Faint Blue Objects at High Galactic Latitude. VII. Spectroscopy of Narrow-Line Objects and the Morphological Selection Criterion of the US Survey (1995)

P.D. Usher, K.J. Mitchell, K-L. Huang https://doi.org/10.1086/176518 http://adsabs.harvard.edu/abs/1995ApJ...454..654U

New spectroscopic results are reported for narrow emission-line galaxies of the US survey. These and other data on extragalactic US objects are used to examine the optical morphological selection criteria for faint blue objects recorded with the Palomar 1.2 m Schmidt camera. An empirical criterion is derived that separates resolved from unresolved extragalactic objects in the Hubble diagram. A working hypothesis for samples selected by surveys with a 1.2 m class Schmidt camera is that the galaxy-quasar interface occurs at about B = 22.0 - 1/z (14.5 ≤ B < 19) mag; objects of a given B magnitude will almost certainly have no resolved component at a greater redshift z, based on imagery of the 1.2 m Palomar Schmidt. A stringent physical definition of an unresolved quasar in the US survey is that it be more luminous than MB = -24.0. Implications for the combining of different quasar surveys are discussed. The quantified morphological selection criterion provides a way of searching for "naked quasars," which will reveal themselves as unresolved objects well below the interface in the Hubble diagram. Further inspection of the morphology of the US spectroscopic samples shows no evidence for such objects in the present sample.