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4. Coordinates

Equatorial coordinates (Right ascension and Declination) are given for two equinoxes 1950 (Besselian coordinates al1950, de1950) and 2000 (Julian coordinates al2000, de2000). Most of published coordinates are B1950 coordinates. Conversion to J2000 has been made according to the "Merits Standards" published in the U.S. Naval Observatory Circular (1983). For computer use, coordinates are expressed as decimal values (hours to 0.00001 and degrees to 0.0001 for Right Ascension and Declination, respectively). The standard deviation of coordinates is generally not known. Thus we are using a flag ipad to tell if the standard deviation is smaller than 10 arcsec or not. We collected systematically accurate coordinates in literature (see Paturel et al. 1989). Recently, we added accurate coordinates directly obtained from images stored in LEDA (Paturel et al. 1996) and from COSMOS database (Rousseau et al. 1996). Among the 100872 galaxies 69165 have accurate coordinates (69 percent).

Galactic coordinates l2, b2 are calculated (in degrees to tex2html_wrap_inline1777) from al1950 and de1950 using the coordinates of the galactic pole al1950(pole) = 12.81667 de1950(pole) = 27.4000 and the coordinates of the origin al1950(origin) = 17.70667 de1950 (origin) = -28.9167 according to Blaauw et al. (1960). These galactic coordinates are used to estimate the galactic extinction ag converted to Burstein-Heiles system (Burstein & Heiles 1984) from the relationship given in the Second Reference Catalog (de Vaucouleurs et al. 1976; p32, rel. 22; hereafter RC2). In fact, for galactic latitude tex2html_wrap_inline1795 the difference between both systems is negligibly small (except for the zero point difference of 0.20 mag due to the fact that Burstein-Heiles give no absorption at the galactic pole). In Fig. 1 (click here) the difference between ag from RC2 and from Burstein-Heiles is plotted vs. the galactic latitude.

  figure259
Figure 1: Difference of galactic extinction from RC2 and Burstein-Heiles vs. the absolute value of the galactic latitude

The conversion from RC2- to BH-system is the following: if tex2html_wrap_inline1799:
equation264
and if tex2html_wrap_inline1801:
equation266

The galactic extinction is higher than the one predicted by RC2 formula for low galactic latitude (tex2html_wrap_inline1803). The adopted galactic extinction is ag=ag(BH)+0.20, where ag(BH) is calculated from Rel. 1 and Rel. 2.

Supergalactic coordinates sgl, sgb are calculated (in degrees to tex2html_wrap_inline1813) from l2, b2 using the coordinates of the supergalactic pole l2(pole) = 47.37 deg b2(pole) = 6.32 deg and the coordinates of the origin l2(origin) = 137.37 deg; b2(origin) = 0   deg according to de Vaucouleurs et al. (1976).


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