Local magnitude estimate at Mt. Etna

In order to verify the duration magnitude MD we calculated local magnitude ML values of 288 earthquakes occurring from October 2002 to April 2003 at Mt. Etna. The analysis was computed at three digital stations of the permanent seismic network of Istituto Nazionale di Geofisica e Vulcanologia of Catania, using the relationship ML = logA+alog?-b, where A is maximum half-amplitude of the horizontal component of the seismic recording measured in mm and the term «+alog?-b» takes the place of the term «-logA0» of Richter relationship. In particular, a = 0.15 for ?<200 km, b=0.16 for ?<200 km. Duration magnitude MD values, moment magnitude MW values and other local magnitude values were compared. Differences between ML and MD were obtained for the strong seismic swarms occurring on October 27, during the onset of 2002-2003 Mt. Etna eruption, characterized by a high earthquake rate, with very strong events (seismogram results clipped in amplitude on drum recorder trace) and high level of volcanic tremor, which not permit us to estimate the duration of the earthquakes correctly. ML and MD relationships were related and therefore a new relationship for MD is proposed. Cumulative strain release calculated after the eruption using ML values is about 1.75E+06 J1/2 higher than the one calculated using MD values

Dust properties at z=6.3 in the host galaxy of GRB 050904

We investigate the dust extinction properties in the host galaxy of the Gamma-Ray Burst (GRB) GRB 050904 at z=6.29 by analyzing simultaneous broad band observations of the optical and UV afterglow at three different epochs. We show that the peculiar afterglow spectral energy distribution (SED) observed at 0.5 days and at 1 day after the burst (1.6 and 3 hours rest frame) cannot be explained with dust reddening with any of the extinction curves observed at low redshift. Yet, the extinction curve recently inferred for the most distant BAL QSO at z=6.2 nicely reproduces the SED of GRB 050904 at both epochs. Our result provides an additional, independent indication that the properties of dust evolve beyond z~6. We discuss the implications of this finding within the context of the dust production mechanisms through the cosmic ages.Comment: 4 pages, 2 figures, accepted for publication in ApJ

The TNG Near Infrared Camera Spectrometer

NICS (acronym for Near Infrared Camera Spectrometer) is the near-infrared cooled camera-spectrometer that has been developed by the Arcetri Infrared Group at the Arcetri Astrophysical Observatory, in collaboration with the CAISMI-CNR for the TNG (the Italian National Telescope Galileo at La Palma, Canary Islands, Spain). As NICS is in its scientific commissioning phase, we report its observing capabilities in the near-infrared bands at the TNG, along with the measured performance and the limiting magnitudes. We also describe some technical details of the project, such as cryogenics, mechanics, and the system which executes data acquisition and control, along with the related software.Comment: 7 pages, 5 figures, compiled with A&A macros. A&A in pres

The 1 keV to 200 keV X-ray Spectrum of NGC 2992 and NGC 3081

The Seyfert 2 galaxies NGC 2992 and NGC 3081 have been observed by INTEGRAL and Swift. We report about the results and the comparison of the spectrum above 10 keV based on INTEGRAL IBIS/ISGRI, Swift/BAT, and BeppoSAX/PDS. A spectrum can be extracted in the X-ray energy band ranging from 1 keV up to 200 keV. Although NGC 2992 shows a complex spectrum below 10 keV, the hard tail observed by various missions exhibits a slope with photon index = 2, independent on the flux level during the observation. No cut-off is detectable up to the detection limit around 200 keV. In addition, NGC 3081 is detected in the INTEGRAL and Swift observation and also shows an unbroken Gamma = 1.8 spectrum up to 150 keV. These two Seyfert galaxies give further evidence that a high-energy cut-off in the hard X-ray spectra is often located at energies E_C >> 100 keV. In NGC 2992 a constant spectral shape is observed over a hard X-ray luminosity variation by a factor of 11. This might indicate that the physical conditions of the emitting hot plasma are constant, while the amount of plasma varies, due to long-term flaring activity.Comment: 8 pages, 4 figures, accepted for publication in Ap

The X-ray absorber of PKS2126-158

BeppoSAX observed the z=3.27 quasar PKS2126-158 on 1999 May 24-28 when its 2-10keV and 0.1-2.5keV fluxes were 1.1E-11 and 4.4E-12 cgs respectively, a factor of 2 higher than in all previous ROSAT and ASCA observations and 40% higher than in two more recent Chandra and XMM-Newton observations. The shortest detected rest frame variability timescale is of a few months, comparable to the causal timescale associated to an emission region of ~10 Schwarzschild radii around a few 1E10 solar masses black hole. The source is detected with a signal to noise ratio S/N>=3 up to ~50 keV, 215 keV rest frame. The BeppoSAX observations confirm the presence of low energy absorption along the line of sight, independent on the continuum model adopted, at high confidence level. Despite the limited spectral resolution of the BeppoSAX LECS and MECS it is possible to put constraints on different absorption and continuum models, but not to unambiguously determine the redshift of the absorber. If the absorber is not significantly ionized the BeppoSAX data do prefer an absorber at z<=2.7. Strong and complex metal line systems along the line of sight to PKS2126-158 have been found at z=0.6631 and at 2.64<z<2.82. They could well be associated to the X-ray absorption. Conversely, an ionized (``warm'') absorber at the quasar redshift provides a good fit only if the iron abundance is smaller than ~0.3 solar, while that of the other elements is fixed to the solar value.Comment: A&A in pres

Sismicità all’Etna dal 1989 al 2010: evidenze sull’evoluzione spazio-temporale dell’attività sismica

Il Monte Etna, uno dei più attivi vulcani basaltici tra i più monitorati al mondo, è sede di una notevole attività sismica e vulcanica. Esso è ubicato in Sicilia orientale in un complesso quadro geodinamico, dove le principali strutture tettoniche regionali giocano un ruolo chiave nei processi dinamici del vulcano. La sismicità dell’Etna si manifesta con un elevato rate di terremoti di bassa e moderata energia che, a volte, a causa dell’estrema superficialità della sorgente, provocano danni ai centri abitati prossimi all’area epicentrale. Il monitoraggio sistematico dell’attività sismica etnea è effettuato sin dal 1989, mediante una rete sismica locale permanente che nel tempo è stata oggetto di importanti miglioramenti. La prima configurazione di rete era costituita da circa 10 stazioni analogiche con sensori a corto periodo gestita dall’Istituto Internazionale di Vulcanologia (IIV-CNR). Nel 1994, una rete sismica costituita da circa 40 stazioni (analogiche con sensori a corto periodo) fu installata sull’Etna nell’ambito del Progetto Poseidon. Nel 2001, le reti gestite dall’IIV-CNR e dal Progetto Poseidon confluirono nell’Istituto Nazionale di Geofisica e Vulcanologia (INGV); attualmente la rete sismica, costituita da circa 50 stazioni digitali equipaggiate con sismometri broadband a tre componenti, è gestita dalla Sezione di Catania dell’INGV. Nel periodo 1989-1999, il catalogo dei terremoti risulta costituito da circa 2000 eventi con soglia di completezza per magnitudo pari a 2.0; dal 1999 ad oggi contiene circa 6000 terremoti con soglia di completezza per magnitudo 1.5. La capacità di detezione della rete è migliorata nel tempo permettendo di registrare e localizzare anche gli eventi meno energetici (M≥1.0). In questo lavoro, vengono presentati i caratteri predominanti della sismicità etnea negli ultimi 20 anni, con un maggiore dettaglio della distribuzione spazio-temporale della sismicità verificatasi dal 1999. L’analisi della attività sismica rappresenta un utile strumento per l’interpretazione delle dinamiche che hanno contraddistinto numerose ed importanti eruzioni (2001, 2002-03, 2004, 2006, 2008-09). In particolare, la variazione del rilascio energetico della sismicità ha contribuito in maniera significativa ad identificare i probabili processi geodinamici legati alla ricarica del sistema magmatico del vulcano. La distribuzione spaziale della sismicità ha consentito di evidenziare inoltre l’esistenza di diverse aree sismogenetiche caratterizzate da un differente rate sismico, profondità focali e cinematica delle strutture associate. Infine, osservando le caratteristiche della sismicità nel lungo periodo, differenti settori del vulcano sono risultati maggiormente attivi in relazione ai più importanti recenti eventi eruttivi

Metallicity evolution, metallicity gradients and gas fractions at z~3.4

We used near-infrared integral field spectroscopic observations from the AMAZE and LSD programs to constrain the metallicity in a sample of 40 star forming galaxies at 3<z<5 (most of which at z~3.4). We measure metallicities by exploiting strong emission line diagnostics. We found that a significant fraction of star-forming galaxies at z~3.4 deviate from the Fundamental Metallicity Relation (FMR), with a metallicity up to a factor of ten lower than expected according to the FMR. This deviation does not correlate with the dynamical properties of the galaxy or with the presence of interactions. To investigate the origin of the metallicity deviations in more detail, we also infer information on the gas content, by inverting the Schmidt-Kennicutt relation. In agreement with recent CO observational data, we found that, in contrast with the steeply rising trend at 0<z<2, the gas fraction in massive galaxies remains constant, with indication of a marginal decline, at 2<z<3.5. When combined with the metallicity information, we infer that to explain both the low metallicity and gas content in z~3.4 galaxies, both prominent outflows and massive pristine gas inflows are needed. In ten galaxies we can also spatially resolve the metallicity distribution. We found that the metallicity generally anticorrelates with the distribution of star formation and with the gas surface density. We discuss these findings in terms of pristine gas inflows towards the center, and outflows of metal rich gas from the center toward the external regions. (Abridged)Comment: Replaced to match the published versio

Molecular depletion times and the CO-to-H2 conversion factor in metal-poor galaxies

Tracing molecular hydrogen content with carbon monoxide in low-metallicity galaxies has been exceedingly difficult. Here we present a new effort, with IRAM 30-m observations of 12CO(1-0) of a sample of 8 dwarf galaxies having oxygen abundances ranging from 12+logO/H=7.7 to 8.4. CO emission is detected in all galaxies, including the most metal-poor galaxy of our sample (0.1 Zsun); to our knowledge this is the largest number of 12CO(1-0) detections ever reported for galaxies with 12+logO/H<=8 (0.2 Zsun) outside the Local Group. We calculate stellar masses (Mstar) and star-formation rates (SFRs), and analyze our results by combining our observations with galaxy samples from the literature. Extending previous results for a correlation of the molecular gas depletion time, tau(dep), with Mstar and specific SFR (sSFR), we find a variation in tau(dep) of a factor of 200 or more (from <50 Myr to 10 Gyr) over a spread of 1000 in sSFR and Mstar. We exploit the variation of tau(dep) to constrain the CO-to-H2 mass conversion factor alpha(CO) at low metallicity, and assuming a power-law variation find alpha(CO) \propto (Z/Zsun)^1.9, similar to results based on dust continuum measurements compared with gas mass. By including HI measurements, we show that the fraction of total gas mass relative to the baryonic mass is higher in galaxies that are metal poor, of low mass, and of high sSFR. Finally, comparisons of the data with star-formation models of the molecular gas phases suggest that, at metallicities Z/Zsun<=0.2, there are some discrepancies with model predictions.Comment: 18 pages, 15 figures, accepted for publication in A&

ALMA observations of cool dust in a low-metallicity starburst, SBS0335-052

We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 Band 7 observations of an extremely metal-poor dwarf starburst galaxy in the Local Universe, SBS0335-052 (12+log(O/H)~7.2). With these observations, dust is detected at 870micron (ALMA Band 7), but 87% of the flux in this band is due to free-free emission from the starburst. We have compiled a spectral energy distribution (SED) of SBS0335-052 that spans almost 6 orders of magnitude in wavelength and fit it with a spherical dust shell heated by a single-age stellar population; the best-fit model gives a dust mass of (3.8+/-0.6)x10^4 Msun. We have also constructed a SED including Herschel archival data for IZw18, another low-metallicity dwarf starburst (12+log(O/H)=7.17), and fit it with a similar model to obtain a dust mass of (3.4+/-1.0)x10^2 Msun. Compared with their atomic gas mass, the dust mass of SBS0335-052 far exceeds the prediction of a linear trend of dust-to-gas mass ratio with metallicity, while IZw18 falls far below. We use gas scaling relations to assess a putative missing gas component in both galaxies and find that the missing, possibly molecular, gas in SBS0335-052 is a factor of 6 times higher than the value inferred from the observed HI column density; in IZw18 the missing component is 4 times smaller. Ultimately, despite their similarly low metallicity, the differences in gas and dust column densities in SBS0335-052 and IZw18 suggest that metal abundance does not uniquely define star-formation processes. At some level, self-shielding and the survival of molecules may depend just as much on gas and dust column density as on metallicity. The effects of low metallicity may at least be partially compensated for by large column densities in the interstellar medium.Comment: 15 pages, 11 figures, accepted for publication in A&

Strongly star-forming rotating disks in a complex merging system at z = 4,7 as revealed by ALMA

We performed a kinematical analysis of the [CII] line emission of the BR 1202-0725 system at z~4,7 using ALMA observations. The most prominent sources of this system are a quasar and a submillimeter galaxy, separated by a projected distance of about 24 kpc and characterized by very high SFR, higher than 1000 Msun/yr. However, the ALMA observations reveal that these galaxies apparently have undisturbed rotating disks, which is at variance with the commonly accepted scenario in which strong star formation activity is induced by a major merger. We also detected faint components which, after spectral deblending, were spatially resolved from the main QSO and SMG emissions. The relative velocities and positions of these components are compatible with orbital motions within the gravitational potentials generated by the QSO host galaxy and the SMG, suggesting that they are smaller galaxies in interaction or gas clouds in accretion flows of tidal streams. We did not find any clear spectral evidence for outflows caused by AGN or stellar feedback. This suggests that the high star formation rates might be induced by interactions or minor mergers with these companions, which do not affect the large-scale kinematics of the disks, however. Our kinematical analysis also indicates that the QSO and the SMG have similar Mdyn, mostly in the form of molecular gas, and that the QSO host galaxy and the SMG are seen close to face-on with slightly different disk inclinations: the QSO host galaxy is seen almost face-on (i~15), while the SMG is seen at higher inclinations (i~25). Finally, the ratio between the black hole mass of the QSO, obtained from XShooter spectroscopy, and the Mdyn of the host galaxy is similar to value found in very massive local galaxies, suggesting that the evolution of black hole galaxy relations is probably better studied with dynamical than with stellar host galaxy masses.Comment: Accepted for publication in Astronomy and Astrophysic