A search for stars in the intergalactic neutral hydrogen cloud H I 1225 + 01: The star formation threshold in galaxies

H I 1225 + 01 is an intergalactic neutral hydrogen cloud in Virgo, made up of two main clumps. The north-east (NE) clump has a dwarf galaxy optical counterpart consisting of knots of stars about 5 × 5 arcsec² and larger. Although the south-west (SW) clump has a peak H I column density similar to that of the NE clump, past V-band searches of the SW clump reveal no stars. However, stars remaining from a previous epoch of star formation would be more easily detected at infrared wavelengths. An I-band image of the SW clump is obtained and modal-filtered on a 5 × 5 arcsec² box. The filtering yields a surface brightness limit for extended emission 2.8 mag arcsec^-2 fainter than that in the original image. No extended emission is seen to a limiting surface brightness of 27.27 mag arcsec^-2. This is about 4 I mag arcsec^-2 fainter than the highest surface brightness portion of the dwarf galaxy in the NE clump. We discuss several ways to prevent the SW clump from forming stars, including photodissociation of its molecular hydrogen and gravitational interaction with the NE clump.