122151847
New infrared observations of seyfert 2 galaxies and unification theories
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.
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.
15600541
The Luminosity Function of Magnitude and Proper-Motion-selected Samples: The Case of White Dwarfs
The luminosity function of white dwarfs is a powerful tool for studies of the evolution and formation of the Milky Way. The (theoretical) white dwarf cooling sequence provides a useful indicator of the evolutionary timescales involved in the chronometry and star formation history of the galactic disk; therefore, intrinsically faint (and old) white dwarfs in the immediate solar neighborhood can be used to determine an upper limit for the age of the galactic disk. Most determinations of the white dwarf luminosity function have relied on the use of Schmidt's 1/Vmax (Schmidt) method for magnitude- and proper-motion-selected samples, the behavior of which has been demonstrated to follow a minimum variance maximum-likelihood pattern for large samples. Additionally, recent numerical simulations have also demonstrated that the 1/Vmax provides a reliable estimator of the true LF, even in the case of small samples (Wood & Oswalt; Garcia-Berro et al.). However, the conclusions from all these previous studies have been based on noise-free data, where errors in the derived LF have been either assumed to follow a Poisson distribution (valid only for large samples) or where other simple estimates for the uncertainties have been used. In this paper we examine the faint-end (MV > +14) behavior of the disk white dwarf luminosity function using the 1/Vmax method but fully including the effects of realistic observational errors in the derived luminosity function. We employ a Monte Carlo approach to produce many different realizations of the luminosity function from a given data set with prespecified and reasonable errors in apparent magnitude, proper motions, parallaxes and bolometric corrections. These realizations allow us to compute both a mean and an expected range in the luminosity function that is compatible with the observational errors. We find that current state-of-the-art observational errors, mostly in the bolometric corrections and trigonometric parallaxes, play a major role in obliterating (real or artificial) small-scale fluctuations in the luminosity function. We also find that a better estimator of the true luminosity function seems to be the median oversimulations, rather than the mean. When using the latter, an age of 10 Gyr or older can not be ruled out from the sample of Leggett, Ruiz, & Bergeron.
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.
14952037
THE CALIBRATION OF THE O/H ABUNDANCE INDICATORS FOR EXTRAGALACTIC H II REGIONS BASED ON O II RECOMBINATION LINES
Based on O II recombination lines we present a new calibration (called O IIRL) of Pagel’s O23 indicator to determine the O/H abundance ratio in extragalactic H II regions and emission line galaxies. The O IIRL calibration produces O/H abundances about a factor of two higher than those derived from the T (4363) method with t 2 = 0.00. The O IIRL calibration has implications for the study of dieren t properties of emission line galaxies such as their metallicity, star formation rate, and initial mass function. The O IIRL calibration also aects the abundance determinations based on other O/H indicators, that include collisionally excited lines, like those known as O3N2, N2, S23, Ar3O3, and S3O3. We argue that the controversy between the T (4363) method and the photoionization models method to derive O/H values is mainly due to temperature variations inside the observed H II regions.
13941541
The Chemical Composition of the Small Magellanic Cloud H II Region NGC 346 and the Primordial Helium Abundance
Spectrophotometry in the λλ3400-7400 range is presented for 13 areas of the brightest H II region in the SMC: NGC 346. The observations were obtained at CTIO with the 4 m telescope. Based on these observations, its chemical composition is derived. The helium and oxygen abundances by mass are given by Y(SMC) = 0.2405 ± 0.0018 and O(SMC) = 0.00171 ± 0.00025. From models and observations of irregular and blue compact galaxies it is found that ΔY/ΔO = 3.5 ± 0.9 and, consequently, that the primordial helium abundance by mass is given by Yp = 0.2345 ± 0.0026 (1 σ). This result is compared with values derived from big bang nucleosynthesis and with other determinations of Yp.
122038173
Vector algebra on a lattice
Prescriptions are introduced for the operations of vector algebra on a random lattice. Some vector identities are preserved exactly. Others receive new terms, described as evanescent because they vanish in the continuum limit.