The StEllar Counterparts of COmpact high velocity clouds (SECCO) survey II. Sensitivity of the survey and the atlas of synthetic dwarf galaxies

The searching for StEllar Counterparts of COmpact high velocity clouds (SECCO) survey is devoted to the search for stellar counterparts within ultra compact high velocity clouds that are candidate low-mass, low-luminosity galaxies. We present the results of a set of simulations aimed at the quantitative estimate of the sensitivity of the survey as a function of the total luminosity, size, and distance of the stellar systems we are looking for. For all of our synthetic galaxies we assumed an exponential surface brightness profile and an old and metal-poor population. The synthetic galaxies are simulated both on the images and on the photometric catalogues, taking all the observational effects into account. In the fields where the available observational material is of top quality (similar or equal to 36% of the SECCO fields), we detect synthetic galaxies as >= 5 sigma over-densities of resolved stars down to mu(v,h) similar or equal to 30.0 mag/arcsec(2), for D <= 1.5 Mpc, and down to mu(v,h) similar or equal to 29.5 mag/arcsec(2), for D <= 2.5 Mpc. In the field with the worst observational material of the whole survey, we detect synthetic galaxies with mu(v,h) <= 28.8 mag/arcsec(2) out to D <= 1.0 Mpc, and those with mu(v,h) <= 27.5 mag/arcsec(2) out to D <= 2.5 Mpc. Dwarf galaxies with M-V = 10.0, with sizes in the range spanned by known dwarfs, are detected by visual inspection of the images up to D = 5 Mpc independent of the image quality. In the best quality images, dwarfs are partially resolved into stars up to D = 3.0 Mpc and completely unresolved at D = 5 Mpc. As an independent test of the sensitivity of our images to low surface brightness galaxies, we report on the detection of several dwarf spheroidal galaxies probably located in the Virgo cluster with M-V less than or similar to -8.0 and mu(v,h) less than or similar to 26.8 mag/arcsec(2). The nature of the previously discovered SECCO 1 stellar system, also likely located in the Virgo cluster, is rediscussed in comparison with these dwarfs. While specific for the SECCO survey, our study may also provide general guidelines for the detection of faint stellar systems with 8 m class telescopes.