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THE ROLE OF HOST GALAXY KINEMATICS ON NUCLEAR ACTIVITY
Abstract
Ce travail de thèse s'articule autour de deux questions scientifiques importantes à propos des galaxies actives : quels sont les mécanismes transportant le gaz et quel est le role de la galaxie sur l'activité nucléaire ? Nous avons donc mené une étude observationnelle approfondie et statistique du gaz et des étoiles, pour comparer la morphologie et cinématique des galaxies actives et non-actives sur differentes échelles spatiales, en utilisant des données spectroscopiques optique et radio. Nos résultats montrent que dans les régions centrales des galaxies actives la cinématique des étoiles est régulière alors que le gaz est perturbé. Ces perturbations suggèrent un lien entre la dynamique au centre des galaxies et les mécanismes d'alimentation du noyau actif. Enfifin les données radio et optique sont combinées pour analyser la cinématique galactique dans son ensemble. Cette étude nous
permet de sonder à differentes échelles spatiales les perturbations liées à l'alimentation du noyau actif.
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We study a complete, distance-limited sample of 25 LINERs, 21 of which have been imaged with the Hubble Space Telescope. In nine objects we detect an unresolved nucleus. To study their physical properties, we compare the radio and optical properties of the nuclei of LINERs with those of other samples of local active galactic nuclei (AGNs), namely, Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our results show that the LINER population is not homogeneous, as there are two subclasses: (1) the first class is similar to the LLRG class, as it extends the population of radio-loud nuclei to lower luminosities; (2) the second is similar to Seyfert galaxies and extends the properties of radio-quiet nuclei toward the lowest luminosities. The objects are optimally discriminated in the plane formed by the black hole mass versus nuclear radio loudness: all radio-loud LINERs have MBH 108M☉, while Seyfert galaxies and radio-quiet LINERs have MBH 108M☉. The different nature of the various classes of local AGNs are best understood when the fraction of the Eddington luminosity they irradiate, Lo/LEdd, is plotted against the nuclear radio-loudness parameter: Seyfert galaxies are associated with relatively high radiative efficiencies Lo/LEdd 10-4 (and high accretion rates onto low-mass black holes); LLRGs are associated with low radiative efficiencies (and low accretion rates onto high-mass black holes); all LINERs have low radiative efficiency (and accretion rates) and can be radio-loud or radio-quiet, depending on their black hole mass.
We report here on the first observations in the Hα line to investigate the kinematics of the spiral galaxy NGC 3627, the brightest member of the Leo Triplet. The velocity field observed at high angular resolution shows signatures of non-circular motion. They can be attributed to dynamical perturbations due to the bar and to external interactions. In particular, we confirm the presence of an inner ring recently detected in CO observations (Regan et al. 2002). We find a corotation radius significantly smaller than that derived in previous studies and we show that the inner ring location is consistent with the ultra-harmonic resonance induced by the bar pattern speed. We also show that a warped tilted-ring model can be fitted to the outer parts of the southern, receding half of the disc, which is inclined by some 15◦ relative to the inner parts of the galaxy.
Results on the VLA observations (with 18 arcsec resolution) of all 114 objects from the Palomar Bright Quasar Survey (BQS) are presented, and the relation between the radio luminosity and optical luminosity is discussed. It was found that most of the BQS quasars are radio quiet and have a radio flux density close to that of the optical flux density; however, 15-20 percent of the quasars are radio loud and are much brighter at radio than at optical wavelengths. The radio luminosity function was derived. It is shown that the radio emission from high-red-shift (z greater than 0.5) quasars is dominated by compact components; most quasars with R above 100 have small red shifts. 39 refs.
The recent development of unified theories of active galactic nuclei (AGN)
has indicated that there are two physically distinct classes of these objects -
radio-loud and radio-quiet. The primary observational distinctions between the
two types are: (1) The radio-loud objects produce large scale radio jets and
lobes, with the kinetic power of the jets being a significant fraction of the
total bolometric luminosity. On the other hand, the weak radio ejecta of the
radio-quiet objects are energetically insignificant. (2) The radio-loud objects
are associated with elliptical galaxies which have undergone recent mergers,
while the radio-quiets prefer spiral hosts. (3) The space density of the
radio-louds at a given optical luminosity is 10 times lower than that
of the radio-quiets. Despite these differences, the (probably) thermal
emissions from the AGN (continua and lines from X-ray to infrared wavelengths)
are quite similar in the two classes of object. We argue that this last result
suggests that the black hole masses and mass accretion rates in the two classes
are not greatly different, and that the difference between the classes is
associated with the spin of the black hole.
We assume that the normal process of accretion through a disk does not lead
to rapidly spinning holes, and propose instead that galaxies (e.g. spirals)
which have not suffered a recent major merger event contain non-rotating or
only slowly rotating black holes. When two such galaxies merge, the two black
holes are known to form a binary and we assume that they eventually
coalesce. In the small fraction of mergers in which the two ``parent'' galaxies
contain very massive holes of roughly equal mass, a rapidly spinning, very
massive hole results. It is proposed that such mergers are the progenitors of
powerful radio
We present the completed results of a high resolution radio imaging survey of all (~200) low-luminosity active galactic nuclei (LLAGNs) and AGNs in the Palomar Spectroscopic Sample. The high incidences of pc-scale radio nuclei, with T(brightness) >=10^7 K, and sub-parsec jets argue for accreting black holes in >=50% of all LINERs and low-luminosity Seyferts; there is no evidence against all LLAGNs being mini-AGNs. The radio luminosity function (RLF) of Palomar Sample LLAGNs and AGNs extends three orders of magnitude below, and is continuous with, that of `classical' AGNs. We find marginal evidence for a low-power turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of present day mass accretion. Adopting the jet model of Falcke & Biermann, we show that the accretion energy output in LLAGNs is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. The Palomar LLAGNs and AGNs follow the same scaling between jet power and narrow line region (NLR) luminosity as the pc to kpc jets in powerful radio galaxies. Eddington ratios of <= 10^{-1} - 10^{-5} are implied in jet models of the radio emission. We find evidence that, in analogy to Galactic black hole candidates, LINERs are in a `low/hard' state (gas poor nuclei, low Eddington ratio, ability to launch collimated jets) while low-luminosity Seyferts are in a `high' state (gas rich nuclei, higher Eddington ratio, less likely to launch collimated jets). The jets are energetically more significant than supernovae in the LLAGN host galaxies, and are potentially able to deposit sufficient energy into the innermost parsecs to significantly slow the gas supply to the accretion disk.
The dependence of the (N II)/H-alpha ratio of 180 Seyfert 2 and LINER
nuclei on the aperture used in the observations is investigated and
quantified. When the aperture effect is taken into account it can be
concluded that the majority of Seyfert 2 and LINER galaxies present (N
II)/H-alpha above 1.5, which means an overabundance of nitrogen in the
nucleus, according to results of previous works. The relationship of (N
II)/H-alpha to nuclear compact radio emission, absolute magnitude and
morphological type of parent galaxy are also investigated.
We present a 5-GHz MERLIN map of the central arcsecond of the Seyfert 2 galaxy NGC 1068. We detect the four-component structure
reported in earlier observations. The higher angular resolution and sensitivity of the MERLIN measurements show extended emission
consistent with a flaring jet which appears to originate from the vicinity of a 100-mas compact double source. One of the
components in the 100-mas double has an inverted spectral index, and hence we identify this as a ‘radio core’ at the active
nucleus of the galaxy. We have evaluated previous positions, and by use of improved calibrator positions determine the J2000
position of the inverted spectrum core/nucleus to be at RA= 02h42m40s7098, Dec. = − 00°00′47″!938, with an error of ± 20 mas in each coordinate.
We compare the MERLIN image with post-COSTAR HST [O iii] and continuum images, and discuss the radio and optical positions for the active nucleus. However, it appears unlikely that
much progress can be made until significant improvements are made in the astrometric precision of the HST images.
An observation performed by the SIGMA experiment on board the GRANAT
satellite has given a negative result for the detection of hard X-rays
from the Seyfert galaxy NGC 1068. We discuss the implications of this
result for the 'Unified Scheme' of Active Galactic Nuclei (AGN) and for
this galaxy in particular. It imposes conditions on the absorbing
material, if one assumes that a typical Seyfert 1 hard X-ray continuum
is indeed hidden in the nucleus.
The evolution of an SNR in a high-density medium has been studied using
analytical and numerical hydrodynamic and photoionization models. The
models show that SNR properties (luminosity, density, size, kinematics,
emission line ratios, etc.) are essentially identical to those
considered typical of the broad-line region (BLR) of radio-quiet active
galactic nuclei (AGN). A model is presented in which the only free
parameter is the density of the circumstellar medium. This model
combines analytical and numerical hydrodynamic simulations with static
photoionization computations to reproduce most of the basic properties
of the BLR in a low-luminosity AGN. The computed line intensity ratios
are found to be in excellent agreement with those observed in the BLR,
and the model output shows the following features: bolometric luminosity
of more than 10 exp 43 erg/s; emission line width of about 5000 km/s;
size of about 0.01 pc; and total mass of ionized gas in the BLR from
about 1 to 10 solar masses.
We examine four models for the optical/UV/soft X-ray emission of AGN: a
mechanically heated plasma, either optically thin or thick for electron
scattering, and a radiatively heated plasma, either optically thin or
thick for electron scattering. We present detailed computations of the
emitted spectra, and we discuss the total energy budget (including the
requirement for the mass accretion flow), and the interaction with the
hot optically thin medium responsible for hard X-ray emission. We show
that in no case the spectrum can be considered as due to a pure
free-free process: lines and/or free-bound continuum are always
important. We can eliminate mechanical heating on the basis of
variability time scales and of a detailed fit of the spectrum in the
optical/UV/X-ray range. If the system of clouds is radiatively heated,
the energy budget requires that it is optically thick to electron
scattering. Finally, we conclude that a system of radiatively heated
clouds, optically thick for electron scattering and having a covering
factor close to unity, is a viable model for Seyfert galaxies. The same
model is applicable for quasars although one cannot rule out in their
case a small contribution from mechanical heating. Therefore we propose
a new model for the emission of the optical to X-ray spectrum in AGN,
made of a quasi-spherical (or a thick disk) distribution of mildly
optically thick clouds. Smaller covering factor, 0.5-0.9, and smaller
optical depth for electron scattering, ~50, are more appropriate for
Seyfert galaxies while larger values (>0.9 and ~500 respectively) are
deduced for quasars. As a consequence, we expect higher Eddington ratios
in quasars than in Seyfert galaxies. One peculiarity of the model is
that UV is due to pure emission, and X-rays to a combination of
reflection and primary spectrum and that absorption features are weak in
the X-ray range in spite of the large covering factor. Although it is
attractive, this cloud scenario is not unique for the UV-X emission of
AGN, with two others being: reprocessing clouds radiating as black
bodies and accretion disks with hot coronae. We discuss the prospects of
differentiation between these possibilities.
Neutral hydrogen observations of 22.7 × 18.5 arcsec2 resolution are presented for the Seyfert type 1 galaxy NGC 5033. Our measurements show that the total H i mass of the galaxy is 7.0 × 109 M⊙ and that it is receding at 870 km s−1. The column density map features prominent H i spiral arms and an asymmetric plume of H i which extends ∼ 6 kpc south of the galaxy. We find a well-ordered velocity field and the large-scale kinematical warp in
the galactic disc has been modelled. The inner region of the velocity field is well fitted by a flat, inclined disc with inclination
68° and position angle 352°, but due to warping these fall to 64° and 342°, respectively, at large radii.
Attention is paid to factors that may be related to Seyfert activity in the nucleus and we find some evidence for a previous
tidal encounter but little evidence for a bar, or significant non-circular motions in the central regions. Large concentrations
of H i at the galactic centre appear to show no evidence of interaction with the active nucleus.
The relative positions of the high and low brightness regions in the extra-galactic sources in the 3CR complete sample are
found to be correlated with the luminosity of these sources.
We have used the VLA to image the neutral hydrogen emission from the Seyfert galaxy GC 3227, with angular resolutions ranging from 12 to 60 arcsec. We detect plumes of H I extending to ~ 70 kpc north and ~ 31 kpc south of the galaxy which may be a consequence of interaction. Complex motions in the galactic disc have been resolved into emission from the disc, which is in approximate solid body rotation, and an anomalous velocity cloud, situated north-west of the disc at the base of the nothern plume. The cloud has a mean velocity of ~ 150 km s–1 above the systemic velocity of NGC 3227 and shows evidence of rotation. We suggest that this cloud is a gas-rich dwarf galaxy that either is one of the bodies responsible for the interaction, or has formed as a consequence of the interaction.
No neutral hydrogen emission is associated with the companion galaxy NGC 3226, and the presence of low velocity (500 km s–1) gas reported in single dish measurements is not detected in the present observations.
The continuum image of NGC 3227 shows an extended component, in addition to the compact core, that may be enhanced disc emission due to the interaction. In the central 15 arcsec of the disc we detect evidence of H I absorption against the nuclear continuum source. Higher resolution observations are required to confirm this.
Although the velocities in the disc are remarkably close to solid body velocities (considering the presence of a disturbing companion), the integrated neutral hydrogen emission shows a bar of enhanced emission crossing the nucleus, in a north-west to south-east direction, that has a ‘Z’ shaped morphology and trails in the same sense as the optical spiral arms. This HI bar seems to be a continuation of the CO bar and we suggest that it may play an important role in the fuelling of the AGN.
From our H I observations we deduce a heliocentric systemic velocity of 1135±10 km s–1 for NGC 3227. We derive a value of 56° for the inclination of the galactic disc and 158°±2° for the PA of the major axis. Neutral hydrogen masses of 5.7 × 10⁸ Mʘ for the disc, 2.1×10⁸ Mʘ for the cloud, 1.3×10⁸ Mʘ in the southern plume and 1.8×10⁸ Mʘ in the northern plume were found, resulting in a total H I mass for NGC 3227 of 1.1×10⁹ Mʘ.
H I rotation curves have been presented and analyzed for 21 bright
spirals in the center of the Virgo Cluster. The curves look similar to
those of spirals in the field in that the great majority of them are
flat or still rising at the Holmberg radius. A detailed comparison with
synthetic curves derived from field spirals shows good agreement. The
main conclusion is that the mass distribution in spirals is not affected
in an obvious way by their environment. The authors' sample contains the
most hydrogen-deficient spirals in the Virgo Cluster. A few galaxies
very close to the cluster center have lost their H I to within almost
the turnover point of the rotation curve. In spite of this, the
Tully-Fisher relation derived for the sample does not differ
significantly from the ones derived from larger samples containing fewer
H I-deficient galaxies; thus the relation can probably be used safely
for cluster spirals.
Multiconfiguration VLA observations of Cygnus A at 1.4 and 5 GHz have
been used to produce detailed maps of exceptional dynamic range. A radio
jet extends from the core into the northwest lobe. The broad radio lobes
reveal an unexpected wealth of filamentary structure. Two new hot spots
are identified. The pressure of the cluster gas in which Cyg A is
embedded appears to be insufficient to confine either the jet or the
filaments. From the necessarily high mechanical luminosity of the jet
and the high energy densities of the hot spots, it is concluded that the
jet is probably relativistic. The surface brightness of the jet is high
in absolute terms, but the total radiated power of the jet is only 0.001
of that of the whole source.
We report broad-band imaging observations which reveal a circum-nuclear
gas ring in NGC 4151. The ring is delineated by two arc-like features
probably resulting from dust extinction of the background stellar
continuum. These features are distributed along opposite quadrants of an
elliptical ring which has a semi-major axis of length 1.8kpc and is
approximately aligned with the NE-SW axis of elongation of the extended
emission line region. We discuss this ring in relation to the spatial
distribution and kinematics of both ionized and neutral gas components
as revealed by published optical spectroscopy and 21-cm observations.
These data allow us to form a coherent picture of the gas flows in the
circum-nuclear environment of NGC 4151 which is strikingly similar to
numerical simulations of streaming induced by barred potentials in which
the x2 family of orbits is well developed. On this basis, we propose
that the distribution of gaseous material in the galactic bar of NGC
4151 traces the path by which the gas is transported towards the active
nucleus. The alignment between the circum-nuclear ring and the ENLR can
then be seen as a strong indication that the collimation of the ionizing
radiation field of the active nucleus is related to the dynamical
parameters of the galactic bar. Specifically, we propose that bar-driven
gas flows create the opaque torus which is believed to collimate the
radiation field.
High-sensitivity neutral hydrogen observations of the Seyfert galaxy NGC
4151 with a linear resolution of 800 pc are reported. Despite the
intense activity in the nucleus, little evidence of large-scale peculiar
motions is found, although there are significant departures from
coplanar circular motion in a central 2 x 3 sq arcmin bar. The total H I
mass of the galaxy is 3 x 10 exp 9 solar masses, of which 5 x 10 exp 8
solar masses is in the bar. In addition to the central bar there is a
region of high column density in the arm 3.5 arcmin southwest of the
nucleus with a mass of about 10 exp 8 solar masses. An average
heliocentric recession velocity of 997 +/- 3 km/s is determined. There
are local minima in the H I distribution to the northwest and southeast
of the active nucleus, where the column densities are less than 10 exp
20/sq cm. There is close agreement between the velocities of the optical
extended narrow-line region (ENLR) and the neutral hydrogen in the bar,
implying that the ENLR is simply a component of the neutral gas close to
the plane of the galaxy, photoionized by anisotropic UV continuum
emission from the nucleus.
It is demonstrated that the H I in the large, nearby spiral NGC 4258
behaves very much as expected for a normal SAB galaxy, notwithstanding
the presence of a pair of anomalous arms indicating some kind of violent
activity. The large angular dimensions of NGC 4258 and the good
signal-to-noise ratio of the observations allow a detailed study of the
properties of the underlying, unperturbed part of the galaxy. Strong
evidence for the presence of a bar is found, as well as indications of a
high pattern speed, putting corotation well inside the optically
observable disk.
We present new narrow-band images of NGC 4151, taken in excellent seeing conditions with the 4.2-m William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos on La Palma. These reveal a wealth of detail that can be used to test models for the anisotropy of the narrow-line region. From an [Om] (λλ4959, 5007)/Hα line ratio map we identify three important features: (i) a jet-like linear structure that extends in a series of relatively high-ionization blobs along PA~ 50° out to a radius of more than 15 arcsec to the south-west and 5 arcsec to the north-east; (ii) a lower ionization 'halo' surrounding the linear structure and (iii) a band of lower ionization or higher obscuration through the nucleus, almost perpendicular to the linear structure. We propose here that the latter can be identified with the central obscuring disc or torus responsible for collimating the nuclear radiation field which is believed to ionize the extended linear structure. The other features, however, do not fit in with a simple model in which a cone of radiation from the nucleus illuminates a uniform ISM in the host galaxy.
Powerful emissions from the centres of nearby galaxies may represent dead quasars.
The dynamics of gas are modeled in a merger of two disk/halo galaxies of equal mass using a hybrid N-body/gasdynamics code. Violent tidal forces acting on the disks draw out extended tails and trigger the formation of central bars. As such bars form, gas in the inner half of each disk loses most of its angular momentum through gravitational torques and falls into a compact cloud within the center of the galaxy. These nuclear gas clouds merge when their parent galaxies do, resulting in the rapid assembly of about 5 billion solar masses of gas, which may plausibly be identified with the large central clouds seen in CO observations of galaxies such as NGC 520. Violent star formation in such central gas clouds, which seems inevitable, offers a likely explanation for luminous IRAS galaxies and may contribute significantly to the central stellar populations of merger remnants. If some of the nuclear gas can continue to lose angular momentum, it may be able to fuel or even form a central black hole, resulting in a radio galaxy or possibly even a quasar.
We examine the properties of the host galaxies of 22 623 narrow-line
active galactic nuclei (AGN) with 0.02 < z < 0.3 selected from a
complete sample of 122 808 galaxies from the Sloan Digital Sky Survey.
We focus on the luminosity of the [OIII]λ5007 emission line as a
tracer of the strength of activity in the nucleus. We study how AGN host
properties compare with those of normal galaxies and how they depend on
L[OIII]. We find that AGN of all luminosities reside almost exclusively
in massive galaxies and have distributions of sizes, stellar surface
mass densities and concentrations that are similar to those of ordinary
early-type galaxies in our sample. The host galaxies of low-luminosity
AGN have stellar populations similar to normal early types. The hosts of
high-luminosity AGN have much younger mean stellar ages. The young stars
are not preferentially located near the nucleus of the galaxy, but are
spread out over scales of at least several kiloparsecs. A significant
fraction of high-luminosity AGN have strong Hδ absorption-line
equivalent widths, indicating that they experienced a burst of star
formation in the recent past. We have also examined the stellar
populations of the host galaxies of a sample of broad-line AGN. We
conclude that there is no significant difference in stellar content
between type 2 Seyfert hosts and quasars (QSOs) with the same [OIII]
luminosity and redshift. This establishes that a young stellar
population is a general property of AGN with high [OIII] luminosities.
Optical and radio observations of the nucleus of the galaxy M51 are presented. The observations show that M51 appears to have a bidirectional jet which emanates from the nucleus and interacts with the surrounding interstellar medium. H-alpha + forbidden N II monochromatic photographs and VLA 20 cm and 6 cm radio maps reveal a bright, resolved cloud and an extraordinary ring paired across the nucleus. The spectrophotometry shows that the nuclear emission-line characteristics change abruptly at the edges of the ring and the cloud, and that the cloud has internal velocities larger than the nucleus and a spectrum similar to some galactic supernova remnants. The physical characteristics of the nuclear emission regions are derived and used to discuss the ionization and excitation of the ring and cloud. It is argued that these features have their origin in a jet flowing from the nucleus. 75 references.
We present 2D mapping of the stellar velocity field within the inner 5 arcsec of six nearby active galaxies, using spectra obtained with the Integral Field Unit (IFU) of the Gemini Multi-Object Spectrograph instrument at the Gemini North telescope. The sampling of the observations is 0.2 arcsec, corresponding at the galaxies to spatial extents ranging from 10 to 30 pc. The spatial resolution range from 20 to about 180 pc, and the observed field of view covers a few hundred parsecs around the nuclei. The Ca ii triplet absorption features at ≈8500 Å were used to measure the stellar radial velocities and velocity dispersions. The radial velocity fields are dominated by rotation in all galaxies. A simple kinematical model assuming a purely rotating system with circular orbits in a plane was fitted to the radial velocity data. The turnover of the rotation curve is at only ≈50 pc for NGC 4051 and between 200 and 700 pc for the other five galaxies. The velocity dispersion (σ) maps show the largest values (100 ≥σ≥ 150 km s−1) at the centre. In the cases of NGC 2273 and 3227, there is a decrease to σ≈ 70–80 km s−1 at ≈200–300 pc from the nucleus, delineating partial rings of low-σ values. A similar broken ring seems to be present at ≈400 pc from the nucleus also in NGC 4593. We interpret these low-σ rings as traces of recently formed stars that partially keep the cold kinematics of the original gas from which they have formed. In NGC 3516 there is a decrease of σ outwards with the steepest gradient observed along the direction of the galaxy major axis, where σ reaches ≈80–90 km s−1 at ≈400 pc from the nucleus.
The main novelty of the present work is the unprecedented spatial resolution reached by a 2D study of stellar kinematics of Seyfert galaxies using an IFU. The few similar IFU studies available in the literature for Seyfert galaxies have a much poorer spatial resolution and/or are restricted to the study of emission-line kinematics.
Spectrophotometric measurements show five galaxies with strong narrow emission lines combined with weak, broad H-alpha and H-beta components to have line and continuum spectral properties intermediate between those of Seyfert 1 and Seyfert 2 galaxies. Because they are closer to the latter, they are called Seyfert 1.8 and Seyfert 1.9, depending on the presence of broad H-beta emissions in the scans. The observed intensity ratios of the broad H-alpha/H-beta emission lines are large compared with those of typical Seyfert 1 galaxies, suggesting the importance of dust in the broad-line regions. Narrow-line spectra of the Seyfert 1.8 and 1.9 are found to have low ionization, implying that their narrow-line region is reached by few high-energy photons. Emphasis is placed on the correlation between a strong, featureless continuum and broad H I emission lines in Seyfert and radio galaxies.
We present interferometric observations of CO and H I in the prototypical flocculent spiral NGC 5055, which has recently been shown to exhibit weak, two-arm spiral structure in the near-infrared. The morphology and kinematics described by VLA observations of H I emission and FCRAO and Berkeley-Illinois-Maryland Association (BIMA) Array observations of CO emission provide evidence for the presence of low-amplitude density waves in NGC 5055. The distribution of CO and H I emission suggests enhanced gas surface densities along the NIR spiral arms, and structures similar to the giant molecular associations found in the grand design spirals M51 and M100 are detected. An analysis of H I and H alpha velocity fields shows the kinematic signature of streaming motions similar in magnitude to those of M100 in both tracers. The lesser degree of organization along the spiral arms of NGC 5055 may be due to the lower overall gas surface density, which in the arms of NGC 5055 is a factor of 2 lower than in M100 and a factor of 6 lower than in M51; an analysis of gravitational instability shows the gas in the arms is only marginally unstable and the interarm gas is marginally stable. The limited extent of the spiral arm pattern is consistent with an isolated density wave with a relatively high pattern speed; other possible formation mechanisms are also discussed.
H I maps, velocity fields, position velocity plots, mosaics of the line channel, and H I proflies are presented for the 25 brightest spiral galaxies located in the center of the Virgo Cluster. A synthetic map presents the maps of all the individual galaxies. It confirms that the galaxies located within 3 deg of M87 have H I disks that are significantly smaller than the optical disks. Further away, a number of galaxies have a very asymmetric H I distribution and a sharp edge on the side which points toward M87. The anemic galaxies usually show a ring structure and a patchy aspect. Several gas-rich dwarf galaxies have been found close to H I deficient galaxies, and one has been found close to the cluster center. The galaxies located in the western part are larger than in the eastern part. Furthermore the most severely stripped spirals show signs of asymmetries in their gas distribution combined with enhanced star formation at the compressed side of the gas disk. This might suggest that the Virgo Cluster exhibits a mild form of ram-pressure enhanced star formation, thought to be an important process in clusters at higher redshifts.
It is argued that supermassive accretion in disks may explain the flat optical continuum observed from some QSOs and Seyfert galaxies. With the aid of energy considerations it is shown that a luminous QSO may be powered by accretion onto a black hole with a mass corresponding to that of 100 million solar masses. It is proposed that the blue continuum component of 3C 273 is thermal continuum emission from the inner part of an accretion disk that powers 3C 273. The present model suggests that many QSOs and Seyfert galaxies may show a power law continuum plus thermal emission from the disk at optical or UV frequencies. Many such objects show indeed a flattening or upturn of their continua in the blue. The QSO PHL 957 shows a flat continuum and indications of a cutoff at the highest observed frequencies.
The idea that interactions between galaxies can lead to enhanced galactic activity has a long and noble lineage. Since the early days of Baade and Minkowski (1954), through the seminal paper of Toomre and Toomre (1977) with its “stoking the furnace” imagery, to the similarly colorful and influential “feeding the monster” paper of Gunn (1979), and the landmark work by Larson and Tinsley (1978), this idea was elaborated upon and generalized. The pace of development of this hypothesis has accelerated greatly on both the observational and theoretical fronts during the 1980’s, due in part to a growing perception that the AGN phenomenon could only be understood when the role of the environment in triggering or nurturing the activity was understood (cf. Balick and Heckman 1982). In recent years, the impact of the extragalactic IRAS database (on the observational side) and of the new generation of supercomputers and innovative software (on the numerical side) have led to an almost explosive growth in the number of papers written about this subject.
A survey was made of typical galactic nuclei in a statistically complete
sample of roughly 90 bright galaxies in order to detect compact
nonthermal radio sources and emission-line regions not excited by normal
stars. A class of Low Ionization Nuclear Emission-line Regions (Liners)
occurring commonly in typical galactic nuclei is defined, whose
principal characteristic is the relative strength of emission lines from
species of low ionization state, and whose luminosity (L(H alpha) of
about 10 to the 38th to 10 to the 40th ergs/s) is more similar to that
of a typical giant H II region than to the Seyfert galaxies and QSOs. In
addition, 13 Compact Nuclear Radio Sources (CNRS) have been detected in
the sample; the presence and strength of a Liner is found to be
statistically related to the presence and strength of a CNRS.
Groups of nearby, loose galaxies are the object of a search for H I
streams, the neutral hydrogen appendages extending outward from the
disks of member galaxies. Galaxy parameter tables are presented which
summarize observational data gathered with the Arecibo telescope on 15
nearby groups, containing at least one pair of galaxies separated by
less than 250 kpc. Of the 15 groups, nine show no evidence of neutral
hydrogen appendages, while in the remaining six H I streams associated
with at least one member of the aggregate are identified. While
previously recognized H I streams have been almost exclusively
associated with late-type spiral galaxies, several of those newly
identified extend in the regions of early-type galaxies. The 15 nearby
groups considered are comprised of 47 galaxies observed in the course of
the H I stream search.
Inner and outer velocity gradients, residuals from synthetic rotation
curves for field spirals, and M/L gradients are used to study the
rotation curves of spiral galaxies. A good correlation is found between
the outer gradient of the rotation curve and the galaxy's distance from
the cluster center, with a similar effect noted in the residuals between
the observed and synthetic rotation curves. A correlation is also shown
to exist between the M/L gradient across a galaxy and the galaxy's
position in the cluster. The results suggest that the inner cluster
environment may strip away some fraction of the mass in the outer halo
of a spiral galaxy or may not allow the halo to form.
We have performed two-dimensional spectroscopy of the Seyfert galaxies
NGC 5728 and 4151 in the 4500-7500A spectral range. The data (especially
the [OIII] lambdalambda4959, 5007 lines), obtained using a new optical
fibre system, complement previous observational work carried out by
means of this technique. The two gaseous components found in NGC 5728
seem to be ionized by the same source, although they have different
kinematics. They coexist (projected) in a strip-shaped zone aligned in
the NE-SW direction. We propose that one of the components, probably
related to outflow, energies at the north-west edge of the obscuring
material which hides the nucleus and gives rise to the biconical
structure of ionization observed in NGC 5728. The cones of ionizing
radiation seem to be tilted with respect to the galaxy's plane. The
velocity field of NGC 4151 derived from the ionized gas is complex,
showing two receding poles. Hence the location of the kinematical centre
is not straight-forward and the active nucleus could be off-centre. The
maxima of the line-intensity and several continuum maps are in
positional agreement within an uncertainty of +/-0.13 arcsec, however,
in contrast with previous results. In addition, from the elliptical
fitting of the surface brightness distribution in the I band, it may be
inferred that the optical nucleus lies at the centre of a remarkably
regular and spherical galactic bulge. This seems to establish the
identification of the optical nucleus with the galaxy's mass centroid.
One result common to these galaxies is that similar kinematical
properties are inferred from the [OIII] and Hα narrow emission
lines. This contrasts with the differences found in other Seyfert
galaxies like NGC 7469 or 3227 where two ionization sources
(starburst-like and AGN-like) associated with different kinematical
systems seem to coexist.
Environments of 104 Seyferts and 138 comparison galaxies have been
analyzed based on measurements on the Palomar Sky Survey plates. Our
basic idea was to have large enough measuring circles to enable good
elimination of background galaxies and to compare the environments of
Seyfert 1 and 2 galaxies. Seyferts were also compared with normal
galaxies. The Seyferts were found to have on the average about two times
more companions than the control galaxies. The satellites are
concentrated mainly around late-type and non-compact peculiar Seyfert 2
galaxies, which may each have several companions. On the other hand,
Seyfert 1 galaxies of all morphological types have on the average the
same number of companions as normal galaxies. Seyferts as a whole do not
appear in interacting systems more frequently than normal galaxies
(Dahari-type tests) giving further evidence that the satellites are
concentrated around a few objects only. Of type 2 Seyferts only peculiar
galaxies are preferentially in interacting systems, whereas type 1
Seyfert neighbourhoods are normal or even deficient of companions. The
difference in the environments of Seyfert 1 and Seyferts 2 galaxies
disagrees with the Unified Model of AGN's according to which these two
Seyferts types should represent similar objects seen from different
viewing angles. While comparing Seyferts with normal galaxies our
conclusions disagree with the previous studies as shown in the
discussion. However, the apparent incompatibilites of the results in
different works can be understood on the basis of morphological sample
selection, problems in background galaxy elimination or bias in redshift
between the compared samples. Particularly we tested the methods by
Dahari (1984) and Fuentes-Williams & Stocke (1988).
Measurements from 12 to 100 microns of 179 quasars observed with the
IRAS satellite are presented; of these 74 have detections with
signal-to-noise ratios greater than four in at least one wavelength
band. The infrared flux densities of the quasars with flat radio spectra
generally lie on a smooth interpolation between measurements at visible
and radio wavelengths. For the radio quiet quasars the peak in the flux
densities appears in the far-infrared. The luminosities near 60 microns
of the flat spectrum radio sources extend to significantly higher values
than do those of the radio quiet quasars or those with steep radio
spectral indexes. From these observations there are no infrared
properties of quasars that are strongly correlated with their radio
properties. There is strong circumstantial evidence from the
observations that the infrared emission from the quasars whose radio
emission has a flat spectrum is predominantly nonthermal in origin.
We used the VLA-D configuration to synthesize the spatial distribution
of λ 21 cm emission from the well-known Seyfert galaxies NGC 4051
and 6814. Both systems are characterized by an annular distribution of
atomic gas but that in 4051 is generally much more regular, and more
nearly contiguous with the optical image; still, it may be warped at
large radii, as evidenced by the presence of a second, weaker kinematic
component along the major axis. The gas disk of NGC 6814 is much larger
than the optical image, which sits entirely within the central H I
depression, and is substantially kinematically warped outside the
optical image in the main body of the galaxy.
Deep imaging of a sample of 25 powerful radio galaxies in the redshift
range between 0.15 and 0.82 is presented. Amplitudes of the galaxy
cross-correlation function about each source are derived. The powerful
radio galaxies in this sample at redshifts above 0.3 occupy environments
nearly as rich on average as Abell class 0 clusters of galaxies, about
three times richer than the environments of the redshifts below 0.3
radio galaxies. The imaging confirms that the most powerful radio
galaxies do inhabit rich environments.
We present bidimensional spectroscopy of the circumnuclear region of the
Seyfert galaxy NGC 3227. The physical and kinematical characteristics of
the two gaseous components reported recently by Mediavilla & Arribas
are discussed. One of the components seems to be directly related to
activity. We propose that it represents gas streaming from the nucleus
toward the north. The other component has more regular properties,
appearing to be kinematically connected with the galactic mean velocity
field. The broadening distribution inferred from this component is not
centered around the broad-line region but next to the kinematical center
derived from the ionized gas. This fact gives independent support to the
offset location of the active nucleus with respect to the galaxy mass
centroid. Despite this anomaly, the velocity field of NGC 3227 exhibits
a rotational pattern. Fitting a simple model to the observed velocity
field we found that the resulting parameters agree fairly well with
those expected for spirals if warps on the galactic plane are allowed.
This result is obtained assuming the kinematical center derived from the
ionized gas. On the contrary, the broad-line region (BLR) occupies a
fairly asymmetrical position in the velocity field and could hardly be
taken as the kinematical center. The ionization structure is rather
complex in NGC 3227. Analysis of the line flux ratios shows Seyfert and
low-ionization nuclear emission-line region (LINER) emission
characteristics in most of the central regions. This suggests that an
active galactic nuclei (AGN)-like continuum (with varying ionization
parameter) is likely to be the main source of ionization of the gas in
these regions. However, H II-type emission is found over an extended
area at about 300 pc from the optical nucleus, suggesting that young
stars are responsible of the ionization of the gas in this region, which
could be identified as an extranuclear starburst. Other zones, where
thermal and AGN-like continua seem to contribute in a comparable manner
to the ionization, are also found. In this respect, NGC 3227 is another
example of a galaxy with composite AGN/H II ionization.
We present broadband BVI and H-alpha photometry and spectroscopy along
the major and minor axis of the active, isolated galaxy NGC 6951. All
the results point to the conclusion that, apart from its nuclear
activity, it is a normal barred Sbc galaxy. The only peculiarity is the
too low present star formation rate, with a narrow, well-delineated ring
5 arcsec from the center surrounded by dust. The morphology and
properties of the inner regions could be the result of the inflow of gas
caused by bar-induced instabilities, as predicted by models. We argue
that the nuclear activity in isolated galaxies could also be triggered
by those same mechanisms and therefore related to the presence of a bar.
We study the gas dynamics in barred galaxies using time-dependent
hydrodynamic simulations. To achieve high resolution near the galaxy's
center, the simulations are performed in cylindrical coordinates using a
nonuniform radial grid. The gravitational potential of the bar is
assumed to be time-independent and is modeled using a Ferrers ellipsoid.
We find that the gas flow evolves to a quasi-steady state in roughly
five bar orbits, and the general features of this steady state are
similar to previous studies. However, we also find that if the
gravitational potential has two inner Lindblad resonances, and if along
the major and minor axes the extremum of the Ω - κ/2 curve
between these resonances is at the same radial position, then the gas
flow forms a dense nuclear ring located at the position of the extremum,
or approximately 1 kpc for the models studied here. These two
requirements are met by most models which have low axial ratio, i.e.,
thick bars. We study the development, evolution, and properties of the
nuclear rings observed in our simulations in detail. We also study the
effect of the bar on mass inflow into the nucleus of the galaxy. We find
this inflow is highest for models with high axial ratio, i.e., thin,
bars (which do not produce nuclear rings), where we find mass inflows of
0.25 Msun yr-1 into the inner 0.1 kpc.
A new technique to analyze the kinematics of early-type galaxies is
presented. The method is shown to be superior to classical methods
because of its low sensitivity to template mismatching and its ability
to unravel the line-of-sight integrated velocity distributions of
early-type galaxies. The Fourier correlation quotient method is used to
analyze the kinematics of NGC4621 along the projected major axis. The
ratio of projected rotation velocity to projected velocity dispersion
indicates that the flattening of NGC4621 is due to rotation.
Explanations for the observed asymmetry of the velocity distributions
along the line of sight are considered.
An indirect method is outlined for mapping the emission line regions of
Seyfert galaxies and quasars having variable line intensities and
profiles, provided that (1) the galactic nuclei in question comprise a
compact, central continuum source surrounded by an extended distribution
of line-emitting gas, and (2) that the covering factor of the gas be
small and the entire region optically thin to Thomson scattering so that
most continuum and line photons may escape directly from the nucleus.
The results obtained are demonstrated by means of models in which the
line luminosity of an element of gas is directly proportional to the
incident observed continuum flux. Additional assumptions justified on
physical grounds are rapid reprocessing and the stability of the gas
distribution over several light crossing times.
New interferometric observations of the molecular gas within the early
type barred spiral galaxy NGC 3351 (M95) have revealed a nuclear
molecular gas bar about 1 kpc long, that is aligned perpendicular to the
large scale stellar bar. The mass of gas associated with the nuclear
molecular gas bar is about 1.8 x 10 exp 8 solar masses representing
about 25 percent of the total molecular gas mass. NGC 3351 is associated
with other distinguishing morphological characteristics including a
nuclear ring of H II regions, which brought NGC 3351 early notoriety as
a 'hotspot nucleus', and a much larger ring of H II regions that
encircles the stellar bar. Collectively, the distinctive morphology of
the neutral and ionized medium within NGC 3351 can be understood in
terms of resonances associated with a stellar bar pattern rotating at an
angular velocity of about 60 km/s kpc.
The results of a search for close companions in two magnitude limited
samples (mB less than or equal to 15.5) of 99 Seyfert 1 and
98 Seyfert 2 galaxies are presented. It is found that there is an excess
of physical companions in both samples of galaxies, compared with two
control samples of normal field spiral galaxies. The calculated lower
limit percentages of physical companions are (12 +/- 3)% and (12 +/- 4)%
for Seyfert 1 and Seyfert 2, respectively, while the upper limits on the
percentage of galaxies with physical companions are in the range (0%-5%)
in both control samples. The excess is confirmed by analysis of the
redshifts for all pairs found in the complete, magnitude limited (B(O)
less than or equal to 14.5) CfA sample of Seyfert galaxies.