Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
``Hidden'' Seyfert 2 Galaxies and the X-Ray Background
Obscured active galactic nuclei, which are classified optically as type2 (narrow line) Seyfert galaxies in the local universe, are by far themost promising candidates for the origin of the hard (2-10 keV) X-raybackground radiation. However, optical follow-up observations of faintX-ray sources in deep Chandra images have revealed surprising numbers ofapparently normal galaxies at modest redshift. Such objects represent~40%-60% of the sources classified in deep Chandra surveys, raising thepossibility that the X-ray galaxy population has evolved with cosmictime. Alternatively, most of the faint X-ray galaxies in question are sodistant that their angular diameters are comparable to the slit widthsused in ground-based spectroscopic observations; thus, their nuclearspectral features may be overwhelmed (``hidden'') by host galaxy light.To test this hypothesis, we have obtained integrated spectra of a sampleof nearby, well-studied Seyfert 2 galaxies. The data, which accuratelysimulate observations of distant Chandra sources, demonstrateconvincingly that the defining spectral signatures of Seyfert 2s can behidden by light from their host galaxies. In fact, 60% of the observedobjects would not be classified as Seyfert 2s on the basis of theirintegrated spectra, similar to the fraction of faint X-ray sourcesidentified with ``normal'' galaxies. Thus, the numbers of narrow-lineactive galaxies in deep Chandra surveys (and perhaps all ground-basedspectroscopic surveys of distant galaxies) are likely to have beenunderestimated.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxies
Using images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC).
| An image database. II. Catalogue between δ=-30deg and δ=70deg.
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
| A survey of the Pisces-Perseus supercluster. V - The declination strip +33.5 deg to +39.5 deg and the main supercluster ridge
Measurements of 544 radial velocities, 229 optical and 315 in the 21 cmH I line, are presented for galaxies, mostly in the declination stripbetween +33.5 and +39.5 deg in the region of the Pisces-Perseussupercluster. These are combined with other available data toinvestigate the linear structure identified as the main superclusterridge. The main ridge of the supercluster extends at least 50/h Mpcbefore it disappears into the zone of avoidance east of Perseus.Confinement both on the plane of the sky and in the velocity dimensionimply an axial ratio of greater than 10:1 and an inclination withrespect to the plane of the sky of less than about 12 degrees. Therelative proximity, low inclination to the plane of the sky, and highcontrast relative to the foreground and background, help to make thePisces-Perseus filament one of the most prominent features in theextragalactic sky on large scales.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
Kataloglar ve belirtme:
|
