ALBERT

Beschreibung: We present the results from the Very Large Array radio continuum total power and polarized intensity observations of Stephan's Quintet at 1.43 and 4.86 GHz, along with complementary 4.85- and 8.35-GHz Effelsberg observations. Our study shows a large envelope of radio emission encompassing all the member galaxies and hence a large volume of intergalactic matter. Infall of the galaxy NGC 7318B produces a ridge of intergalactic, polarized emission, for which the magnetic field strength has been estimated as 11.0 ± 2.2 μG, with an ordered component of 2.6 ± 0.8 μG. The energy density of the field within the ridge area is of the same order as estimates of the thermal component, implying that the magnetic field has a significant role in the dynamics of the intergalactic matter. We also report that the tidal dwarf galaxy candidate SQ-B possesses a strong and highly anisotropic magnetic field, with a total strength equal to 6.5 ± 1.9 μG and an ordered component reaching 3.5 ± 1.2 μG, which is comparable to that found in normal-sized galaxies.

Beschreibung: Faint undetected sources of radio-frequency interference (RFI) might become visible in long radio observations when they are consistently present over time. Thereby, they might obstruct the detection of the weak astronomical signals of interest. This issue is especially important for Epoch of Reionization (EoR) projects that try to detect the faint redshifted H  i signals from the time of the earliest structures in the Universe. We explore the RFI situation at 30–163 MHz by studying brightness histograms of visibility data observed with Low-Frequency Array (LOFAR), similar to radio-source-count analyses that are used in cosmology. An empirical RFI distribution model is derived that allows the simulation of RFI in radio observations. The brightness histograms show an RFI distribution that follows a power-law distribution with an estimated exponent around –1.5. With several assumptions, this can be explained with a uniform distribution of terrestrial radio sources whose radiation follows existing propagation models. Extrapolation of the power law implies that the current LOFAR EoR observations should be severely RFI limited if the strength of RFI sources remains strong after time integration. This is in contrast with actual observations, which almost reach the thermal noise and are thought not to be limited by RFI. Therefore, we conclude that it is unlikely that there are undetected RFI sources that will become visible in long observations. Consequently, there is no indication that RFI will prevent an EoR detection with LOFAR.

Beschreibung: We investigate the effect of propagation of cosmic ray electrons (CRE) on the non-thermal (NTH; synchrotron)–far-infrared correlations in M 31 and M 33. The thermal (TH) and NTH emission components of the radio continuum emission at 1.4 GHz and one higher frequency are compared with dust emission from M 31 and M 33 using Spitzer data. In both galaxies the TH emission is linearly correlated with the emission from warm dust (24 μ m, 70 μ m), but the power laws of the NTH–FIR correlations have exponents b  〈 1 that increase with increasing frequency. Furthermore, the values of b for M 33 are significantly smaller ( b  ~= 0.4) than those for M 31 ( 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 M 31 only has a thin NTH disc with a scale height of h  = 0.3–0.4 kpc at 1.4 GHz, whereas M 33 has a similar thin disc as well as a thick disc with scale height h thick  ~= 2 kpc. In the thin discs, the (deprojected) diffusion length at 1.4 GHz is ~=1.5 kpc, yielding a diffusion coefficient of ~=2 10 28 cm 2 s –1 . The structure, strength and regularity of the magnetic field in a galaxy as well as the existence of a thick disc determine the diffusion of the CRE, and hence, the power-law exponent of the NTH–FIR correlations.

Beschreibung: How a galaxy regulates its supernovae (SNe) energy into different interstellar/circumgalactic medium components strongly affects galaxy evolution. Based on the JVLA D-configuration C- (6 GHz) and L -band (1.6 GHz) continuum observations, we perform statistical analysis comparing multiwavelength properties of the Continuum Haloes in Nearby Galaxies – an EVLA Survey galaxies. The high-quality JVLA data and edge-on orientation enable us for the first time to include the halo into the energy budget for a complete radio-flux-limited sample. We find tight correlations of L radio with the mid-IR-based star formation rate (SFR). The normalization of our I 1.6 GHz /W Hz –1 –SFR relation is ~2–3times of those obtained for face-on galaxies, probably a result of enhanced IR extinction at high inclination. We also find tight correlations between L radio and the SNe energy injection rate $\dot{E}_{\rm SN(Ia+CC)}$ , indicating the energy loss via synchrotron radio continuum accounts for ~1 of $\dot{E}_{\rm SN}$ , comparable to the energy contained in cosmic ray electrons. The integrated C -to- L -band spectral index is α ~ 0.5–1.1 for non-active galactic nucleus galaxies, indicating a dominance by the diffuse synchrotron component. The low-scatter L radio –SFR/ $L_{\rm radio}\text{-}\dot{E}_{\rm SN (Ia+CC)}$ relationships have superlinear logarithmic slopes at ~2 in L band (1.132 ± 0.067/1.175 ± 0.102) while consistent with linear in C band (1.057 ± 0.075/1.100 ± 0.123). The superlinearity could be naturally reproduced with non-calorimeter models for galaxy discs. Using Chandra halo X-ray measurements, we find sublinear L X – L radio relations. These results indicate that the observed radio halo of a starburst galaxy is close to electron calorimeter, and a galaxy with higher SFR tends to distribute an increased fraction of SNe energy into radio emission (than X-ray).

Beschreibung: We present the methodology and data behind the photometric redshift data base of the Sloan Digital Sky Survey (SDSS) Data Release 12. We adopt a hybrid technique, empirically estimating the redshift via local regression on a spectroscopic training set, then fitting a spectrum template to obtain K -corrections and absolute magnitudes. The SDSS spectroscopic catalogue was augmented with data from other, publicly available spectroscopic surveys to mitigate target selection effects. The training set is comprised of 1976 978 galaxies, and extends up to redshift z 0.8, with a useful coverage of up to z 0.6. We provide photometric redshifts and realistic error estimates for the 208 474 076 galaxies of the SDSS primary photometric catalogue. We achieve an average bias of $\overline{\Delta z_{\mathrm{norm}}} = {5.84 \times 10^{-5}}$ , a standard deviation of ( z norm ) = 0.0205, and a 3 outlier rate of P o  = 4.11 per cent when cross-validating on our training set. The published redshift error estimates and photometric error classes enable the selection of galaxies with high-quality photometric redshifts. We also provide a supplementary error map that allows additional, sophisticated filtering of the data.

Beschreibung: The spectral index of synchrotron emission is an important parameter in understanding the properties of cosmic ray electrons (CREs) and the interstellar medium (ISM). We determine the synchrotron spectral index (α nt ) of four nearby star-forming galaxies, namely NGC 4736, NGC 5055, NGC 5236 and NGC 6946 at sub-kpc linear scales. The α nt was determined between 0.33 and 1.4 GHz for all the galaxies. We find the spectral index to be flatter ( –0.7) in regions with total neutral (atomic + molecular) gas surface density, gas 50 M pc – 2 , typically in the arms and inner parts of the galaxies. In regions with gas 50 M pc – 2 , especially in the interarm and outer regions of the galaxies, the spectral index steepens sharply to 〈–1.0. The flattening of α nt is unlikely to be caused due to thermal free–free absorption at 0.33 GHz. Our result is consistent with the scenario where the CREs emitting at frequencies below ~0.3 GHz are dominated by bremsstrahlung and/or ionization losses. For denser medium ( gas 200 M pc – 2 ), having strong magnetic fields ( ~ 30 μG), α nt is seen to be flatter than –0.5, perhaps caused due to ionization losses. We find that, due to the clumpy nature of the ISM, such dense regions cover only a small fraction of the galaxy (5 per cent). Thus, the galaxy-integrated spectrum may not show indication of such loss mechanisms and remain a power law over a wide range of radio frequencies (between ~0.1 to 10 GHz).

Beschreibung: We analyse the correlations between continuum properties and emission line equivalent widths of star-forming and active galaxies from the Sloan Digital Sky Survey. Since upcoming large sky surveys will make broad-band observations only, including strong emission lines into theoretical modelling of spectra will be essential to estimate physical properties of photometric galaxies. We show that emission line equivalent widths can be fairly well reconstructed from the stellar continuum using local multiple linear regression in the continuum principal component analysis (PCA) space. Line reconstruction is good for star-forming galaxies and reasonable for galaxies with active nuclei. We propose a practical method to combine stellar population synthesis models with empirical modelling of emission lines. The technique will help generate more accurate model spectra and mock catalogues of galaxies to fit observations of the new surveys. More accurate modelling of emission lines is also expected to improve template-based photometric redshift estimation methods. We also show that, by combining PCA coefficients from the pure continuum and the emission lines, automatic distinction between hosts of weak active galactic nuclei (AGNs) and quiescent star-forming galaxies can be made. The classification method is based on a training set consisting of high-confidence starburst galaxies and AGNs, and allows for the similar separation of active and star-forming galaxies as the empirical curve found by Kauffmann et al. We demonstrate the use of three important machine learning algorithms in the paper: k -nearest neighbour finding, k -means clustering and support vector machines.

Beschreibung: The galaxy pair NGC 5580 and NGC 5588 is part of a loose group of galaxies. They are surrounded by steep-spectrum, extended radio emission which was previously suggested to be a down-scaled example of Mpc-size radio haloes present in galaxies clusters. We present a multifrequency study of the radio emission aimed to clarify its nature. The source has been observed with the Giant Metrewave Radio Telescope at 235, 325 and 610 MHz and the images obtained were combined with archival data to cover the frequency range 150–1400 MHz. The new observations revealed the presence of a second, fainter lobe on the south-east of NGC 5580. The spectral index study of the source shows a flattening of the spectrum (which implies a younger particle population) close to the two galaxies. We argue that the extended radio emission is the remnant of a past activity cycle of the active galactic nucleus present in NGC 5580 and therefore a notable example of a dying radio galaxy located outside dense environments.

Beschreibung: We present 1D cosmic ray transport models, numerically solving equations of pure advection and diffusion for the electrons and calculating synchrotron emission spectra. We find that for exponential halo magnetic field distributions advection leads to approximately exponential radio continuum intensity profiles, whereas diffusion leads to profiles that can be better approximated by a Gaussian function. Accordingly, the vertical radio spectral profiles for advection are approximately linear, whereas for diffusion they are of ‘parabolic’ shape. We compare our models with deep Australia Telescope Compact Array observations of two edge-on galaxies, NGC 7090 and 7462, at 22 and 6 cm. Our result is that the cosmic ray transport in NGC 7090 is advection dominated with $V=150^{+80}_{-30} \,\rm km\,s^{-1}$ , and that the one in NGC 7462 is diffusion dominated with $D=3.0\pm 1.0 \times 10^{28}E_{\rm GeV}^{0.5} \,\rm cm^2\,s^{-1}$ . NGC 7090 has both a thin and thick radio disc with respective magnetic field scale heights of h B1 = 0.8 ± 0.1 kpc and h B2 = 4.7 ± 1.0 kpc. NGC 7462 has only a thick radio disc with h B2 = 3.8 ± 1.0 kpc. In both galaxies, the magnetic field scale heights are significantly smaller than what estimates from energy equipartition would suggest. A non-negligible fraction of cosmic ray electrons can escape from NGC 7090, so that this galaxy is not an electron calorimeter.

Beschreibung: We report on C -band (5–7 GHz) observations of the galaxy, NGC 2992, from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES) sample. This galaxy displays an embedded nuclear double-lobed radio morphology within its spiral disc, as revealed in linearly polarized emission but not in total intensity emission. The radio lobes are kpc-sized, similar to what has been observed in the past for other Seyfert galaxies, and show ordered magnetic fields. NGC 2992 has shown previous evidence for AGN-related activity, but not the linearly polarized radio features that we present here. We draw attention to this galaxy as the first clear example (and prototype) of bipolar radio outflow that is revealed in linearly polarized emission only. Such polarization observations, which are unobscured by dust, provide a new tool for uncovering hidden weak active galactic nucleus (AGN) activity which may otherwise be masked by brighter unpolarized emission within which it is embedded. The radio lobes observed in NGC 2992 are interacting with the surrounding interstellar medium (ISM) and offer new opportunities to investigate the interactions between nuclear outflows and the ISM in nearby galaxies. We also compare the radio emission with a new CHANDRA X-ray image of this galaxy. A new CHANG-ES image of NGC 3079 is also briefly shown as another example as to how much more obvious radio lobes appear in linear polarization as opposed to total intensity.