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1.
For LAMOST, the largest sky survey program in China, the solution of the problem of automatic discrimination of stars from galaxies by spectra has shown that the results of the PSF test can be significantly refined. However, the problem is made worse when the redshifts of galaxies are not available. We present a new automatic method of star/(normal) galaxy separation, which is based on Statistical Mixture Modeling with Radial Basis Function Neural Networks (SMM-RBFNN). This work is a continuation of our previous one, where active and non-active celestial objects were successfully segregated. By combining the method in this paper and the previous one, stars can now be effectively separated from galaxies and AGNs by their spectra-a major goal of LAMOST, and an indispensable step in any automatic spectrum classification system. In our work, the training set includes standard stellar spectra from Jacoby's spectrum library and simulated galaxy spectra of EO, SO, Sa, Sb types with redshift ranging from 0 to 1  相似文献   

2.
Difference imaging is a technique for obtaining precise relative photometry of variable sources in crowded stellar fields and, as such, constitutes a crucial part of the data reduction pipeline in surveys for microlensing events or transiting extrasolar planets. The Optimal Image Subtraction (OIS) algorithm of Alard & Lupton (1998) permits the accurate differencing of images by determining convolution kernels which, when applied to reference images with particularly good seeing and signal‐to‐noise (S/N), provide excellent matches to the point‐spread functions (PSF) in other images of the time series to be analysed. The convolution kernels are built as linear combinations of a set of basis functions, conventionally bivariate Gaussians modulated by polynomials. The kernel parameters, mainly the widths and maximal degrees of the basis function model, must be supplied by the user. Ideally, the parameters should be matched to the PSF, pixel‐sampling, and S/N of the data set or individual images to be analysed. We have studied the dependence of the reduction outcome as a function of the kernel parameters using our new implementation of OIS within the IDL‐based TRIPP package. From the analysis of noise‐free PSF simulations of both single objects and crowded fields, as well as the test images in the ISIS OIS software package, we derive qualitative and quantitative relations between the kernel parameters and the success of the subtraction as a function of the PSF widths and sampling in reference and data images and compare the results to those of other implementations found in the literature. On the basis of these simulations, we provide recommended parameters for data sets with different S/N and sampling. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
We have surveyed 188 ROSAT Position Sensitive Proportional Counter (PSPC) fields for X-ray sources with hard spectra ( α <0.5); such sources must be major contributors to the X-ray background at faint fluxes. In this paper we present optical identifications for 62 of these sources: 28 active galactic nuclei (AGN) which show broad lines in their optical spectra (BLAGN), 13 narrow emission line galaxies (NELGs), five galaxies with no visible emission lines, eight clusters and eight Galactic stars.
The BLAGN, NELGs and galaxies have similar distributions of X-ray flux and spectra. Their ROSAT spectra are consistent with their being AGN obscured by columns of 20.5< log( N H/cm−2)<23 . The hard spectrum BLAGN have a distribution of X-ray to optical ratios which is similar to that found for AGN from soft X-ray surveys (1< α OX<2) . However, a relatively large proportion (15 per cent) of the BLAGN, NELGs and galaxies are radio loud. This could be because the radio jets in these objects produce intrinsically hard X-ray emission, or if their hardness is caused by absorption, it could be because radio-loud objects are more X-ray luminous than radio-quiet objects. The eight hard sources identified as clusters of galaxies are the brightest, and softest group of sources and hence clusters are unlikely to be an important component of the hard, faint population.
We propose that BLAGN are likely to constitute a significant fraction of the faint, hard, 0.5–2 keV population and could be important to reproducing the shape of the X-ray background, because they are the most numerous type of object in our sample (comprising almost half the identified sources), and because all our high redshift ( z >1) identified hard sources have broad lines.  相似文献   

4.
We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars.
Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos /Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples.  相似文献   

5.
We present spectroscopic observations of a sample of faint gigahertz peaked‐spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS). Redshifts have been determined for 19 (40 per cent) of the objects. The optical spectra of the GPS sources identified with low‐redshift galaxies show deep stellar absorption features. This confirms previous suggestions that their optical light is not significantly contaminated by active galactic nucleus-related emission, but is dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (probably) young radio sources as probes of galaxy evolution. The optical spectra of GPS sources identified with quasars are indistinguishable from those of flat-spectrum quasars, and clearly different from the spectra of compact steep‐spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples presented in the literature, peaking at z ∼2–3. It is unlikely that a significant population of low-redshift GPS quasars is missed as a result of selection effects in our sample. We therefore claim that there is a genuine difference between the redshift distributions of GPS galaxies and quasars, which, because it is present in both the radio-faint and bright samples, cannot be caused by a redshift–luminosity degeneracy. It is therefore unlikely that the GPS quasars and galaxies are unified by orientation, unless the quasar opening angle is a strong function of redshift. We suggest that the GPS quasars and galaxies are unrelated populations and just happen to have identical observed radio spectral properties, and hypothesize that GPS quasars are a subclass of flat-spectrum quasars.  相似文献   

6.
We describe the results of our spectroscopy for a sample of barred galaxies whose inner regions exhibit an isophotal twist commonly interpreted as a secondary bar. The line-of-sight velocity fields of the ionized gas and stars and the light-of-sight velocity dispersion fields of the stars were constructed from two-dimensional spectroscopy with the 6-m Special Astrophysical Observatory telescope. We detected various types of noncircular motions of ionized gas: radial flows within large-scale bars, counterrotation of the gas and stars at the center of NGC 3945, a polar gaseous disk in NGC 5850, etc. Our analysis of the optical and near-infrared galaxy images (both ground-based and those from the Hubble Space Telescope) revealed circumnuclear minispirals in five objects. The presence of an inner (secondary) bar in the galaxy images is shown to have no effect on the circumnuclear kinematics of the gas and stars. Thus, contrary to popular belief, the secondary bar is not a dynamically decoupled galactic structure. We conclude that the so-called double-barred galaxies are not a separate type of galaxy but are a combination of objects with distinctly different morphologies of their circumnuclear regions.  相似文献   

7.
Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars.  相似文献   

8.
We employ an Artificial Neural Network (ANN) based technique to develop a pipeline for automated segregation of stars from the galaxies to be observed by Tel-Aviv University Ultra-Violet Experiment (TAUVEX). We use synthetic spectra of stars from UVBLUE library and selected International Ultraviolet Explorer (IUE) low-resolution spectra for galaxies in the ultraviolet (UV) region from 1250 to 3220 Å as the training set and IUE low-resolution spectra for both the stars and the galaxies as the test set. All the data sets have been pre-processed to get band integrated fluxes so as to mimic the observations of the TAUVEX UV imager. We also perform the ANN based segregation scheme using the full length spectral features (which will also be useful for the ASTROSAT mission). Our results suggest that, in the case of the non-availability of full spectral features, the limited band integrated features can be used to segregate the two classes of objects; although the band data classification is less accurate than the full spectral data classification.  相似文献   

9.
We present a multicolour catalogue of faint galaxies situated close to bright stars,   V ≲ 15  , with the aim of identifying high-redshift galaxies suitable for study with adaptive optics-equipped near-infrared imagers and spectrographs. The catalogue is constructed from archival calibration observations of the United Kingdom Infrared Telescope (UKIRT) Faint Standard stars with the UKIRT Fast Track Imager (UFTI) camera on UKIRT. We have analysed the deepest 16 fields from the archive to provide a catalogue of galaxies brighter than   K ∼ 20.3  lying between 3 and 25 arcsec of the guide stars. We identify 111 objects in a total survey area of  8.7 arcmin2  . Of these, 87 are classified as galaxies based on their light profiles in our ∼0.5 arcsec median seeing K -band images. 12 of the galaxies have  ( J − K ) ≥ 2.0  consistent with them lying at high redshifts,   z ≳ 2  . These 12 very red galaxies have K -band magnitudes of   K = 18.1–20.1  and separations from the guide stars of 4–20 arcsec and hence are very well suited to adaptive optics studies to investigate their morphologies and spectral properties on sub-kpc scales. We provide coordinates and JHK photometry for all catalogued objects.  相似文献   

10.
In the Kepler/K2 data analysis performed to date, aperture photometry has generally been used, but this method has not given any reasonable results when examining nearby sources due to the insufficient number of pixels on Kepler's CCD camera. The point-spread function (PSF) technique prepared by Libralato et al. (2016a), which presents promising results in undersampled pixels even in crowded star groups, was used in this study. Open cluster stars M 35 and NGC 2158 in the K2 Campaign 0 were selected as the target field. By applying the PSF method to the high angular resolution input catalogue prepared using the Asiago Schmidt telescope, M 35 and NGC 2158 provided the opportunity to examine the light curves of faint stars (Kp > 15.5) in crowded regions and gave better results compared to aperture photometry. In this study, I detrended ~50 000 raw light curves from systematic effects and spacecraft motion effects. I ultimately found two candidate host stars that could have possible planetary transits.  相似文献   

11.
This paper reviews the current state of the problem of magnetism in massive Main Sequence stars. Chemically peculiar Bp stars with enhanced silicon lines and anomalous helium lines in their spectra are shown to be the most promising targets for the observational verification of various mechanisms of the formation and subsequent evolution of magnetic fields in CP stars. A catalog of magnetic Bp stars, containing 125 objects is prepared. Applying different criteria, we compiled a variety of magnetic star samples, which were then used to analyze magnetic fields in objects of different ages. The results of this analysis show that massive stars generally have stronger fields in all the samples studied, and thus confirm earlier results based on smaller star samples. No tight relation is observed and the parameters of individual objects show a very large scatter about the mean relation. The strongest and most complex fields are found in the youngest Bp stars with ages below 30 Myr. Magnetic Bp stars generally rotate slower than normal B-type stars, except for the hottest objects with enhanced helium lines, which have normal rotation velocities. No systematic differences are found between the angular rotation velocities of Bp stars with anomalous helium and silicon lines. We discuss various criteria, which can be used to observationally test the alternative mechanisms of formation and evolution of magnetic fields in CP stars and, in particular, to quantitatively compare not only the magnitudes, but also the topology of fields in objects of different ages.  相似文献   

12.
A new method for deconvolution of the nuclear component of active galaxies is presented, along with some initial results with several nuclear-type objects. Unlike the majority of procedures currently in use, this method is based on the following steps: (a) derivation of seeing-free bulge and disc parameters, (b) construction of a two-dimensional model of the galaxy, (c) derivation of a point-spread function (PSF) from star profiles and convolution of the bulge+disc model with the obtained PSF and (d) extraction of the seeing-convolved model radial profile and subtraction from the observed one. The residual gives the nuclear component. Only standard image processing packages are used.  相似文献   

13.
The new DECA software package, which is designed to perform photometric analysis of the images of disk and elliptical galaxies having a regular structure, is presented. DECA is written in Python interpreted language and combines the capabilities of several widely used packages for astronomical data processing such as IRAF, SExtractor, and the GALFIT code used to perform two-dimensional decomposition of galaxy images into several photometric components (bulge+disk). DECA has the advantage that it can be applied to large samples of galaxies with different orientations with respect to the line of sight (including edge-on galaxies) and requires minimum human intervention. Examples of using the package to study a sample of simulated galaxy images and a sample of real objects are shown to demonstrate that DECA can be a reliable tool for the study of the structure of galaxies.  相似文献   

14.
Results are presented from a detailed spectrophotometric study of a unique system of physically coupled galaxies whose spectra contain identical emission lines. This system consists of four galaxies, three of which have a double or multiple structure in their central portions and are, by definition, galaxies with two or multiple nuclei. Two of these objects are the galaxies with an ultraviolet excess (UVE), Mark 261 and Mark 262, while one is a galaxy made up of two identical starlike nuclei that are referred to as the “twin objects.” In the DSS2 charts, the latter show up as two adjacent stars without any surroundings. However, there are two condensations between these starlike nuclei. The fourth object has a triplet structure. It is shown here that all these objects have the same emission (line) spectra and red shifts. All the physical characteristics that can be determined from the spectra are determined: red shifts, relative intensities of emission lines, their equivalent widths, distances to the galaxies, etc. It is concluded that the members of this system have a common origin, more or less consistent with Ambartsumyan's idea of the fragmentation of an isolated, high density body. __________ Translated from Astrofizika, Vol. 48, No. 4, pp. 503–514 (November 2005).  相似文献   

15.
Efficient spectrographs at large telescopes have made it possible to obtain high-resolution spectra of stars with high signal-to-noise ratio and advances in model atmosphere analyses have enabled estimates of high-precision differential abundances of the elements from these spectra, i.e. with errors in the range 0.01–0.03 dex for F, G, and K stars. Methods to determine such high-precision abundances together with precise values of effective temperatures and surface gravities from equivalent widths of spectral lines or by spectrum synthesis techniques are outlined, and effects on abundance determinations from using a 3D non-LTE analysis instead of a classical 1D LTE analysis are considered. The determination of high-precision stellar abundances of the elements has led to the discovery of unexpected phenomena and relations with important bearings on the astrophysics of galaxies, stars, and planets, i.e. (i) Existence of discrete stellar populations within each of the main Galactic components (disk, halo, and bulge) providing new constraints on models for the formation of the Milky Way. (ii) Differences in the relation between abundances and elemental condensation temperature for the Sun and solar twins suggesting dust-cleansing effects in proto-planetary disks and/or engulfment of planets by stars; (iii) Differences in chemical composition between binary star components and between members of open or globular clusters showing that star- and cluster-formation processes are more complicated than previously thought; (iv) Tight relations between some abundance ratios and age for solar-like stars providing new constraints on nucleosynthesis and Galactic chemical evolution models as well as the composition of terrestrial exoplanets. We conclude that if stellar abundances with precisions of 0.01–0.03 dex can be achieved in studies of more distant stars and stars on the giant and supergiant branches, many more interesting future applications, of great relevance to stellar and galaxy evolution, are probable. Hence, in planning abundance surveys, it is important to carefully balance the need for large samples of stars against the spectral resolution and signal-to-noise ratio needed to obtain high-precision abundances. Furthermore, it is an advantage to work differentially on stars with similar atmospheric parameters, because then a simple 1D LTE analysis of stellar spectra may be sufficient. However, when determining high-precision absolute abundances or differential abundance between stars having more widely different parameters, e.g. metal-poor stars compared to the Sun or giants to dwarfs, then 3D non-LTE effects must be taken into account.  相似文献   

16.
We present the results of observations of a sample of isolated lenticular galaxies, performed at the SCORPIO and SCORPIO-2 spectrographs of the 6-meter BTA telescope of the SAO RAS in the long-slit mode. By direct spectra approximation, using the evolutionary synthesis models, we have measured the radial profiles of the rotation velocity as well as the dispersions of velocities, average age, and average metallicity of stars in 12 objects. The resulting average ages of the stellar population in bulges and discs fill an entire range of possible values from 1.5 to 15 Gyr which indicates the absence in the isolated lenticular galaxies, unlike in the members of groups and clusters, of a certain epoch when the structural components are formed: they could have been formed at a redshift of z > 2 as well as only several billion years ago. Unlike the S0 galaxies in a more dense environment, isolated galaxies typically have the same age of stars in the bulges and discs. The lenses and rings of increased stellar brightness, identified from the photometry of 7 of 11 galaxies, do not significantly differ from the stellar discs by the properties of stellar populations and velocity dispersion of stars. We draw a conclusion that the final arrangement of the morphological type of a lenticular galaxy in complete isolation is critically dependent on the possible modes of accretion of the cold external gas.  相似文献   

17.
Sloan Digital Sky Survey   总被引：1，自引：0，他引：1  
The Sloan Digital Sky Survey is a project which will produce a detailed digital phometric map of half the northern sky to about 23 magnitude using a special purpose wide field telescope of 2.5 meter aperture. This map will be used to select about a million galaxies and 100,000 quasars, for which high resolution spectra will be obtained using the same telescope. A catalog will be produced of all the detected objects, about 100 million galaxies and a similar number of stars, and a million quasar candidates.  相似文献   

18.
Two of the basic building blocks of galaxies are stars and the interstellar medium. The evolution of the abundance composition in the latter and especially the enrichment of heavy elements as a function of space and time reflects in turn the history of star formation and the lifetimes of the diverse contributing stellar objects. Therefore, the understanding of stellar evolution and its endpoints (mainly planetary nebulae, supernovae of type Ia and type II/Ib/Ic) is essential. Despite many efforts, a full and self-consistent understanding of supernovae (the main contributors to nucleosynthesis in galaxies) is not existing, yet. However, they leave fingerprints, seen either in spectra, lightcurves, radioactivities/decay gamma-rays or in galactic evolution. Here we want to address the composition of ejecta, their model uncertainties and relate them to constraints from abundance observations in galactic evolution. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

19.
We present an X-ray spectral analysis of a sample of eight bona fide Seyfert 2 galaxies, selected on the basis of their high [O  iii ] λ 5007 flux, from the Ho et al. spectroscopic sample of nearby galaxies. We find that, in general, the X-ray spectra of our Seyfert 2 galaxies are complex, with some of our objects having spectra different from the 'typical' spectrum of X-ray selected Seyfert 2 galaxies. Two (NGC 3147 and 4698) show no evidence for intrinsic absorption. We suggest that this is a result of the fact that when the torus suppresses the intrinsic medium and hard energy flux, underlying emission from the host galaxy, originating in circumnuclear starbursts, and scattering from warm absorbers contributes in these energy bands more significantly. Our ASCA data alone cannot discriminate whether low-absorption objects are Compton-thick active galactic nuclei (AGNs) with a strong scattered component or lack an obscuring torus. The most striking example of our low absorption Seyfert 2 is NGC 4698. Its spectrum could be explained by either a dusty warm absorber or a lack of broad-line clouds so that its appearance as a Seyfert 2 is intrinsic and not a result of absorption.  相似文献   

20.
IRAF软件中DAOPHOT包的应用   总被引：1，自引：0，他引：1  
为了检验威海天文台1 m望远镜的测光精度,2009年10月7日晚用该望远镜观测了M39星团,并对观测数据进行测光归算。由于M39星团中个别星较密集,使用IRAF中的DAOPHOT包进行星像测光。详细介绍了IRAF软件中的DAOPHOT包及其使用。数据归算结果表明,在UCAC3星表中匹配到的星,稀疏区域亮星的星等内部精度达到0.003mag,较密集区域的暗星内部精度约为0.012 8 mag。  相似文献   

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