Completeness of the FBS

The Catalogue of mean UBV data on stars [44] contains 102 stars in the FBS area, with 11.0 < V < 16.5 (bright stars are saturated on the FBS plates and are therefore missed) and U-B < -0.50; 53 are included in the FBS catalogue suggesting that, in this magnitude interval, the completeness of the FBS is 52% (53/102). Only 9% (2/23) of the stars weaker than V = 16.5 appear in this catalogue. The survey is insensitive to objects with U-B > -0.5 (0/9); its completeness increases from $\sim$20% at U-B $\sim$ -0.6 to $\sim$80% for U-B < -1.0.

The QSO U-B colour changes with z; but most of the changes, at least for z < 2.2, are due to the presence of an emission line in one of the two filters. The U-B colour of the continuum is in the range -0.9 < U-B < -0.7. Slitless spectroscopic surveys are sensitive to the colour of the continuum, unaffected by the emission lines. There are 58 known stars with 14.0 < V < 16.5 and U-B < -0.70 in the FBS area; 39 (67%) have been found by the FBS; we shall adopt this value as the completeness of this survey for QSOs brighter than V = 16.5.

The PG survey does not cover the region at galactic latitudes lower than 30$^\circ$. The PG and FBS samples have about 2250 deg² in common. Out of the 1103 FBS objects, 618 are within the PG fields, 276 being in the PG sample (FBS 0854+385 is PG 0854+385, but the original FBS R.A. is affected by a printing error; FBS 0935+395 is not a PG object; PG 0752+770, 0836+619, 1047+694, 1335+369, 1551+719, 1600+369, 1606+627, 1620+648 and 1722+353 are FBS objects, but their original PG positions are affected by errors reaching several arcminutes; the declination of FBS 1559+369 is affected by a printing error of 1$^\prime$; it is G180-23 [21] and PG 1600+369; the declination of FBS 1619+648 is also affected by a printing error of 1$^\prime$. Accurate positions for these objects have been measured and are given in Table 1).

 

$$b\:$$ FBS Name Ref. Mag Type Date Exp. time Classification (min)

0019+348 -27.4 (2) 15.0 B2 27.10.97 10 CV

0028+441 -18.3 (11) 14.5 B2e: 27.10.97 15 sdB

0140+427 -18.9 (11) 16.5 N1e 27.10.97 20 sdB

0306+333 -21.1 (2) 14.7 B1 27.10.97 20 CV:

0632+663 23.3 (5) 16.0 N3e: 28.10.97 15 sdB

0649+716 25.9 (6) 17.1 N1 28.10.97 20 featureless

0732+396 25.1 (9) 16. B2 28.10.97 20 Sey1 z = 0.118

1112+668 47.9 (5) 17. B2a: 26.05.98 20 QSO z = 0.544

1150+334 76.0 (2) 16.2 N2e: 26.05.98 20 QSO z = 1.40

1235+699 47.4 (6) 17.9 N1e 26.05.98 20 QSO z = 0.522?

1255+447 72.6 (11) 16.5 B1 25.05.98 20 QSO z = 0.300

1315+447 71.9 (11) 17.0 N1 25.05.98 20 DZ:

1324+448 71.1 (11) 17.0 B1 26.05.98 20 QSO z = 0.331?

1401+865 30.7 (8) 16.2 N1e: 25.05.98 20 DZ

1449+440 75.6 (12) 16.0 N1 26.05.98 20 F0

1452+762 39.0 (7) 16.0 N2e 25.05.98 20 sdB

1500+752 39.5 (7) 16.9 B2a 26.05.98 20 DA:

1523+363 56.3 (3) 16.1 N1 26.05.98 20 F0

1557+448 48.9 (12) 16.5 de: 25.05.98 20 Hii z = 0.0417

1607+439 47.1 (12) 16.0 s1e: 26.05.98 20 F0:

1715+406 34.5 (6) 16.0 sd3e 25.05.98 20 sdF:

1716+394 34.2 (10) 17.0 N1e: 25.05.98 20 F0

1755+663 30.2 (5) 16.3 N2 26.05.98 20 F0

1810+374 23.4 (3) 15.7 B2 26.05.98 20 sdA

1819+348 20.9 (3) 14.8 B1e: 15.06.98 20 sdA

1822+414 22.4 (12) 14.5 B1 25.05.98 20 sdB-O

1822+352 20.5 (3) 15.8 B2 25.05.98 20 DA

1833+447 21.5 (12) 15.5 B1a 25.05.98 20 F0

2149+425 -8.7 (12) 13.5 B1 27.10.97 5 sdB

2249+391 -17.9 (10) 16.5 N1e: 27.10.97 20 F5

2315+443 -15.2 (12) 17. N2e: 27.10.97 20 F5

Forty-six PG objects have not been found in the FBS, but ten are Markarian objects, i.e. belong to the first part of the FBS. So 88% (276/312) of the PG objects have been discovered. The 36 undiscovered objects have been examined on the FBS plates; 24 have a weak UV excess (the PG survey finds a significant fraction of stars with U-B $\sim$ -0.4, while the FBS is relatively unsensitive for U-B > -0.7; the others are fainter than B$\sim$16 and are near the magnitude limit of rather poor plates). From this, we conclude that the FBS survey is $\sim$ 90% complete for U-B < -0.5. This is significantly larger than the 67% success rate obtained from the UBV stars; it is probably due to the fact that, in principle, the PG survey contains only objects brighter than B = 16.2.

There are 25 PG QSOs in the FBS area (listed in Table 4), 23 of them have been found; the two exceptions are PG 0953+414 and PG 1112+431; the first is on the very edge of the FBS plate, while the second is weak on the original plate and has been missed; its APS magnitude is also quite weak (O = 17.03). This confirms that the FBS is very efficient in discovering bright QSOs. However, at low galactic latitudes, there is only one FBS QSO, suggesting a very low success rate which could be due in part to Galactic extinction and reddening and in part to crowding on the objective prism plates.