2943456
A near‐infrared polarized bipolar cone in the Circinus galaxy
We present near–infrared broad–band polarization images of the nuclear regions of the Circinus galaxy in the J, H and K bands. For the first time the south–eastern reflection cone is detected in polarized light, which is obscured at optical wavelengths behind the galactic disk. This biconical structure is clearly observed in J and H band polarized flux whilst in the K band a more compact structure is detected. Total flux J–K and H–K colour maps reveal a complex colour gradient toward the south–east direction (where the Circinus galactic disk is nearer to us). We find enhanced extinction in an arc shaped structure, at about 200pc from the nucleus, probably part of the starformation ring. We model the polarized flux images with the scattering and torus model of Young et al. , with the same basic input parameters as used by Alexander et al. in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of �16pc, and a visual extinction AV , through the torus, to the near–infrared emission regions of >66 mags.
120151930
New constraints on the unified model of Seyfert galaxies
9424089
Constraints on the nuclear emission of the Circinus galaxy: the torus
In the context of the unified model of Seyfert galaxies, we use observations from the literature and a radiative transfer model to investigate the near–IR to mm emission produced by the presumed torus in the Circinus galaxy, from 2µm to 1.3mm. From the infrared SED modelling, we find that the total luminosity (LIR) in this wavelength range, consists of similar contributions from the torus and starburst with a ratio of nuclear luminosity to starburst luminosity (LNUC/LSB) � 0.8. By using a similar torus model to that of NGC1068, but without the conical dust, we find an upper limit to the outer torus radius of �12pc with a best fit of �2pc. The upper limit torus size estimated from the radiative transfer modelling is consistent with the 16pc torus radius estimated from near–IR imaging polarimetry of Circinus.
119183225
First T dwarfs in the VISTA Hemisphere Survey
School of Physics & Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, United KingdomOctober 19, 2012; October 19, 2012ABSTRACTAims.The aim of the project is to improve our current knowledge of the density of T dwarfs and the shape of the substellar initialmass function by identifying a magnitude-limited sample ofT dwarfs in the full southern sky.Methods. We present the results of a photometric search aimed at discovering cool brown dwarfs in the Southern sky imaged atinfrared wavelengths by the Visible and Infrared Survey Telescope for Astronomy (VISTA) and the Wide Infrared Survey Explorer(WISE) satellite mission. We combined the ﬁrst data release (DR1) of the VISTA Hemisphere Survey (VHS) and the WISE prelimi-nary data release to extract candidates with red mid-infrared colours and near- to mid-infrared colours characteristics of cool browndwarfs.Results.The VHS DR1 vs. WISE search returned tens of T dwarf candidates, 13 of which are presented here, including two previ-ously published in the literature and ﬁve new ones conﬁrmed s pectroscopically with spectral types between T4.5 and T8. We estimatethat the two T6 dwarfs lie within 16 pc and the T4.5 within 25 pc. The remaining three are 30–50 pc distant. The only T7 dwarf inour sample is the faintest of its spectral class with J =19.28 mag. The other six T dwarf candidates remain without spectroscopicfollow-up. We also improve our knowledge on the proper motion accuracy for three bright T dwarfs by combining multi-epoch datafrom public databases (DENIS, 2MASS, VHS, WISE, Spitzer).Key words. Stars: low-mass stars and brown dwarfs — techniques: photom etric — techniques: spectroscopic — Infrared: Stars —surveys
17086551
Chemical Composition of Two H II Regions in NGC 6822 Based on VLT Spectroscopy
We present long-slit spectrophotometry of regions V and X of the Local Group irregular galaxy NGC 6822. The data consist of VLT FORS observations in the 3450-7500 A range. We have obtained electron temperatures and densities using different line intensity ratios. We have derived the He, C, and O abundances relative to H based on recombination lines; the abundance ratios among these elements are almost independent of the temperature structure of the nebulae. We have also determined the N, O, Ne, S, Cl, and Ar abundances based on collisionally excited lines; the ratios of these abundances relative to that of H depend strongly on the temperature structure of the nebulae. The chemical composition of NGC 6822 V is compared with those of the Sun, the Orion Nebula, NGC 346 in the SMC, and 30 Doradus in the LMC. The value of O/H derived from recombination lines is in good agreement with that derived by Venn and coworkers from two A-type supergiants in NGC 6822.
18172579
Evolution of the nuclear accretion disk emission in NGC 1097 : getting closer to the black hole
We study the evolution of the broad, double-peaked Hα emission-line profile of the LINER/Seyfert 1 nucleus of NGC 1097, using 24 spectra obtained over a time span of 11 years—from 1991 November through 2002 October. While in the first 5 years the main variation was in the relative intensity of the blue and red peaks, in the last years we have also observed an increasing separation between the two peaks, at the same time as the integrated flux in the broad line has decreased. We propose a scenario in which the emission originates in an asymmetric accretion disk around a supermassive black hole, whose source of ionization is getting dimmer, causing the region of maximum emission to come closer to the center (and thus to regions of higher projected velocity). We use the observations to constrain the evolution of the accretion disk emission and to evaluate two models: the elliptical-disk model previously found to reproduce the observations from 1991 to 1996 and a model of a circular disk with a single spiral arm. In both models the peak emissivity of the disk drifts inward with time, while the azimuthal orientation of the elliptical-disk or the spiral pattern varies with time. In the case of the spiral-arm model, the whole set of data is consistent with a monotonic precession of the spiral pattern, which has completed almost two revolutions since 1991. Thus, we favor the spiral-arm model, which, through the precession period, implies a black hole mass that is consistent with the observed stellar velocity dispersion. In contrast, the elliptical-disk model requires a mass that is an order of magnitude lower. Finally, we have found tentative evidence of the emergence of an accretion disk wind, which we hope to explore further with future observations.
14792267
The chemical composition of the galactic H II regions M8 and M17. A revision based on deep vlt echelle spectrophotometry
We present new echelle spectrophotometry of the Galactic H II regions M8 and M17. The data have been taken with the VLT UVES echelle spectrograph in the 3100 to 10400 A range. We have measured the intensities of 375 and 260 emission lines in M8 and M17 respectively, increasing signicativ ely the number of emission lines measured in previous spectrophotometric studies of these nebulae. Most of the detected lines are permitted lines. Electron temperatures and densities have been determined using dieren t diagnostics. We have derived He + , C ++ , O + and O ++ ionic abundances from pure recombination lines. We have also derived abundances from collisionally excited lines for a large number of ions of dieren t elements. Highly consistent estimations of t 2 have been obtained by using dieren t independent indicators; the values are moderate and very similar to those obtained in other Galactic H II regions. We report the detection of deuterium Balmer emission lines, up to D , in M8 and show that their intensities are consistent with continuum uorescence as their main excitation mechanism.
118914948
Optical and near-infrared observations of the peculiar type ia supernova 1999ac
We present 39 nights of optical photometry, 34 nights of IR photometry, and 4 nights of optical spectroscopy of the Type Ia supernova 1999ac. This supernova was discovered 2 weeks before maximum light, and observations were begun shortly thereafter. At early times its spectra resembled the unusual SN 1999aa and were characterized by very high velocities in the Ca II H and K lines but very low velocities in the Si II λ6355 line. The optical photometry showed a slow rise to peak brightness but, quite peculiarly, was followed by a more rapid decline from maximum. Thus, the B- and V-band light curves cannot be characterized by a single stretch factor. We argue that the best measure of the nature of this object is not the decline rate parameter Δm15(B). The B - V colors were unusual from 30 to 90 days after maximum light in that they evolved to bluer values at a much slower rate than normal Type Ia supernovae. The spectra and bolometric light curve indicate that this event was similar to the spectroscopically peculiar slow decliner SN 1999aa.