Circumnuclear Starburst Regions Research Articles

Solving the Multimessenger Puzzle of the AGN-starburst Composite Galaxy NGC 1068

Multiwavelength observations indicate that some starburst galaxies show a dominant nonthermal contribution from their central region. These active galactic nuclei (AGN)-starburst composites are of special interest, as both phenomena on their own are potential sources of highly energetic cosmic rays and associated γ-ray and neutrino emission. In this work, a homogeneous, steady-state two-zone multimessenger model of the nonthermal emission from the AGN corona as well as the circumnuclear starburst region is developed and subsequently applied to the case of NGC 1068, which has recently shown some first indications of high-energy neutrino emission. Here, we show that the entire spectrum of multimessenger data—from radio to γ-rays including the neutrino constraint—can be described very well if both, starburst and AGN corona, are taken into account. Using only a single emission region is not sufficient.

DETECTION OF A METHANOL MEGAMASER IN A MAJOR-MERGER GALAXY

We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and 7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of the closest ultra-luminous infrared galaxy Arp 220. The emission in both the methanol transitions show narrow spectral features and have luminosities approximately 8 orders of magnitude stronger than that observed from typical class I methanol masers observed in Galactic star formation regions. The emission is also orders of magnitude stronger than the expected intensity of thermal emission from these transitions and based on these findings we suggest that the emission from the two transitions are masers. These observations provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz transitions and represents only the second detection of a methanol megamaser, following the recent report of an 84 GHz methanol megamaser in NGC1068. We find the methanol megamasers are significantly offset from the nuclear region and arise towards regions where there is Ha emission, suggesting that it is associated with starburst activity. The high degree of correlation between the spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests that the production of strong extragalactic class I methanol masers is related to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH and H2O megamasers which originate close to the nucleus, methanol megamasers provide a new probe of feedback (e.g. outflows) processes on larger-scales and of star formation beyond the circumnuclear starburst regions of active galaxies.

Probing the nuclear and circumnuclear activity of NGC 1365 in the infrared

We present new far-infrared (70-500micron) Herschel PACS and SPIRE imaging observations as well as new mid-IR Gemini/T-ReCS imaging (8.7 and 18.3micron) and spectroscopy of the inner Lindblad resonance (ILR) region (R<2.5kpc) of the spiral galaxy NGC1365. We complemented these observations with archival Spitzer imaging and spectral mapping observations. The ILR region of NGC1365 contains a Seyfert 1.5 nucleus and a ring of star formation with an approximate diameter of 2kpc. The strong star formation activity in the ring is resolved by the Herschel/PACS imaging data, as well as by the Spitzer 24micron continuum emission, [NeII]12.81micron line emission, and 6.2 and 11.3micron PAH emission. The AGN is the brightest source in the central regions up to lambda~24micron, but it becomes increasingly fainter in the far-infrared when compared to the emission originating in the infrared clusters (or groups of them) located in the ring. We modeled the AGN unresolved infrared emission with the CLUMPY torus models and estimated that the AGN contributes only to a small fraction (~5%) of the infrared emission produced in the inner ~5kpc. We fitted the non-AGN 24-500micron spectral energy distribution of the ILR region and found that the dust temperatures and mass are similar to those of other nuclear and circumnuclear starburst regions. Finally we showed that within the ILR region of NGC1365 most of the on-going star formation activity is taking place in dusty regions as probed by the 24micron emission.

DETECTIONS OF C 2 H, CYCLIC-C 3 H 2 , AND H 13 CN IN NGC 1068

We used the Nobeyama 45-m telescope to conduct a spectral line survey in the 3-mm band (85.1-98.4 GHz) toward one of the nearest galaxies with active galactic nucleus NGC 1068 and the prototypical starburst galaxy NGC 253. The beam size of this telescope is ~18", which was sufficient to spatially separate the nuclear molecular emission from the emission of the circumnuclear starburst region in NGC 1068. We detected rotational transitions of C2H, cyclic-C3H2, and H13CN in NGC 1068. These are detections of carbon-chain and carbon-ring molecules in NGC 1068. In addition, the C2H N = 1-0 lines were detected in NGC 253. The column densities of C2H were determined to be 3.4 x 10^15 cm^-2 in NGC 1068 and 1.8 x 10^15 cm^-2 in NGC 253. The column densities of cyclic-C3H2 were determined to be 1.7 x 10^13 cm^-2 in NGC 1068 and 4.4 x 10^13 cm^-2 in NGC 253. We calculated the abundances of these molecules relative to CS for both NGC 1068 and NGC 253, and found that there were no significant differences in the abundances between the two galaxies. This result suggests that the basic carbon-containing molecules are either insusceptible to AGN, or are tracing cold (T_rot ~10 K) molecular gas rather than X-ray irradiated hot gas.

ULTRAVIOLET SPECTROSCOPY OF CIRCUMNUCLEAR STAR CLUSTERS IN M83

We analyze archival HST/STIS/FUV-MAMA imaging and spectroscopy of 13 compact star clusters within the circumnuclear starburst region of M83, the closest such example. We compare the observed spectra with semi-empirical models, which are based on an empirical library of Galactic O and B stars observed with IUE, and with theoretical models, which are based on a new theoretical UV library of hot massive stars computed with WM-Basic. The models were generated with Starburst99 for metallicities of Z=0.020 and Z=0.040, and for stellar IMFs with upper mass limits of 10, 30, 50, and 100 M_sol. We estimate the ages and masses of the clusters from the best fit model spectra, and find that the ages derived from the semi-empirical and theoretical models agree within a factor of 1.2 on average. A comparison of the spectroscopic age estimates with values derived from HST/WFC3/UVIS multi-band photometry shows a similar level of agreement for all but one cluster. The clusters have a range of ages from about 3 to 20 Myr, and do not appear to have an age gradient along M83's starburst. Clusters with strong P-Cygni profiles have masses of a few times 10^4 M_sol, seem to have formed stars more massive than 30 M_sol, and are consistent with a Kroupa IMF from 0.1-100 M_sol. Field regions in the starburst lack P-Cygni profiles and are dominated by B stars.

The central region of M83

We combine VLT/ISAAC NIR spectroscopy with archival HST/WFPC2 and HST/NICMOS imaging to study the central 20x20 of M83. Our NIR indices for clusters in the circumnuclear star-burst region are inconsistent with simple instantaneous burst models. However, models of a single burst dispersed over a duration of 6 Myrs fit the data well and provide the clearest evidence yet of an age gradient along the star forming arc, with the youngest clusters nearest the north-east dust lane. The long slit kinematics show no evidence to support previous claims of a second hidden mass concentration, although we do observe changes in molecular gas velocity consistent with the presence of a shock at the edge of the dust lane.

The stellar mass spectrum in warm and dusty gas: deviations from Salpeter in the Galactic centre and in circumnuclear starburst regions

Abstract Understanding the origin of stellar masses is a key problem in astrophysics. In the solar neighbourhood, the mass distribution of stars follows a seemingly universal pattern. In the centre of the Milky Way, however, there are indications for strong deviations and the same may be true for the nuclei of distant starburst galaxies. Here we present the first numerical hydrodynamical calculations of stars formed in a molecular region with chemical and thermodynamic properties similar to those of warm and dusty circumnuclear starburst regions. The resulting initial mass function is top-heavy with a peak at ∼15 M⊙, a sharp turn-down below ∼7 M⊙ and a power-law decline at high masses. We find a natural explanation for our results in terms of the temperature dependence of the Jeans mass, with collapse occurring at a temperature of ∼100 K and an H2 density of a few ×105 cm−3, and discuss possible implications for galaxy formation and evolution.

Magnetic fields in barred galaxies

We present λ3.5 cm and A6.2 cm radio continuum maps in total and polarized intensity of the barred galaxies NGC 1097 (at 2-15 resolution) and NGC 1365 (at 9-25 resolution). A previously unknown radio galaxy southwest of NGC 1097 is reported. Apart from a smooth faint envelope and a bright central region, both galaxies exhibit radio ridges roughly overlapping with the massive dust lanes in the bar region. The contrast in total intensity across the radio ridges is compatible with compression and shear of an isotropic random magnetic field, where the gas density compression ratio is approximately equal to 4 and the cosmic ray density is constant across the ridges. The contrast in polarized intensity is significantly smaller than that expected from compression and shearing of the regular magnetic field: this could be the result of decoupling of the regular field from the dense molecular clouds. The regular field in the ridge is probably strong enough to reduce significantly shear in the diffuse gas (to which it is coupled) and hence to reduce magnetic field amplification by shearing. This contributes to the misalignment of the observed field orientation with respect to the velocity vectors of the dense gas. Our observations, for the first time, indicate that magnetic forces can control the flow of the diffuse inlerstellar gas at kilopursec scales. The total radio intensity reaches its maximum in the circumnuclear starburst regions, where the equipartition field strength is about 60μG, amongst the strongest fields detected in spiral galaxies so far. The regular field in the inner region has a spiral shape with large pitch angle, indicating the action of a dynamo. Magnetic stress leads to mass inflow towards the centre, sufficient to feed the active nucleus in NGC 1097. - We detected diffuse X-ray emission, possibly forming a halo of hot gas around NGC 1097.

Molecular Gas Structure around an Active Galactic Nucleus with Nuclear Starburst: Three‐dimensional Non‐LTE Calculations of CO Lines

We have performed three-dimensional, non-LTE (non-local thermodynamic equilibrium) radiative transfer calculations for 12CO and 13CO lines, applying them to our high-resolution hydrodynamic models of the torus around a supermassive black hole in an active galactic nucleus. The hydrodynamic simulations reveal inhomogeneous and turbulent gas structure on a subparsec scale in a circumnuclear starburst region. Thick disks interlaced with filaments, clumps, and holes are naturally formed as a result of the interplay among energy feedback from supernovae, self-gravity of the gas, galactic rotation, and radiative cooling. The intensity maps of the molecular lines for the circumnuclear disks show a clumpy structure, reflecting the intrinsic inhomogeneity and turbulent motion of the gas disk. The fine structure of the torus could be resolved in nearby active galaxies using the Atacama Large Millimeter Array. We also found that the CO-to-H2 conversion factor (X-factor) is not uniformly distributed in the central 100 pc region. The X-factor derived for 12CO (J = 1-0) intensity depends strongly on the intensity, whereas the X for 12CO (J = 3-2) is nearly constant over 2 orders of magnitude of the intensity. The suggested conversion factor for the molecular gas mass is XCO(J=3-2) ~ 0.27 ? 1020 cm-2 (K km s-1)-1. The line ratio of high-J transitions of CO is not uniformly distributed in the nuclear disk, and the apparent ratio depends on the beam sizes. Observed 12CO(J = 4-3)/12CO(J = 2-1) can differ from the intrinsic ratio by as much as 20%.

Obscuration of Active Galactic Nuclei by Circumnuclear Starbursts

We examine the possibility of active galactic nucleus (AGN) obscuration by dusty gas clouds that spurt out from circumnuclear starburst regions. For this purpose, the dynamical evolution of gas clouds is pursued, including the effects of radiation forces by an AGN as well as a starburst. Here we solve the radiative transfer equations for clouds, taking into consideration the growth of clouds by inelastic cloud-cloud collisions and the resultant change in optical depth. As a result, if the starburst is more luminous than the AGN, gas clouds are distributed extensively above the galactic disk with the assistance of radiation pressure from the starburst. The total covering factor of gas clouds reaches a maximum of about 20%. After several 107 yr, gas clouds with larger optical depth form by cloud-cloud collisions; thereafter the clouds fall back as a result of weakened radiation pressure. The larger clouds undergo runaway growth and are eventually distributed around the equatorial plane on the inner sides of circumnuclear starburst regions. These clouds have an optical depth of several tens. The result is qualitatively consistent with the putative tendency that Seyfert 2 galaxies appear more frequently associated with starbursts than Seyfert 1 galaxies. On the other hand, if the AGN luminosity overwhelms that of the starburst, almost all clouds are ejected from the galaxy as a result of the radiation pressure from the AGN, resulting in the formation of a quasar-like object. The origin of obscuration of AGNs is discussed with relevant observations.

Formation of obscuring clouds by circumnuclear Starbursts

We pursue the dynamics of dusty gas clouds that are blown out of circumnuclear starburst regions, taking into consideration the growth of clouds by inelastic cloud-cloud collision and the resultant change of the optical depth. As a result, we find that the clouds originating in the circumnuclear starburst can contribute to the obscuration to produce type 2 Seyfert galaxies. Also, it is shown that, if the AGN luminosity overwhelms the starburst one, almost all clouds are ejected from the galaxy by the radiation pressure of the AGN, resulting in the formation of quasar-like objects.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

NGC 5953/54:BVRIHαJKphotometry and [N II] Fabry-Perot interferometry

We present new BVRIHαJK imaging and [N II] scanning Fabry-Perot observations for the interacting galaxy pair NGC 5953/54. Morphology is reviewed using a combination of contrast-enhanced images, color-index maps, and geometric (radial μ, , PA and ) profile parameters. Our data are complemented by archived V/R/H images from HST. The Seyfert 2 NGC 5953 shows an underlying featureless disk (≥1.5 kpc) in all the observed bands and traces of faint external fan-like features in Hα and [N II]. A compact flocculent spiral pattern (≤1.5 kpc), and a ~60 pc bar-like central structure are also observed. The Liner galaxy NGC 5954 is an inclined and distorted spiral with a strong circumnuclear starburst region and star-forming regions throughout the disk. A tidal bridge or distorted arm appears to link the two galaxies, extending to the northwest as a linear feature (plume). NGC 5954 also shows blue filaments delineating bubble-like features on the north side of the nucleus where our Fabry-Perot observations reveal double emission line profiles. We derive BVRIJK magnitudes and surface brightness profiles for the galaxies. HST V/R and H-band surface brightness profiles are also presented for NGC 5953. Fabry-Perot observations yield an [N II] velocity field and rotation curves for the components. The observations coupled with results from a simulation atlas of tidal features allow us 1) to suggest a tentative geometry of the encounter in NGC 5953/54, and 2) to comment about a suspected secular transformation in NGC 5953/54 via the interaction process. A forthcoming paper is devoted to a more detailed analysis of our observations including HI synthesis observations and n-body numerical simulations.

Mid-Infrared observations of NGC 1068 with the Infrared Space Observatory

We report on Mid-Infrared (MIR) observations of the Seyfert 2 galaxy NGC 1068, obtained with ISOCAM in low-resolution spectro-imaging mode. The spatial resolution (around 5 arcsec) allows us to disentangle the circumnuclear starburst regions from the emission of the active galactic nucleus (AGN). The global spatial distribution of the Unidentified Infrared Bands (UIBs) is similar to the cold dust component, traced by the 450 microns emission and the gaseous component obtained from the CO(1-0) map. However, a shift between the maximum of the UIB and 450 microns emission is clearly seen in our maps. The UIBs in the MIR (5-16 microns) originate almost exclusively from the starburst regions in the galactic disk with an emission peaking at the extremity of the stellar/ gaseous bar at a distance of 1 kpc from the AGN. The spectrum of the nucleus is characterized over the whole 5-16 microns range by a strong continuum which can be fitted with a power law of index -1.7. Moreover, the high [NeIII]/[NeII] ratio (>2.5) in the nuclear region argues for a hard radiation field from the AGN. Observations indicate that the AGN in NGC 1068 contributes less than 5% to the total integrated UIB emission even though its hot dust continuum contributes as much as 75% to the total MIR flux. On the contrary, the nuclear contribution to the cold dust emission decreases considerably at submillimeter wavelengths and does not represent more than 25% of the total integrated emission at 450 microns.

Kinematics of Ionized and Molecular Hydrogen in the Core of M100

We present high angular and velocity resolution two-dimensional kinematic observations in the spectral lines of Hα and CO J = 1 → 0 of the circumnuclear starburst region in the barred spiral galaxy M100, and compare them with kinematics derived from our previously published numerical modeling. The Hα data, fully sampled and at subarcsecond resolution, show a rotation curve that is rapidly rising in the central ~140 pc, and stays roughly constant, at the main disk value, further out. Noncircular motions are studied from the Hα and CO data by detailed consideration of the velocity fields, residual velocity fields after subtraction of the rotation curve, and sets of position-velocity diagrams. These motions are interpreted as the kinematic signatures of gas streaming along the inner part of the bar, and of density wave streaming motions across a two-armed minispiral. Comparison with a two-dimensional velocity field and rotation curve derived from our 1995 dynamical model shows good qualitative and quantitative agreement for the circular and noncircular kinematic components. Both morphology and kinematics of this region require the presence of a double inner Lindblad resonance in order to explain the observed twisting of the near-infrared isophotes and the gas velocity field. These are compatible with the presence of a global density wave driven by the moderately strong stellar bar in this galaxy. We review recent observational and modeling results on the circumnuclear region in M100, and discuss the implications for bar structure and gas dynamics in the core of M100 and other disk galaxies.

Radiation Hydrodynamical Mass Accretion onto Galactic Nuclei Driven by Circumnuclear Starbursts

Abstract We explore the radiation hydrodynamical evolution of a galactic nuclear gas disk exposed to intense radiation from circumnuclear starbursts. The evolution is characterized by two stages. First, the disk is radially contracted by the radiation force. Subsequently, the surface stratum of the disk avalanches onto the center due to the removal of angular momenta by radiation drag. The mass accretion via a radiatively-driven avalanche was analyzed quantitatively while assuming a torus shape of the circumnuclear starburst regions. As a result, it was found that the mass-accretion rate is sensitively dependent upon the extension of the starburst regions. For an optically thick disk, the mass-accretion rate is roughly in proportion to the ratio of the thickness to the curvature radius of the torus. The rate is also dependent upon the rotation law of the disk. We investigated the evolution of the disk surface density resulting from radiatively-driven mass accretion. It was found that for a more intensive starburst the central concentration of surface density becomes more conspicuous. The present results seem to be significant when considering the evolution of starburst nuclei, for instance, in luminous IRAS galaxies.

Radiative Avalanche Driven by Spherical Starbursts

As a solid physical mechanism to link between starburst activity and AGNs, we have proposed a radiative avalanche, where the mass accretion is driven by radiation drag exerted by stellar radiation from circumnuclear (ring-like) starburst regions (Umemura et al. 1997a,b). We here examine a radiative avalanche for a case of spherically distributed sources (Fukue et al. 1997). The results of the present spherical case are qualitatively similar to those of the ring-like case. For example, the drift timescale tdrift is found to be tdrift = 1.86×109 yr (m+1)(b/100 pc)2 (L∗/1012L⊙)−1g(r)−1, where m is the power of rotational velocity (vϕ ∝ rm), L∗ the starburst luminosity, b the radius of the starburst region, and g(r) a function of the order unity. This timescale is sufficiently shorter than the viscous timescale of the relevant region. In contrast to ring-like starbursts, the advantage of spherical starbursts is the large solid angle subtended by the starburst region. Hence, the luminosity of the starburst region is fully available to avalanche on the surface of the disk. Furthermore, the radiation field of the starburst region has no angular momentum, which is in favor for the avalanche.

Spectroscopy of Mrk 231 and NGC 4151 in the 3 μm Region

We present the results of spectroscopic studies of Mrk 231 and NGC 4151 in the 3 μm region. In the spectrum of Mrk 231, the strengths of the 3.3 μm emission feature and the 3.4 μm carbonaceous absorption feature are smaller than expected. The weakness of the 3.3 μm emission feature implies that large fractions of extended H2 emission detected by Krabbe et al. (1997, AAA 67.158.058) are excited by SMBH-driven activities, and/or the 3.3 μm emission carriers in the circumnuclear starburst region are destroyed by being exposed to energetic radiation from SMBH-driven activities. The weakness of the 3.4 μm carbonaceous absorption feature may be due to the destruction of C–H stretching bonds in dust grains and/or lower abundance of C relative to Si, O, and Mg than that in our Galaxy, consistent to the fact that Mrk 231 is now undergoing starburst activities. In the spectrum of NGC 4151, we detect the broad Bralpha emission line, but do not detect the 3.3 μm emission feature nor the 3.4 μm carbonaceous absorption feature. The non-detection of the 3.3 μm emission feature implies that nuclear starburst activities just outside the obscuring torus are weak (LFIR&amp;lt; 3.7 × 109 L⊙).

Radiative Avalanche Driven by Spherical Starbursts

We examine a radiative avalanche for the case of spherically distributed sources. A radiative avalanche is a new mechanism, in which the mass-accretion of the nuclear disk is driven via radiation drag exerted by stellar radiation from circumnuclear starburst regions, and which is expected to be a link between starburst activities and AGNs. The results of the present spherical case are qualitatively similar to those of the ring-like case. However, the solid angle subtended by the starburst region is large and the radiation field of the starburst region has no angular momentum, which are in favor for the avalanche model. In addition, an avalanche can take place continuously from the outside to the inside of the starburst region in the spherical case.

Does the Radiative Avalanche Fueling Work in Any Active Galactic Nuclei?

Recently Umemura, Fukue, & Mineshige (1997) proposed the radiative avalanche fueling to active galactic nuclei; gas accretion is driven by radiation drag exerted by stellar radiation from circumnuclear starburst regions. This mechanism is also interesting in terms of starburst-AGN connections. We therefore present observational tests for the radiative avalanche fueling. Our tests, however, show that gas accretion rates driven by the radiative avalanche are significantly lower than those expected from the standard accretion theory applied for typical active galactic nuclei with the circumnuclear starburst regions. Instead we propose an alternative, possible starburst-AGN connection; a minor merger with a nucleated satellite drives circumnuclear starbursts and then leads to gas fueling onto the central engine as the merger proceeds.

Radiative Avalanche: Starburst-induced Fueling to Active Galactic Nuclei

We propose a novel mechanism for fueling active galactic nuclei (AGNs), i.e., a radiative avalanche, which is mass accretion driven by radiation drag exerted by stellar radiation from circumnuclear starburst regions. If a surface layer of a rotating gas disk is irradiated by intensive starlight, then it could lose angular momentum via radiation drag, which would result in an avalanche of the layer as an inevitable consequence. Analyses show that in an optically thick regime, the mass accretion rate via this radiative avalanche is independent of not only the extinction coefficient of photon-absorbing matter but also the density distribution of the disk, and the maximal mass accretion rate is simply described as ${u{M}{705F}}$ --> ~ L*/c2 with L* being the bolometric luminosity of a starburst. An extraordinarily intensive starburst of 3 ? 1012 L?, for example, could produce ${u{M}{705F}}$ --> ~ 0.2 M? yr-1. In a disk containing dust, the accretion timescale could be as short as less than 108 yr. If the radiative avalanche feeds an accretion disk around a supermassive black hole, gravitational energy can be liberated with a luminosity of the same order as L*. In the present context, the intensity of radiative avalanche determines whether a starburst galaxy is destined to posses an AGN or not. The present model provides a solid physical mechanism deduced from first principles to account for the possible link between starburst activities and AGNs, which has been suggested by a number of observations.