ULTRAVIOLET QUASI-STELLAR OBJECTS
- Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States)
- National Research Council of Herzberg Institute of Astrophysics (Canada)
- Center for Astrophysical Sciences, Johns Hopkins University, Baltimore, MD (United States)
- INAF-Astronomical Observatory of Padova, Padova (Italy)
- California Institute of Technology, Pasadena, CA (United States)
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 A, NUV band: 1771-2831 A) 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 Ly{alpha} transits the GALEX FUV band from z = 0.1 to z = 0.47. Synthetic QSO templates constructed with Ly{alpha} 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 H{alpha} 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, Ly{alpha} 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 {approx}2, where the GALEX bands sample rest-frame {approx}450-590 A (FUV) and {approx}590-940 A (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 Ly{alpha} 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.
- OSTI ID:
- 21269263
- Journal Information:
- Astronomical Journal (New York, N.Y. Online), Vol. 137, Issue 4; Other Information: DOI: 10.1088/0004-6256/137/4/3761; Country of input: International Atomic Energy Agency (IAEA); ISSN 1538-3881
- Country of Publication:
- United States
- Language:
- English
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