Other
Scientific paper
Sep 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...192.5522d&link_type=abstract
American Astronomical Society, 192nd AAS Meeting, #55.22; Bulletin of the American Astronomical Society, Vol. 30, p.1153
Other
Scientific paper
The Digital POSS-II (DPOSS) and the resulting Palomar-Norris Sky Catalog (PNSC) provide an excellent, large data set in which one can search for rare types of objects, distinguished from the majority of detected sources through some set of properties, such as the colors. We use gri color selection to pick out quasar candidates at z > 4 from the large number of stellar objects detected in the survey. Down to the magnitude limit used for this search (r < 19.5(m) ), there is roughly 1 quasar at z > 4 per million foreground stars, at moderate to high Galactic latitude. Our selection procedures produce only about 10 false-alarm candidates per quasar, with the majority of contaminants being compact, high-surface brightness galaxies at modest redshifts. To date, almost 40 quasars at z > 4 have been found in DPOSS. They are used for a variety of follow-up studies by ourselves and other groups, including searches for protogalaxies and protoclusters at z > 4, as probes of the intergalactic medium at very high redshifts, etc. One still tentative, but potentially very exciting result is a hint of clustering among these quasars on ~ 100 h(-1) comoving Mpc scale. In addition to quasar candidates, we look for stellar objects with extreme red (r-i) colors, ostensibly stars at the bottom of the main sequence or isolated brown dwarfs, very blue stars, and other objects with peculiar colors. In some cases, apparently unusual colors were an artifact of a strong variability over the time baseline between the different plate exposures. In a data set this large (ultimately > 10(9) sources), there is even an exciting possibility of discovering some previously unknown, rare type of objects. For example, we have found one object with an extremely unusual spectrum, whose nature we still do not understand.
Darling Jeremy
de Carvalho Reinaldo R.
Desai Vandana
Djorgovski Stanislav G.
Gal Roy R.
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