THE DETECTION OF ULTRA-FAINT LOW SURFACE BRIGHTNESS DWARF GALAXIES IN THE VIRGO CLUSTER: A PROBE OF DARK MATTER AND BARYONIC PHYSICS

We have discovered 11 ultra-faint (r less than or similar to 22.1) low surface brightness (LSB, central surface brightness 23 less than or similar to mu(r) less than or similar to 26) dwarf galaxy candidates in one deep Virgo field of just 576 arcmin(2) obtained by the Large Binocular Camera at the Large Binocular Telescope. Their association with the Virgo cluster is supported by their distinct position in the central surface brightness-total magnitude plane with respect to the background galaxies of similar total magnitude. They have typical absolute magnitudes and scale sizes, if at the distance of Virgo, in the range -13 less than or similar to M-r less than or similar to -9 and 250 less than or similar to rs less than or similar to 850 pc, respectively. Their colors are consistent with a gradually declining star formation history with a specific star formation rate of the order of 10(-11) yr(-1), i.e., 10 times lower than that of main sequence star-forming galaxies. They are older than the cluster formation age and appear to be regular in morphology. They represent the faintest extremes of the population of low luminosity LSB dwarfs that has recently been detected in wider surveys of the Virgo cluster. Thanks to the depth of our observations, we are able to extend the Virgo luminosity function down to M-r similar to -9.3 (corresponding to total masses M similar to 10(7) M-circle dot), finding an average faint-end slope alpha similar or equal to 1.4. This relatively steep slope puts interesting constraints on the nature of the dark matter and, in particular, on warm dark matter (WDM) often invoked to solve the overprediction of the dwarf number density by the standard cold dark matter scenario. We derive a lower limit on the WDM particle mass >1.5 keV.