Bibcode
Arp, H.; Gutiérrez, C. M.; López-Corredoira, M.
Bibliographical reference
Astronomy and Astrophysics, v.418, p.877-883 (2004)
Advertised on:
5
2004
Journal
Citations
22
Refereed citations
18
Description
We present optical spectroscopic observations of three Ultra Luminous
X-ray sources (ULXs). Two of them are very close to the active galaxy
NGC 720 and the other is near NGC 1073. The two around NGC 720 turn out
to be quasars at z= 2.216 and z= 0.959, the one near NGC 1073 seems to
be associated to an HII region at the redshift of NGC 1073. We
concentrate our analysis on the two quasars and analyze them in
conjunction with a set of 22 additional X-ray sources close to nearby
galaxies which also fit the criteria of ULXs and which also have been
identified as quasars of medium to high redshift. This sample shows an
unusually large fraction of rare BL Lac type objects. The high redshifts
of these ULXs and their close proximity to their low redshift,
supposedly parent galaxies is a surprising result in the light of
standard models. We describe the main properties of each of these
objects and their parent galaxy, and briefly discuss possible
interpretations.