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Register a new userThe Central Starburst and Ionization Mechanism in the LINER/H II Region Transition Nucleus in NGC 4569
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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.
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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.
We study the CO and the radiocontinuum emission in an active galaxy to analyze the interplay between the central activity and the molecular gas. We present new high-resolution observations of the CO(1-0) and CO(2-1) emission lines, and 3.5 cm and 20 cm radio continuum emission in the central region of the LINER/starburst galaxy NGC 6764. The galaxy has an outflow morphology in radio continuum, spatially coincident with the CO and H$alpha$ emission, and centered slightly off the radio continuum peak at the LINER nucleus. The total molecular gas mass in the center is about 7x10^8 msun, using a CO luminosity to total molecular gas conversion factor that is three times lower than the standard one. CO(1-0) emission is found near the boundaries of the radio continuum emission cone. The outflow has a projected expansion velocity of 25 km/s relative to the systemic velocity of NGC6764. About 4x 10^6 msun of molecular gas is detected in the outflow. The approximate location (~1 kpc) of the dynamical inner Lindblad resonance has been derived from the rotation curve. The peak of the CO emission is slightly (< 200 pc) offset from the peak of the radio continuum. The molecular gas has most likely been ejected by the stellar winds from the recent starburst, but the CO line ratios show indication of an interaction with the AGN. The energy released by the nuclear starburst is sufficient to explain the observed outflow, even if the data cannot exclude the AGN from being the major energy source. Comparison of the outflow with hydrodynamical simulations suggests that the nuclear starburst is 3--7 Myr old and the bubble-like outflow is still confined and not freely expanding.
(abridged) NGC 1569 is an ideal test environment to understand the impact of feedback from massive stars on the surrounding ISM. We present HST WFPC2 narrowband imagery of NGC 1569 in an attempt to understand the underlying ionizing emission mechanisms on a 3 pc scale and to generate a H II region size distribution and luminosity function. We use [O III]/Hbeta and [S II]/Halpha ratio maps to find that non-photoionizing mechanisms (e.g. shocks) are responsible for 10%+/-3% of the Halpha emission, 2.3-3.3 times larger than results from similar galaxies. Our results for NGC 1569 indicate that these non-photoionized areas do not lie in low surface brightness regions exclusively. To explain this large percentage of non-photoionized emission, we suggest that NGC 1569 is, indeed, in a post-starburst phase as previous authors have claimed. We also derived slopes for the H II region luminosity function (-1.00+/-0.08) and size distribution (-3.02+/-0.27). The luminosity slope, though shallow, is similar to previous work on this galaxy and other irregular galaxies. The size distribution slope is shallower than previous slopes found for irregular galaxies, but our slope value fits into their confidence intervals and vice versa.
A high angular resolution, multi-wavelength study of the LINER galaxy NGC1614 has been carried out. OVRO CO 1-0 observations are presented together with extensive multi-frequency radio continuum and HI absorption observations with the VLA and MERLIN. Toward the center of NGC1614, we have detected a ring of radio continuum emission with a radius of 300 pc. This ring is coincident with previous radio and Paschen-alpha observations. The dynamical mass of the ring based on HI absorption is 3.1 x 10E9 Msun. The peak of the integrated CO 1-0 emission is shifted by 1 to the north-west of the ring center and a significant fraction of the CO emission is associated with a crossing dust lane. An upper limit to the molecular gas mass in the ring region is 1.7 x 10E9 Msun. Inside the ring, there is a north to south elongated 1.4GHz radio continuum feature with a nuclear peak. This peak is also seen in the 5GHz radio continuum and in the CO. We suggest that the R=300 pc star forming ring represents the radius of a dynamical resonance - as an alternative to the scenario that the starburst is propagating outwards from the center into a molecular ring. The ring-like appearance probably part of a spiral structure. Substantial amounts of molecular gas have passed the radius of the ring and reached the nuclear region. The nuclear peak seen in 5GHz radio continuum and CO is likely related to previous star formation, where all molecular gas was not consumed. The LINER-like optical spectrum observed in NGC1614 may be due to nuclear starburst activity, and not to an Active Galactic Nucleus (AGN). Although the presence of an AGN cannot be excluded.
We present a kinematic study of the nuclear stellar disk in M31 at infrared wavelengths using high spatial resolution integral field spectroscopy. The spatial resolution achieved, FWHM = 0.12 (0.45 pc at the distance of M31), has only previously been equaled in spectroscopic studies by space-based long-slit observations. Using adaptive optics-corrected integral field spectroscopy from the OSIRIS instrument at the W. M. Keck Observatory, we map the line-of-sight kinematics over the entire old stellar eccentric disk orbiting the supermassive black hole (SMBH) at a distance of r<4 pc. The peak velocity dispersion is 381+/-55 km/s , offset by 0.13 +/- 0.03 from the SMBH, consistent with previous high-resolution long-slit observations. There is a lack of near-infrared (NIR) emission at the position of the SMBH and young nuclear cluster, suggesting a spatial separation between the young and old stellar populations within the nucleus. We compare the observed kinematics with dynamical models from Peiris & Tremaine (2003). The best-fit disk orientation to the NIR flux is [$theta_l$, $theta_i$, $theta_a$] = [-33 +/- 4$^{circ}$, 44 +/- 2$^{circ}$, -15 +/- 15$^{circ}$], which is tilted with respect to both the larger-scale galactic disk and the best-fit orientation derived from optical observations. The precession rate of the old disk is $Omega_P$ = 0.0 +/- 3.9 km/s/pc, lower than the majority of previous observations. This slow precession rate suggests that stellar winds from the disk will collide and shock, driving rapid gas inflows and fueling an episodic central starburst as suggested in Chang et al. (2007).
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