Ultraviolet Quasi-Stellar Objects

We present a sample of spectroscopically confirmed quasi-stellar objects (QSOs) with FUV-NUV color (as measured by Galaxy Evolution Explorer (GALEX) photometry, FUV band: 1344-1786 angstrom, NUV band: 1771-2831 angstrom) bluer than canonical QSO templates and than the majority of known QSOs. We analyze their FUV to NIR colors, luminosities, and optical spectra. The sample includes a group of 150 objects at low redshift (z < 0.5), and a group of 21 objects with redshift 1.7 < z < 2.6. For the low-redshift objects, the "blue" FUV-NUV color may be caused by enhanced Ly alpha emission, since Lya transits the GALEX FUV band from z = 0.1 to z = 0.47. Synthetic QSO templates constructed with Lya up to three times stronger than in standard templates match the observed UV colors of our low-redshift sample. Optical photometric and spectroscopic properties of these QSOs are not atypical. The Ha emission increases, and the optical spectra become bluer, with increasing absolute UV luminosity. The lack of selected objects at intermediate redshift is consistent with the fact that for z = 0.48-1.63, Lya is included in the GALEX NUV band, making the observed FUV-NUV redder than the limit of our sample selection. The UV-blue QSOs at redshift similar to 2, where the GALEX bands sample rest-frame approximate to 450-590 angstrom (FUV) and similar to 590-940 angstrom (NUV), are fainter than the average of UV-normal QSOs at similar redshift in NUV, while they have comparable luminosities in other bands. Therefore, we speculate that their observed FUV NUV color may be explained by a combination of steep flux rise toward short wavelengths and dust absorption below the Lyman limit, such as from small grains or crystalline carbon (nanodiamonds). The ratio of Lya to C IV could be measured in 10 objects; it is higher (30% on average) than for UV-normal QSOs, and close to the value expected for shock or collisional ionization. However, optical spectra are taken at different times than the UV photometry, which may bias the comparison if lines are variable. These QSO groups are uniquely set apart by the GALEX photometry within larger samples, given that their optical properties are not unusual.