha}{\sim}0.56(S_{\nu}{\propto}{\nu}^{-{\alpha}})$</TEX> around G192.8-1.1, while of the radio continuum emission is thermal. Also, our Arecibo <TEX>$H_I$</TEX> data reveal an <TEX>$H_I$</TEX> shell, expanding with an expansion velocity of <TEX>$20-60km\;s^{-1}$</TEX>, that has an excellent morphological correlation with the radio continuum emission. The estimated physical parameters of the <TEX>$H_I$</TEX> shell and the possible association of non-thermal radio continuum emission with it suggest G192.8-1.1 to be an~0.3 Myr-old SNR. However, the presence of thermal radio continuum implies the presence of early-type stars in the same region. One possibility is that a massive star is ionizing the interior of an old SNR. If it is the case, the electron distribution assumed by the centrally-peaked surface brightness of thermal emission implies that G192.8-1.1 is a "thermal-composite" SNR, rather than a typical shell-type SNR, where the central hot gas that used to be bright in X-rays has cooled down. Therefore, we propose that G192.8-1.1 is an old evolved thermal-composite SNR showing recurring emission in the radio continuum due to a nearby massive star. The infrared image supports that the <TEX>$H_I$</TEX> shell of G192.8-1.1 is currently encountering a nearby star forming region that possibly contains an early type star(s).

AN ESTIMATE OF THE MAGNETIC FIELD STRENGTH ASSOCIATED WITH A SOLAR CORONAL MASS EJECTION FROM LOW FREQUENCY RADIO OBSERVATIONS

We report ground based, low frequency heliograph (80 MHz), spectral (85-35 MHz) and polarimeter (80 and 40 MHz) observations of drifting, non-thermal radio continuum associated with the `halo' coronal mass ejection (CME) that occurred in the solar atmosphere on 2013 March 15. The magnetic field strengths ($B$) near the radio source were estimated to be $B \approx 2.2 \pm 0.4$ G at 80 MHz and $B \approx 1.4 \pm 0.2$ G at 40 MHz. The corresponding radial distances ($r$) are $r \approx 1.9~R_{\odot}$ (80 MHz) and $r \approx 2.2~R_{\odot}$ (40 MHz).

INTERSTELLAR PROTONS IN THE TeV γ-RAY SNR HESS J1731-347: POSSIBLE EVIDENCE FOR THE COEXISTENCE OF HADRONIC AND LEPTONIC γ-RAYS

HESS J1731-347 (G353.6-0.7) is one of the TeV gamma-ray SNRs which shows the shell-like morphology. We have made a new analysis of the interstellar protons toward the SNR by using both the 12CO(J=1-0) and HI datasets. The results indicate that the TeV gamma-ray shell shows significant spatial correlation with the interstellar protons at a velocity range from -90 km/s to -75 km/s, and the distance corresponding to the velocity range is ~5.2 kpc, a factor of 2 larger than the previous figure 3 kpc. The total mass of the interstellar protons is estimated to be 6.4x10^4 M_sun, 25 % of which is atomic gas. We have identified the cold HI gas observed as self-absorption which shows significant correspondence with the northeastern gamma-ray peak. While the good correspondence between the interstellar protons and TeV gamma-rays in the north of the SNR lends support to the hadronic scenario for the TeV gamma-rays, the southern part of the shell shows a break in the correspondence; in particular, the southwestern rim of the SNR shell shows a significant decrease of the interstellar protons by a factor of 2. We argue that this discrepancy can be explained as due to leptonic gamma-rays, because this region well coincides with the bright shell which emit non-thermal radio continuum emission and non-thermal X-rays, suggesting that the gamma-rays of HESS J1713-347 consist of both the hadronic and leptonic components. The leptonic contribution then corresponds to ~20 % of the total gamma-rays. The total energy of cosmic-ray protons is estimated to be 10^49 erg for the gamma-ray energy range of 1 GeV - 100 TeV by assuming that 80 % of the total gamma-ray is of the hadronic origin.

Thermal and non-thermal emission in the Cygnus X region

Radio continuum observations detect non-thermal synchrotron and thermal bremsstrahlung radiation. Separation of the two different emission components is crucial to study the properties of diffuse interstellar medium. The Cygnus X region is one of the most complex areas in the radio sky which contains a number of massive stars and HII regions on the diffuse thermal and non-thermal background. More supernova remnants are expected to be discovered. We aim to develop a method which can properly separate the non-thermal and thermal radio continuum emission and apply it to the Cygnus X region. The result can be used to study the properties of different emission components and search for new supernova remnants in the complex. Multi-frequency radio continuum data from large-scale surveys are used to develop a new component separation method. Spectral analysis is done pixel by pixel for the non-thermal synchrotron emission with a realistic spectral index distribution and a fixed spectral index of beta = -2.1 for the thermal bremsstrahlung emission. With the new method, we separate the non-thermal and thermal components of the Cygnus X region at an angular resolution of 9.5arcmin. The thermal emission component is found to comprise 75% of the total continuum emission at 6cm. Thermal diffuse emission, rather than the discrete HII regions, is found to be the major contributor to the entire thermal budget. A smooth non-thermal emission background of 100 mK Tb is found. We successfully make the large-extent known supernova remnants and the HII regions embedded in the complex standing out, but no new large SNRs brighter than Sigma_1GHz = 3.7 x 10^-21 W m^-2 Hz^-1 sr^-1 are found.

LOW-FREQUENCY RADIO-FIR CORRELATION IN NORMAL GALAXIES AT ∼1 kpc SCALES

We study the radio--FIR correlation between the nonthermal (synchrotron) radio continuum emission at \lambda 90 cm (333 MHz) and the far infrared emission due to cool (~20 K) dust at \lambda 70\mu m in spatially resolved normal galaxies at scales of ~1 kpc. The slope of the radio--FIR correlation significantly differs between the arm and interarm regions. However, this change is not evident at a lower wavelength of \lambda 20 cm (1.4 GHz). We find the slope of the correlation in the arm to be 0.8 \pm 0.12 and we use this to determine the coupling between equipartition magnetic field (B_{eq}) and gas density (\rho_{gas}) as B_{eq} \propto \rho_{gas}^{0.51 \pm 0.12}. This is close to what is predicted by MHD simulations of turbulent ISM, provided the same region produces both the radio and far infrared emission. We argue that at 1 kpc scales this condition is satisfied for radio emission at 1.4 GHz and may not be satisfied at 333 MHz. Change of slope observed in the interarm region could be caused by propagation of low energy (~1.5 GeV) and long lived (~ 10^8 yr) cosmic ray electrons at 333 MHz.

FeII] as a tracer supernova rate

Supernovae play an integral role in the feedback of processed material into the ISM of galaxies and are responsible for most of the chemical enrichment of the universe. The rate of supernovae can also reveal the star formation histories. Supernova rate is usually measured through the non-thermal radio continuum luminosity, but in this paper we establish a quantitative relationship between the [FeII]1.26 luminosity and supernova rate in a sample of 11 near-by starburst galaxies. SINFONI data cubes are used to perform a pixel pixel analysis of this correlation. Using Bry equivalent width and luminosity as the only observational inputs into Starburst 99, the supernova rate is derived at each pixel and a map of supernova rate is created. This is then compared morphologically and quantitatively to [FeII]1.26 luminosity map. We find a strong linear and morphological correlation between supernova rate and [FeII]1.26 on a pixel-pixel basis: The Starburst 99 derived supernova rates are also in good agreement with the radio derived supernova rates, which further demonstrates the strength of [FeII] as a tracer of supernova rate. With the strong correlation found in this sample of galaxies, we now qualitatively use [FeII]1.26 to derive supernova rate on either a pixel-pixel or integrated galactic basis.

Non-thermal insights on mass and energy flows through the Galactic Centre and into the Fermi bubbles

We construct a simple model of the star-formation- (and resultant supernova-) driven mass and energy flows through the inner ~200 pc (in diameter) of the Galaxy. Our modelling is constrained, in particular, by the non-thermal radio continuum and {\gamma}-ray signals detected from the region. The modelling points to a current star-formation rate of 0.04 - 0.12 M\msun/year at 2{\sigma} confidence within the region with best-fit value in the range 0.08 - 0.12 M\msun/year which - if sustained over 10 Gyr - would fill out the ~ 10^9 M\msun stellar population of the nuclear bulge. Mass is being accreted on to the Galactic centre (GC) region at a rate ~0.3M\msun/year. The region's star-formation activity drives an outflow of plasma, cosmic rays, and entrained, cooler gas. Neither the plasma nor the entrained gas reaches the gravitational escape speed, however, and all this material fountains back on to the inner Galaxy. The system we model can naturally account for the recently-observed ~> 10^6 'halo' of molecular gas surrounding the Central Molecular Zone out to 100-200 pc heights. The injection of cooler, high-metallicity material into the Galactic halo above the GC may catalyse the subsequent cooling and condensation of hot plasma out of this region and explain the presence of relatively pristine, nuclear-unprocessed gas in the GC. The plasma outflow from the GC reaches a height of a few kpc and is compellingly related to the recently-discovered Fermi Bubbles. Our modelling demonstrates that ~ 10^9 M\msun of hot gas is processed through the GC over 10 Gyr. We speculate that the continual star-formation in the GC over the age of the Milky Way has kept the SMBH in a quiescent state thus preventing it from significantly heating the coronal gas, allowing for the continual accretion of gas on to the disk and the sustenance of star formation on much wider scales in the Galaxy [abridged].