28 research outputs found
Peering through the dust: Evidence for a supermassive Black Hole at the nucleus of Centaurus A from VLT IR spectroscopy
We used the near infrared spectrometer ISAAC at the ESO 'Very Large
Telescope' to map the velocity field of Centaurus A (NGC 5128) at several
position angles and locations in the central 20" of the galaxy. The high
spatial resolution (~0.5") velocity fields from both ionized and molecular gas
(PaBeta, [FeII], BrGamma, and H2) are not compromised by either excitation
effects or obscuration. We identify three distinct kinematical systems: (i) a
rotating 'nuclear disk' of ionized gas, confined to the inner 2", the
counterpart of the PaAlpha feature previously revealed by HST/NICMOS imaging;
(ii) a ring-like system with a ~6" inner radius detected only in H2, likely the
counterpart of the 100pc-scale structure detected in CO by other authors; (iii)
a normal extended component of gas rotating in the galactic potential. The
nuclear disk is in keplerian rotation around a central mass concentration, dark
(M/L>20 Msun/LsunK) and point-like at the spatial resolution of the data
(R<0.25" ~4pc). We interpret this mass concentration as a supermassive black
hole. Its dynamical mass based on the line velocities and disk inclination
(i>15deg) is M=2(+3.0;-1.4) 10^8 Msun. The ring-like system is probably
characterized by non-circular motions; a 'figure-of-8' pattern observed in the
H2 position-velocity diagram might provide kinematical evidence for the
presence of a nuclear bar.Comment: 43 pages, 19 figures, Astrophysical Journal in press, higher quality
figures available at http://www.arcetri.astro.it/~marconi/pubs.htm
Is There an Advection-Dominated Accretion Flow in Radio Galaxies with Double-Peaked Balmer Lines?
In order to examine the prediction that the broad-line radio galaxies (BLRGs)
with double-peaked Balmer lines harbor an accretion disk characterized by an
advection-dominated accretion flow (ADAF) in their nuclei, we investigate
narrow emission-line flux ratios of the narrow-line regions which are
photoionized by the nuclear continuum radiation. We compile data from the
literature and confirm the pioneering work of Eracleous & Halpern that the
BLRGs with the double-peaked Balmer emission exhibit larger flux ratios of both
[O I]6300/[O III]5007 and [O II]3727/[O III]5007 than the BLRGs without the
double-peaked Balmer emission. To examine whether or not these properties are
attributed to the difference in the spectral energy distribution (SED) of the
ionizing radiation between the BLRGs with and without the double-peaked Balmer
emission, we perform photoionization model calculations using two types of
input continuum radiation; one has the strong big blue bump which is expected
for standard optically-thick accretion disks and another does not exhibit a
strong big blue bump as expected for optically-thin ADAFs. We find that the
data of the BLRGs with the double-peaked Balmer lines are consistent with the
models adopting the SED without a strong big blue bump while the data of the
BLRGs without the double-peaked emission lines are well described by the models
adopting the SED with a strong big blue bump. On the other hand, the observed
differences in the NLR emission is hard to be explain by the difference in the
contribution of shocks. These results support the idea that the double-peaked
Balmer lines arise at an outer region of an accretion disk which is illuminated
by an inner, geometrically-thick ADAF.Comment: 12 pages including figures, to appeare in the Astrophysical Journa
Sphingomyelinase D Activity in Model Membranes: Structural Effects of in situ Generation of Ceramide-1-Phosphate
The toxicity of Loxosceles spider venom has been attributed to a rare enzyme, sphingomyelinase D, which transforms sphingomyelin to ceramide-1-phosphate. The bases of its inflammatory and dermonecrotic activity, however, remain unclear. In this work the effects of ceramide-1-phosphate on model membranes were studied both by in situ generation of this lipid using a recombinant sphingomyelinase D from the spider Loxosceles laeta and by pre-mixing it with sphingomyelin and cholesterol. The systems of choice were large unilamellar vesicles for bulk studies (enzyme kinetics, fluorescence spectroscopy and dynamic light scattering) and giant unilamellar vesicles for fluorescence microscopy examination using a variety of fluorescent probes. The influence of membrane lateral structure on the kinetics of enzyme activity and the consequences of enzyme activity on the structure of target membranes containing sphingomyelin were examined. The findings indicate that: 1) ceramide-1-phosphate (particularly lauroyl ceramide-1-phosphate) can be incorporated into sphingomyelin bilayers in a concentration-dependent manner and generates coexistence of liquid disordered/solid ordered domains, 2) the activity of sphingomyelinase D is clearly influenced by the supramolecular organization of its substrate in membranes and, 3) in situ ceramide-1-phosphate generation by enzymatic activity profoundly alters the lateral structure and morphology of the target membranes
The interaction of bacterial pathogens with platelets.
In recent years, the frequency of serious cardiovascular infections such as endocarditis has increased, particularly in association with nosocomially acquired antibiotic-resistant pathogens. Growing evidence suggests a crucial role for the interaction of bacteria with human platelets in the pathogenesis of cardiovascular infections. Here, we review the nature of the interactions between platelets and bacteria, and the role of these interactions in the pathogenesis of endocarditis and other cardiovascular diseases