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Register a new userA systematic observational study of radio properties of H2O megamaser Seyfert-2 galaxies
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A systematic study is performed on radio properties of H$_{2}$O megamaser host Seyfert 2 galaxies, through multi-band radio continuum observations (at 11cm, 6.0 cm, 3.6 cm, 2.0 cm and 1.3 cm) with the Effelsberg 100-m radio telescope within a total time duration of four days. For comparison, a control Seyfert 2 galaxy sample without detected maser emission was also observed. Spectral indices were determined for those sources for which measurements exist at two adjacent bands assuming a power-law dependence S$_ u propto u^{-alpha}$, where S is the flux density and $ u$ is the frequency. Comparisons of the radio continuum properties between megamaser and non-masing Seyfert 2s show no difference in spectral indices. However, a difference in radio luminosity is statistically significant, i.e. the maser galaxies tend to have higher radio luminosities by a factor of 2 to 3 than the non-masing ones, commonly reaching values above a critical threshold of 10$^{29}$ ergs$^{-1}$Hz$^{-1}$. This result confirms an earlier conclusion by Zhang et al. (2012), but is based on superior data with respect to the time interval within which the data were obtained, with respect to the observational facility (only one telescope used), the number of frequency bands.
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We examine the radio properties of the Brightest Cluster Galaxies (BCGs) in a large sample of X-ray selected galaxy clusters comprising the Brightest Cluster Sample (BCS), the extended BCS (eBCS) and ROSAT-ESO Flux Limited X-ray (REFLEX) cluster catalogues. We have multi-frequency radio observations of the BCG using a variety of data from the Australia Telescope Compact Array (ATCA), Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA) telescopes. The radio spectral energy distributions (SEDs) of these objects are decomposed into a component attributed to on-going accretion by the active galactic nuclei (AGN) that we refer to as the core, and a more diffuse, ageing component we refer to as the non-core. These BCGs are matched to previous studies to determine whether they exhibit emission lines (principally H-alpha), indicative of the presence of a strong cooling cluster core. We consider how the radio properties of the BCGs vary with cluster environmental factors. Line emitting BCGs are shown to generally host more powerful radio sources, exhibiting the presence of a strong, distinguishable core component in about 60% of cases. This core component more strongly correlates with the BCGs [OIII]5007A line emission. For BCGs in line-emitting clusters, the X-ray cavity power correlates with both the extended and core radio emission, suggestive of steady fuelling of the AGN over bubble-rise time-scales in these clusters.
In recent years, several Radio-Loud Narrow-Line Seyfert 1 galaxies (RL-NLS1) possessing relativistic jets have come into attention with their detections in Very Large Baseline Array (VLBA) and in $gamma$-ray observations. In this paper we attempt to understand the nature of radio-jets in NLS1s by examining the kpc-scale radio properties of, hitherto, the largest sample of 11101 optically-selected NLS1s. Using 1.4 GHz FIRST, 1.4 GHz NVSS, 327 MHz WENNS, and 150 MHz TGSS catalogues we find the radio-detection of merely $sim$ 4.5 per cent (498/11101) NLS1s, with majority (407/498 $sim$ 81.7 per cent) of them being RL-NLS1s. Our study yields the highest number of RL-NLS1s and it can only be a lower limit. We find that the most of our radio-detected NLS1s are compact ($<$ 30 kpc), exhibit both flat as well as steep radio spectra, and are distributed across a wide range of 1.4 GHz radio luminosities (10$^{22}$ $-$ 10$^{27}$ W Hz$^{-1}$). At the high end of radio luminosity our NLS1s often tend to show blazar-like properties such as compact radio-size, flat/inverted radio spectrum, radio variability and polarization. The diagnostic plots based on the mid-IR colours suggest that the radio emission in NLS1s is mostly powered by AGN, while nuclear star-formation may have a significant contribution in NLS1s of low radio luminosities. The radio luminosity versus radio-size plot infers that the radio-jets in NLS1s are either in the early evolutionary phase or possibly remain confined within the nuclear region due to low-power or intermittent AGN activity.
We present a new investigation of the dependence of H2O maser detection rates and properties on the mid-IR AGN luminosity, L_AGN, and the obscuring column density, N_H, based on mid-IR and hard X-ray photometry. Based on spectral energy distribution fitting that allows for decomposition of the black hole accretion and star-formation components in the mid-infrared, we show that the megamaser (disk maser) detection rate increases sharply for galaxies with 12 micron AGN luminosity L^{AGN}_{12 micron} greater than 10^42 erg/s, from ~<3%(~<2%) to ~12%(~5%). By using the ratio of the observed X-ray to mid-IR AGN luminosity as an indicator of N_H, we also find that the megamaser (disk maser) detection rates are boosted to 15%(7%) and 20%(9%) for galaxies with N_H >= 10^23 cm^{-2} and N_H >= 10^{24} cm^{-2}, respectively. Combining these column density cuts with a constraint for high L^{AGN}_{12 micron} (>=10^42 erg/s) predicts further increases in the megamaser (disk maser) detection rates to 19%(8%) and 27%(14%), revealing unprecedented potential boosts of the megamaser and disk maser detection rates by a factor of 7-15 relative to the current rates, depending on the chosen sample selection criteria. A noteworthy aspect of these new predictions is that the completeness rates are only compromised mildly, with the rates remaining at the level of ~95%(~50%) for sources with N_H >= 10^{23} cm^{-2} (N_H >= 10^24 cm^-2). Applying these selection methods to the current X-ray AGN surveys predicts the detection of >~15 new megamaser disks.
We revisit the relation between H2O maser detection rate and nuclear obscuration for a sample of 114 Seyfert galaxies, drawn from the CfA, 12um and IRAS F25/F60 catalogs. These sources have mid-infrared spectra from the Spitzer Space Telescope and they are searched for X-ray and [O III], 5007Angstrom fluxes from the literature. We use the strength of the [O IV], 25.9um emission line as tracer for the intrinsic AGN strength. After normalization by [O IV] the observed X-ray flux provides information about X-ray absorption. The distribution of X-ray / [O IV] flux ratios is significantly different for masers and non-masers: The maser detected Seyfert-2s (Sy 1.8-2.0) populate a distinct X-ray / [O IV] range which is, on average, about a factor four lower than the range of Seyfert-2 non-masers and about a factor of ten lower than the range of Seyfert-1s (Sy 1.0-1.5). Non-masers are almost equally distributed over the entire X-ray / [O IV] range. This provides evidence that high nuclear obscuration plays a crucial role for the probability of maser detection. Furthermore, after normalization with [O IV], we find a similar but weaker trend for the distribution of the maser detection rate with the absorption of the 7um dust continuum. This suggests that the obscuration of the 7 um continuum occurs on larger spatial scales than that of the X-rays. Hence, in the AGN unified model, at moderate deviation from edge-on, the 7um dust absorption may occur without proportionate X-ray absorption. The absorption of [O III] appears unrelated to maser detections. The failure to detect masers in obscured AGN is most likely due to insufficient observational sensitivity.
Narrow line Seyfert 1 (NLSy1) galaxies constitute a class of active galactic nuclei characterized by the full width at half maximum (FWHM) of the H$beta$ broad emission line < 2000 km/s and the flux ratio of [O III] to H$beta$ < 3. Their properties are not well understood since only a few NLSy1 galaxies were known earlier. We have studied various properties of NLSy1 galaxies using an enlarged sample and compared them with the conventional broad-line Seyfert 1 (BLSy1) galaxies. Both the sample of sources have z $le$ 0.8 and their optical spectra from SDSS-DR12 that are used to derive various physical parameters have a median signal to noise (S/N) ratio >10 per pixel. Strong correlations between the H$beta$ and H$alpha$ emission lines are found both in the FWHM and flux. The nuclear continuum luminosity is found to be strongly correlated with the luminosity of H$beta$, H$alpha$ and [O III] emission lines. The black hole mass in NLSy1 galaxies is lower compared to their broad line counterparts. Compared to BLSy1 galaxies, NLSy1 galaxies have a stronger FeII emission and a higher Eddington ratio that place them in the extreme upper right corner of the $R_{4570}$ - $xi_{Edd}$ diagram. The distribution of the radio-loudness parameter (R) in NLSy1 galaxies drops rapidly at R > 10 compared to the BLSy1 galaxies that have powerful radio jets. The soft X-ray photon index in NLSy1 galaxies is on average higher (2.9 $pm$ 0.9) than BLSy1 galaxies (2.4 $pm$ 0.8). It is anti-correlated with the H$beta$ width but correlated with the Fe II strength. NLSy1 galaxies on average have a lower amplitude of optical variability compared to their broad lines counterparts. These results suggest Eddington ratio as the main parameter that drives optical variability in these sources.
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