AGN candidates were selected with the criterion from 3 different
sources: [1] the OMHR survey of UV-excess objects at the North Galactic pole
[(Moreau & Reboul 1995)], [2] the Hamburg Quasar Survey (HQS) by
[Hagen etal. (1995)] and [3] the ROSAT All-Sky Survey (RASS)
using identifications on HQS plates [(Bade etal. 1998)].
We also selected the object US 370 [(Usher 1981)], which was classified as a QSO candidate with V=18 and U-V=-0.55 by [Berger etal. (1991)]. The proper motion of this target could not be measured due to the proximity of a bright star as mentioned by Moreau & Reboul (1995). A slit-less observation was performed by [Weedman (1985)] and we provide here a spectrum of US 370 to confirm its nature and measure its redshift properly.
[Berger etal. (1991)] performed a photometric (U, B, V) and
astrometric analysis of 1221 UV-excess unresolved objects in a 40.5field almost centred on the selected area SA 57:
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A recent comparison of the MAMA catalogues extracted from the Schmidt
plates taken in 1962 with those obtained in the same way from OCA
(Observatoire de la Côte d'Azur) Schmidt plates taken in 1991 and 1994
provides measures of proper motions of the above OMHR quasar candidates,
among many other objects (Moreau & Reboul, in preparation). The limited field
of view of the OCA telescope constrained this proper motion survey to a
26.7 field within:
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The combined requirements of [1] unresolved morphology, [2] UV-excess and [3] absence of proper motion larger than about 10 mas yr-1 (i.e. a transverse velocity less than 25 km s-1 at 500 pc) give a priori high-quality quasar candidates. However, the constraint imposed by medium resolution spectroscopy with the HST restricted our choice to objects brighter than magnitude 18. Quasars this bright are drastically scarcer (and consequently samples are far more polluted by stars) than for visual magnitudes fainter than V=18 where the best and most numerous quasar candidates are expected to lie.
The sample of OMHR objects selected for observation finally contained the 18
brightest and bluest targets (,
) with no proper motion
detected at the 0.68 confidence level.
Quasar candidates were selected from the HQS digitized objective-prism plates
in two steps (Hagen et al. 1995, 1999). In the first
step, prism spectra with a blue continuum are selected automatically from the
digitized data which are available only at low resolution. The selected
spectra are rescanned with the Hamburg PDS machine to yield high-resolution
spectra having a dispersion of 139 nm mm-1 at H. These spectra
are interactively classified on a vector graphics screen. Non-blue spectra
with stellar absorption features are discarded while the others are kept in an
archive.
Blue stellar-like spectra are classified as "QSO/HS", meaning quasar or hot star candidate. Spectra with apparent emission lines or unusual continua are classified as "QSO" or "Narrow Emission". If the spectra are blue and featureless they enter the category "Unid" for "unidentified". The number of entries in these classes increases towards the "unidentified" sources, while the efficiency to find quasars in these classes decreases from "QSO" towards the other classes.
We selected from the archive all the objects with contained in the
above mentioned classes within about
from the centre of the Coma
cluster. Hagen et al. (1999) estimate that for z>0.1 and
less than 10%
of the quasars are missed. Objects already known from the literature were
discarded by cross-correlation with the NED and SIMBAD databases. We checked
for overlaps with the OMHR survey, and discarded as well the objects with a
significant proper motion (confidence level >0.68) or colour U-V>0. The
final HQS list used to obtain follow-up spectroscopy contains 31 objects.
The HQS list was augmented by two AGN candidates from the RASS. X-ray sources from this survey are identified on the Schmidt plates of the HQS. The confirmation rate of the AGN candidates is very high, about >90% [(Bade etal. 1992)], but most of them are optically fainter than our selection limit or were already known. The first RASS source RXJ1252.6+3002 will be contained in the RASS Faint Source Catalog (Voges et al., in preparation) while the second one RXJ1303.7+2633 is published in the RASS Bright Source Catalogue [(Voges et al. 1997)]. Also, HS 1312+2735 is listed as X-ray source in the latter catalogue (namely RXJ1314.3+2719).
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