We present the result of the cross-matching between UV-excess sources selected from the UV-excess survey of the Northern Galactic Plane (UVEX) and several infrared surveys (2MASS, UKIDSS and WISE). From the position in the (J-H) vs. (H-K) colour-colour diagram we select UV-excess candidate white dwarfs with an M-dwarf type companion, candidates that might have a lower mass, brown-dwarf type companion, and candidates showing an infrared-excess only in the K-band, which might be due to a debris disk. Grids of reddened DA+dM and sdO+MS/sdB+MS model spectra are fitted to the U,g,r,i,z,J,H,K photometry in order to determine spectral types and estimate temperatures and reddening. From a sample of 964 hot candidate white dwarfs with (g-r)<0.2, the spectral energy distribution fitting shows that ~2-4% of the white dwarfs have an M-dwarf companion, ~2% have a lower-mass companion, and no clear candidates for having a debris disk are found. Additionally, from WISE 6 UV-excess sources are selected as candidate Quasi-Stellar Objects (QSOs). Two UV-excess sources have a WISE IR-excess showing up only in the mid-IR W3 band of WISE, making them candidate Luminous InfraRed Galaxies (LIRGs) or Sbc star-burst galaxies.
We present the results of the first spectroscopic follow-up of 132 optically blue UV-excess sources selected from the UV-excess survey of the Northern Galactic Plane (UVEX). The UV-excess spectra are classified into different populations and grids of model spectra are fit to determine spectral types, temperatures, surface gravities and reddening. From this initial spectroscopic follow-up 95% of the UV-excess candidates turn out to be genuine UV-excess sources such as white dwarfs, white dwarf binaries, subdwarfs type O and B, emission line stars and QSOs. The remaining sources are classified as slightly reddened main-sequence stars with spectral types later than A0V. The fraction of DA white dwarfs is 47% with reddening smaller than E(B-V)<0.7 mag. Relations between the different populations and their UVEX photometry, Galac- tic latitude and reddening are shown. A larger fraction of UVEX white dwarfs is found at magnitudes fainter than g>17 and Galactic latitude smaller than |b|<4 compared to main-sequence stars, blue horizontal branch stars and subdwarfs.
We carefully reconsider the problem of classifying broad absorption line quasars (BALQSOs) and derive a new, unbiased estimate of the intrinsic BALQSO fraction from the SDSS DR3 QSO catalogue. We first show that the distribution of objects selected by the so-called ``absorption index (AI) is clearly bimodal in log(AI), with only one mode corresponding to definite BALQSOs. The surprisingly high BALQSO fractions that have recently been inferred from AI-based samples are therefore likely to be overestimated. We then present two new approaches to the classification problem that are designed to be more robust than the AI, but also more complete than the traditional ``balnicity index (BI). Both approaches yield observed BALQSO fractions around 13.5 per cent, while a conservative third approach suggests an upper limit of 18.3 per cent. Finally, we discuss the selection biases that affect our observed BALQSO fraction. After correcting for these biases, we arrive at our final estimate of the intrinsic BALQSO fraction. This is f_BALQSO = 0.17 +/- 0.01 (stat) +/- 0.03 (sys), with an upper limit of f_BALQSO = 0.23. We conclude by pointing out that the bimodality of the log(AI) distribution may be evidence that the BAL-forming region has clearly delineated physical boundaries.
