Overdensity of VVV galaxies behind the Galactic bulge
  We studied a region of 1.636 square degrees corresponding to the VVV tile
$b204$. Using SExtractor, we analysed photometric data generating a catalogue
of extended sources in this area. In order to confirm these sources as galaxy
candidates we visually inspected RGB images looking for typical galaxy
features. Using 2MASX and GCMW catalogued sources we tested completeness and
contamination of our catalogue and define suitable colour cuts to select
galaxies. We also compared the observational results with those obtained from
two semi-analytical models on Dark Matter simulations. One galaxy catalogue was
constructed with the SAG semi-analytic model of galaxy formation, and the other
one was constructed with the L-Galaxies semi-analytic model.By adopting
CLASS-STAR$< 0.5$, $r_{1/2} > 0.7$ arcsec and specific colour cuts
(J-Ks$>$0.97, J-H$>$0 and H-Ks$>$0) we generated an automatic catalogue of
extended sources. After visual inspection we identified 624 sources with
10$<$Ks$<$17 as galaxy candidates. The contamination of the automatic catalogue
is 28% when considering visually confirmed galaxies as reliable objects. The
estimated completeness is 87% up to magnitude Ks=13.5. We analysed the spatial
distribution of galaxy candidates, finding a high concentration of galaxies in
a small region of 15 arcmin radius. This region has three times higher density
than similar areas in the tile. We compared the number of galaxies in this
small area with the mean density values obtained from a suitable sample of
galaxies from semi-analytic models finding that our results are consistent with
an overdensity region. Using VVV near-infrared data and mock catalogues we
detect new extragalactic sources that have not been identified by other
catalogues. We demonstrate the potentiality of the VVV survey studying a large
number of galaxy candidates and extragalactic structures obscured by the Milky
Way.
