The hyperluminous infrared quasar 3C 318 and its implications for interpreting submm detections of high-redshift radio galaxies

Abstract

We present near‐infrared spectroscopy and imaging of the compact steep‐spectrum radio source 3C 318, which shows it to be a quasar at redshift z=1.574 (the z=0.752 value previously reported is incorrect). 3C 318 is an IRAS, ISO and SCUBA source so its new redshift makes it the most intrinsically luminous far‐infrared (FIR) source in the 3C catalogue (there is no evidence of strong gravitational lensing effects). Its bolometric luminosity greatly exceeds the 1013 L⊙ level above which an object is said to be hyperluminous. Its spectral energy distribution (SED) requires that the quasar heats the dust responsible for the FIR flux, as is believed to be the case in other hyperluminous galaxies, and contributes (at the >10 per cent level) to the heating of the dust responsible for the submm emission. We cannot determine whether a starburst makes an important contribution to the heating of the coolest dust, so evidence for a high star formation rate is circumstantial, being based on the high dust, and hence gas, mass required by its submm detection. We show that the current submm and FIR data available for the highest‐redshift radio galaxies are consistent with SEDs similar to that of 3C 318. This indicates that at least some of this population may be detected in the submm because of dust heated by the quasar nucleus, and that interpreting submm detection as evidence for very high (≳1000 M⊙ yr−1) star formation rates may not always be valid. We show that the 3C 318 quasar is slightly reddened (AV≈0.5), the most likely cause of which is SMC‐type dust in the host galaxy. If very distant radio galaxies are reddened in a similar way then we show that only slightly greater amounts of dust could obscure the quasars in these sources. We speculate that the low fraction of quasars amongst the very high redshift (z≳3) objects in low‐frequency radio‐selected samples is the result of such obscuration. The highest‐z objects might be preferentially obscured because like 3C 318 they are inevitably observed very shortly after the jet‐triggering event, or because their host galaxies are richer in dust and gas at earlier cosmic epochs, or because of some combination of these two effects.

Molecular Gas Observations of the Reddened Quasar 3C 318

“…These values are comparable with the mean of a sample of CO-detected SMGs (Greve et al 2005). We note that there is no evidence for gravitational lensing of 3C 318 (Willott et al 2000).…”

“…The weather conditions were generally excellent, particularly the seeing (0:74 00 and 1:01 00 on April 19 and 22, respectively), leading to very good phase stability. The central frequency was tuned to 89.60 GHz, close to 89.5641 GHz, which is the frequency of the CO(2-1) transition at a redshift of z ¼ 1:574 (the redshift of the narrow [O iii] emission line detected in near-IR spectroscopy by Willott et al 2000). The maximum bandwidth of 560 MHz was used, equivalent to 1875 km s À1 .…”

“…As discussed in Willott et al (2000), 3C 318 is interesting because (1) it is one of the most distant far-IR sources detected with IRAS and therefore has a very high far-IR luminosity, (2) it has a high submillimeter luminosity measured with SCUBA, (3) the quasar ultraviolet continuum is reddened by dust, and (4) it is a very compact (i.e., young) radio source. Here we report the discovery of CO(2-1) emission in this quasar and discuss the implications.…”

Feii/MgiiEmission‐Line Ratio in High‐Redshift Quasars

“…It is assumed that the dust is distributed in a kiloparsec-scale warped disk (Sanders et al 1989), which is illuminated by the central AGN. The observed dust emission spectra from 3 to 30 lm can be explained by model such as that by Andreani et al (1999) and Willott, Rawlings, & Jarvis (2000). However, since the presented spectra are typical quasar spectra with prominent broad emission lines, it is unlikely that the broad-line region is significantly blocked by dust (e.g., Netzer & Laor 1993).…”

The X-ray emission of the most luminous 3CR radio sources

“…In particular, the galaxy 3C 241 has been found to exhibit a broad line in the near-infrared observer-frame spectrum (Hirst et al 2003), and the broad line object 3C 318 has a very red, underluminous optical continuum (Willott et al 2000). All the objects in Table 1 are steep spectrum radio sources; this is usually interpreted as evidence of viewing angles substantially larger than the jet beaming.…”