Updated Analysis of a "Dark" Galaxy and its Blue Companion in the Virgo
  Cloud HI 1225+01
  HI 1225+01 is an intergalactic gas cloud located on the outskirts of Virgo
cluster. Its main components are two large clumps of comparable HI masses (M_HI
~ 10^9 Msun) separated by about 100 kpc. One of the clumps hosts a blue
low-surface-brightness galaxy J1227+0136, while the other has no identified
stellar emission and is sometimes referred to as a promising candidate of a
"dark galaxy", an optically invisible massive intergalactic system. We present
a deep optical image covering the whole HI 1225+01 structure for the first
time, as well as a collection of archival data from ultraviolet to far-infrared
(IR) spectral region of the brightest knot "R1" in J1227+0136. We find that R1
has a young stellar population of age 10-100 Myr and mass ~ 10^6 Msun, near-IR
excess brightness which may point to the presence of hot dust with color
temperature ~ 600 K, and relatively faint mid- to far-IR fluxes corresponding
to the dust mass of up to ~ 100 Msun. Overall, it seems to share the general
properties with low-metallicity blue compact dwarf galaxies. On the other hand,
no optical counterpart to the other clump is found in our deepest-ever image.
Now the limiting surface brightness reaches down to R_AB > 28 mag/arcsec2 for
any emission extended over 10" (comparable to R1), which is more than one
hundred times fainter than the brightest part of the companion galaxy
J1227+0136.
