The Circumnuclear Molecular Gas in the Seyfert Galaxy NGC 4945 (original) (raw)
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The dusty torus in the Circinus galaxy: a dense disk and the torus funnel
Astronomy & Astrophysics, 2014
Context. With infrared interferometry it is possible to resolve the nuclear dust distributions that are commonly associated with the dusty torus in active galactic nuclei (AGN). The Circinus galaxy hosts the closest Seyfert 2 nucleus and previous interferometric observations have shown that its nuclear dust emission is particularly well resolved. Aims. The aim of the present interferometric investigation is to better constrain the dust morphology in this active nucleus. Methods. To this end, extensive new observations were carried out with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer, leading to a total of 152 correlated flux spectra and differential phases between 8 and 13 µm.
The Astrophysical Journal, 1999
We present high resolution (3-5) observations of CO(J=1−0) and HCN(J=1−0) emission from the circumnuclear star forming ring in the barred spiral galaxy NGC 6951, a host of a type-2 Seyfert nucleus, using the Nobeyama Millimeter Array and the Nobeyama 45 m telescope. We find that the distribution of the HCN emission is different from that of CO in the circumnuclear region of NGC 6951; it is confirmed that CO emission is dominated by "twin peaks" morphology with two spiral arms, which are connected to the dust lanes, as reported by Kenney et al. (1992). On the other hand, although the HCN emission also shows a twin peaks morphology, the HCN peaks are spatially shifted downstream compared with the CO peaks. Most of the HCN emission is associated with the circumnuclear ring, where vigorous star formation occurs. The HCN to CO integrated intensity ratio in the brightness temperature scale, R HCN/CO , is also enhanced in the star forming ring. The peak value of the R HCN/CO is about 0.16-0.18, which is comparable to the R HCN/CO in the starburst regions of NGC 253 and M82. Consequently, the HCN emission spatially correlates better with the massive star forming regions than the CO emission in the circumnuclear region of NGC 6951. The formation mechanism of dense molecular gas has been investigated. No significant enhancement of R HCN/CO is observed at the CO peaks, which are interpreted as x 1 /x 2 orbit crowding regions. This suggests that the shocks along the orbit crowding do not promote the formation of dense molecular gas effectively but enhance the presence of low density GMCs in NGC 6951. Instead, gravitational instability can account for the dense molecular gas formation in the circumnuclear star forming ring because Toomre's Q value is below unity there. The R HCN/CO toward the type-2 Seyfert nucleus of NGC 6951 is 0.086 averaged over central r < 120 pc region. This is a rather normal value compared with non-active galaxies such as the Milky Way, and quite different from other type-2 Seyfert galaxies NGC 1068 and M51 where extremely high R HCN/CO of ∼ 0.5 have been reported. The variety of R HCN/CO values in these Seyfert nuclei would be attributed to the different physical conditions of the molecular gas around the nuclei.
CIRCUMNUCLEAR MOLECULAR GAS IN MEGAMASER DISK GALAXIES NGC 4388 AND NGC 1194
The Astrophysical Journal, 2014
We explore the warm molecular and ionized gas in the centers of two megamaser disk galaxies using K−band spectroscopy. Our ultimate goal is to determine how gas is funneled onto the accretion disk, here traced by megamaser spots on sub-pc scales. We present NIR IFU data with a resolution of ∼ 50 pc for two galaxies: NGC 4388 with VLT/SINFONI and NGC 1194 with Keck/OSIRIS+AO. The high spatial resolution and rich spectral diagnostics allow us to study both the stellar and gas kinematics as well as gas excitation on scales only an order of magnitude larger than the maser disk. We find a drop in the stellar velocity dispersion in the inner ∼ 100 pc of NGC 4388, a common signature of a dynamically cold central component seen in many active nuclei. We also see evidence for non-circular gas motions in the molecular hydrogen on similar scales, with the gas kinematics on 100-pc scales aligned with the megamaser disk. In contrast, the high ionization lines and Brγ trace outflow along the 100 pc-scale jet. In NGC 1194, the continuum from the accreting black hole is very strong, making it difficult to measure robust two-dimensional kinematics, but the spatial distribution and line ratios of the molecular hydrogen and Brγ have consistent properties between the two galaxies.
The Astrophysical Journal, 2013
We present millimeter molecular-line complemented by optical observations, along with a reanalysis of archival centimeter H I and continuum data, to infer the global dynamics and determine where dense molecular gas and massive stars preferentially form in the circumnuclear starburst ring of the barred-spiral galaxy NGC 7552. We find diffuse molecular gas in a pair of dust lanes each running along the large-scale galactic bar, as well as in the circumnuclear starburst ring. We do not detect dense molecular gas in the dust lanes, but find such gas concentrated in two knots where the dust lanes make contact with the circumnuclear starburst ring. When convolved to the same angular resolution as the images in dense gas, the radio continuum emission of the circumnuclear starburst ring also exhibits two knots, each lying downstream of an adjacent knot in dense gas. The results agree qualitatively with the idea that massive stars form from dense gas at the contact points, where diffuse gas is channeled into the ring along the dust lanes, and later explode as supernovae downstream of the contact points. Based on the inferred rotation curve, however, the propagation time between the respective pairs of dense gas and centimeter continuum knots is about an order of magnitude shorter than the lifetimes of OB stars. We discuss possible reasons of this discrepancy, and conclude that either the initial mass function is top-heavy or massive stars in the ring do not form exclusively at the contact points where dense molecular gas is concentrated.
The effect of stellar feedback on the formation and evolution of gas and dust tori in AGN
Monthly Notices of the Royal Astronomical Society, 2009
Recently, the existence of geometrically thick dust structures in Active Galactic Nuclei (AGN) has been directly proven with the help of interferometric methods in the mid-infrared. The observations are consistent with a two-component model made up of a geometrically thin and warm central disk, surrounded by a colder, fluffy torus component. Within the framework of an exploratory study, we investigate one possible physical mechanism, which could produce such a structure, namely the effect of stellar feedback from a young nuclear star cluster on the interstellar medium in centres of AGN. The model is realised by numerical simulations with the help of the hydrodynamics code TRAMP. We follow the evolution of the interstellar medium by taking discrete mass loss and energy ejection due to stellar processes, as well as optically thin radiative cooling into account. In a post-processing step, we calculate observable quantities like spectral energy distributions and surface brightness distributions with the help of the radiative transfer code MC3D. The interplay between injection of mass, supernova explosions and radiative cooling leads to a two-component structure made up of a cold geometrically thin, but optically thick and very turbulent disk residing in the vicinity of the angular momentum barrier, surrounded by a filamentary structure. The latter consists of cold long radial filaments flowing towards the disk and a hot tenuous medium in between, which shows both inwards and outwards directed motions. With the help of this modelling, we are able to reproduce the range of observed neutral hydrogen column densities of a sample of Seyfert galaxies as well as the relation between them and the strength of the silicate 10 µm spectral feature. Despite being quite crude, our mean Seyfert galaxy model is even able to describe the SEDs of two intermediate type Seyfert galaxies observed with the Spitzer Space Telescope.
A SINFONI view of the nuclear activity and circumnuclear star formation in NGC 4303
Monthly Notices of the Royal Astronomical Society, 2016
We present new maps of emission-line flux distributions and kinematics in both ionized (traced by H I and [Fe II] lines) and molecular (H 2) gas of the inner 0.7 × 0.7 kpc 2 of the galaxy NGC 4303, with a spatial resolution 40-80 pc and velocity resolution 90-150 km s −1 obtained from near-IR integral field spectroscopy using the Very Large Telescope instrument SINFONI. The most prominent feature is a 200-250 pc ring of circumnuclear star-forming regions. The emission from ionized and molecular gas shows distinct flux distributions: while the strongest H I and [Fe II] emission comes from regions in the west side of the ring (ages ∼ 4 Myr), the H 2 emission is strongest at the nucleus and in the east side of the ring (ages > 10 Myr). We find that regions of enhanced hot H 2 emission are anti-correlated with those of enhanced [Fe II] and H I emission, which can be attributed to post-starburst regions that do not have ionizing photons anymore but still are hot enough (≈2000 K) to excite the H 2 molecule. The line ratios are consistent with the presence of an active galactic nucleus at the nucleus. The youngest regions have stellar masses in the range 0.3-1.5 × 10 5 M and ionized and hot molecular gas masses of ∼0.25-1.2 × 10 4 M and ∼2.5-5 M , respectively. The stellar and gas velocity fields show a rotation pattern, with the gas presenting larger velocity amplitudes than the stars, with a deviation observed for the H 2 along the nuclear bar, where increased velocity dispersion is also observed, possibly associated with non-circular motions along the bar. The stars in the ring show smaller velocity dispersion than the surroundings, which can be attributed to a cooler dynamics due to their recent formation from cool gas.
Proceedings of the International Astronomical Union, 2013
We suggest a new formation mechanism for the inclined, sub-parsec scale and counterrotating stellar disks observed around the central black hole in the Milky Way Galactic center. The simulation of a single molecular cloud crashing into a circumnuclear ring of gas leads to the inflow of multiple streams of gas towards the central parsec region. The time delayed arrival of those streams forms multiple, sub-parsec scale accretion disks, with angular momentum depending on the ratio of cloud and circumnuclear ring material. These accretion disks could then be the progenitors which fragmented into the observed stellar disks. A similar event might have also led to the creation of the so-called minispiral in the Galactic center.
Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy
Astronomy & Astrophysics, 2007
To test the dust torus model for active galactic nuclei directly, we study the extent and morphology of the nuclear dust distribution in the Circinus galaxy using high resolution interferometric observations in the mid-infrared with the MIDI instrument at the Very Large Telescope Interferometer. We find that the dust distribution in the nucleus of Circinus can be explained by two components, a dense and warm disk-like component of 0.4 pc size and a slightly cooler, geometrically thick torus component with a size of 2.0 pc. The disk component is oriented perpendicular to the ionisation cone and outflow and seems to show the silicate feature at 10 micron in emission. It coincides with a nuclear maser disk in orientation and size. From the energy needed to heat the dust, we infer a luminosity of the accretion disk corresponding to 20% of the Eddington luminosity of the nuclear black hole. We find that the interferometric data are inconsistent with a simple, smooth and axisymmetric dust emission. The irregular behaviour of the visibilities and the shallow decrease of the dust temperature with radius provide strong evidence for a clumpy or filamentary dust structure. We see no evidence for dust reprocessing, as the silicate absorption profile is consistent with that of standard galactic dust. We argue that the collimation of the ionising radiation must originate in the geometrically thick torus component. Our findings confirm the presence of a geometrically thick, torus-like dust distribution in the nucleus of Circinus, as required in unified schemes of Seyfert galaxies. Several aspects of our data require that this torus is irregular, or "clumpy".
Molecular gas and dust in NGC 4550
Astronomy and Astrophysics, 2001
We report the detection of 1 × 10 7 M of molecular gas in the central region of the S0/E7 galaxy NGC 4550, inferred from observations of CO(1-0) emission. Dust is detected in HST WFPC2 images and found to be asymmetrically distributed around the nucleus, only extending to a galactocentric distance of 7 (600 pc). The shape of the CO emission profile is consistent with a molecular gas distribution following the dust. The distribution of the dust and gas in the center could be the result of an m = 1 instability, which is the fastest growing unstable mode in counterrotating stellar disks. On a global scale the molecular gas in NGC 4550 is stable against gravitational collapse but nevertheless star formation appears to be ongoing with normal star formation efficiency and gas consumption time scales. The stellar velocity dispersion in NGC 4550 resembles that of elliptical galaxies. It is therefore likely that a hot X-ray emitting plasma limits the lifetime of the molecular gas, that must arise from a recent (1 Gyr) accretion event.
Molecular gas in NUclei of GAlaxies (NUGA) XIII. The interacting Seyfert 2/LINER galaxy NGC 5953
Astronomy and Astrophysics, 2010
We present 12 CO(1-0) and 12 CO(2-1) maps of the interacting Seyfert 2/LINER galaxy NGC 5953 obtained with the IRAM interferometer at resolutions of 2. ′′ 1 × 1. ′′ 4 and 1. ′′ 1 × 0. ′′ 7, respectively. We also present single-dish IRAM 30 m observations of the central region of NGC 5953 for the 12 CO(1-0), 12 CO(2-1), and HCN(1-0) transitions at resolutions of 22 ′′ , 12 ′′ , and 29 ′′ , respectively. The CO emission is distributed over a disk of diameter ∼16 ′′ (∼2.2 kpc), within which are several, randomly distributed peaks. The strongest peak does not coincide with the nucleus, but is instead offset from the center, ∼2-3 ′′ (∼340 pc) toward the west/southwest. The kinematics of the molecular component are quite regular, as is typical of a rotating disk. We also compared the 12 CO distribution of NGC 5953 with observations at other wavelengths in order to study correlations between different tracers of the interstellar medium. The HST/F606W WFPC2 images show flocculent spiral structures and an "S-shape" feature > ∼ 60 pc in radius, possibly associated with a nuclear bar or with the radio jet. A two-dimensional bulge/disk decomposition of the H-band (HST/F160W) and 3.6 µm (Spitzer/IRAC) images reveals a circumnuclear "ring" ∼10-14 ′′ in diameter, roughly coincident in size with the CO disk and with a star-forming ring previously identified in ionized gas. This ring is not present in the near-infrared (NIR) J − K color image, nor is it present in the "dust-only" image constructed from the 8 µm IRAC map. The implication is that the excess residual ring is stellar, with colors similar to the surrounding disk. We interpret this ring, visible in ionized gas, which appears as stars in the NIR, and with no sign of hot dust, as due to a red super giant population at least 10-15 Myr old. However, star formation is still ongoing in the disk and in the ring itself. Using NIR images, we computed the gravity torques exerted by the stellar potential on the gas. The torques are predominantly positive in both 12 CO(1-0) and 12 CO(2-1), suggesting that gas is not flowing into the center, and less than 5% of the gas angular momentum is exchanged in each rotation. This comes from the regular and almost axisymmetric total mass and gas distributions in the center of the galaxy. In NGC 5953, the AGN is apparently not being actively fueled in the current epoch.