Ultraviolet and optical spectrophotometry of the Seyfert 1.8 galaxy Markarian 609

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Abstract

We present simultaneous ultraviolet and optical observations of the Seyfert 1.8 galaxy Markarian 609. The broad hydrogen lines are in the ratio Lyα/Hβ/Hα = 16/1.0/5.0. The Hβ(broad)/[O III] ratio has increased since the 1976 measurements of Osterbrock, and the Hα/Hβ ratio is no longer as large as when he classified Mrk 609 as a type 1.8 Seyfert galaxy. The large value of Lyα relative to Hβ and Hα rules out reddening as the source of the steep Balmer decrement. It is probable that Hβ has increased, and the Balmer decrement has decreased, in response to a brightening of the ionizing continuum. The enhancement of Hβ relative to Hα and the steep decrement observed by Osterbrock can be understood with the aid of radiative transfer calculations if the Lyman continuum optical depth in the region where the lines form is significantly less than believed typical for Seyfert 1 galaxies. The small equivalent widths of the Balmer lines and the weakness of the optical Fe II emission support this contention. Smaller than normal optical depths are likely to be the causes of the larger Balmer decrements in many Seyfert 1.8 and 1.9 galaxies. In addition, there is evidence for a sizable population of early-type stars in Mrk 609. A deficit of nuclear ultraviolet radiation relative to the total infrared emission suggests that these young stars power the bulk of the considerable far-infrared luminosity detected by IRAS.

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