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Register a new userLINER/H II Transition Nuclei and the Nature of NGC 4569
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Motivated by the discovery of young, massive stars in the nuclei of some LINER/H II ``transition nuclei such as NGC 4569, we have computed photoionization models to determine whether some of these objects may be powered solely by young star clusters rather than by accretion-powered active nuclei. The models were calculated with the photoionization code CLOUDY, using evolving starburst continua generated by the the STARBURST99 code of Leitherer et al. (1999). We find that the models are able to reproduce the emission-line spectra of transition nuclei, but only for instantaneous bursts of solar or higher metallicity, and only for ages of ~3-5 Myr, the period when the extreme-ultraviolet continuum is dominated by emission from Wolf-Rayet stars. For clusters younger than 3 Myr or older than 6 Myr, and for models with a constant star-formation rate, the softer ionizing continuum results in an emission spectrum more typical of H II regions. This model predicts that Wolf-Rayet emission features should appear in the spectra of transition nuclei. While such features have not generally been detected to date, they could be revealed in observations having higher spatial resolution. Demographic arguments suggest that this starburst model may not apply to the majority of transition nuclei, particularly those in early-type host galaxies, but it could account for some members of the transition class in hosts of type Sa and later. The starburst models during the Wolf-Rayet-dominated phase can also reproduce the narrow-line spectra of some LINERs, but only under conditions of above-solar metallicity and only if high-density gas is present (n_e >~ 10^5 cm^{-3}). This scenario could be applicable to some ``Type 2 LINERs which do not show any clear signs of nonstellar activity.
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We present a comprehensive study to determine if the LINER/H II region transition spectrum in NGC 4569 can be generated solely by photoionization by the nuclear starburst. A review of the multiwavelength data from the literature reveals no additional sources that contribute to the ionization. We find that the young starburst dominating the UV emission is distinct from the nuclear population of A supergiants identified in the optical spectrum by Keel (1996). Spectral synthesis analysis provides constraints on the physical nature of the starburst, revealing a 5-6 Myr, approximately instantaneous starburst with subsolar metallicity. These results are used to model the spectral energy distribution of the ionizing continuum. Luminosity constraints place limits on the steepness of the extinction curve for the young starburst. The Savage & Mathis (1979) curve satisfies all luminosity constraints and the derived reddening is similar to the emission line reddening. These results imply extreme conditions in the nuclear starburst, with ~5x10^4 O and B stars compacted in the inner 9 x 13 region of the nucleus. Using photoionization analysis and employing all observational constraints on the emission line gas, we find very specific conditions are required if the spectrum is generated solely by stellar photoionization. At least two spatially distinct components are required - a compact region with strong O III emission and an extended, low density component emitting most of the S II flux. A high density component is also needed to generate the O I flux. Additionally, a limited contribution from Wolf-Rayet stars to the ionizing SED is necessary, consistent with the results of Barth & Shields (2000). We present a physical interpretation for the multi-component emission line gas.
Stellar archeology of nearby LINER galaxies may reveal if there is a stellar young population that may be responsible for the LINER phenomenon. We show results for the classical LINER galaxies NGC 4579 and NGC 4736 and find no evidence of such populations.
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 2farcs1 $times$ 1farcs4 and 1farcs1 $times$ 0farcs7, respectively. The CO emission is distributed over a disk of diameter $sim$16arcsec ($sim$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, $sim2-3^{primeprime}$ ($sim$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. 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.
We present CO(1-0) and CO(2-1) maps of the interacting barred LINER/Seyfert 2 galaxy NGC 3627 obtained with the IRAM interferometer at resolutions of 2.1 x 1.3 and 0.9 x 0.6, respectively. The molecular gas emission shows a nuclear peak, an elongated bar-like structure of ~18 (~900 pc) diameter in both CO maps and, in CO(1-0), a two-arm spiral feature from r~9 (~450 pc) to r~16 (~800 pc). The inner ~18 bar-like structure, with a north/south orientation (PA = 14{deg}), forms two peaks at the extremes of this elongated emission region. The kinematics of the inner molecular gas shows signatures of non-circular motions associated both with the 18 bar-like structure and the spiral feature detected beyond it. The 1.6 micron H-band 2MASS image of NGC 3627 shows a stellar bar with a PA = -21{deg}, different from the PA (= 14{deg}) of the CO bar-like structure, indicating that the gas is leading the stellar bar. The torques computed with the HST-NICMOS F160W image and our PdBI maps are negative down to the resolution limit of our images, ~60 pc in CO(2-1). If the bar ends at ~3 kpc, coincident with corotation (CR), the torques are negative between the CR of the bar and the nucleus, down to the resolution limit of our observations. This scenario is compatible with a recently-formed rapidly rotating bar which has had insufficient time to slow down because of secular evolution, and thus has not yet formed an inner Lindblad resonance (ILR). The presence of molecular gas inside the CR of the primary bar, where we expect that the ILR will form, makes NGC 3627 a potential smoking gun of inner gas inflow. The gas is fueling the central region, and in a second step could fuel directly the active nucleus.
Using the short-high module of the Infrared Spectrograph on the Spitzer Space Telescope, we have measured the [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56, and [S III] 18.71-micron emission lines in nine H II regions in the dwarf irregular galaxy NGC 6822. These lines arise from the dominant ionization states of the elements neon (Ne$^{++}$, Ne$^+$) and sulphur (S$^{3+}$, S$^{++}$), thereby allowing an analysis of the neon to sulphur abundance ratio as well as the ionic abundance ratios Ne$^+$/Ne$^{++}$ and S$^{3+}$/S$^{++}$. By extending our studies of H II regions in M83 and M33 to the lower metallicity NGC 6822, we increase the reliability of the estimated Ne/S ratio. We find that the Ne/S ratio appears to be fairly universal, with not much variation about the ratio found for NGC 6822: the median (average) Ne/S ratio equals 11.6 (12.2$pm$0.8). This value is in contrast to Asplund et al.s currently best estimated value for the Sun: Ne/S = 6.5. In addition, we continue to test the predicted ionizing spectral energy distributions (SEDs) from various stellar atmosphere models by comparing model nebulae computed with these SEDs as inputs to our observational data, changing just the stellar atmosphere model abundances. Here we employ a new grid of SEDs computed with different metallicities: Solar, 0.4 Solar, and 0.1 Solar. As expected, these changes to the SED show similar trends to those seen upon changing just the nebular gas metallicities in our plasma simulations: lower metallicity results in higher ionization. This trend agrees with the observations.
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