Nuclear Activity in Circumnuclear Ring Galaxies

We have analyzed the frequency and properties of the nuclear activity in a sample of galaxies with circumnuclear rings and spirals (CNRs). This sample was compared with a control sample of galaxies with very similar global properties but without circumnuclear rings. We discuss the relevance of the results in regard to the AGN feeding processes and present the following results: (i) bright companion galaxies seem not to be important for the appearance of CNRs, which appear to be more related to intrinsic properties of the host galaxies or to minor merger processes; (ii) the proportion of strong bars in galaxies with an AGN and a CNR is somewhat higher than the expected ratio of strongly barred AGN galaxies from the results of Ho and co-workers; (iii) the incidence of Seyfert activity coeval with CNRs is clearly larger than the rate expected from the morphological distribution of the host galaxies; (iv) the rate of Sy 2 to Sy 1 type galaxies with CNRs is about three times larger than the expected ratio for galaxies without CNRs and is opposite to that predicted by the geometric paradigm of the classical unified model for AGNs, although it does support the hy-pothesis that Sy 2 activity is linked to circumnuclear star formation. The possible selection effects of the sample are discussed, and we conclude that the detected trends are strong enough to justify high quality observations of as large as possible sets of galaxies with circumnuclear rings and their matched control samples.Comment: Submitted to International Journal of Astronomy and Astrophysic

Morphologies and ages of star cluster pairs and multiplets in the Small Magellanic Cloud

An isophotal atlas of 75 star cluster pairs and multiplets in the Small Magellanic Cloud is presented, comprising 176 objects. They are concentrated in the SMC main body. The isophotal contours were made from Digitized Sky Survey images and showed relevant structural features possibly related to interactions in about 25% of the sample. Previous N-body simulations indicate that such shapes could be due to tidal tails, bridges or common envelopes. The diameter ratio between the members of a pair is preferentially in the range 1 - 2, with a peak at 1. The projected separation is in the range ~ 3 - 22 pc with a pronounced peak at ~ 13 pc. For 91 objects it was possible to derive ages from Colour-Magnitude Diagrams using the OGLE-II photometric survey. The cluster multiplets in general occur in OB stellar associations and/or HII region complexes. This indicates a common origin and suggests that multiplets coalesce into pairs or single clusters in a short time scale. Pairs in the SMC appear to be mostly coeval and consequently captures are a rare phenomenon. We find evidence that star cluster pairs and multiplets may have had an important role in the dynamical history of clusters presently seen as large single objects

Capture of field stars by globular clusters in dense bulge regions

The recent detection of a double Red Giant Branch in the optical color-magnitude diagram (CMD) of the bulge globular cluster HP1 (Ortolani et al. 1997), a more populated metal-poor steep one corresponding to the cluster itself, and another metal-rich curved, led us to explore in the present Letter the possibility of capture of field stars by a globular cluster orbiting in dense bulge regions over several gigayears. Analytical arguments, as well as N-body calculations for a cluster model of 10^5 solar masses in a bulge-like environment, suggest that a significant fraction of cluster stars may consist of captures. Metal-poor globular clusters in the inner bulge, like HP1, contrasting at least in Delta [Fe/H] = 1.0 dex with respect to the surrounding metal-rich stars, are ideal probes to further test the capture scenario. In turn, if this scenario is confirmed, the double RGB of HP1 could provide direct estimates of blanketing amounts, which is fundamental for the photometric calibration of metal-rich stellar populations.Comment: 6 pages, 2 included figures, aas2pp4,sty Latex style. To be published in Astrophysical Journal Letter

Analysis of flash flood scenarios in an urbanized catchment using a two-dimensional hydraulic model

Abstract. In Italy, growing urbanization is leading to a higher risk of flooding of small water courses, especially in steep catchments of limited area, where severe flash flood events can occur. The assessment of flash flood hazard requires new modelling tools that can reproduce both the rainfall–runoff processes in the catchment, and the flow processes in the drainage network. In this paper we propose the use of a simple two-dimensional hydraulic model for analysing a flood scenario in a small valley within the urban area of the city of Bologna, Italy. Historically this area has been prone to severe flood events, the most recent of which occurred in 1955 and 1932. Since then there has been a significant increase in urbanization of the lower portion of the catchment, while the natural stream bed has been partially replaced by a culvert. The two-dimensional hydraulic model was therefore applied at catchment scale, in order to simulate the possible effects of historical scenarios in the present catchment configuration. Rainfall and runoff data measured during recent rainfall events were used to calibrate model parameters. Model results show that the current culvert section would be insufficient to drain the runoff produced by intense rainfall events, with potential inundation of surrounding urban areas

High performance liquid chromatography of long-chain acylcarnitine and phospholipids in fatty acid turnover studies.

In this paper we describe a rapid, isocratic high performance liquid chromatography (HPLC) method for the study of radioactive fatty acid incorporation into complex lipids of human erythrocytes, which allows the simultaneous separation of the major phospholipid classes and long-chain acylcarnitines. The lipid extract of erythrocytes pulsed with radioactive fatty acids was injected into an HPLC system equipped with a silica column. The individual components eluted were monitored by ultraviolet absorption and radioactive emission. With respect to the UV profile, the radioactive profile showed an additional peak between phosphatidyl-choline and phosphatidylethanolamine, which was identified as long-chain acylcarnitine by different experimental approaches. The radioactivity recovered in the long-chain acylcarnitines contains essential information enabling definition of acyl trafficking in red cells

Ages and Metallicities of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud

We present Washington system C,T_1 CMDs of 13 star clusters and their surrounding fields which lie in the outer parts of the LMC disk. Ages are determined by means of the magnitude difference between the giant branch clump and the turnoff, while metallicities are derived from the location of the giant and subgiant branches as compared to fiducial star clusters. We find that in most cases the stellar population of each star cluster is quite similar to that of the field where it is embedded. Three particular fields present remarkable properties: (i) The so far unique cluster ESO121-SC03 at ~9 Gyr has a surrounding field which shares the same properties. (ii) The field surrounding the far eastern intermediate age cluster OHSC37 is noteworthy in the sense that we do not detect any evidence of LMC stars. (iii) The fields of SL388 and SL509 present CMDs with a secondary clump ~0.45 mag fainter than the dominant intermediate age clump, suggesting a stellar population component located behind the LMC disk at a distance comparable to that of the SMC. The mean metallicity derived for the intermediate age outer disk clusters is =-0.7 and for their surrounding fields =-0.6. These values are significantly lower than found by Olszewski et al. (1991, AJ, 101, 515) for a sample of clusters of similar age, but are in good agreement with several recent studies. A few clusters stand out in the age--metallicity relation in the sense that they are intermediate age clusters at relatively low metallicity ([Fe/H]~-1).Comment: LaTeX, to be published in July, 1998 Astronomical Journa