We have conducted B, g, V, and R-band imaging in a 45x40 arcmin^2 field containing part of the high Galactic latitude translucent cloud MBM32, and correlated the intensity of diffuse optical light S_ u(lambda) with that of 100 micron emission S_ u(10
0um). A chi^2 minimum analysis is applied to fit a linear function to the measured correlation and derive the slope parameter b(lambda)= Delta S_ u(lambda) / Delta S_ u(100um) of the best-fit linear function. Compiling a sample by combining our b(lambda) and published ones, we show that the b(lambda) strength varies from cloud to cloud by a factor of 4. Finding that b(lambda) decreases as S_ u(100um) increases in the sample, we suggest that a non-linear correlation including a quadratic term of S_ u(100um)^2 should be fitted to the measured correlation. The variation of optical depth, which is A_V = 0.16 - 2.0 in the sample, can change b(lambda) by a factor of 2 - 3. There would be some contribution to the large b(lambda) variation from the forward-scattering characteristic of dust grains which is coupled to the non-isotropic interstellar radiation field (ISRF). Models of the scattering of diffuse Galactic light (DGL) underestimate the b(lambda) values by a factor of 2. This could be reconciled by deficiency in UV photons in the ISRF or by a moderate increase in dust albedo. Our b(lambda) spectrum favors a contribution from extended red emission (ERE) to the diffuse optical light; b(lambda) rises from B to V faster than the models, seems to peak around 6000 AA, and decreases towards long wavelengths. Such a characteristic is expected from the models in which the DGL is combined with ERE.
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.
We present the new constraints on the cosmic optical background (COB) obtained from an analysis of the Pioneer 10/11 Imaging Photopolarimeter (IPP) data. After careful examination of data quality, the usable measurements free from the zodiacal light
are integrated into sky maps at the blue (~0.44 um) and red (~0.64 um) bands. Accurate starlight subtraction is achieved by referring to all-sky star catalogs and a Galactic stellar population synthesis model down to 32.0 mag. We find that the residual light is separated into two components: one component shows a clear correlation with thermal 100 um brightness, while another betrays a constant level in the lowest 100 um brightness region. Presence of the second component is significant after all the uncertainties and possible residual light in the Galaxy are taken into account, thus it most likely has the extragalactic origin (i.e., the COB). The derived COB brightness is (1.8 +/- 0.9) x 10^(-9) and (1.2 +/- 0.9) x 10^(-9) erg/s/cm2/sr/A at the blue and red band, respectively, or 7.9 +/- 4.0 and 7.7 +/- 5.8 nW/m2/sr. Based on a comparison with the integrated brightness of galaxies, we conclude that the bulk of the COB is comprised of normal galaxies which have already been resolved by the current deepest observations. There seems to be little room for contributions of other populations including first stars at these wavelengths. On the other hand, the first component of the IPP residual light represents the diffuse Galactic light (DGL) - scattered starlight by the interstellar dust. We derive the mean DGL-to-100 um brightness ratios of 2.1 x 10^(-3) and 4.6 x 10^(-3) at the two bands, which are roughly consistent with the previous observations toward denser dust regions. Extended red emission in the diffuse interstellar medium is also confirmed.
