starburst-AGN Connection Research Articles

The Starburst‐Active Galactic Nucleus Connection in Active Galaxies: The MassiveNuclear Star‐Forming Disk in NGC 4303

Two-dimensional spectroscopy of the low-redshift active galaxy NGC 4303 obtained with the Two-Dimensional Fiber ISIS System is presented. The ionization structure and velocity field of the nuclear region of this galaxy, where a compact UV-bright spiral structure connected with the UV-bright core has been detected with the Hubble Space Telescope (HST; Colina et al. 1997a), are investigated in detail. The ionized gas shows a structure similar to that observed in the high spatial resolution HST UV-continuum image. The Hβ and Hα emission is dominated by the nuclear star-forming regions, while the core of the galaxy is the brightest region in the [O III] λ5007 and [N II] λ6584 emission lines. The optical emission line ratios of the nuclear star-forming regions are characteristic of young (ages 2-3 Myr) star-forming regions, while the core of the galaxy shows the emission-line ratios of a low-luminosity AGN in between an [O I]-weak LINER and a low-excitation Seyfert 2. The nature of the ionizing source located at the core of the galaxy is still unsolved. Its luminosity and optical emission line ratios are compatible with the presence of a young (~3.5 Myr) massive (~8×104 M☉) cluster of stars, but are also consistent with the existence of a power-law nonthermal ionizing source. Whatever the nature of this ionizing source, the nuclear star-forming spiral dominates the ionizing radiation, contributing about 90% of the total ionizing flux. The velocity field of the ionized gas, as measured by the Hβ emission line, is consistent with that of a massive rotating disk characterized by a radius of 300 pc, a rotation velocity of 85 km s-1, an inclination of 45° with respect to the line of sight, and a kinematic major axis oriented along P.A. 130°. The inferred dynamical mass inside a radius of 300 pc is 5.0×108 M☉. The mass and size of this nuclear rotating disk is similar to those recently detected in ultraluminous infrared galaxies like Mrk 231 and Arp 220. The observed line profiles of the high-excitation [O III]-emitting gas show the presence of two kinematically distinct gaseous components. The main component follows the velocity pattern of the massive rotating disk. The secondary component has an amplitude of ±350 km s-1, and a minor axis oriented, in projection, almost perpendicular to the minor axis of the low-ionization gas. This second[O III] velocity component could represent gas located in a nuclear ionizing cone, i.e., gas flowing outward outside the plane of the low-excitation gas disk, and being ionized by the UV-bright source located at the core of the galaxy. The proximity of NGC 4303 together with the detection of (1) an AGN-like nucleus, (2) a compact nuclear star-forming spiral structure connected to the nucleus, (3) a massive rotating nuclear disk, and (4) radially flowing high-excitation gas makes this galaxy an ideal candidate for the study of the fueling of active galaxies and the starburst-AGN connection.

Ultraviolet‐Optical Observations of the Seyfert 2 Galaxies NGC 7130, NGC 5135, and IC 3639: Implications for the Starburst–Active Galactic Nucleus Connection

We present and discuss HST (WFPC2 and FOC) images and UV GHRS spectra plus ground-based near UV through to near IR spectra of three Seyfert 2 nuclei (NGC 7130, NGC 5135 and IC 3639). These galaxies, together to Mrk 477, were selected from a bigger sample that comprises the 20 brightest Seyfert 2 nuclei, with the goal to study the origin of the UV-optical-near IR featureless continuum in Seyfert 2 nuclei. These four galaxies have bolometric luminosities, as computed with the four IRAS bands, of 10^11 Lsol. They are close enough to be resolved with HST the nuclear zone. This makes these Seyfert 2 galaxies benchmarks to study the Starburst-AGN connection in more distant galaxies. The data provide direct evidence of the existence of a central nuclear starburst that dominates the UV light, and that seem to be responsible for the origin of the so called featureless continuum. These starbursts are dusty and compact. They have sizes (from less than 100 pc to a few hundred pc) much smaller and closer to the nucleus than that seen in the prototype Seyfert 2 galaxy NGC 1068. The bolometric luminosity of these starbursts is similar to the estimated bolometric luminosities of their obscured Seyfert 1 nuclei, and thus they contribute in the same amount to the overall energetics of these galaxies.

Seyfert Activity and Nuclear Star Formation in the Circinus Galaxy

We present high angular resolution (0.15-0.5) near infrared images and spectroscopy of the Circinus galaxy, the closest Seyfert 2 galaxy known. The data reveal a non-stellar nuclear source at 2.2 microns. The coronal line region and the hot molecular gas emission extend for 20-50 pc in the ionization cone. The data do not show evidence for a point-like concentration of dark mass; we set an upper limit of 4*10^6 Mo to the mass of a putative black hole. We find evidence for a young nuclear stellar population, with typical ages between 4*10^7 and 1.5*10^8 yrs. The luminosity of the starburst inside a few hundred pc is comparable to the intrinsic luminosity of the Seyfert nucleus, and the two of them together account for most of the observed bolometric luminosity of the galaxy. Within the central 12 pc the starburst has an age of about 7*10^7 yrs and radiates about 2% of the luminosity of the active nucleus. We discuss the implications of these results for models that have been proposed for the starburst-AGN connection.

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.