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Astron. Astrophys. 333, 877-881 (1998)

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1. Introduction

NGC 5252 (1335+04) is a S0 galaxy at a redshift of z [FORMULA] 0.023. Nuclear spectra have shown it to be a Seyfert 2 galaxy (Véron-Cetty & Véron 1986; Huchra & Burg 1992). However, Osterbrock & Martel (1993) have found a weak broad H [FORMULA] component in the nuclear region; Acosta-Pulido et al. (1996) have confirmed the presence of this broad component, with a measured FWHM of 2 485 [FORMULA] 78 km s-1. Using a [FORMULA] -diameter aperture, Ruiz et al. (1994) detected the He i [FORMULA] 1.083 µm emission line in the nucleus; in addition to a narrow component (527 km s-1 FWHM), this line shows a broad feature (1 043 km s-1 FWHM), which was interpreted as the signature of a Seyfert 1 cloud. Goodrich et al. (1994) have reported a marginal detection of a broad Pa [FORMULA] line.

Unger et al. (1987), having obtained a high resolution (0.75 Å) slit spectrum of the nuclear region of NGC 5252, found the [O iii][FORMULA] 4959, 5007 lines to be double with a velocity separation of [FORMULA] 180 km s-1. Acosta-Pulido et al. (1996) obtained a 2.0 Å resolution spectrum centered on the nucleus; this spectrum, covering the red spectral region, was extracted on a length of 4:006 along the slit, which was 1:000 wide. The [S ii][FORMULA] 6716, 6731 and H [FORMULA] +[N ii][FORMULA] 6548, 6584 complexes were fitted with two Gaussians for each line, the velocity difference between the two components being [FORMULA] 200 km s-1 ; the need for an additional broad H [FORMULA] component was already mentioned above.

Images taken in the light of the [O iii][FORMULA] 5007 line reveal a sharply defined biconical structure extending to a maximum radius of [FORMULA], corresponding to 32 kpc, if H [FORMULA] = 50 km s-1 Mpc-1 (Tadhunter & Tsvetanov 1989). Spectra (Durret & Warin 1990) and images in the light of [O iii][FORMULA] 5007 and H [FORMULA] +[N ii] (Haniff et al. 1991; Prieto & Freudling 1996) show that the gas outside the nucleus has a very high [FORMULA] 5007 [FORMULA] H [FORMULA] ratio. HST narrow band images show that three bright knots dominate the line emission in the innermost [FORMULA] ; they are aligned along PA [FORMULA] [FORMULA] with a total separation of [FORMULA] 0:007 and are embedded in fainter diffuse gas (Tsvetanov et al. 1996).

J, H and K imaging with a seeing of 1:005-1:007 suggests the presence, in the nucleus, of a heavily reddened ([FORMULA] 6 mag) non stellar source (Kotilainen & Prieto 1995). ASCA observations show that NGC 5252 is a relatively strong X-ray source ([FORMULA] = 2.6 1043 erg s-1). A description of the spectrum with a single power law is ruled out; there is evidence for a strong soft excess. The best-fit partial covering model results in a flat ([FORMULA] 1.45 [FORMULA] 0.2) power-law continuum emitted by a source almost completely covered (at [FORMULA] 94-97%) by neutral matter ([FORMULA] 4.3 1022 cm-2) (Cappi et al. 1996). For galactic X-ray sources, the hydrogen column density [FORMULA] and the visual extinction [FORMULA] follow the relation: [FORMULA] 5 10 [FORMULA] (Gorenstein 1975; Reina & Tarenghi 1973). If this is valid for galactic nuclei, the observed X-ray extinction would imply a visual extinction of [FORMULA] 20 mag. Although this is significantly larger than the value derived from near IR observations, it confirms that the nuclear source is heavily reddened. Spectropolarimetry allowed only a marginal detection of a broad H [FORMULA] component (Young et al. 1996).

The presence of a broad H [FORMULA] component in a Seyfert galaxy in which the nucleus is heavily obscured was surprising and induced us to observe this object.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998