Extremely Metal-Poor Galaxies: The Environment
  We have analyzed bibliographical observational data and theoretical
predictions, in order to probe the environment in which extremely metal-poor
dwarf galaxies (XMPs) reside. We have assessed the HI component and its
relation to the optical galaxy, the cosmic web type (voids, sheets, filaments
and knots), the overdensity parameter and analyzed the nearest galaxy
neighbours. The aim is to understand the role of interactions and cosmological
accretion flows in the XMP observational properties, particularly the
triggering and feeding of the star formation. We find that XMPs behave
similarly to Blue Compact Dwarfs; they preferably populate low-density
environments in the local Universe: ~60% occupy underdense regions, and ~75%
reside in voids and sheets. This is more extreme than the distribution of
irregular galaxies, and in contrast to those regions preferred by elliptical
galaxies (knots and filaments). We further find results consistent with
previous observations; while the environment does determine the fraction of a
certain galaxy type, it does not determine the overall observational
properties. With the exception of five documented cases (four sources with
companions and one recent merger), XMPs do not generally show signatures of
major mergers and interactions; we find only one XMP with a companion galaxy
within a distance of 100 kpc, and the HI gas in XMPs is typically well-behaved,
demonstrating asymmetries mostly in the outskirts. We conclude that metal-poor
accretion flows may be driving the XMP evolution. Such cosmological accretion
could explain all the major XMP observational properties: isolation, lack of
interaction/merger signatures, asymmetric optical morphology, large amounts of
unsettled, metal-poor HI gas, metallicity inhomogeneities, and large specific
star formation.
