1999A&A...352L..57P

The photometric properties of the extremely metal-deficient (Z_☉/24)i0-BCD Tol 65 are investigated using broad-band data obtained with VLT/FORS. A prime objective of this study is the detection and analysis of the structural properties of the putative low-surface brightness (LSB) stellar continuum underlying the extended starburst component. Our deep (µ_B>28mag/arcsec²) VLT data reveal an unresolved irregular LSB host, dominating the light for surface brightness levels fainter than 25 B mag/arcsec². The intensity distribution as determined in its outskirts can be well approximated by an exponential fitting law and a flattening inside a radius of ∼0.6kpc, equivalent to ∼2.4 exponential scale lengths. The age of this stellar component as inferred from radially averaged colour profiles is of the order of 10⁸yr. The luminosity of the starburst component of Tol 65 is contributed primarily by an assembly of at least five distinct luminous knots arranged over a projected linear scale of 1kpc and giving rise to ample gaseous emission on scales of several hundred pc. The properties of a faint LSB source discovered by the present observations in the immediate vicinity of Tol 65 are intriguing. Provided that it lies at the distance of the BCD, its absolute B magnitude and exponential scale length make it one of the most compact LSB dwarf galaxies detected so far. The properties of this source then prove remarkably similar to those inferred for a virtually inactive companion of another extremely metal-deficient young galaxy candidate, the i0-BCD I Zw 18. It will be important to explore whether such faint companions are common and may have formed in a preceding activity period from the same HI envelope as the currently active BCD.