Surface photometry and spectral analysis of Mk50 : a Seyfert 1 nucleus in an early type galaxy

Abstract

We analyze Markarian 50 using V and I CCD frames and IPCS and CCD spectra, respectively, obtained in June 1985 (low state of activity) and May 1990 (high state), covering the range 3200 Å<λ<10 100 Å. The surface brightness profiles derived from the images indicate that we are dealing with an early type, elliptical or SO galaxy, which is supported by the spectral analysis of the stellar population. We study the emission properties of the Seyfert 1 nucleus, after subtraction of the stellar population. In the low state we detect a strong, broad Hα with FWHM ≈ 4500 km s-ˡ, whereas Hβ is stronger than [O III]λ5007 by a factor ≈ 2 (in flux). The stellar population is clear by the presence of Ca II H and K, G band, and Mg I, and its continuum contribution is more than 92% for λ> 4000 Å. In the high state Hα broadens to FWHM≈5400 km s-ˡ, Hβ becomes a factor≈7 (in flux) stronger than [O III]λ5007, and He Iλ5876 is prominent. The stellar population fractions in the high state continuum are 48% at λ 3800 Å, 71% at λ 5000 Å, and 82% at λ 8900 Å. Permitted Fe II emission has similar intensities in the high and low states, showing that its formation zone does not coincide with that of the broad H I and He I lines, i.e., it is farther than five light years from the central source. The off-nuclear spectra centered at 8" (≈ 5 kpc) have [O II], [O III], [N II], Hα, and H β at least in part produced by radiation from the strong central source, and the stellar population is dominated by a typical red strong­lined bulge population while the contribution of young components (age < 5 X 108 yr) is ≈7% at λ 5870 Å. The spectral analysis also shows that the point source at 17" northeast of Mk50 is not a companion, but a foreground star of spectral type K7V-MOV.