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NOAO Newsletter - NOAO Highlights - March 2000 - Number 61


The Most Distant Quasar Known

Based on a Solicited Contribution from Daniel Stern

A team of astronomers, led by Daniel Stern (JPL), has recently discovered a quasar at redshift z = 5.50, using the KPNO 4-m, Palomar 5-m, and Keck 10-m telescopes. The quasar, now the most distant known, was picked out as a high-redshift candidate from a deep, multicolor (RIz) imaging survey.


Caption: KPNO 4-m PFCCD images used to identify "Red-Dropout" objects. The left panel is an R-band exposure; the right panel is in I. The z=5.50 QSO, RD300, is only visible at the center of the right panel.

The team of astronomers, which also includes Hyron Spinrad (Berkeley), Peter Eisenhardt (JPL), Andrew Bunker (Cambridge), Steve Dawson (Berkeley), Adam Stanford (Davis, IGPP), and Richard Elston (Florida), has begun a deep imaging survey to probe the first Gyr of galaxy formation through rest-frame UV-dropout selection of z ~ 5 galaxies. Broad-band RIz images have been obtained using PFCCD on the KPNO 4-m and the COSMIC camera at the prime focus of the Palomar 5-m telescope. Targets are selected on the basis of the strong signature of the Lyman- forest at z ~ 5, causing objects to disappear in the R-band. For z ~ 5, the Lyman break is inconsequential relative to the thick jungle of the Lyman- forest. The aim of the survey is primarily to study high-redshift galaxies, but it is also sensitive to high-redshift quasars.

The newly discovered R-drop quasar, RD300, was the brightest candidate from the current sample, which covered 74 square arcminutes. The quasar is undetected in R (3- limit in 3" diameter aperture is 26.3 mag) and has I = 23.8 and z = 23.4. Figure 1 shows the R and I images centered on the quasar.

Slit-mask spectroscopy was obtained using LRIS on the Keck II telescope in January 2000, revealing RD300 to be an extremely high-redshift quasar. The spectrum shows broad Lyman-/NV emission and sharp absorption decrements from the highly redshifted hydrogen forests. The fractional continuum depression due to the Lyman- forest is D = 0.9, comparable to values determined from distant galaxies in the Hubble Deep Field and from models of evolution in the Lyman- forest.


Caption: Keck II LRIS spectrum of RD300.

Imaging surveys provide one of our most robust tools for probing the evolution of galaxies and quasars. Wide-field, "shallow'' surveys such as the Palomar Digital Sky Surveys and Sloan Digital Sky Survey have been very successful at identifying bright, high-redshift quasars at redshifts between z = 4 and z = 5 (e.g., Djorgovski et al. 1999; Fan et al. 1999). Such discoveries are useful for studying the conditions of the early Universe. In particular, distant quasars have shown that the IGM remains largely ionized out to z ~ 5. Deep, smaller field surveys have proved successful at probing star-forming galaxies at early cosmic epoch (e.g., Steidel et al. 1996; Hu et al. 1999), with interesting implications, for example, regarding the cosmic star formation history of the Universe and the morphological evolution of galaxies.

Potentially the most interesting aspect of the current discovery is its implications for the faint end of the quasar luminosity function at high redshift. RD300, at MB = -22.7, is the faintest quasar known at z > 4. Its luminosity is comparable to the lower luminosity objects in local quasar samples. The expected surface density of such quasars is therefore uncertain, but extrapolating the z = 2 (Boyle et al. 1991) quasar luminosity function to high redshift, with the Schmidt, Schneider, and Gunn (1995) pure density evolution (quasar space density falls off by a factor of 2.7 per unit redshift beyond z = 3), would suggest that only ~ 0.15 R-drop (4.3 <= z <= 5.8), faint (MB < -22.5) quasars should appear in this survey. Was the current record-breaking discovery yet another example of the infamous Spinradian luck (cf. McCarthy et al. 1988; Dey et al. 1998), or are faint, distant quasars more common than expected? The latter possibility could have important consequences for the early ionization of the IGM and bode well for future, moderately deep surveys for high-redshift quasars.


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