Narrow band survey for intragroup light in the Leo HI cloud: Constraints on the galaxy background contamination in imaging surveys for intracluster planetary nebulae

We have observed emission line objects located in a 0.26 deg² field in the M 96 (Leo) group, coincident with the intergalactic HI cloud. The emission line objects were selected using the same procedure as used for the search for intracluster planetary nebulae in the Virgo cluster. 29 emission line objects were identified, with [OIII] filter magnitudes brighter than m₅₀₀₇ = 28. Their m₅₀₀₇ luminosity function has a bright cut-off ≃1.2 mag fainter than for the luminosity function of the planetary nebulae (PN) associated with the elliptical galaxies in the M 96 group. Therefore the vast majority of these emission line objects are compatible with not being intragroup planetary nebulae at the Leo group distance of 10 Mpc. Spectroscopic follow-up of two emission line objects in this Leo field showed that indeed these do not have the [OIII] λ4959, λ5007 Å doublet expected for a real PN. The brighter source is identified as an AGN at redshift z = 3.128, because of a second emission in the near infrared, identified as FeII (λ2220 Å). From these data we derive three main results: (i) from the absence of PN we can determine a more stringent upper limit to the surface brightness in any old stellar population associated with the Leo HI cloud at the surveyed position, μ_B.* < 32.8 mag arcsec^-2. (ii) This translates to an upper limit of 1.6% for the fraction of luminosity in a diffuse intragroup component in the densest 3° × 2° area of the Leo group, relative to the light in galaxies. (iii) Using this Leo field as a blank field, we derive an average fraction of 13.6% background emission line objects that enter in surveys of Virgo intracluster PN. This is an agreement with an earlier estimate (15%) obtained from the Ly break galaxy population at z = 3.13. The small fraction confirms the validity of the selection criteria for intracluster PN candidates in Virgo.