No description available.

Observations of neutral hydrogen (HI) absorption in the central regions of eight active galaxies are reported. These observations represent the most detailed studies of the neutral hydrogen component so far made of these sources, providing information on the HI gas distribution and kinematics on scales of 0.2 arcsec, corresponding to linear scales of tens of parsecs in the nearest galaxies observed. The sample of active galaxies presented here consists of the radio galaxy 3C293, two Seyfert galaxies NGC7674 and NGC7469, and five luminous infrared galaxies; NGC6240, IIIZw35, UGC2369, IR0335+1523 and Zw049.0. In each of these galaxies MERLIN has been used to provide the highest angular resolution observations of the neutral hydrogen content in these sources made to date. In addition to these high resolution HI studies the newly operational Giant Meterwavelength Radio Telescope has also been used to provide arcsecond resolution observations of the HI gas content in two of the MERLIN sources, 3C293 and NGC7674.Against the central few kiloparsecs of the radio galaxy 3C293 both the neutral hydrogen distribution and the radio continuum emission is imaged on linear scales of 150pc. The distribution and velocity structure of the HI gas against this radio galaxy is attributed to two gas structures. One of these gas structures is indicative of quiescent gas lying 10kpc from the galaxy centre and is physically associated with both ionised gas and dust lanes in the galaxy. The second HI velocity structure is situated within 600pc of the active galactic nucleus (AGN) and is observed to form a circumnuclear disc with a velocity gradient of 179km/s/arcsec. This circumnuclear disc of gas is shown to enclose a mass of M_dyn<600pc)leq3.9*10^9(sin^{-2}i) Msolar, where i represents the inclination of the disc. Extremely localised HI absorption is observed against the unresolved nuclei of the Seyfert galaxies NGC7674 and NGC7469. In the case of the Seyfert-2 galaxy NGC7674, it is shown that the HI is distributed within a 100pc of the AGN. Against the unresolved Seyfert-1 nucleus of NGC7469 the HI absorption is thought to either result from quiescent gas in the outer parts of this galaxy or from gas close to the nucleus. The HI absorption distribution in both of these sources is consistent with the absorption arising in circumnuclear dusty tori. Of the five luminous infrared galaxies observed, HI absorption is detected in four cases. The observations of IIIZw35, IR0335+015 and Zw49.0 all revealed unresolved 1.4GHz radio continuum structure against which HI absorption is detected. However, against the prototypical luminous infrared merger NGC6240 extensive radio continuum is resolved and imaged. It is concluded that the radio continuum structure of NGC6240 is consistent with the radio emission arising from both a starburst region and one or two AGN existing in the remnant nuclei of the merging galaxies. Extremely broad HI absorption is detected against the two dominant radio continuum components. The HI gas structure is consistent with a neutral gas disc forming between the two merging nuclei as they orbit around each other.

In the framework of active galactic nuclei (AGNs), a galaxy’s supermassive black hole is surrounded by a dusty torus whose clumpy configuration allows for either direct or obscured views toward the central engine. Viewing AGNs from different angles gives rise to a variety of AGN classifications; for example, the generic Type 1 AGN class requires the detection of optically broad emission lines, which arise from quickly moving material within the torus, whereas Type 2 AGNs lack these observations. While these viewing angles are not directly observable, synthetic torus models generated with CLUMPY provide a means to determine them along with other parameters that describe the nature and characteristics of the torus in general. Employing CLUMPY models with mid-infrared spectroscopic observations of a large sample of both Type 1 and Type 2 AGNs allows us to acquire a further understanding of the clumpy torus structure and its viewing angles.

Recent research has lead to the possible discovery of a new class of gamma-ray emitting ac- tive galactic nuclei (AGN). These objects – the very radio-loud narrow-line Seyfert 1 galaxies (NLSy1s)– demonstrate observational features suggesting that they are similar to blazars. One of the key characteristics of blazars is the presence of high-amplitude optical microvariability. While this phenomenon has been investigated in individual objects, no study of the intra-night variability of radio-loud NLSy1s as a class has previously been available. This dissertation presents a sys- tematic search for optical variability in a sample of 33 radio-loud NLSy1s. It was found that 26 objects demonstrated microvariations. However, only 9 objects did so with duty cycles comparable to blazars, and only 7 of these 9 objects — J0706+3901, J0849+5108, J0948+0022, J1246+0238, PKS 1502+036, J1644+2619, and IRAS 20181-2244 — demonstrated microvariability at compa- rable amplitudes. Two objects stand out as exceptional sources. J0849+5108 was found to have a duty cycle of ~90% and was observed to undergo an enormous 4-magnitude optical flare in a two-month time span. The object has not been reported to have undergone such an event since 1975. The second object, J0948+0022, is the class prototype. High cadence data indicates that J0948+0022 has a remarkably rapid doubling time scale of ~40 minutes, and it was seen to vary by over 0.9 magnitudes within an individual night. Attempts to correlate microvariability to radio loudness, gamma-ray loudness, and other parameters were largely unsuccessful. However, it was found that only radio-loud NLSy1s that were detected at gamma-ray energies demonstrated microvariability at blazar-like duty cycles. Additionally, an analysis of the frequency of microvariations at various amplitudes suggests that the sample of radio-loud NLSy1s presented in this study share a parent population identical to low energy peaked BL Lac-type (LBL) blazars. This is in agreement with the work of astronomers such as Abdo et al. 2009, who have created spectral energy distributions for a few radio-loud NLSy1s and found them to resemble those of LBLs. Blazar-like variability was found in multiple objects with radio loudnesses of log(R) < 2, suggesting that even moderately radio-loud NLSy1s may be blazar-like objects.

No description available.

We present new optical and radio spectroscopic observations of the remarkable galaxy Malin 1. This galaxy has unique features that include an extremely low surface brightness disk with an enormous mass of neutral hydrogen, and a low luminosity Seyfert nucleus. Malin 1 is exceptional in its values of MHO, LB, and MHI /Ln, and modest in its surface mass density of gas and stars. Spirals with large Min /LB tend to have low mean column densities of HI, and are close to the threshold for star formation due to instabilities in a rotating gas disk. In these terms, Malin 1 has a disk with extremely inefficient star formation. The bulge spectrum is dominated by the absorption features of an old, metal rich stellar population, although there is some evidence for hot (young) stars. The emission line excitations and widths in the nucleus are typical of a Seyfert galaxy; but Malin 1 is in the lowest 5% of the luminosity function of Seyferts, despite a copious fuel supply. Malin 1 is in a low density region of the universe. We propose it as an unevolving disk galaxy, where the surface mass density is so low that the chemical composition and mass fraction in gas change very slowly over a Hubble time. Its properties are similar to those of the damped Lyman -a absorption systems seen in the spectra of high redshift quasars. We emphasize that there are strong observational selection effects against finding gas -rich galaxies that are both massive and diffuse. Finally, we suggest that large and massive HI disks may have formed as early as z - 2, and remained quiescent to the present day. Subject headings : individual (Malin 1) - galaxies : photometry - galaxies : Seyfert - galaxies : stellar content - radio sources : 21 cm radiation - stars : formation

A study of the Galactic Center stellar population is continuing with a sensitive 2μm CCD camera. Using a 64 x 64 detector array, background limited images are recorded with modest amounts of observing time (tob, 20 sec to reach K =13). Magnitudes have been extracted using DAOPHOT from repeated imaging of the central 5' x 5' to search among approximately 1500 stars for long period variables (LPV's, P > 200d), particularily Miras. Miras have a well defined period - luminosity relationship as well as one in period -mass. This program investigates the nature of highly luminous stars at the Galactic Center. Presently 12 variables have been found and have several characteristics consistant with Miras. They have a maximum bolometric luminosity of -4.4 mag which supports the case that high luminosity stars in the central 6 pc are young supergiants.

During an intensive Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) UV monitoring campaign of the Seyfert 1 galaxy NGC 5548 performed from 2014 February to July, the normally highly correlated far UV continuum and broad emission line variations decorrelated for similar to 60-70 days, starting similar to 75 days after the first HST/COS observation. Following this anomalous state, the flux and variability of the broad emission lines returned to a more normal state. This transient behavior, characterized by significant deficits in flux and equivalent width of the strong broad UV emission lines, is the first of its kind to be unambiguously identified in an active galactic nucleus reverberation mapping campaign. The largest corresponding emission line flux deficits occurred for the high ionization, collisionally excited lines C IV and Si IV(+O IV]), and also He II(+O III]), while the anomaly in Ly alpha was substantially smaller. This pattern of behavior indicates a depletion in the flux of photons with E-ph > 54 eV relative to those near 13.6 eV. We suggest two plausible mechanisms for the observed behavior: (i) temporary obscuration of the ionizing continuum incident upon broad line region (BLR) clouds by a moving veil of material lying between the inner accretion disk and inner (BLR), perhaps resulting from an episodic ejection of material from the disk, or (ii) a temporary change in the intrinsic ionizing continuum spectral energy distribution resulting in a deficit of ionizing photons with energies > 54 eV, possibly due to a transient restructuring of the Comptonizing atmosphere above the disk. Current evidence appears to favor the latter explanation.

Swift monitoring of NGC 4151 with an similar to 6 hr sampling over a total of 69 days in early 2016 is used to construct light curves covering five bands in the X-rays (0.3-50 keV) and six in the ultraviolet (UV)/optical (1900-5500 angstrom). The three hardest X-ray bands (> 2.5 keV) are all strongly correlated with no measurable interband lag, while the two softer bands show lower variability and weaker correlations. The UV/optical bands are significantly correlated with the X-rays, lagging similar to 3-4 days behind the hard X-rays. The variability within the UV/optical bands is also strongly correlated, with the UV appearing to lead the optical by similar to 0.5-1 days. This combination of greater than or similar to 3 day lags between the X-rays and UV and less than or similar to 1 day lags within the UV/optical appears to rule out the "lamp-post" reprocessing model in which a hot, X-ray emitting corona directly illuminates the accretion disk, which then reprocesses the energy in the UV/optical. Instead, these results appear consistent with the Gardner & Done picture in which two separate reprocessings occur: first, emission from the corona illuminates an extreme-UV-emitting toroidal component that shields the disk from the corona; this then heats the extreme-UV component, which illuminates the disk and drives its variability.

Active Galactic Nuclei (AGN) are axisymmetric systems to first order; their observed properties are likely strong functions of inclination with respect to our line of sight. However, except for a few special cases, the specific inclinations of individual AGN are unknown. We have developed a promising technique for determining the inclinations of nearby AGN by mapping the kinematics of their narrow-line regions (NLRs), which are easily resolved with Hubble Space Telescope (HST) [O III] imaging and long-slit spectra from the Space Telescope Imaging Spectrograph (STIS). Our studies indicate that NLR kinematics dominated by radial outflow can be fit with simple biconical outflow models that can be used to determine the inclination of the bicone axis, and hence the obscuring torus, with respect to our line of sight. We present NLR analysis of 52 Seyfert galaxies and resultant inclinations from models of 17 individual AGN with clear signatures of biconical outflow. From these AGN, we can for the first time assess the effect of inclination on other observable properties in radio-quiet AGN, including the discovery of a distinct correlation between AGN inclination and X-ray column density.