The halo of M105 and its group environment as traced by planetary nebula
  populations: I. Wide-field photometric survey of planetary nebulae in the Leo
  I group
  M105 (NGC 3379) is an early-type galaxy in the Leo I group. This group is the
nearest group that contains all main galaxy types and can thus be used as a
benchmark to study the properties of the intra-group light (IGL) in low-mass
groups. We use PNe as discrete stellar tracers of the diffuse light around
M105. PNe were identified on the basis of their bright [OIII]5007 AA emission
and the absence of a broad-band continuum. We compare the PN number density
profile with the galaxy surface-brightness profile decomposed into metallicity
components using published HST photometry in two halo fields. We identify 226
PNe candidates within a limiting magnitude of mlim = 28.1 from our
Subaru-SuprimeCam imaging, covering 67.6 kpc along the major axis of M105 and
the halos of NGC 3384 and NGC 3398. We find an excess of PNe at large radii
compared to the stellar surface brightness profile from broad-band surveys.
This excess is related to a variation in the luminosity-specific PN number
$\alpha$ with radius. The $\alpha$-parameter value of the extended halo is more
than 7 times higher than that of the inner halo. We also measure an increase in
the slope of the PN luminosity function at fainter magnitudes with radius. We
infer that the radial variation of the PN population properties is due to a
diffuse population of metal-poor stars ([M/H] < -1.0) following an exponential
profile, in addition to the M105 halo. The spatial coincidence between the
number density profile of these metal-poor stars and the increase in the
$\alpha$-parameter value with radius establishes the missing link between
metallicity and the post-AGB phases of stellar evolution. We estimate that the
total bolometric luminosity associated with the exponential IGL population is
2.04x10^9 Lsun as a lower limit, corresponding to an IGL fraction of 3.8%. This
work sets the stage for kinematic studies of the IGL in low-mass groups.
