The magnetic field of M31 from multi-wavelength radio polarization observations

The configuration of the regular magnetic field in M31 is deduced from radio polarization observations at the wavelengths 6, 11 and 20 cm. By fitting the observed azimuthal distribution of polarization angles, we find that the regular magnetic field, averaged over scales 1--3 kpc, is almost perfectly axisymmetric in the radial range 8 to 14 kpc, and follows a spiral pattern with pitch angles of p\simeq -19\degr to p\simeq -8\degr. In the ring between 6 and 8 kpc a perturbation of the dominant axisymmetric mode may be present, having the azimuthal wave number m=2. A systematic analysis of the observed depolarization allows us to identify the main mechanism for wavelength dependent depolarization -- Faraday rotation measure gradients arising in a magneto-ionic screen above the synchrotron disk. Modelling of the depolarization leads to constraints on the relative scale heights of the thermal and synchrotron emitting layers in M31; the thermal layer is found to be up to three times thicker than the synchrotron disk. The regular magnetic field must be coherent over a vertical scale at least similar to the scale height of the thermal layer, estimated to be h\therm\simeq 1 kpc. Faraday effects offer a powerful method to detect thick magneto-ionic disks or halos around spiral galaxies.Comment: 17 pages, 16 figures, accepted for publication in A&

Density PDFs of diffuse gas in the Milky Way

The probability distribution functions (PDFs) of the average densities of the diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal, especially when lines of sight at |b|5 degree are considered separately. Our results provide strong support for the existence of a lognormal density PDF in the diffuse ISM, consistent with a turbulent origin of density structure in the diffuse gas.Comment: 4 pages, 1 figure. To be published in the proceedings of the August 2008 conference (held in Eapinho, Portugal) "The Role of Disk-Halo Interaction in Galaxy Evolution: Outflow vs Infall?", Ed. M. A. de Avillez, EAS Publications Serie

Probability distribution functions of gas in M31 and M51

We present probability distribution functions (PDFs) of the surface densities of ionized and neutral gas in the nearby spiral galaxies M31 and M51, as well as of dust emission and extinction Av in M31. The PDFs are close to lognormal and those for HI and Av in M31 are nearly identical. However, the PDFs for H2 are wider than the HI PDFs and the M51 PDFs have larger dispersions than those for M31. We use a simple model to determine how the PDFs are changed by variations in the line-of-sight (LOS) pathlength L through the gas, telescope resolution and the volume filling factor of the gas, f_v. In each of these cases the dispersion sigma of the lognormal PDF depends on the variable with a negative power law. We also derive PDFs of mean LOS volume densities of gas components in M31 and M51. Combining these with the volume density PDFs for different components of the ISM in the Milky Way (MW), we find that sigma decreases with increasing length L with an exponent of -0.76 +/- 0.06, which is steeper than expected. We show that the difference is due to variations in f_v. As f_v is similar in M31, M51 and the MW, the density structure in the gas in these galaxies must be similar. Finally, we demonstrate that an increase in f_v with increasing distance to the Galactic plane explains the decrease in sigma with latitude of the PDFs of emission measure and FUV emission observed for the MW.Comment: 15 pages, 7 figures, 7 tables, accepted for publication in Monthly Notices of the Royal Astronomical Societ

The radio-infrared correlation in galaxies

The radio-infrared correlation was explained as a direct and linear relationship between star formation and IR emission. However, one fact making the IR-star formation linkage less obvious is that the IR emission consists of at least two emission components, cold dust and warm dust. The cold dust emission may not be directly linked to the young stellar population. Furthermore, understanding the origin of the radio-IR correlation requires to discriminate between the two main components of the radio continuum emission, free-free and synchrotron emission. Here, we present a multi-scale study of the correlation of IR with both the thermal and non-thermal (synchrotron) components of the radio continuum emission from the nearby galaxies M33 and M31.Comment: To appear in Highlights of Astronomy, Volume 15, XXVIIth IAU General Assembly, August 200

The Brightness of the Galactic Radio Loops at 1420 MHz: Some Indications for the Existence of Loops V and VI

In this article we use 1420 MHz data to demonstrate the likely reality of Galactic radio Loops V and VI. We further estimate distances and spectral indices for both these and the four main radio loops. In the cases of Loops I - IV, radio spectral indices are calculated from the mean brightnesses at 1420 and 820/404 MHz. The spectral indices of Loops V and VI are obtained from $T - T$ plots between 1420 and 408 MHz. Using the supernova remnant (SNR) hypothesis for the origin of radio loops, distances are calculated from the surface brightnesses and the angular diameters at 1420 MHz. We also study how results for brightnesses and distances of radio loops agree with current theories of SNR evolution. For this purpose, the ambient density and initial explosion energy of the loops are discussed. We also discuss applications of different $\Sigma - D$ relations. The results obtained confirm a non-thermal origin and nearby locations for the Galactic radio loops. Therefore, we have indications that they are very old SNRs that evolve in low ambient densities, with high initial explosion energies.Comment: 6 pages, 4 tables, 4 figures. Accepted for publication in Astron. Nach

An absolutely calibrated survey of polarized emission from the northern sky at 1.4 GHz

A new polarization survey of the northern sky at 1.41 GHz is presented. The observations were carried out using the 25.6m telescope at the Dominion Radio Astrophysical Observatory in Canada, with an angular resolution of 36 arcmin. The data are corrected for ground radiation to obtain Stokes U and Q maps on a well-established intensity scale tied to absolute determinations of zero levels, containing emission structures of large angular extent, with an rms noise of 12 mK. Survey observations were carried out by drift scanning the sky between -29 degr and +90 degr declination. The fully sampled drift scans, observed in steps of 0.25 degr to 2.5 degr in declination, result in a northern sky coverage of 41.7% of full Nyquist sampling. The survey surpasses by a factor of 200 the coverage, and by a factor of 5 the sensitivity, of the Leiden/Dwingeloo polarization survey (Spoelstra 1972) that was until now the most complete large-scale survey. The temperature scale is tied to the Effelsberg scale. Absolute zero-temperature levels are taken from the Leiden/Dwingeloo survey after rescaling those data by the factor of 0.94. The paper describes the observations, data processing, and calibration steps. The data are publicly available at http://www.mpifr-bonn.mpg.de/div/konti/26msurvey or http://www.drao.nrc.ca/26msurvey.Comment: 18 pages, 11 figures, accepted for publication in Astronomy and Astrophysic

The distance to the SNR CTB109 deduced from its environment

We conducted a study of the environment around the supernova remnant CTB109. We found that the SNR is part of a large complex of HII regions extending over an area of 400 pc along the Galactic plane at a distance of about 3 kpc at the closer edge of the Perseus spiral arm. At this distance CTB109 has a diameter of about 24 pc. We demonstrated that including spiral shocks in the distance estimation is an ultimate requirement to determine reliable distances to objects located in the Perseus arm. The most likely explanation for the high concentration of HII regions and SNRs is that the star formation in this part of the Perseus arm is triggered by the spiral shock.Comment: 6 pages, 6 figures. accepted for publication in the Astrophysical Journa

Electron Distribution in the Galactic Disk - Results From a Non-Equilibrium Ionization Model of the ISM

Using three-dimensional non-equilibrium ionization (NEI) hydrodynamical simulation of the interstellar medium (ISM), we study the electron density, $n_{e}$, in the Galactic disk and compare it with the values derived from dispersion measures towards pulsars with known distances located up to 200 pc on either side of the Galactic midplane. The simulation results, consistent with observations, can be summarized as follows: (i) the DMs in the simulated disk lie between the maximum and minimum observed values, (ii) the log derived from lines of sight crossing the simulated disk follows a Gaussian distribution centered at \mu=-1.4 with a dispersion \sigma=0.21, thus, the Galactic midplane =0.04\pm 0.01$cm$^{-3}$, (iii) the highest electron concentration by mass (up to 80%) is in the thermally unstable regime (200<T<10^{3.9} K), (iv) the volume occupation fraction of the warm ionized medium is 4.9-6%, and (v) the electrons have a clumpy distribution along the lines of sight.Comment: Letter accepted for publication in Monthly Notices of the Royal Astronomical Societ

Magnetic fields and spiral arms in the galaxy M51

(Abridged) We use new multi-wavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on scales from 200\pc to several \kpc. Interferometric and single dish data are combined to obtain new maps at \wwav{3}{6} in total and polarized emission, and earlier \wav{20} data are re-reduced. We compare the spatial distribution of the radio emission with observations of the neutral gas, derive radio spectral index and Faraday depolarization maps, and model the large-scale variation in Faraday rotation in order to deduce the structure of the regular magnetic field. We find that the \wav{20} emission from the disc is severely depolarized and that a dominating fraction of the observed polarized emission at \wav{6} must be due to anisotropic small-scale magnetic fields. Taking this into account, we derive two components for the regular magnetic field in this galaxy: the disc is dominated by a combination of azimuthal modes, $m=0+2$, but in the halo only an $m=1$ mode is required to fit the observations. We disuss how the observed arm-interarm contrast in radio intensities can be reconciled with evidence for strong gas compression in the spiral shocks. The average arm--interam contrast, representative of the radii r>2\kpc where the spiral arms are broader, is not compatible with straightforward compression: lower arm--interarm contrasts than expected may be due to resolution effects and \emph{decompression} of the magnetic field as it leaves the arms. We suggest a simple method to estimate the turbulent scale in the magneto-ionic medium from the dependence of the standard deviation of the observed Faraday rotation measure on resolution. We thus obtain an estimate of 50\pc for the size of the turbulent eddies.Comment: 21 pages, 18 figures (some at lower resolution than submitted version), accepted for publication in MNRA

Filling factors and scale heights of the DIG in the Milky Way

The combination of dispersion measures of pulsars, distances from the model of Cordes and Lazio (2002) and emission measures from the WHAM survey enabled a statistical study of electron densities and filling factors of the diffuse ionized gas (DIG) in the Milky Way. The emission measures were corrected for absorption and contributions from beyond the pulsar distance. For a sample of 157 pulsars at |b| > 5 degrees, mainly located in interarm regions within about 3 kpc from the Sun, we find that: (1) The average volume filling factor along the line of sight is inversely proportional to the mean electron density in clouds. (2) The average volume filling factor increases towards larger distances from the Galactic plane. (3) The local volume filling factor may reach a maximum near |z| = 0.9 kpc, whereas the local electron density continues to decrease at higher |z|, thus causing the observed flattening in the distribution of dispersion measures perpendicular to the plane above this height. (4) The scale heights of the electron density, the volume filling factor and the emission measure are the same and in the range 250-500 pc.Comment: 16 pages, 13 figures. Accepted for publication in A