Circular No. 4042 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION Postal Address: Central Bureau for Astronomical Telegrams Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. TWX 710-320-6842 ASTROGRAM CAM Telephone 617-495-7244/7440/7444 UNUSUAL OBJECT IN NGC 4618 A. V. Filippenko, University of California at Berkeley; and W. L. W. Sargent, California Institute of Technology, report the discovery of an object with a unique spectrum in the galaxy NGC 4618 (R.A. = 12h39m1, Decl. = +41deg26', equinox 1950.0). Two-dimensional CCD spectra of NGC 4618 were obtained with the Hale 5-m telescope on Feb. 28.52 UT. A long (2" x 120") slit was placed along the prominent bar in the center of the galaxy at p.a. 64 deg and passing through a starlike object of mag 16 situated 14" northeast of the center. Spectra covering the ranges 425-512 and 562-685 nm at resolutions of ~ 0.43 and 0.26 nm, respectively, were obtained. In addition to the usual sharp nebular emission lines (H-alpha, H-beta, H-gamma, [N II], [S II] and [O III]), which are also seen elsewhere along the slit, the object has very strong, broad emission lines (FWHM = 3000-5000 km/s, FWZI = 10 000-15 000 km/s) on a smooth continuum. The most intense (equivalent width 200 nm) broad feature is what appears to be a blended pair of lines at 631.2 and 637.5 nm. This is almost undoubtedly the [O I] 630.0/636.4-nm doublet at the redshift (z = 0.0018) of NGC 4618. A second strong line at 590.5 nm nm is then interpreted as the blended Na D lines at 589.3 nm. The only strong line in the blue region is asymmetrical with a shoulder on the blue side and an extended red wing. The wavelength of the line peak is 457.5 nm, corresponding to 456.7 nm at z = 0.0018. Weaker but certain lines are also found at observed wavelengths of 479.5, 492.1, 503.1, 566.9, 660.6 and 668.9 nm. These lines cannot be readily identified, although Fe II, Mg I (457.1 nm) and [Ca I] (491.3, 491.8 nm) are possibilities. Broad emission lines of H and He are definitely absent. The object is almost certainly a supernova in an advanced stage, although its spectrum does not conform to published supernova spectra. Based on the present brightness and on the distance modulus of NGC 4618, it is estimated that the object reached maximum about 160 days ago and has faded by 5 to 6 mag, if it was initially a normal Type I or Type II supernova. It is noteworthy that Minkowski (1939, Ap.J. 89, 156) observed the [O I] 630.0/636.4-nm doublet to be strong after 184 days past maximum in the spectrum of the Type I supernova 1937C in IC 4182. The feature was not present in the spectrum of SN 1972E in NGC 5253 some 400 days after maximum (Kirshner and Oke 1975, Ap.J. 200, 574). Prediscovery data on the brightness of the object and future observations of the evolution of its spectrum would be of great interest. 1985 March 13 (4042) Brian G. Marsden
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