An infrared study of Centaurus A

Deep, narrow-band Halpha observations taken with the TAURUS Tunable Filter (TTF) on the 4.2m WHT telescope are presented for two nearby radio galaxies with strong jet-cloud interactions. Although the brightest emission line components are closely aligned with the radio jets --- providing nearby examples of the ``alignment effect'' most commonly observed in high redshift (z > 0.5) radio galaxies --- lower surface brightness emission line structures are detected at large distances (10's of kpc) from the radio jet axis. These latter structures cannot be reconciled with anisotropic illumination of the ISM by obscured quasar-like sources, since parts of the structures lay outside any plausible quasar ionization cones. Rather, the distribution of the emission lines around the fringes of the extended radio lobes suggests that the gas is ionized either by direct interaction with the radio components, or by the diffuse photoionizing radiation fields produced in the shocks generated in such interactions. These observations serve to emphasise that the ionizing effects of the radio components can extend far from the radio jet axes, and that deep emission line imaging observations are required to reveal the true distribution of warm gas in the host galaxies. We expect future deep imaging observations to reveal similar structures perpendicular to the radio axes in the high-z radio galaxies.

We study the ionization and kinematics of the ionized gas in the nuclear region of the barred Seyfert 2 galaxy NGC 5643 using MUSE integral field observations in the framework of the Measuring Active Galactic Nuclei Under MUSE Microscope (MAGNUM) survey. The data were used to identify regions with different ionization conditions and to map the gas density and the dust extinction. We find evidence for a double-sided ionization cone, possibly collimated by a dusty structure surrounding the nucleus. At the center of the ionization cone, outflowing ionized gas is revealed as a blueshifted, asymmetric wing of the [OIII] emission line, up to projected velocity v10 ~ -450 km s-1. The outflow is also seen as a diffuse, low-luminosity radio and X-ray jet, with similar extension. The outflowing material points in the direction of two clumps characterized by prominent line emission with spectra typical of HII regions, located at the edge of the dust lane of the bar. We propose that the star formation in the clumps is due to positive feedback induced by gas compression by the nuclear outflow, providing the first candidate for outflow-induced star formation in a Seyfert-like, radio-quiet AGN. This suggests that positive feedback may be a relevant mechanism in shaping the black hole-host galaxy coevolution.

We report on the results of infrared spectroscopic mapping observations carried out in the nuclear region of Centaurus A (NGC5128). The 500 pc bipolar dust shell discovered by Quillen et al.(2006) is even more clearly seen in the 11.3 micron dust emission feature than previous broad band imaging. The pure rotational lines of molecular hydrogen other than the S(0) line are detected above the dusty disk and associated with the oval dust shell. The molecular hydrogen transitions indicate the presence of warm gas at temperatures 250--720K. The ratio of the dust emission features at 7.7 and 11.3 micron and the ratio of the [NeII](12.8) and 11.3 dust emission feature are lower in the 500 pc dust shell than in the star forming disk. The clearer shell morphology at 11.3 micron, warm molecular hydrogen emission in the shell, and variation in line ratios in the shell compared to those in the disk, confirm spectroscopically that this shell is a separate coherent entity and is unlikely to be a chance superposition of dust filaments. The physical conditions in the shell are most similar to Galactic supernova remnants where blast waves encounter molecular clouds. The lines requiring the highest level of ionization, [NeV] and [OIV], are detected 20--25 arcsec north-east and south-west of the nucleus and at position angles near the radio jet axis. Fine structure line ratios and limits from this region suggest that the medium is low density and illuminated by a hard radiation field at low ionization parameter. These higher S molecular hydrogen pure rotational transitions are also particularly bright in the same region as the [OIV] and [NeV] emission. This suggests that the gas associated with the dust shell has been excited near the jet axis and is part of an ionization cone.

We present near-infrared integral field spectroscopy data obtained with VLT/SINFONI of "the Teacup galaxy". The nuclear K-band (1.95-2.45 micron) spectrum of this radio-quiet type-2 quasar reveals a blueshifted broad component of FWHM~1600-1800 km/s in the hydrogen recombination lines (Pa$\alpha$, Br$\delta$, and Br$\gamma$) and also in the coronal line [Si VI]$\lambda$1.963 micron. Thus the data confirm the presence of the nuclear ionized outflow previously detected in the optical and reveal its coronal counterpart. Both the ionized and coronal nuclear outflows are resolved, with seeing-deconvolved full widths at half maximum of 1.1$\pm$0.1 and 0.9$\pm$0.1 kpc along PA$\sim$72-74 deg. This orientation is almost coincident with the radio axis (PA=77 deg), suggesting that the radio jet could have triggered the nuclear outflow. In the case of the H$_2$ lines we do not require a broad component to reproduce the profiles, but the narrow lines are blueshifted by ~50 km/s on average from the galaxy systemic velocity. This could be an indication of the presence of a nuclear molecular outflow, although the bulk of the H$_2$ emission in the inner ~2 arcsec (~3 kpc) of the galaxy follows a rotation pattern. We find evidence for kinematically disrupted gas (FWHM>250 km/s) at up to 5.6 kpc from the AGN, which can be naturally explained by the action of the outflow. The narrow component of [Si VI] is redshifted with respect to the systemic velocity, unlike any other emission line in the K-band spectrum. This indicates that the region where the coronal lines are produced is not co-spatial with the narrow line region.

In this paper, we report a detailed study with a variety of data from optical, near-infrared, X-ray, and radio telescopes of the nuclear region of the galaxy NGC 613 with the aim of understanding its complexity. We detected an extended stellar emission in the nucleus that, at first, appears to be, in the optical band, two stellar nuclei separated by a stream of dust. The active galactic nucleus (AGN) is identified as a variable point-like source between these two stellar components. There is a central hard X-ray emission and an extended soft X-ray emission that closely coincides with the ionization cone, as seen in the [O iii]λ5007 emission. The centroid of the [O i]λ6300 emission does not coincide with the AGN, being shifted by 0.24 arcsec towards the ionization cone; this shift is probably caused by a combination of differential dust extinction together with emission and reflection in the ionization cone. The optical spectra extracted from the central region are typical of low-ionization nuclear emission-line regions. We also identify 10 H ii regions, eight of them in a star-forming ring that is visible in Br γ, [Fe ii]λ16436, and molecular CO(3-2) images observed in previous studies. Such a ring also presents weak hard X-ray emission, probably associated with supernova remnants, not detected in other studies. The position of the AGN coincides with the centre of a nuclear spiral (detected in previous works) that brings gas and dust from the bar to the nucleus, causing the high extinction in this area.

We present the first VLBI observations of the compact source S1 in the radio jet of NGC 1068. Roughly 1 pc in length and 0.2 pc wide, S1 resolves into clumps aligned perpendicular to the local radio jet axis. The radio continuum emission arises from a hot (Te ~ 106 K), dense (ne ~ 106 cm-3) plasma, and the source of the radio emission is either thermal free-free emission or Thomson-reflected synchrotron emission. The clouds comprising S1 are opaque to soft X-radiation, and we therefore propose that S1 is the inner, ionized region, or ‘hot zone,’ of the obscuring medium surrounding the active nucleus. The covering fraction of the hot zone is small, Cf ⋦ 10%. Since the covering fraction of the dusty, obscuring medium is probably much larger, the obscuring disk must either flare or warp outside of the hot zone. That the radio jet and hot zone axes are perpendicular also suggests that the hot zone may be considered to trace the outermost extent of the accretion disk.

The present work is based on a spectroscopically selected sample of Seyfert galaxies and consists of the observation, reduction, and analysis of three-dimensional data, obtained through the MPFS, the integral field spectrograph designed and developed by the scientists of the Special Astrophysical Observatory (Russia) for the 6 m telescope. Our aim is focused on a study of the circumnuclear and extranuclear environment of our sample of Seyfert galaxies, within the more general frame of finding clues to the “unified model,” such as the existence of the ionization cones, by applying a modern technique that has proved to be very powerful and advantageous in the case of extended sources. In chapter 1 an overview of active galactic nuclei (AGNs) and the unified model is presented and in particular a description of the main characteristics, similarities, and differences of the Seyfert galaxies. Chapter 2 contains a description of the radiation processes in AGNs, the mechanisms at the origin of the formation of the emission lines, and the diagnostic line ratios that allow one to discriminate between photoionization by thermal and nonthermal sources. Chapter 3 is an introduction to the method of two-dimensional spectroscopy and describes the properties of our observed data, the necessary starting point for developing a reduction procedure. In particular, we can appreciate the advantages and disadvantages in using a simple lenslet array or a more complex combination of a lenslet array and a bundle of optical fibers to decompose the image of a source into several elements. The following chapters present the results obtained through the analysis of the reduced data of three Seyfert galaxies: Mrk 917, NGC 4388, and IRAS 04502 0317. Mrk 917 shows a thick luminous bar, evident both in the images and in the distorted pattern of the velocity field of the gas. The nucleus of the galaxy has a typical spectrum of a Seyfert 2, with index of the power-law continuum . a p 1.5 The excitation map [O iii] l5007/Ha does not show any indication of possible ionization cones. Moreover, the emissionline spectrum of a region close to the active nucleus, classified as AGN-like but showing a high star formation rate, is probably produced by the combined effects of a local starburst and of not absorbed nonthermal radiation from the nucleus. In NGC 4388 we have observed part of one of the two ionization cones known in this galaxy, the so-called southwest cone. The regions inside the cone show spectra of an AGNlike nature. The FWHM of the lines is ∼500 km s , typical of the narrow-line region of the Seyfert galaxies, even if a broad Ha component (FWHM ∼2000 km s ) can be seen, suggesting the idea of scattered photons from the broad-line region. A high degree of ionization, almost constant even at large distances from the active nucleus, cannot be explained only by nuclear photons but is also probably due to the interaction between the known radio jet, aligned with the cone, and the gas within the cone. In IRAS 04502 0317 we have revealed a previously unknown one-sided ionization cone. In spite of its narrow emission lines (FWHM ∼ 300 km s ) more common in a LINER, the nuclear spectrum is typical of a Seyfert galaxy and is modeled by a nonthermal continuum with . A bright a p 1.2 region very close to the nucleus and partially crossed by the ionization cone is present in both the images and the spectra. Some evidence, such as tidal tails (or faint spiral arms), two non-coplanar dust lanes, an offset of the kinematical center with respect to the nucleus, and a negligible circumnuclear star formation, suggests that this bright region could be a secondary nucleus resulting from a minor merger between a gas-poor galaxy and a nucleated companion.

LINERs exist in the nuclei of a large fraction of luminous galaxies, but their connection with the AGN phenomenon has remained elusive. We present Hubble Space Telescope narrowband [O III]5007 and H-alpha+[N II] emission-line images of the central regions of 14 galaxies with LINER nuclei. The compact, ~1 arcsec-scale, unresolved emission that dominates the line flux in ground-based observations is mostly resolved by HST. The bulk of this emission comes from regions with sizes of tens to hundreds of parsecs that are resolved into knots, filaments, and diffuse gas whose morphology differs from galaxy to galaxy. Most of the galaxies do not show clear linear structures or ionization cones analogous to those often seen in Seyfert galaxies. An exception is NGC 1052, the prototypical LINER, in which we find a 3 arcsec-long (~ 250 pc) biconical structure that is oriented on the sky along the galaxy's radio jet axis. Seven of the galaxies have been shown in previously published HST images to have a bright compact ultraviolet nuclear source, while the other seven do not have a central UV source. Our images find evidence of dust in the nuclear regions of all 14 galaxies, with clear indications of nuclear obscuration in most of the UV-dark cases. The data suggest that the line-emitting gas in most LINERs is photoionized by a central source (which may be stellar, nonstellar, or a combination thereof) but that this source is often hidden from direct view. We find no obvious morphological differences between LINERs with detected weak broad H-alpha wings in their spectra and those with only narrow lines. Likewise, there is no clear morphological distinction between objects whose UV spectra are dominated by hot stars (e.g., NGC 4569) and those that are AGN-like (e.g., NGC 4579).

We have observed simultaneously with the VLA in its A configuration the 1.3 cm continuum and H2O maser emission toward the star-forming region W75N(B) with 01 resolution using a powerful cross-calibration technique. Three continuum sources (VLA 1, VLA 2, and VLA 3) were detected in a region of 15. VLA 1 is elongated (043 × 012) approximately in the direction of the bipolar molecular outflow observed at scales of 2'. The frequency dependence of the flux density and size are consistent with an optically partially thick ionized thermal biconical jet. VLA 2 appears unresolved, while VLA 3 shows a bright core plus extended emission. We detected 29 H2O maser spots (spatial components) in a region of 13'' × 7'' around W75N(B). These masers are mainly distributed in two clusters, one associated with VLA 1 (11 maser spots), and the other one associated with VLA 2 (eight maser spots). One H2O maser spot is associated with VLA 3. The masers associated with VLA 1 are distributed along the major axis of the radio jet. We conclude that VLA 1 is the powering source of the extended bipolar molecular outflow and that the water masers along the radio jet axis are delineating the outflow at scales of 1''. On the other hand, the eight masers coincident with VLA 2 are distributed in a shell of 018 × 010, with a rough north-south velocity segregation that could indicate bound motions around this continuum source. From the comparison of H2O and OH maser distribution in the region with respect to the three radio sources, we consider an evolutionary scheme in which H2O masers are excited in gravitationally bound material (e.g., in circumstellar disks) in less evolved young stellar objects (YSOs), while in more evolved YSOs H2O masers preferentially trace outflows.

We present spatially resolved far-UV spectra (912-1840 A) of NGC 1068 obtained using the Hopkins Ultraviolet Telescope (HUT) during the March 1995 Astro-2 mission. Three spectra of this prototypical Seyfert 2 galaxy were obtained through a 12 arcsec diameter aperture centered on different locations near the nucleus. The first pointing (A1) was centered west of the optical nucleus; the nucleus was on the eastern edge of the aperture. The second (A2) was centered southwest of the optical nucleus with the nucleus well inside the aperture. The third (B) was centered on the ionization cone, with the nucleus on the southwestern edge of the aperture. While all three aperture locations have spectra similar to the Astro-1 observations of Kriss et al., these new spatially resolved observations localize the source of the far-UV line and continuum emission. The ionization cone (B) has both brighter emission lines and continuum than the nucleus (A2). A1 is fainter than either A2 or B in both lines and continuum. The far-UV emission lines observed with HUT have a spatial distribution that most similar to [O III] 5007, but appear to be more extended and offset to the northeast along the axis of the radio jet. This supports the previous conclusion of Kriss et al. that the bright C III 977 and N III 991 arises in shock-heated gas. The UV continuum radiation has a more extended spatial distribution than the line-emitting gas. At wavelengths longward of 1200 A the inferred continuum distribution is consistent with that seen in archival HST/WFPC2 F218W images, and it appears to contain a substantial contribution from starlight. At wavelengths shorter than 1200 A, the UV continuum becomes more concentrated in the ionization cone, consistent with nuclear flux scattered by hot electrons and dust.

NGC 4261 (3C 270) is an elliptical galaxy containing a 300 pc–scale nuclear disk of gas and dust imaged by the Hubble Space Telescope (HST), around a central supermassive black hole. Previous VLBI observations of NGC 4261 revealed a gap in emission in the radio counterjet, presumably due to free-free absorption in the inner parsec of the accretion disk. Here we present three 8 GHz VLBA observations of NGC 4261 that allow us to monitor the location and depth of the gap and check for motions in the jet and counterjet. The separation between the brightest peak and the gap is stable, with an upper limit to its motion of 0.01c, supporting the interpretation of the gap as absorption by an accretion disk rather than as an intrinsic jet feature. These observations span a time of order that required for orbiting material in the disk to transit the counterjet, so we are able to search for density changes (clumps) in the disk by monitoring the optical depth of the gap. The optical depth of the gap is stable to within 20% over 5 years at τ = 1.1 ± 0.1, corresponding to an electron density in the disk that is constant to within 10%. We measure an apparent speed in the jet of (0.52 ± 0.07)c. An apparent speed could not be measured for the counterjet because of a lack of identifiable features. From the apparent jet speed and the jet-to-counterjet brightness ratio, we calculate the viewing angle of the jet to be 63° ± 3° and its intrinsic speed to be (0.46 ± 0.02)c. From the inclination and position angles of the parsec-scale radio jet and outer HST disk rotation axis we calculate a difference between the parsec-scale radio jet and outer HST disk rotation axis of 12° ± 2°. Because of its well-defined HST disk and bright parsec-scale radio jet and counterjet, NGC 4261 is ideal for studying the combined disk-jet system, and this is the first case known to us in which both the inclination and position angles of both the disk and jet have been determined.

We present optical images of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C 272.1) obtained with the Wide Field/Planetary Camera 2 aboard the Hubble Space Telescope. Our three images cover the Hα + [N II] emission lines as well as the V and I continuum bands. Analysis of these images confirms that the Hα + [N II] emission in the central 5'' (410 pc) is elongated along position angle (P.A.) ≈ 72°, which is roughly parallel to two nuclear dust lanes. Our high-resolution images reveal that the Hα + [N II] emission has three components, namely, a nuclear gas disk, an "ionization cone," and outer filaments. The nuclear disk of ionized gas has diameter ≈ 1'' = 82 pc and major axis P.A. ≈ 58° ± 6°. On an angular scale of 05, the major axis of this nuclear gas disk is consistent with that of the dust. However, the minor axis of the gas disk (P.A. ≈ 148°) is tilted with respect to that of the filamentary Hα + [N II] emission at distances greater than 2'' from the nucleus; the minor axis of this larger scale gas is roughly aligned with the axis of the kiloparsec-scale radio jets (P.A. ≈ 170°). The ionization cone (whose apex is offset by ≈ 03 south of the nucleus) extends 2'' from the nucleus along the axis of the southern radio jet. This feature is similar to the ionization cones seen in some Seyfert nuclei, which are also aligned with the radio axes.

A new generation of sensitive X-ray measurements are indicating that the existence of X-ray attenuation column densities, NH > 1024 cm-2, is quite common among broad absorption line quasars (BALQSOs). This is significant to the geometry of the broad absorption line (BAL) outflow. In particular, such an X-ray shield also shields equatorial accretion disk winds from the UV, thereby preventing high-velocity equatorial outflows from being launched. By contrast, bipolar winds initiated by continuum radiation pressure from the funnel of a slim accretion disk flare outward (like a trumpet) and offer vastly different absorbing columns to the X-ray and UV emission that are emitted from distinct regions of the disk, ~6M and ~10M-40M, respectively (where M is the radius of the black hole). Recent numerical work indicates that it is also possible to launch bipolar outflows from the inner regions of a thin disk. The recent discovery with VLBI that the Galactic analog of a BALQSO, the X-ray binary Circinus X-1 (with high-velocity P Cygni X-ray absorption lines), is viewed virtually along the radio jet axis (and therefore along the spin axis of the black hole and the normal to the accretion disk) has rekindled interest in the bipolar models of BALQSOs. We explore this possibility by studying the nearest BALQSO, Mrk 231. High-resolution two-dimensional optical spectroscopy and VLBI mappings of the radio jet axis indicate that the BAL outflow is parallel to the parsec-scale radio jet.

In this paper we present Multi Unit Spectroscopic Explorer integral field unit spectroscopic observations of the ∼70 × 30 kpc2 Lyα halo around the radio galaxy 4C04.11 at z = 4.5077. High-redshift radio galaxies are hosted by some of the most massive galaxies known at any redshift and are unique markers of concomitant powerful active galactic nucleus (AGN) activity and star formation episodes. We map the emission and kinematics of the Lyα across the halo as well as the kinematics and column densities of eight H I absorbing systems at −3500 < Δv < 0 km s−1. We find that the strong absorber at Δv ∼ 0 km s−1 has a high areal coverage (30 × 30 kpc2), being detected across a large extent of the Lyα halo, a significant column density gradient along the southwest to northeast direction, and a velocity gradient along the radio jet axis. We propose that the absorbing structure, which is also seen in C IV and N V absorption, represents an outflowing metal-enriched shell driven by a previous AGN or star formation episode within the galaxy and is now caught up by the radio jet, leading to jet-gas interactions. These observations provide evidence that feedback from AGN in some of the most massive galaxies in the early Universe may play an important role in redistributing material and metals in their environments.

Active galaxies are thought to be both fueled and obscured by neutral gas removed from the host galaxy and funneled into a central accretion disk. We performed a VLA imaging survey of 21 cm absorption in Seyfert and starburst nuclei to study the neutral gas in the near-nuclear environment. With the exception of NGC 4151, the absorbing gas traces 100 pc-scale, rotating disks aligned with the outer galaxy disk. These disks appear to be rich in atomic gas relative to nuclear disks in nonactive spirals. We find no strong evidence for rapid infall or outflow of neutral hydrogen, but our limits on the mass infall rates are compatible with that required to feed a Seyfert nucleus. Among the galaxies surveyed here, neutral hydrogen absorption traces parsec-scale gas only in NGC 4151. Based on the kinematics of the absorption line, the disk symmetry axis appears to align with the radio jet axis rather than the outer galaxy axis. The most surprising result is that we detect no 21 cm absorption toward the central radio sources of the hidden Seyfert 1 nuclei Mrk 3, Mrk 348, and NGC 1068. Moreover, 21 cm absorption is commonly observed toward extended radio jet structure but appears to avoid central, compact radio sources in Seyfert nuclei. To explain these results, we propose that 21 cm absorption toward the nucleus is suppressed by either free-free absorption, excitation effects (i.e., enhanced spin temperature), or rapid motion in the obscuring gas. Ironically, the implications of these effects is that the obscuring disks must be small, typically not larger than a few tens of parsecs.