arXiv subjects

Rainer Beck

Publications and source records attributed to Rainer Beck.

At least 73 records · Page 4Linked to original sources

arXiv2015

Probing the nature of Dark Matter with the SKA

Dark Matter (DM) is a fundamental ingredient of our Universe and of structure formation, and yet its nature is elusive to astrophysical probes. Information on the nature and physical properties of the WIMP (neutralino) DM (the leading candidate for a cosmologically relevant DM) can be obtained by studying the astrophysical signals of their annihilation/decay. Among the various e.m. signals, secondary electrons produced by neutralino annihilation generate synchrotron emission in the magnetized atmosphere of galaxy clusters and galaxies which could be observed as a diffuse radio emission (halo or haze) centered on the DM halo. A deep search for DM radio emission with SKA in local dwarf galaxies, galaxy regions with low star formation and galaxy clusters (with offset DM-baryonic distribution, like e.g. the Bullet cluster) can be very effective in constraining the neutralino mass, composition and annihilation cross-section. For the case of a dwarf galaxy, like e.g. Draco, the constraints on the DM annihilation cross-section obtainable with SKA1-MID will be at least a factor $\sim 10^3$ more stringent than the limits obtained by Fermi-LAT in the $γ$-rays. These limits scale with the value of the B field, and the SKA will have the capability to determine simultaneously both the magnetic field in the DM-dominated structures and the DM particle properties. The optimal frequency band for detecting the DM-induced radio emission is around $\sim 1$ GHz, with the SKA1-MID Band 1 and 4 important to probe the synchrotron spectral curvature at low-$ν$ (sensitive to DM composition) and at high-$ν$ (sensitive to DM mass).

astro-ph.HE↗

arXiv2014

Discovery of Carbon Radio Recombination Lines in M82

Carbon radio recombination lines (RRLs) at low frequencies (<=500 MHz) trace the cold, diffuse phase of the interstellar medium, which is otherwise difficult to observe. We present the detection of carbon RRLs in absorption in M82 with LOFAR in the frequency range of 48-64 MHz. This is the first extragalactic detection of RRLs from a species other than hydrogen, and below 1 GHz. Since the carbon RRLs are not detected individually, we cross-correlated the observed spectrum with a template spectrum of carbon RRLs to determine a radial velocity of 219 +- 9 km/s . Using this radial velocity, we stack 22 carbon-α transitions from quantum levels n = 468-508 to achieve an 8.5 sigma detection. The absorption line profile exhibits a narrow feature with peak optical depth of 0.003 and FWHM of 31 km/s. Closer inspection suggests that the narrow feature is superimposed on a broad, shallow component. The total line profile appears to be correlated with the 21 cm H I line profile reconstructed from H I absorption in the direction of supernova remnants in the nucleus. The narrow width and centroid velocity of the feature suggests that it is associated with the nuclear starburst region. It is therefore likely that the carbon RRLs are associated with cold atomic gas in the direction of the nucleus of M82.

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arXiv2014

The magnetic field structure of the central region in M31

The Andromeda Galaxy (M31) is the nearest grand-design spiral galaxy. Thus far most studies in the radio regime concentrated on the 10 kpc ring. The central region of M31 has significantly different properties than the outer parts: The star formation rate is low, and inclination and position angle are largely different from the outer disk. The existing model of the magnetic field in the radial range 6<=r<=14 kpc is extended to the innermost part r<=0.5 kpc to ultimately achieve a picture of the entire magnetic field in M31. We combined observations taken with the VLA at 3.6 cm and 6.2 cm with data from the Effelsberg 100-m telescope to fill the missing spacings of the synthesis data. The resulting polarization maps were averaged in sectors to analyse the azimuthal behaviour of the polarized intensity (PI), rotation measure (RM), and apparent pitch angle (ϕ_obs). We developed a simplified 3-D model for the magnetic field in the central region to explain the azimuthal behaviour of the three observables. Our 3-D model of a quadrupolar or dipolar dynamo field can explain the observed patterns in PI, RM, and ϕ_obs, while a 2-D configuration is not sufficient to explain the azimuthal behaviour. In addition and independent of our model, the RM pattern shows that the spiral magnetic field in the inner 0.5 kpc points outward, which is opposite to that in the outer disk, and has a pitch angle of about 33 degrees, which is much larger than that of 8-19 degrees in the outer disk. The physical conditions in the central region differ significantly from those in the 10 kpc ring. In addition, the orientation of this region with respect to the outer disk is completely different. The opposite magnetic field directions suggest that the central region is decoupled from the outer disk, and we propose that an independent dynamo is active in the central region.

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arXiv2014

Do magnetic fields influence gas rotation in galaxies?

We aim to estimate the contribution of the radial component of the Lorentz force to the gas rotation in several types of galaxies. Using typical parameters for the exponential scale of synchrotron emission and the scale length of HI gas, under the assumption of equipartition between the energies of cosmic rays and total magnetic fields, we derive the Lorentz force and compare it to the gravitational force in the radial component of the momentum equation. We distinguish the different contributions between the large-scale and the small-scale turbulent fields by Reynolds averaging. We compare these findings with a dynamical dynamo model. We find a possible reduction of circular gas velocity in the very outer parts and an increase inside a radius of four times the synchrotron scale length. Sufficiently localized radial reversals of the magnetic field may cause characteristic modulations in the gas rotation curve with typical amplitudes of 10-20 km/s. It is unlikely that the magnetic field contributes to the flat rotation in the outer parts of galaxies. If anything, it will \emph{impede} the gravitationally supported rotation, demanding for an even higher halo mass to explain the observed rotation profile. We speculate that this may have consequences for ram pressure stripping and the truncation of the stellar disc.

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arXiv2014

Enhancement of magnetic fields arising from galactic encounters

Galactic encounters are usually marked by a substantial increase of synchrotron emission of the interacting galaxies compared to the typical emission from similar isolated galaxies. This is believed to be associated with an increase of the star formation rate and the associated turbulent magnetic fields. The regular magnetic field is usually believed to decrease. We consider a simple, however rather realistic, mean-field galactic dynamo model where the effects of small-scale generation are represented by random injections of magnetic field from star forming regions. We represent an encounter by the introduction of large-scale streaming velocities and by an increase in small-scale magnetic field injections. The latter describes the effect of an increase of the star formation rate caused by the encounter. We demonstrate that large-scale streaming, with associated deviations in the rotation curve, can result in an enhancement of the anisotropic turbulent (ordered) magnetic field strength, mainly along the azimuthal direction, leading to a significant temporary increase of the total magnetic energy during the encounter; the representation of an increase in star formation rate has an additional strong effect. In contrast to expectations, the large-scale (regular) magnetic field structure is not significantly destroyed by the encounter. It may be somewhat weakened for a relatively short period, and its direction after the encounter may be reversed. The encounter causes enhanced total and polarized emission without increase of the regular magnetic field strength. The increase of synchrotron emission caused by the large-scale streaming can be comparable to the effect of the increase of the star formation rate, depending on the choice of parameters.The effects of the encounter on the total magnetic field energy last only slightly longer than the duration of the encounter (ca. 1 Gyr).

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arXiv2014

The Radio Continuum-Star Formation Rate Relation in WSRT SINGS Galaxies

We study the spatially resolved Radio Continuum-Star Formation Rate (RC-SFR) relation using state-of-the-art star-formation (SF) tracers in a sample of 17 THINGS galaxies. We use hybrid Sigma_SFR maps (GALEX FUV plus Spitzer 24 mu), RC maps at 22/18 cm from the WSRT SINGS survey, and H-alpha maps to correct for thermal RC emission. We compare azimuthally averaged radial profiles of the RC and FUV/MIR-based Sigma_SFR maps and study pixel-by-pixel correlations at fixed linear scales of 1.2 and 0.7 kpc. The ratio of the integrated SFRs from the RC emission to that of the FUV/MIR-based SF tracers is R_int = 0.78 +/- 0.38, consistent with Condon's relation. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based Sigma_SFR for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based Sigma_SFR agrees with the integrated ratio with only small quasi-random fluctuations as function of radius. Pixel-by-pixel plots show a tight correlation in log-log diagrams of radio to FUV/MIR-based Sigma_SFR, with a typical standard deviation of a factor of two. Averaged over our sample we find (Sigma_SFR)_RC ~ (Sigma_SFR)_hyb^{0.63+/-0.25} implying that data points with high Sigma_SFR are relatively radio dim, whereas the reverse is true for low Sigma_SFR. We interpret this as a result of spectral ageing of CRe, which is supported by the radio spectral index: data points dominated by young CRe are relatively radio dim, those dominated by old CRe are relatively radio bright. The ratio of radio to FUV/MIR-based integrated SFR is independent of global galaxy parameters, suggesting that we can use RC emission as a universal SF tracer for galaxies, if we restrict ourselves to global or azimuthally averaged measurements. A magnetic field-SFR relation, B ~ SFR_hyb^{0.30+/-0.02}, holding both globally and locally, can explain our results. (abridged)

astro-ph.GA↗

arXiv2014

A Wideband Polarization Survey of the Extragalactic Sky at 2-4 GHz: A Science White Paper for the VLA Sky Survey

A VLA Sky Survey of the extragalactic sky at S band (2-4 GHz) with polarization information can uniquely probe the magneto-ionic medium in a wide range of astrophysical environments over cosmic time. For a shallow all-sky survey, we expect to detect over 4 million sources in total intensity $>$ 0.45 mJy beam$^{-1}$ and over 2.2$\times$10$^5$ sources in polarized intensity. With these new observations, we expect to discover new classes of polarized radio sources in very turbulent astrophysical environments and those with extreme values of Faraday depth. Moreover, by determining reliable Faraday depths and by modeling depolarization effects, we can derive properties of the magneto-ionic medium associated with AGNs, absorption line systems and galaxies, addressing the following unresolved questions: (1) What is the covering fraction, the degree of turbulence and the origin of absorption line systems? (2) What is the thermal content in AGNs and radio galaxies? (3) How do AGNs and galaxies evolve over cosmic time? (4) What causes the increase in percentage polarization with decreasing flux densities at the low flux density end of the polarized source count? (5) What is the growth rate of large-scale magnetic fields in galaxies?

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arXiv2013

CHANG-ES III: UGC10288 -- An Edge-on Galaxy with a Background Double-lobed Radio Source

This 3rd paper in the CHANG-ES series shows the first results from our regular data taken with the Karl G. Jansky Very Large Array (JVLA). The edge-on galaxy, UGC 10288, has been observed in the B, C, and D configurations at L-band (1.5 GHz) and in the C and D configurations at C-band (6 GHz) in all polarization products. We show the first spatially resolved images in these bands,the first polarization images, and the first composed image at an intermediate frequency (4.1 GHz) which has been formed from a combination of all data sets. A surprising new result is the presence of a strong, polarized, double-lobed extragalactic radio source ({\it CHANG-ES A}) almost immediately behind the galaxy and perpendicular to its disk. The core of {\it CHANG-ES A} has an optical counterpart at a photometric redshift of $z_{phot}\,=\,0.39$; the southern radio lobe is behind the disk of UGC 10288 and the northern lobe is behind the halo region. This background `probe' has allowed us to do a preliminary Faraday Rotation analysis of the foreground galaxy, putting limits on the regular magnetic field and electron density in the halo of UGC 10288 in regions in which there is no direct detection of a radio continuum halo. We have revised the flux densities of the two sources individually as well as the star formation rate (SFR) for UGC 10288. UGC 10288 would have fallen well below the CHANG-ES flux density cutoff, had it been considered without the brighter contribution of the background source. UGC 10288 shows discrete high-latitude radio continuum features, but it does not have a {\it global} radio continuum halo. The total minimum magnetic field strength at a sample position in the arc is $\sim$ 10 $μ$G. Thus, this galaxy still appears to be able to form substantial high latitude, localized features in spite of its relatively low SFR.

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arXiv2013

Polarized synchrotron radiation from the Andromeda Galaxy M31 and background sources at 350 MHz

Polarization measurements at low radio frequencies allow detection of small Faraday rotation measures caused by regular magnetic fields in galaxies and in the foreground of the Milky Way. The galaxy M31 was observed in two overlapping pointings with the Westerbork Synthesis Radio Telescope (WSRT) resulting in ~4' resolution in total intensity and linearly polarized emission. The frequency range 310-376 MHz was covered by 1024 channels which allowed the application of RM synthesis. We derived a data cube in Faraday depth and compared two symmetric ranges of negative and positive Faraday depths. This new method avoids the range of high instrumental polarization and allows the detection of very low degrees of polarization. For the first time, diffuse polarized emission from a nearby galaxy is detected below 1 GHz. The degree of polarization is only 0.21 +/- 0.05 %, consistent with extrapolation of internal depolarization from data at higher radio frequency. A catalogue of 33 polarized sources and their Faraday rotation in the M31 field is presented. Their average depolarization is DP(90,20) = 0.14 +/- 0.02, 7 times stronger depolarized than at 1.4 GHz. We argue that this strong depolarization originates within the sources, e.g. in their radio lobes, or in intervening galaxies on the line of sight. On the other hand, the Faraday rotation of the sources is mostly produced in the foreground of the Milky Way and varies significantly across the ~9 square degree M31 field. As expected, polarized emission from M31 and extragalactic background sources is much weaker at low frequencies compared to the GHz range. Future observations with LOFAR, with high sensitivity and high angular resolution to reduce depolarization, may reveal diffuse polarization from the outer disks and halos of galaxies.

astro-ph.CO↗

arXiv2013

An observational test for correlations between cosmic rays and magnetic fields

We derive the magnitude of fluctuations in total synchrotron intensity in the Milky Way and M33, from both observations and theory under various assumption about the relation between cosmic rays and interstellar magnetic fields. Given the relative magnitude of the fluctuations in the Galactic magnetic field (the ratio of the rms fluctuations to the mean magnetic field strength) suggested by Faraday rotation and synchrotron polarization, the observations are inconsistent with local energy equipartition between cosmic rays and magnetic fields. Our analysis of relative synchrotron intensity fluctuations indicates that the distribution of cosmic rays is nearly uniform at the scales of the order of and exceeding $100\p$, in contrast to strong fluctuations in the interstellar magnetic field at those scales. A conservative upper limit on the ratio of the the fluctuation magnitude in the cosmic ray number density to its mean value is 0.2--0.4 at scales of order 100\,pc. Our results are consistent with a mild anticorrelation between cosmic-ray and magnetic energy densities at these scales, in both the Milky Way and M33. Energy equipartition between cosmic rays and magnetic fields may still hold, but at scales exceeding 1\,kpc. Therefore, we suggest that equipartition estimates be applied to the observed synchrotron intensity smoothed to a linear scale of kiloparsec order (in spiral galaxies) to obtain the cosmic ray distribution and a large-scale magnetic field. Then the resulting cosmic ray distribution can be used to derive the fluctuating magnetic field strength from the data at the original resolution. The resulting random magnetic field is likely to be significantly stronger than existing estimates.

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arXiv2013

How cosmic-ray electron propagation affects radio-far-infrared correlations in M31 and M33

We investigate the effect of propagation of cosmic-ray electrons (CRE) on the nonthermal (synchrotron) - far-infrared correlations in M31 and M33. The thermal (TH) and nonthermal (NTH) emission components of the radio continuum emission at 1.4 GHz and one higher frequency are compared with dust emission from M31 and M33 using Spitzer data. In both galaxies the TH emission is linearly correlated with the emission from warm dust (24 μm, 70 μm), but the power laws of the NTH-FIR correlations have exponents b < 1 that increase with increasing frequency. Furthermore, the values of b for M33 are significantly smaller (b ~ 0.4) than those for M31 (b ~ 0.6). We interpret the differences in b as differences in the diffusion length of the CRE. We estimate the diffusion length in two ways: (1) by smoothing the NTH emission at the higher frequency until the correlation with NTH emission at 1.4 GHz has b = 1, and (2) by smoothing the TH emission until the correlation with the NTH emission at the same frequency has b = 1, assuming that the TH emission represents the source distribution of the CRE. Our smoothing experiments show that M31 only has a thin NTH disk with a scale height of h = 0.3-0.4 kpc at 1.4 GHz, whereas M33 has a similar thin disk as well as a thick disk with scale height h_thick ~ 2 kpc. In the thin disks, the (deprojected) diffusion length at 1.4 GHz is ~ 1.5 kpc, yielding a diffusion coefficient of ~ 2 10^28 cm^2/s. The structure, strength and regularity of the magnetic field in a galaxy as well as the existence of a thick disk determine the diffusion of the CRE, and hence, the power-law exponent of the NTH-FIR correlations.

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arXiv2013

A new interpretation of the far-infrared - radio correlation and the expected breakdown at high redshift

(Abrigded) Observations of galaxies up to z 2 show a tight correlation between far-infrared and radio continuum emission. We explain the far-infrared - radio continuum correlation by relating star formation and magnetic field strength in terms of turbulent magnetic field amplification, where turbulence is injected by supernova explosions from massive stars. We calculate the expected amount of turbulence in galaxies based on their star formation rates, and infer the expected magnetic field strength due to turbulent dynamo amplification. We estimate the timescales for cosmic ray energy losses via synchrotron emission, inverse Compton scattering, ionization and bremsstrahlung emission, probing up to which redshift strong synchrotron emission can be maintained. We find that the correlation between star formation rate and magnetic field strength in the local Universe can be understood as a result of turbulent magnetic field amplification. If the typical gas density in the interstellar medium increases at high z, we expect an increase of the magnetic field strength and the radio emission, as indicated by current observations. Such an increase would imply a modification of the far-infrared - radio correlation. We expect a breakdown when inverse Compton losses start dominating over synchrotron emission. For a given star formation surface density, we calculate the redshift where the breakdown occurs, yielding z (Sigma_SFR/0.0045 M_solar kpc^{-2} yr^{-1})^{1/(6-alpha/2)}. In this relation, the parameter αdescribes the evolution of the characteristic ISM density in galaxies as (1+z)^α. Both the possible raise of the radio emission at high redshift and the final breakdown of the far-infrared -- radio correlation at a critical redshift will be probed by the Square Kilometre Array (SKA) and its pathfinders, while the typical ISM density in galaxies will be probed with ALMA.

astro-ph.CO↗

arXiv2013

The LOFAR View of Cosmic Magnetism

The origin of magnetic fields in the Universe is an open problem in astrophysics and fundamental physics. Polarization observations with the forthcoming large radio telescopes will open a new era in the observation of magnetic fields and should help to understand their origin. At low frequencies, LOFAR (10-240 MHz) will allow us to map the structure of weak magnetic fields in the outer regions and halos of galaxies, in galaxy clusters and in the Milky Way via their synchrotron emission. Even weaker magnetic fields can be measured at low frequencies with help of Faraday rotation measures. A detailed view of the magnetic fields in the local Milky Way will be derived by Faraday rotation measures from pulsars. First promising images with LOFAR have been obtained for the Crab pulsar-wind nebula, the spiral galaxy M51, the radio galaxy M87 and the galaxy clusters A2255 and A2256. With help of the polarimetric technique of "Rotation Measure Synthesis", diffuse polarized emission has been detected from a magnetic bubble in the local Milky Way. Polarized emission and rotation measures were measured for more than 20 pulsars so far.

astro-ph.IM↗

arXiv2013

Characterizing Magnetized Turbulence in M51

We use previously published high-resolution synchrotron polarization data to perform an angular dispersion analysis with the aim of charactering magnetized turbulence in M51. We first analyze three distinct regions (the center of the galaxy, and the northwest and southwest spiral arms) and can clearly discern the turbulent correlation length scale from the width of the magnetized turbulent correlation function for two regions and detect the imprint of anisotropy in the turbulence for all three. Furthermore, analyzing the galaxy as a whole allows us to determine a two-dimensional Gaussian model for the magnetized turbulence in M51. We measure the turbulent correlation scales parallel and perpendicular to the local mean magnetic field to be, respectively, delta_{para} = 98 +/- 5 pc and delta_{perp} = 54 +/- 3 pc, while the turbulent to ordered magnetic field strength ratio is found to be Bt/B0 = 1.01 +/- 0.04. These results are consistent with those of Fletcher et al. (2011), who performed a Faraday rotation dispersion analysis of the same data, and our detection of anisotropy is consistent with current magnetized turbulence theories.

astro-ph.GA↗

arXiv2013

The Equipartition Magnetic Field Formula in Starburst Galaxies: Accounting for Pionic Secondaries and Strong Energy Losses

Equipartition arguments provide an easy way to find a characteristic scale for the magnetic field from radio emission, by assuming the energy densities in cosmic rays and magnetic fields are the same. Yet most of the cosmic ray content in star-forming galaxies is in protons, which are invisible in radio emission. Therefore, the argument needs assumptions about the proton spectrum, typically that of a constant proton/electron ratio. In some environments, particularly starburst galaxies, the reasoning behind these assumptions does not necessarily hold: secondary pionic positrons and electrons may be responsible for most of the radio emission, and strong energy losses can alter the proton/electron ratio. We derive an equipartition expression that should work in a hadronic loss-dominated environment like starburst galaxies. Surprisingly, despite the radically different assumptions from the classical equipartition formula, numerically the results for starburst magnetic fields are similar. We explain this fortuitous coincidence using the energetics of secondary production and energy loss times. We show that these processes cause the proton/electron ratio to be ~100 for GHz-emitting electrons in starbursts.

astro-ph.CO↗

arXiv2012

Radio Continuum Surveys with Square Kilometre Array Pathfinders

In the lead-up to the Square Kilometre Array (SKA) project, several next-generation radio telescopes and upgrades are already being built around the world. These include APERTIF (The Netherlands), ASKAP (Australia), eMERLIN (UK), VLA (USA), e-EVN (based in Europe), LOFAR (The Netherlands), Meerkat (South Africa), and the Murchison Widefield Array (MWA). Each of these new instruments has different strengths, and coordination of surveys between them can help maximise the science from each of them. A radio continuum survey is being planned on each of them with the primary science objective of understanding the formation and evolution of galaxies over cosmic time, and the cosmological parameters and large-scale structures which drive it. In pursuit of this objective, the different teams are developing a variety of new techniques, and refining existing ones. Here we describe these projects, their science goals, and the technical challenges which are being addressed to maximise the science return.

astro-ph.CO↗

arXiv2012

Future Observations of Cosmic Magnetic Fields with the SKA and its Precursors

The origin of magnetic fields in the Universe is an open problem in astrophysics and fundamental physics. Polarization observations with the forthcoming large radio telescopes, especially the Square Kilometre Array (SKA), will open a new era in the observation of magnetic fields and should help to understand their origin. Low-frequency radio synchrotron emission, to be observed with LOFAR, MWA and the SKA, traces low-energy cosmic ray electrons and allows us to map the structure of weak magnetic fields in the outer regions and halos of galaxies, in halos and relics of galaxy clusters and in the Milky Way. Polarization at higher frequencies (1-10 GHz), to be observed with the SKA and its precursors ASKAP and MeerKAT, will trace magnetic fields in the disks and central regions of galaxies and in cluster relics in unprecedented detail. All-sky surveys of Faraday rotation measures towards a dense grid of polarized background sources with ASKAP (project POSSUM) and the SKA are dedicated to measure magnetic fields in intervening galaxies, clusters and intergalactic filaments, and will be used to model the overall structure and strength of magnetic fields in the Milky Way. "Cosmic Magnetism" is key science for LOFAR, ASKAP and the SKA.

astro-ph.CO↗

arXiv2012

Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- I: Introduction to the Survey

We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky Very Large Array (i.e. the Expanded Very Large Array, or EVLA) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared - radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of AGNs. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.

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