Old open clusters: UBGVRI photometry of NGC 2506

UBGVRI photometry for the open cluster NGC 2506 is presented. From comparison of the observed colour-magnitude diagrams with simulations based on stellar evolutionary models we derive in a self consistent way reddening, distance, and age of the cluster: E(B-V)=0-0.07, (m-M)o = 12.6, age = 1.5-2.2 Gyr. The cluster shows a well definite secondary sequence, suggesting that binary systems constitute about 20 % of the cluster members visible in the colour-magnitude diagram.Comment: 11 pages, 7 figures, MNRAS latex style, accepte

The HST view of the innermost narrow line region

We analyze the properties of the innermost narrow line region in a sample of low-luminosity AGN. We select 33 LINERs (bona fide AGN) and Seyfert galaxies from the optical spectroscopic Palomar survey observed by HST/STIS. We find that in LINERs the [NII] and [OI] lines are broader than the [SII] line and that the [NII]/[SII] flux ratio increases when moving from ground-based to HST spectra. This effect is more pronounced considering the wings of the lines. Our interpretation is that, as a result of superior HST spatial resolution, we isolate a compact region of dense ionized gas in LINERs, located at a typical distance of about 3 pc and with a gas density of about 10$^4$-10$^5$ cm$^{-3}$, which we identify with the outer portion of the intermediate line region (ILR). Instead, we do not observe these kinds of effects in Seyferts; this may be the result of a stronger dilution from the NLR emission, since the HST slit maps a larger region in these sources. Alternatively, we argue that the innermost, higher density component of the ILR is only present in Seyferts, while it is truncated at larger radii because of the presence of the circumnuclear torus. The ILR is only visible in its entirety in LINERs because the obscuring torus is not present in these sources.Comment: 11 pages, 9 figures, A&A in pres

Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei. I. Method and simulations

This is the first in a series of papers in which we study the application of spectroastrometry in the context of gas kinematical studies aimed at measuring the mass of supermassive black holes. The spectroastrometrical method consists in measuring the photocenter of light emission in different wavelength or velocity channels. In particular we explore the potential of spectroastrometry of gas emission lines in galaxy nuclei to constrain the kinematics of rotating gas disks and to measure the mass of putative supermassive black holes. By means of detailed simulations and test cases, we show that the fundamental advantage of spectroastrometry is that it can provide information on the gravitational potential of a galaxy on scales significantly smaller (~ 1/10) than the limit imposed by the spatial resolution of the observations. We then describe a simple method to infer detailed kinematical informations from spectroastrometry in longslit spectra and to measure the mass of nuclear mass concentrations. Such method can be applied straightforwardly to integral field spectra, which do not have the complexities due to a partial spatial covering of the source in the case of longslit spectra.Comment: Accepted for publication in A&

Infrared 3D Observations of Nearby Active Galaxies

We present multi-wavelength imaging observations of three nearby and famous active galaxies obtained with NICMOS, ISOCAM and the MPE near-IR integral field spectrometer. The data reveal a variety of features and properties that are missed in optical studies and in traditional IR monodimensional spectroscopy.Comment: 6 pages, to appear in "Imaging the Universe in Three Dimensions: Astrophysics with Advanced Multi-Wavelength Imaging Devices", eds. W. van Breugel and J. Bland-Hawthorn, needs pasp3D.st

Spine-sheath layer radiative interplay in subparsec-scale jets and the TeV emission from M87

Simple one-zone homogeneous synchrotron self-Compton models have severe difficulties in explaining the TeV emission observed in the radiogalaxy M87. Also the site of the TeV emission region is uncertain: it could be the unresolved jet close to the nucleus, analogously to what proposed for blazars, or an active knot, called HST-1, tens of parsec away. We explore the possibility that the TeV emission of M87 is produced in the misaligned subpc scale jet. We base our modelling on a structured jet, with a fast spine surrounded by a slower layer. In this context the main site responsible for the emission of the TeV radiation is the layer, while the (debeamed) spine accounts for the emission from the radio to the GeV band: therefore we expect a more complex correlation with the TeV component than that expected in one-zone scenarios, in which both components are produced by the same region. Observed from small angles, the spine would dominate the emission, with an overall Spectral Energy Distribution close to those of BL Lac objects with a synchrotron peak located at low energy (LBLs).Comment: 5 pages, 2 figures. Accepted for publication in MNRAS Letter

Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei. III. CRIRES observations of the Circinus galaxy

We present new CRIRES spectroscopic observations of BrGamma in the nuclear region of the Circinus galaxy, obtained with the aim of measuring the black hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an ideal benchmark for the spectroastrometric technique given its proximity and secure BH measurement obtained with the observation of its nuclear H2O maser disk. The kinematical data have been analyzed both with the classical method based on the analysis of the rotation curves and with the new method developed by us and based on spectroastrometry. The classical method indicates that the gas disk rotates in the gravitational potential of an extended stellar mass distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun, concentrated within r < 7 pc. The new method is capable of probing gas rotation at scales which are a factor ~3.5 smaller than those probed by the rotation curve analysis. The dynamical mass spatially unresolved with the spectroastrometric method is a factor ~2 smaller, 7.9 (+1.4 -1.1) 10^6 Msun indicating that spectroastrometry has been able to spatially resolve the nuclear mass distribution down to 2 pc scales. This unresolved mass is still a factor ~4.5 larger than the BH mass measurement obtained with the H2O maser emission indicating that it has not been possible to resolve the sphere of influence of the BH. Based on literature data, this spatially unresolved dynamical mass distribution is likely dominated by molecular gas and it has been tentatively identified with the circum-nuclear torus which prevents a direct view of the central BH in Circinus. This mass distribution, with a size of ~2pc, is similar in shape to that of the star cluster of the Milky Way suggesting that a molecular torus, forming stars at a high rate, might be the earlier evolutionary stage of the nuclear star clusters which are common in late type spirals.Comment: A&A in press. We wish to honor the memory of our great friend and colleague David Axon. He will be greatly missed by all of us. arXiv admin note: text overlap with arXiv:1110.093

Rocking ratchets in 2D Josephson networks: collective effects and current reversal

A detailed numerical study on the directed motion of ac-driven vortices and antivortices in 2D Josephson junction arrays (JJA) with an asymmetric periodic pinning potential is reported. Dc-voltage rectification shows a strong dependence on vortex density as well as an inversion of the vortex flow direction with ac amplitude for a wide range of vortex density around $f$=1/2 ($f$=$Ha^2 / \Phi_0$), in good agreement with recent experiments by Shal\'om and Pastoriza [Phys. Rev. Lett. {\bf 94}, 177001 (2005)]. The study of vortex structures, spatial and temporal correlations, and vortex-antivortex pairs formation gives insight into a purely collective mechanism behind the current reversal effect.Comment: 4 pages, 5 figures. Accepted for publication in Phys. Rev. Let