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Register a new userHI absorption towards low luminosity radio-loud AGNs of different accretion modes and WISE colours
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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.
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Using data from the Wide-field Infrared Survey Explorer (WISE) we show that the mid infrared (MIR) colors of low-luminosity AGNs (LLAGNs) are significanlty different from those of post-asymptotic giant branch stars (PAGBs). This is due to a difference in spectral energy distribution (SEDs), the LLAGNs showing a flat component due to an AGN. Consistent with this interpretation we show that in a MIR color-color diagram the LINERs and the Seyfert~2s follow a power law with specific colors that allow to distinguish them from each other, and from star forming galaxies, according to their present level of star formation. Based on this result we present a new diagnostic diagram in the MIR that confirms the classification obtained in the optical using standard diagnostic diagrams, clearly identifying LINERs and LLAGNs as genuine AGNs.
How Supermassive Blackholes (SMBHs) are spun-up is a key issue of modern astrophysics. As an extension of the study in Wang et al. (2016), we here address the issue by comparing the host galaxy properties of nearby ($z<0.05$) radio-selected Seyfert 2 galaxies. With the two-dimensional bulge+disk decompositions for the SDSS $r$-band images, we identify a dichotomy on various host galaxy properties for the radio-powerful SMBHs. By assuming the radio emission from the jet reflects a high SMBH spin, which stems from the well-known BZ mechanism of jet production, high-mass SMBHs (i.e., $M_{mathrm{BH}}>10^{7.9}M_odot$) have a preference for being spun-up in classical bulges, and low-mass SMBHs (i.e., $M_{mathrm{BH}}=10^{6-7}M_odot$) in pseudo-bulges. This dichotomy suggests and confirms that high-mass and low-mass SMBHs are spun-up in different ways, i.e., a major dry merger and a secular evolution.
We present results from a spectroscopically blind search for associated and intervening HI 21-cm and OH 18-cm absorption lines towards 88 AGNs at $2le zle5$ using the upgraded Giant Metrewave Radio Telescope (uGMRT). The sample of AGNs with 1.4 GHz spectral luminosity in the range, $10^{27 - 29.3}$ W/Hz, is selected using mid-infrared colors and closely resembles the distribution of the underlying quasar population. The search for associated or proximate absorption, defined to be within 3000 km/s of the AGN redshift, led to one HI 21-cm absorption detection (M1540-1453; $z_{abs}$= 2.1139). This is only the fourth known absorption at $z>2$. The detection rate ($1.6^{+3.8}_{-1.4}$%) suggests low covering factor of cold neutral medium (CNM; T$sim$100 K) associated with these powerful AGNs. The intervening absorption line search, with a sensitivity to detect CNM in damped Ly$alpha$ systems (DLAs), has comoving absorption path lengths of $Delta$X = 130.1 and 167.7 for HI and OH, respectively. The corresponding number of absorber per unit comoving path lengths are $le$0.014 and $le$0.011, respectively. The former is at least 4.5 times lower than that of DLAs and consistent with the CNM cross-section estimated using H$_2$ and CI absorbers at $z>2$. Our AGN sample is optically fainter compared to the quasars used to search for DLAs in the past. In our optical spectra obtained using the Southern African Large Telescope (SALT) and the Nordic Optical Telescope (NOT), we detect 5 intervening (redshift path$sim9.3$) and 2 proximate DLAs. This is slightly excessive compared to the statistics based on optically selected quasars. The non-detection of HI 21-cm absorption from these DLAs suggests small CNM covering fraction around galaxies at $z>2$.
Active galactic nuclei (AGNs) are known to cover an extremely broad range of radio luminosities and the spread of their radio-loudness is very large at any value of the Eddington ratio. This implies very diverse jet production efficiencies which can result from the spread of the black hole spins and magnetic fluxes. Magnetic fluxes can be developed stochastically in the innermost zones of accretion discs, or can be advected to the central regions prior to the AGN phase. In the latter case there could be systematic differences between the properties of galaxies hosting radio-loud (RL) and radio-quiet (RQ) AGNs. In the former case the differences should be negligible for objects having the same Eddington ratio. To study the problem we decided to conduct a comparison study of host galaxy properties of RL and RQ AGNs. In this study we selected type II AGNs from SDSS spectroscopic catalogues. Our RL AGN sample consists of the AGNs appearing in the Best & Heckman (2012) catalogue of radio galaxies. To compare RL and RQ galaxies that have the same AGN parameters we matched the galaxies in black hole mass, Eddington ratio and redshift. We compared several properties of the host galaxies in these two groups of objects like galaxy mass, colour, concentration index, line widths, morphological type and interaction signatures. We found that in the studied group RL AGNs are preferentially hosted by elliptical galaxies while RQ ones are hosted by galaxies of later type. We also found that the fraction of interacting galaxies is the same in both groups of AGNs. These results suggest that the magnetic flux in RL AGNs is advected to the nucleus prior to the AGN phase.
We have used the 610 MHz receivers of the Giant Metrewave Radio Telescope (GMRT) to detect associated HI 21cm absorption from the $z = 1.2230$ blazar TXS1954+513. The GMRT HI 21cm absorption is likely to arise against either the milli-arcsecond-scale core or the one-sided milli-arcsecond-scale radio jet, and is blueshifted by $approx 328$ km s$^{-1}$ from the blazar redshift. This is consistent with a scenario in which the HI cloud giving rise to the absorption is being driven outward by the radio jet. The integrated HI 21cm optical depth is $(0.716 pm 0.037)$ km s$^{-1}$, implying a high HI column density, $N_{rm HI} = (1.305 pm 0.067) times ({rm T_s/100: K}) times 10^{20}$ cm$^{-2}$, for an assumed HI spin temperature of 100 K. We use Nickel Telescope photometry of TXS1954+513 to infer a high rest-frame 1216 AA luminosity of $(4.1 pm 1.2) times 10^{23}$ W Hz$^{-1}$. The $z = 1.2230$ absorber towards TXS1954+513 is only the fifth case of a detection of associated HI 21cm absorption at $z > 1$, and is also the first case of such a detection towards an active galactic nucleus (AGN) with a rest-frame ultraviolet luminosity $gg 10^{23}$ W Hz$^{-1}$, demonstrating that neutral hydrogen can survive in AGN environments in the presence of high ultraviolet luminosities.
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