  White dwarfs whose atmospheres are polluted by terrestrial-like planetary
debris have become a powerful and unique tool to study evolved planetary
systems. This paper presents results for an unbiased Spitzer IRAC search for
circumstellar dust orbiting a homogeneous and well-defined sample of 134 single
white dwarfs. The stars were selected without regard to atmospheric metal
content but were chosen to have 1) hydrogen rich atmospheres, 2) 17 000 K <
T_eff < 25 000 K and correspondingly young post main-sequence ages of
15-270Myr, and 3) sufficient far-ultraviolet brightness for a corresponding
Hubble Space Telescope COS Snapshot. Five white dwarfs were found to host an
infrared bright dust disc, three previously known, and two reported here for
the first time, yielding a nominal 3.7% of white dwarfs in this post-main
sequence age range with detectable circumstellar dust. Remarkably,
complementary HST observations indicate that a fraction of 27% show metals in
their photosphere that can only be explained with ongoing accretion from
circumstellar material, indicating that nearly 90% of discs escape detection in
the infrared, likely due to small emitting surface area. This paper also
presents the distribution of disc fractional luminosity as a function of
cooling age for all known dusty white dwarfs, suggesting possible disc
evolution scenarios and indicating an undetected population of circumstellar
discs.
