Search results for "galaxy"
showing 10 items of 1505 documents
Studying the microlenses mass function from statistical analysis of the caustic concentration
2011
The statistical distribution of caustic crossings by the images of a lensed quasar depends on the properties of the distribution of microlenses in the lens galaxy. We use a procedure based in Inverse Polygon Mapping to easily identify the critical and caustic curves generated by a distribution of stars in the lens galaxy. We analyze the statistical distributions of the number of caustic crossings by a pixel size source for several projected mass densities and different mass distributions. We compare the results of simulations with theoretical binomial distributions. Finally we apply this method to the study of the stellar mass distribution in the lens galaxy of QSO 2237+0305.
Comparing estimators of the galaxy correlation function
1999
We present a systematic comparison of some usual estimators of the 2--point correlation function, some of them currently used in Cosmology, others extensively employed in the field of the statistical analysis of point processes. At small scales, it is known that the correlation function follows reasonably well a power--law expression $\xi(r) \propto r^{-\gamma}$. The accurate determination of the exponent $\gamma$ (the order of the pole) depends on the estimator used for $\xi(r)$; on the other hand, its behavior at large scale gives information on a possible trend to homogeneity. We study the concept, the possible bias, the dependence on random samples and the errors of each estimator. Erro…
Observational Cosmology at High Redshift
2007
Summary. I offer a brief review of the evolution and present status of our observational knowledge of the high-redshift Universe. In particular, I focus on the different methods that have been devised to select distant objects, and the observational evidence in hand to support (or else) the standard evolutionary scenario. 1 How High is High? The study of objects at cosmological distances from us started in the 1960s with the discovery and identification of quasi-stellar radiosources. The explanation of the features observed in the optical spectra of these objects as highly redshifted hydrogen lines opened the door to the very distant Universe. For the next 30 years after the discovery of qu…
Empirical determination of Einstein A-coefficient ratios of bright [Fe II] lines
2014
The Einstein spontaneous rates (A-coefficients) of Fe+ lines have been computed by several authors with results that differ from each other by up to 40%. Consequently, models for line emissivities suffer from uncertainties that in turn affect the determination of the physical conditions at the base of line excitation. We provide an empirical determination of the A-coefficient ratios of bright [Fe II] lines that would represent both a valid benchmark for theoretical computations and a reference for the physical interpretation of the observed lines. With the ESO-Very Large Telescope X-shooter instrument between 3000 Å and 24700 Å, we obtained a spectrum of the bright Herbig-Haro object HH 1. …
Impact of Crystal Habit on Cirrus Radiative Properties
2007
The impact of assumed ice crystal morphology of subtropical cirrus on the solar and thermal infrared (IR) radiative field above, within, and below the cirrus is quantified. For this purpose airborne measurements of ice crystal size distribution from the CRYSTAL-FACE campaign and a library of optical properties of nonspherical ice crystal habits are implemented into radiative transfer simulations.Two cirrus cases are studied in detail: a high (cold) cirrus cloud with small visible optical thickness (τ≈1), and a lower (warmer) cirrus cloud of relatively large visible optical thickness (τ≈7). For t+he solar wavelength range the impact of shape characteristics of the crystals was important for …
On the identification of celestial γ-ray sources
1978
THE observations from COS B have provided a new and more detailed picture of the high energy γ-ray emission from the Galaxy. We discuss here the first catalogue with 13 localised sources which has been compiled1,2 and how the list should lengthen in the near future, as the data analysis progresses.
Galactic Magnetic Fields As a Consequence of Inflation
2002
The generation of a magnetic field in the Early Universe is considered, due to the gravitational production of the Z-boson field during inflation. Scaled to the epoch of galaxy formation this magnetic field suffices to trigger the galactic dynamo and explain the observed galactic magnetic fields. The mechanism is independent of the inflationary model.
Disc loss and renewal in A0535+26
1999
This paper presents observations of the Be/X-ray binary system A0535+26 revealing the first observed loss of its circumstellar disc, demonstrated by the loss of its JHK infrared excess and optical/IR line emission. However optical/IR spectroscopy reveals the formation of a new inner disc with significant density and emission strength at small radii; the disc has proven to be stable over 5 months in this intermediate state.
Chandra X-ray observation of the HII region Gum 31 in the Carina Nebula complex
2014
(abridged) We used the Chandra observatory to perform a deep (70 ksec) X-ray observation of the Gum 31 region and detected 679 X-ray point sources. This extends and complements the X-ray survey of the central Carina nebula regions performed in the Chandra Carina Complex Project. Using deep near-infrared images from our recent VISTA survey of the Carina nebula complex, our Spitzer point-source catalog, and optical archive data, we identify counterparts for 75% of these X-ray sources. Their spatial distribution shows two major concentrations, the central cluster NGC 3324 and a partly embedded cluster in the southern rim of the HII region, but majority of X-ray sources constitute a rather homo…
The rich young cluster NGC 6530: a combined X-ray-optical-infrared study
2006
We present a combined X-ray, optical, and IR (2MASS) study of NGC 6530, complementing our previous studies of this cluster. We consider different indicators of IR excesses, which can be taken as an indicator of circumstellar disks and therefore of pre-main-sequence status. We used reddening-free indices to ensure that our results are unaffected by highly irregular, differential reddening. More than the study of the JHK bands alone (33 IR-excess stars found), we found it useful to compare various optical and IR colors, finding overall 333 stars with optical-IR excess. In the field of the previously studied Chandra ACIS-I observation, we found 196 stars with optical-IR excess, of which 120 ha…