We have discovered an extended Lyman- \alpha plume associated with a compact source at redshift z \approx 5.4 in slitless spectroscopic data from the Grism ACS Program for Extragalactic Science ( GRAPES ) project .
The spatial extent of the emission is about 6 \times 1.5 ~ { } \hbox { kpc } ( 1 { { } ^ { \prime \prime } } \times 0.25 { { } ^ { \prime \prime } } ) .
Combining our grism data and the broadband images from the Hubble Ultra Deep Field ( UDF ) images , we find a Lyman- \alpha line flux of \sim 2.2 \times 10 ^ { -17 } { erg cm ^ { -2 } s ^ { -1 } } and surface brightness \sim 7 \times 10 ^ { -17 } { erg cm ^ { -2 } s ^ { -1 } } / { \square ^ { \prime \prime } } .
The UDF images show diffuse continuum emission associated with the Lyman- \alpha plume ( hereafter UDF 5225 ) , with three embedded knots .
The morphology of UDF 5225 is highly suggestive of a galaxy in assembly .
It is moreover possible that the prominent Lyman- \alpha emission from this object is due to an active nucleus , and that we are seeing the simultaneous growth through accretion of a galaxy and its central black hole .
Followup observations at higher spectral resolution could test this hypothesis .