Near-Infrared Adaptive Optics Imaging of the Central Regions of Nearby Sc Galaxies. II. NGC 247 and NGC 2403

J, H, and K' images obtained with the Canada-France-Hawaii Telescope adaptive optics system are used to investigate the star-forming histories of the central regions of the Sc galaxies NGC 247 and NGC 2403. The brightest resolved red stars within 15 arcsec of the nucleus of each galaxy are red supergiants, indicating that the central few hundred parsecs of these galaxies experienced star formation within the last ~ 0.1 Gyr. However, when averaged over Gyr time scales, the star-forming histories of the inner disks of these galaxies have been remarkably similar, as expected if the long-term evolution of disks is defined by local characteristics such as mass density. It is demonstrated that NGC 247 and NGC 2403, like M33, harbour nuclear star clusters with stellar contents that differ from the surrounding central light concentrations. The nucleus of NGC 2403 is significantly bluer than that of the other two galaxies and the K-band surface brightnesses near the centers of NGC 247 and NGC 2403 are 1 -- 2 mag per square arcsec lower than in M33. Finally, it is noted that young or intermediate-age nuclear star clusters are a common occurence in nearby spirals, indicating that nuclear star formation in these objects is either continuous or episodic on time scales of 0.1 - 1 Gyr.Comment: 27 pages of text and 14 figures; to appear in the Astronomical Journa

The Influence of Bars on Nuclear Activity

We test ideas on fueling of galactic nuclei by bar-driven inflow by comparing the detection rate and intensity of nuclear H II regions and AGNs among barred and unbarred galaxies in a sample of over 300 spirals selected from our recent optical spectroscopic survey of nearby galaxies. Among late-type spirals (Sc-Sm), but not early-type (S0/a-Sbc), we observe in the barred group a very marginal increase in the detection rate of H II nuclei and a corresponding decrease in the incidence of AGNs. The minor differences in the detection rates, however, are statistically insignificant, most likely stemming from selection effects and not from a genuine influence from the bar. The presence of a bar seems to have no noticeable impact on the likelihood of a galaxy to host either nuclear star formation or an AGN. The nuclei of early-type barred spirals do exhibit measurably higher star-formation rates than their unbarred counterparts, as indicated by either the luminosity or the equivalent width of H-alpha emission. By contrast, late-type spirals do not show such an effect. Bars have a negligible effect on the strength of the AGNs in our sample, regardless of the Hubble type of the host galaxy. This result confirms similar conclusions reached by other studies based on much smaller samples.Comment: To appear in the Astrophysical Journal. LaTex, 31 pages including 6 postscript figures and 3 tables. AAStex macros include

Galactic bulge formation as a maximum intensity starburst

Properties of normal galactic star formation, including the density dependence, threshold density, turbulent scaling relations, and clustering properties, are applied to the formation of galactic bulges. One important difference is that the bulge potential well is too deep to have allowed self-regulation or blow-out by the pressures from young stars, unlike galactic disks or dwarf galaxies. As a result, bulge formation should have been at the maximum rate, which is such that most of the gas would get converted into stars in only a few dynamical time scales, or ~10^8 years. The gas accretion phase can be longer than this, but once the critical density is reached, which depends primarily on the total virial density from dark matter, the formation of stars in the bulge should have been extremely rapid. Such three-dimensional star formation should also have formed many clusters, like normal disk star formation today. Some of these clusters may have survived as old globulars, but most got dispersed, although they might still be observable as concentrated streams in phase space.Comment: 10 pages, 1 figure, scheduled for ApJ, vol. 517, May 20, 199

Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences

We study the characteristics of molecular gas in the central regions of spiral galaxies on the basis of our CO(J=1-0) imaging survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of molecular gas at galactic centers with sizescales < 1 kpc and CO-derived masses M_gas(R<500pc) = 10^8 - 10^9 M_sun are found to be prevalent in the gas-rich L^* galaxies. Moreover, the degree of gas concentration to the central kpc is found to be higher in barred systems than in unbarred galaxies. This is the first statistical evidence for the higher central concentration of molecular gas in barred galaxies, and it strongly supports the theory of bar-driven gas transport. It is most likely that more than half of molecular gas within the central kpc of a barred galaxy was transported there from outside by the bar. The supply of gas has exceeded the consumption of gas by star formation in the central kpc, resulting in the excess gas in the centers of barred systems. The mean rate of gas inflow is statistically estimated to be larger than 0.1 - 1 M_sun/yr. The correlation between gas properties in the central kpc and the type of nuclear spectrum (HII, LINER, or Seyfert) is investigated. A correlation is found in which galaxies with larger gas-to-dynamical mass ratios tend to have HII nuclear spectra, while galaxies with smaller ratios show spectra indicating AGN. Also, the theoretical prediction of bar-dissolution by condensation of gas to galactic centers is observationally tested. It is suggested that the timescale for bar dissolution is larger than 10^8 - 10^10 yr, or a bar in a L^* galaxy is not destroyed by a condensation of 10^8 - 10^9 M_sun gas in the central kpc.Comment: AASTeX, 20 pages, 8 eps figs, ApJ in press (10 Nov. 1999 issue

The Host Galaxies of Narrow-Line Seyfert 1s: Evidence for Bar-Driven Fueling

We present a study of the host-galaxy morphologies of narrow- and broad-line Seyfert 1 galaxies (NLS1s and BLS1s) based on broad-band optical images from the Hubble Space Telescope archives. We find that large-scale stellar bars, starting at ~1 kpc from the nucleus, are much more common in NLS1s than BLS1s. Furthermore, the fraction of NLS1 spirals that have bars increases with decreasing full-width at half-maximum (FWHM) of the broad component of H-beta. These results suggest a link between the large-scale bars, which can support high fueling rates to the inner kpc, and the high mass-accretion rates associated with the supermassive black holes in NLS1s.Comment: 19 pages, 4 figures (1a, 1b, 2, and 3), Accepted for publication in the Astronomical Journa

Volatile organic emissions from the distillation and pyrolysis of vegetation

International audienceLeaf and woody plant tissue (Pinus ponderosa, Eucalyptus saligna, Quercus gambelli, Saccharum officinarum and Oriza sativa) were heated from 30 to 300°C and volatile organic compound (VOC) emissions were identified and quantified. Major VOC emissions were mostly oxygenated and included acetic acid, furylaldehyde, acetol, pyrazine, terpenes, 2,3-butadione, phenol and methanol, as well as smaller emissions of furan, acetone, acetaldehyde, acetonitrile and benzaldehyde. Total VOC emissions from distillation and pyrolysis were on the order of 10 gC/kgC dry weight of vegetation, as much as 33% and 44% of CO2 emissions (gC(VOC)/gC(CO2)) measured during the same experiments, in air and nitrogen atmospheres, respectively. The emissions are similar in identity and quantity to those from smoldering combustion of woody tissue and of different character than those evolved during flaming combustion. VOC emissions from the distillation of pools and endothermic pyrolysis under low turbulence conditions may produce flammable concentrations near leaves and may facilitate the propagation of wildfires. VOC emissions from charcoal production are also related to distillation and pyrolysis; the emissions of the highly reactive VOCs from production are as large as the carbon monoxide emissions

3-Phase 4-wire matrix converter-based voltage sag/ swell generator to test low-voltage ride through in wind energy conversion systems

The high penetration of energy from wind energy conversion systems (WECSs) can have a significant influence on the stability, power quality and reliability of power systems. Therefore several countries have developed stringent grid codes in recent years in order to enhance the overall stability of power systems. In these grid codes, the capacity to fulfil low-voltage ride through (LVRT) requirements is considered an important issue for the control of WECSs. Therefore in this study, a novel voltage sag/ swell generator (VSG) based on a 4-leg matrix converter is presented. This VSG can be used to generate the symmetrical and asymmetrical faults required to test LVRT algorithms in a laboratory environment. The performance of the VSG is experimentally demonstrated and compared with the operation of other VSGs conventionally used for LVRT studies

Numerical Simulations of Globular Cluster Formation

We examine various physical processes associated with the formation of globular clusters by using the three-dimensional Smoothed Particle Hydrodynamics (SPH) code. Our code includes radiative cooling of gases, star formation, energy feedback from stars including stellar winds and supernovae, and chemical enrichment by stars. We assume that, in the collapsing galaxy, isothermal cold clouds form through thermal condensations and become proto-globular clouds. We calculate the size of proto-globular clouds by solving the linearized equations for perturbation. We compute the evolution of the inner region of the proto-cloud with our SPH code for various initial radius and initial composition of gases. When the initial gases contain no heavy elements, the evolution of proto-clouds sensitively depends on the initial radius. For a smaller initial radius, the initial star burst is so intense that the subsequent star formation occurs in the central regions to form a dense star cluster as massive as the globular cluster. When the initial gases contain some heavy elements, the metallicity of gases affects the evolution and the final stellar mass. If the initial radius of the proto-globular clouds was relatively large, the formation of a star cluster as massive as the globular clusters requires the initial metallicity as high as [Fe/H] $\geq -2$. The self-enrichment of heavy elements in the star cluster does not occur in all cases.Comment: Accpeted for publication in the ApJ. Correctiong errors in Table

The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular Evolution

Despite compelling evidence that stellar bars drive gas into the inner 1--2 kpc or circumnuclear (CN) region of galaxies, there are few large, high resolution studies of the CN molecular gas and star formation (SF). We study a sample of local barred non-starbursts and starbursts with high-resolution CO, optical, Ha, RC, Br-gamma, and HST data, and find the following. (1) The inner kpc of bars differs markedly the outer disk and hosts molecular gas surface densities Sigma-gas-m of 500-3500 Msun pc-2, gas mass fractions of 10--30 %, and epicyclic frequencies of several 100--1000 km s-1 kpc-1.Consequently, gravitational instabilities can only set in at high gas densities and grow on a short timescale (few Myr). This high density, short timescale, `burst' mode may explain why powerful starbursts tend to be in the CN region of galaxies. (2) We suggest that the variety in CO morphologies is due to different stages of bar-driven inflow. At late stages, most of the CN gas is inside the outer inner Lindblad resonance (OILR), and has predominantly circular motions. Across the sample, we find bar pattern speeds with upper limits of 43 to 115 km s-1 kpc-1 and OILR radii of > 500 pc. (3) Barred starbursts and non-starbursts have CN SFRs of 3--11 and 0.1--2 Msun yr-1, despite similar CN gas mass. Sigma-gas-m in the starbursts is larger (1000--3500 Msun pc-2) and close to the Toomre critical density over a large region. (4) Molecular gas makes up 10%--30% of the CN dynamical mass (6--30 x 10^9 Msun).In the starbursts, it fuels CN SFRs of 3--11 Msun yr-1, building young, massive, high V/sigma components. We present evidence for such a pseudo-bulge in NGC 3351. Implications for secular evolution along the Hubble sequence are discussed.Comment: Accepted by the Astrophysical Journal. Paper length reduced to fit within APJ page limits. Version of paper with high resolution figures is at http://www.as.utexas.edu/~sj/papers/ms-hires-sj05a.ps.g

A Kinematic Link between Boxy Bulges, Stellar Bars, and Nuclear Activity in NGC 3079 & NGC 4388

We present direct kinematic evidence for bar streaming motions in two active galaxies with boxy stellar bulges. The Hawaii Imaging Fabry-Perot Interferometer was used on the Canada-France-Hawaii 3.6-m telescope and the University of Hawaii 2.2-m telescope to derive the two-dimensional velocity field of the line-emitting gas in the disks of the Sc galaxy NGC 3079 and the Sb galaxy NGC 4388. In contrast to previous work based on long-slit data, the detection of the bar potential from the Fabry-Perot data does not rely on the existence of inner Lindblad resonances or strong bar-induced shocks. Simple kinematic models which approximate the intrinsic gas orbits as nonintersecting, inclined elliptical annuli that conserve angular momentum characterize the observed velocity fields. Box-shaped bulges in both NGC 3079 and NGC 4388 are confirmed using new near-infrared images to reduce dust obscuration. Morphological analysis of starlight in these galaxies is combined with the gas kinematics derived from the Fabry-Perot spectra to test evolutionary models of stellar bars that involve transitory boxy bulges, and to quantify the importance of such bars in fueling active nuclei. Our data support the evolutionary bar models, but fail to prove convincingly that the stellar bars in NGC 3079 and NGC 4388 directly trigger or sustain the nuclear activity. (abridged)Comment: 31 pages, 18 figures, Latex, requires aaspp4.sty. Accepted for the Astronomical Journal (November issue