The Optical/Near-IR Colours of Red Quasars

Abstract

Abstract:We present quasi-simultaneous multi-colour optical/near-IR photometry for 157 radio selected quasars, forming an unbiassed sub-sample of the Parkes Flat-Spectrum Sample. Data are also presented for 12 optically selected QSOs, drawn from the Large Bright QSO Survey. The spectral energy distributions of the radio-and optically-selected sources are quite different. The optically selected QSOs are all very similar: they have blue spectral energy distributions curving downwards at shorter wavelengths. Roughly 90% of the radio-selected quasars have roughly power-law spectral energy distributions, with slopes ranging from Fν ∝ ν 0 to F ν ∝ ν −2 . The remaining 10% have spectral energy distributions showing sharp peaks: these are radio galaxies and highly reddened quasars. Four radio sources were not detected down to magnitude limits of H ∼ 19 · 6. These are probably high redshift (z > 3) galaxies or quasars. We show that the colours of our red quasars lie close to the stellar locus in the optical: they will be hard to identify in surveys such as the Sloan Digital Sky Survey. If near-IR photometry is added, however, the red power-law sources can be clearly separated from the stellar locus: IR surveys such as 2MASS should be capable of finding these sources on the basis of their excess flux in the K-band.

Quasars in the 2MASS Second Incremental Data Release

“…This detection rate of 36% suggests that the Ðnal 2MASS catalog will yield matches for approximately 4760 of the 13214 VV00 quasars. Our sample of 2277 quasars with IR data is nearly 10 times larger than the largest previous such sample (Srianand & Kembhavi 1997) and more than 10 times larger than the largest previous samples with homogeneous IR data (Warren et al 2000 ;Francis et al 2000).…”

“…Dust obscuration seems the simplest explanation for Figure 4, although we note that Francis, Whiting, & Webster (2000) concluded that synchrotron emission was the best explanation for the red colors of the many RLQs in the Parkes Half-Jansky sample of Webster et al (1995).…”

“…As noted by Francis et al (2000) and Cutri et al (2000), sampling the full parameter space open to obscured quasars will be difficult based on B[R versus J[K colors. However, Warren, Hewett, & Foltz (2000, hereafter WHF) have proposed a promising criterion for selecting K-band Ñux-limited samples of quasars from the V [J versus J[K color-color diagram, which they dub the KX method.…”

“…However, we have also plotted a subsample of radio-selected quasars from the completely identiÐed Parkes Half-Jansky sample (triangles ; Francis et al 2000). There is no strong correlation between J[K and B[J or B[K for these objects either.…”

Atomic and molecular absorption in redshifted radio sources

“…To ensure sufficient flux against which to detect the absorption (Sect. 2.2), sources were selected from the Parkes Half-Jansky Flat-spectrum Sample (PHFS, Drinkwater et al 1997;Francis et al 2000), giving a total of ten targets with redshifts of z = 0.219 − 0.405. Lastly, the sources were prioritised by faintness, for which we chose B > ∼ 19 (as quoted in the PHFS, Table 1), since this gave the ten faintest targets for which the estimated flux density at the redshifted H I 21-cm absorption frequency, Sest, was confirmed to exceed 0.5 Jy.…”