Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...476...98k&link_type=abstract
Astrophysical Journal v.476, p.98
Astronomy and Astrophysics
Astronomy
22
Galaxies: Individual Name: Markarian 231, Galaxies: Ism, Galaxies: Seyfert, Galaxies: Structure, Infrared: Galaxies
Scientific paper
The ultraluminous infrared Seyfert 1 galaxy Mrk 231 has been spectrally imaged in the K band with the new three-dimensional MPE integral field spectrometer. The combined images of the H2 emission lines show, for the first time in an ultraluminous infrared galaxy, the presence of an extended circumnuclear structure of hot molecular gas. The H2 emitting region has a size of ~2.4 kpc and a hot molecular gas mass M ^{{hot}}_{{H}2} ~2 x 104 M&sun;. The H2 emission-line ratios indicate that the gas is most likely thermally excited. If as in NGC 7469 star formation is associated with the H2 emission, the starburst would have a far-IR luminosity LFIR ~ 1 x 1012 L&sun;. This value represents an upper limit, since a fraction of the hot molecular gas may be excited by the radiation field emerging from the nucleus. The K-band three-dimensional data cube also shows for the first time the presence of extended narrow Pa alpha emission blueshifted by ~1400 km s-1 with respect to the systemic velocity, and located ~0.6 kpc northwest of the nucleus. The detection of CO absorption bands with a spatial distribution peaking on the K-band continuum provides evidence for a central stellar concentration. The low CO spectroscopic index indicates, however, dilution by hot dust emission or by a nonthermal active galactic nucleus. The Pa alpha /H alpha ratio confirms previous extinction measurements based on Balmer line ratios, i.e., visual extinction of AV ~ 2.0--6.6 mag. The quasar-type nucleus of Mrk 231 should then be transparent at 2 mu m and also in hard X-rays. A weak nuclear He I lambda 2.058 mu m (He I/Pa alpha = 0.032) is detected, and no detection of [Si VI] lambda 1.962 mu m is made, placing an upper limit of 4 x 10-18 Wm-2 for the coronal gas emission. The ionizing source could either be a far-UV and X-ray quiet quasar or else a nuclear starburst with an upper mass limit >=60 M&sun;.
Colina Luis
Krabbe Alfred
Kroker Harald
Thatte Niranjan
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