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Register a new userThe Magnetic Field and Polarization Properties of Radio Galaxies in Different Accretion States
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Shane P. O'Sullivan
Publication date
2015
fields
Physics
and research's language is
English
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We use the integrated polarized radio emission at 1.4 GHz ($Pi_{rm 1.4,GHz}$) from a large sample of AGN (796 sources at redshifts $z<0.7$) to study the large-scale magnetic field properties of radio galaxies in relation to the host galaxy accretion state. We find a fundamental difference in $Pi_{rm 1.4,GHz}$ between radiative-mode AGN (i.e. high-excitation radio galaxies, HERGs, and radio-loud QSOs) and jet-mode AGN (i.e. low-excitation radio galaxies, LERGs). While LERGs can achieve a wide range of $Pi_{rm 1.4,GHz}$ (up to $sim$$30%$), the HERGs and radio-loud QSOs are limited to $Pi_{rm 1.4,GHz} lesssim 15%$. A difference in $Pi_{rm 1.4,GHz}$ is also seen when the sample is divided at 0.5% of the total Eddington-scaled accretion rate, where the weakly accreting sources can attain higher values of $Pi_{rm 1.4,GHz}$. We do not find any clear evidence that this is driven by intrinsic magnetic field differences of the different radio morphological classes. Instead, we attribute the differences in $Pi_{rm 1.4,GHz}$ to the local environments of the radio sources, in terms of both the ambient gas density and the magnetoionic properties of this gas. Thus, not only are different large-scale gaseous environments potentially responsible for the different accretion states of HERGs and LERGs, we argue that the large-scale magnetised environments may also be important for the formation of powerful AGN jets. Upcoming high angular resolution and broadband radio polarization surveys will provide the high precision Faraday rotation measure and depolarization data required to robustly test this claim.
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Young radio galaxies (YRGs) provide an ideal laboratory to explore the connection between accretion disk and radio jet thanks to their recent jet formation. We investigate the relationship between the emission-line properties, the black hole accretion rate, and the radio properties using a sample of 34 low-redshift (z < 0.4) YRGs. We classify YRGs as high-excitation galaxies (HEGs) and low-excitation galaxies (LEGs) based on the flux ratio of high-ionization to low-ionization emission lines. Using the H{alpha} luminosities as a proxy of accretion rate, we find that HEGs in YRGs have sim1 dex higher Eddington ratios than LEGs in YRGs, suggesting that HEGs have higher mass accretion rate or higher radiative efficiency than LEGs. In agreement with previous studies, we find that the luminosities of emission lines, in particular H{alpha}, are correlated with radio core luminosity, suggesting that accretion and young radio activities are fundamentally connected.
We present a study of the radio properties of 870$mu$m-selected submillimetre galaxies (SMGs), observed at high resolution with ALMA in the Extended Chandra Deep Field South. From our initial sample of 76 ALMA SMGs, we detect 52 SMGs at $>3sigma$ significance in VLA 1400MHz imaging, of which 35 are also detected at $>3sigma$ in new 610MHz GMRT imaging. Within this sample of radio-detected SMGs, we measure a median radio spectral index $alpha_{610}^{1400} = -0.79 pm 0.06$, (with inter-quartile range $alpha=[-1.16,-0.56]$) and investigate the far-infrared/radio correlation via the parameter $q_{rm IR}$, the logarithmic ratio of the rest-frame 8-1000$mu$m flux and monochromatic radio flux. Our median $q_{rm IR} = 2.56 pm 0.05$ (inter-quartile range $q_{rm IR}=[2.42,2.78]$) is higher than that typically seen in single-dish 870$mu$m-selected sources ($q_{rm IR} sim 2.4$), which may reflect the fact that our ALMA-based study is not biased to radio-bright counterparts, as previous samples were. Finally, we search for evidence that $q_{rm IR}$ and $alpha$ evolve with age in a co-dependent manner, as predicted by starburst models: the data populate the predicted region of parameter space, with the stellar mass tending to increase along tracks of $q_{rm IR}$ versus $alpha$ in the direction expected, providing the first observational evidence in support of these models.
We observed total and polarized radio continuum emission from the spiral galaxy M 101 at 6.2 cm and 11.1 cm wavelengths with the Effelsberg telescope. We use these data to study various emission components in M 101 and properties of the magnetic field. Separation of thermal and non-thermal emission shows that the thermal emission is closely correlated with the spiral arms, while the non-thermal emission is more smoothly distributed indicating diffusion of cosmic ray electrons away from their places of origin. The radial distribution of both emissions has a break near R=16 kpc, where it steepens to an exponential scale length of about 5 kpc, which is about 2.5 times smaller than at R<16 kpc. The distribution of the polarized emission has a broad maximum near R=12 kpc and beyond R=16 kpc also decreases with about 5 kpc scalelength. It seems that near R=16 kpc a major change in the structure of M 101 takes place, which also affects the distributions of the strength of the random and ordered magnetic field. Beyond R=16 kpc the radial scale length of both fields is about 20 kpc, which implies that they decrease to about 0.3 mu G at R=70 kpc, which is the largest optical extent. The equipartition strength of the total field ranges from nearly 10 mu G at R<2 kpc to 4 mu G at R=22-24 kpc. As the random field dominates in M 101, wavelength-independent polarization is the main polarization mechanism. We show that energetic events causing HI shells of mean diameter <625 pc could partly be responsible for this. At radii <24 kpc, the random magnetic field depends on the star formation rate per area with a power-law exponent of 0.28+-0.02. The ordered magnetic field is generally aligned with the spiral arms with pitch angles that are about 8{deg} larger than those of HI filaments.
H{sc i} absorption studies of active galaxies enable us to probe their circumnuclear regions and the general interstellar medium, and study the supply of gas which may trigger the nuclear activity. In this paper, we investigate the detection rate of H{sc i} absorption on the nature of radio galaxies based on their emission-line spectra, nature of the host galaxies based on the textit{WISE} colours and their radio structure, which may help understand the different accretion modes. We find significant difference in distributions of W2$-$W3 colour for sources with H{sc i} absorption detections and non-detections. We report a high detection rate of H{sc i} absorption in the galaxies with textit{WISE} infrared colours W2$-$W3 $>$ 2, which is typical of gas-rich systems, along with a compact radio structure. The H{sc i} detection rate for low-excitation radio galaxies (LERGs) with W2$-$W3 $>$ 2 and compact radio structure is high (70.6$pm$20.4 %). In HERGs, compact radio structure in the nuclear or circumnuclear region could give rise to absorption by gas in the dusty torus in addition to gas in the interstellar medium. However, higher specific star formation rate (sSFR) for the LERGs with W2$-$W3 $>$ 2 suggests that H{sc i} absorption may be largely due to star-forming gas in their hosts. LERGs with extended radio structure tend to have significantly lower values of W2$-$W3 compared to those with compact structure. Extended radio sources and those with W2$-$W3 $<$ 2 have low H{sc i} detection rates.
Understanding the evolution of accretion activity is fundamental to our understanding of how galaxies form and evolve over the history of the Universe. We analyse a complete sample of 27 radio galaxies which includes both high-excitation (HEGs) and low excitation galaxies (LEGs), spanning a narrow redshift range of 0.9 < z < 1.1 and covering a factor of ~1000 in radio luminosity. Using data from the Spitzer Space Telescope combined with ground-based optical and near-infrared imaging, we show that the host galaxies have masses in the range of 10.7 < log (M /M_sun) < 12.0 with HEGs and LEGs exhibiting no difference in their mass distributions. We also find that HEGs accrete at significantly higher rates than LEGs, with the HEG/LEG division lying at an Eddington ratio of ~0.04, which is in excellent agreement with theoretical predictions of where the accretion rate becomes radiatively inefficient, thus supporting the idea of HEGs and LEGs being powered by different modes of accretion. Our study also shows that at least up to L_151MHz ~3x10^27 W /Hz /sr, HEGs and LEGs are indistinguishable in terms of their radio properties. From this result we infer that, at least for the lower radio luminosity range, another factor besides accretion rate must play an important role in the process of triggering jet activity.
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