Massive Stellar Content of the Galactic Supershell GSH 305+01-24

The distribution of OB stars along with that of H$\alpha$, $^{12}$CO, dust infrared emission, and neutral hydrogen is carried out in order to provide a more complete picture of interactions of the young massive stars and the observed supershell GSH 305+01-24. The studied field is located between $299^\circ \le l \le 311^\circ$ and $-5^\circ \le b \le 7^\circ$. The investigation is based on nearly 700 O-B9 stars with $uvby\beta$ photometry currently available. The derived stellar physical parameters were used to establish a homogeneous scale for the distances and extinction of light for major apparent groups and layers of foreground and background stars in Centaurus and study the interaction with the surrounding interstellar medium. The distance to the entire Centaurus star-forming complex is revised and a maximum of the OB-star distance distribution is found at 1.8$\pm$0.4 (r.m.s) kpc. The massive star component of GSH 305+01-24 is identified at about 85-90 % completeness up to 11.5-12 mag. The projected coincidence of the OB stars with the shell and the similarities between the shell's morphology and the OB-star distribution indicate a strong interaction of the stellar winds with the superbubble material. We demonstrate that these stars contribute a sufficient wind injection energy in order to explain the observed size and expansion velocity of the supershell. The derived stellar ages suggest an age gradient over the Coalsack Loop. A continuous star-formation might be taking place within the shell with the youngest stars located at its periphery and the open cluster NGC 4755 being the oldest. A layer of very young stars at 1 kpc is detected and its connection to both GSH 305+01-24 and the foreground GSH 304-00-12 H I shells is investigated.Comment: Accepted for publication in A&A. Paper consists of 11 pages, 3 tables and 9 figures. Table 1 and Table 3 will only be available from CD

Globular cluster systems II: On the formation of old globular clusters and their sites of formation

We studied the metal-poor globular cluster (GC) populations of a large variety of galaxies (47 galaxies spanning about 10mag in absolute brightness) and compared their mean [Fe/H] with the properties of the host galaxies. The mean [Fe/H] of the systems lie in the -1.65<[Fe/H]<-1.20 range (74% of the population). Using only GC systems with more than 6 objects detected, 85% of the population lie within -1.65<[Fe/H]<-1.20. The relation between the mean [Fe/H] of the metal-poor GC systems and the Mv of their host galaxies presents a very low slope which includes zero. An analysis of the correlation of the mean [Fe/H] with other galaxy properties also leads to the conclusion that no strong correlation exists. The lack of correlation suggests a formation of all metal-poor GC in similar gas fragments. A weak correlation might exist between mean [Fe/H] of the metal-poor GC and host galaxy metallicity. This would imply that some fragments in which metal-poor GC formed were already embedded in the larger dark matter halo of the final galaxy (as oppose to being independent satellites that were accreted later). Our result suggests a homogeneous formation of metal-poor GC in all galaxies, in typical fragments of masses around 10^9-10^10 solar masses with very similar metallicities, compatible with hierarchical formation scenarios for galaxies. We compared the mean [Fe/H] of the metal-poor GC populations with the typical metallicities of high-z objects. If we add the constraint that GC need a high column density of gas to form, DLAs are the most likely sites for the formation of metal-poor GC populations.Comment: accepted for publication in AJ, scheduled for the May 2001 issu

Morphology and properties of ZnO films obtained by repeated spin coating on porous silicon substrates

Layers of porous silicon (PS), multilayered ZnO films, and heterostructures based on them are obtained. The surface morphology, chemical and phase composition of the PS layers and ZnO films, and the transverse cleavage of ZnO-PS nanocomposite, are investigated via energy-dispersive X-ray spectral analysis (EDX), X-ray diffraction (XRD), and scanning electron microscopy (SEM

Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra

We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa galaxy NGC1291. The X-ray emission from the bulge resembles the X-ray emission from a sub-class of elliptical and S0 galaxies with low L_X/L_B luminosity ratios. The X-ray emission is composed of a central point-like nucleus, ~50 point sources that are most likely low mass X-ray binaries (LMXBs), and diffuse gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global temperature of 0.32^{+0.04}_{-0.03} keV and metallicity of 0.06 +/- 0.02 solar, and both quantities marginally decrease with increasing radius. The hot gas fills the hole in the HI distribution, and the softening of the spectrum of the X-ray gas with radius might indicate a thermal coupling of the hot and cold phases of the interstellar medium as previously suggested. The integrated X-ray luminosity of the LMXBs, once normalized by the optical luminosity, is a factor of 1.4 less than in the elliptical galaxy NGC4697 or S0 galaxy NGC1553. The difference in L_{X,stellar}/L_B between the galaxies appears to be because of a lack of very bright sources in NGC1291. No sources above 3 x 10^38 ergs/s were found in NGC1291 when ~7 were expected from scaling from NGC4697 and NGC1553. The cumulative L_{X,stellar}/L_B value including only sources below 1.0 x 10^38 ergs/s is remarkably similar between NGC1291 and NGC4697, if a recent surface brightness fluctuation-determined distance is assumed for NGC4697. If this is a common feature of the LMXB population in early-type systems, it might be used as a distance indicator. Finally, a bright, variable (1.6-3.1 x 10^39 ergs/s) source was detected at the optical center of the galaxy. Its spectrum shows excess soft emission superimposed on a highly absorbed power law component, similar to what has been found in several other low luminosity AGN (ABRIDGED).Comment: 13 pages in emulateapj5 style with 11 embedded Postscript figures; minor revisions since last version; accepted by Ap

Dust and the Infrared Kinematic Properties of Early-Type Galaxies

We have obtained spectra and measured the stellar kinematics in a sample of 25 nearby early-type galaxies (with velocity dispersions from less than 100 km/s to over 300 km/s) using the near-infrared CO absorption bandhead at 2.29 microns. Our median uncertainty for the dispersions is ~10%. We examine the effects of dust on existing optical kinematic measurements. We find that the near-infrared velocity dispersions are in general smaller than optical velocity dispersions, with differences as large as 30%. The median difference is 11% smaller, and the effect is of greater magnitude for higher dispersion galaxies. The lenticular galaxies (18 out of 25) appear to be causing the shift to lower dispersions while the classical ellipticals (7 out of 25) are consistent between the two wavelength regimes. If uniformly distributed dust causes these differences, we would expect to find a correlation between the relative amount of dust in a galaxy and the fractional change in dispersion, but we do not find such a correlation. We do see correlations both between velocity dispersion and CO bandhead equivalent width, and velocity dispersion and Mg2 index. The differences in dispersion are not well explained by current models of dust absorption. The lack of correlation between the relative amount of dust and shift in dispersion possibly suggets that dust does not have a similar distribution from galaxy to galaxy. The CO equivalent widths of these galaxies are quite high (>10 angstroms for almost all), requiring the light at these wavelengths to be dominated by very cool stars.Comment: 17 pages, 14 figures, accepted to The Astronomical Journa

Extended High-Ionization Nuclear Emission-Line Region in the Seyfert Galaxy NGC 4051

We present an optical spectroscopic analysis of the well-known Seyfert galaxy NGC 4051. The high-ionization nuclear emission-line region (HINER) traced by [Fe X]6374 is found to be spatially extended to a radius of 3a rcseconds (150 pc) west and southwest from the nucleus; NGC 4051 is the third example which has an extended HINER. The nuclear spectrum shows that the flux of [Fe X]6374 is stronger than that of [Fe VII] 6087 in our observation. This property cannot be interpreted in terms of a simple one-zone photoionization model. In order to understand what happens in the nuclear region in NGC 4051, we investigate the physical condition of the nuclear emission-line region in detail using new photoionization models in which the following three emission-line components are taken into account; (1) optically thick, ionization-bounded clouds; (2) optically thin, matter-bounded clouds; and (3) a contamination component which emits H$\alpha$ and H$\beta$ lines. Here the observed extended HINER is considered to be associated with the low-density, matter-bounded clouds. Candidates of the contamination component are either the broad-line region (BLR) or nuclear star forming regions or both. The complexity of the excitation condition found in NGC 4051 can be consistently understood if we take account of these contamination components.Comment: 16 pages, including figures. To Appear in the Astronomical Journal February 2000 Issu

Intensive HST, RXTE and ASCA Monitoring of NGC 3516: Evidence Against Thermal Reprocessing

During 1998 April 13-16, NGC 3516 was monitored almost continuously with HST for 10.3 hr in the UV and 2.8 d in the optical, and simultaneous RXTE and ASCA monitoring covered the same period. The X-rays were strongly variable with the soft (0.5-2 keV) showing stronger variations (~65% peak-to-peak) than the hard (2-10 keV; ~50% peak-to-peak). The optical continuum showed much smaller but highly significant variations: a slow ~2.5% rise followed by a faster ~3.5% decline. The short UV observation did not show significant variability. The soft and hard X-ray light curves were strongly correlated with no significant lag. Likewise, the optical continuum bands (3590 and 5510 A) were also strongly correlated with no measurable lag above limits of <0.15 d. However no significant correlation or simple relationship could be found for the optical and X-ray light curves. These results appear difficult to reconcile with previous reports of correlations between X-ray and optical variations and of measurable lags within the optical band for some other Seyfert 1s. These results also present serious problems for "reprocessing" models in which the X-ray source heats a stratified accretion disk which then reemits in the optical/ultraviolet: the synchronous variations within the optical would suggest that the emitting region is <0.3 lt-d across, while the lack of correlation between X-ray and optical variations would indicate, in the context of this model, that any reprocessing region must be >1 lt-d in size. It may be possible to resolve this conflict by invoking anisotropic emission or special geometry, but the most natural explanation appears to be that the bulk of the optical luminosity is generated by some other mechanism than reprocessing.Comment: 23 pages including 6 figures, accepted for publication in Ap