The velocity dispersion of the ultra diffuse galaxy NGC1052-DF2 was found to be \sigma _ { gc } = 7.8 ^ { +5.2 } _ { -2.2 } km s ^ { -1 } , much lower than expected from the stellar mass – halo mass relation and nearly identical to the expected value from the stellar mass alone .
This result was based on the radial velocities of ten luminous globular clusters that were assumed to be associated with the galaxy .
A more precise measurement is possible from high resolution spectroscopy of the diffuse stellar light .
Here we present an integrated spectrum of the diffuse light of NGC1052-DF2 obtained with the Keck Cosmic Web Imager , with an instrumental resolution of \sigma _ { instr } \approx 12 km s ^ { -1 } .
The systemic velocity of the galaxy is v _ { sys } = 1805 \pm 1.1 km s ^ { -1 } , in very good agreement with the average velocity of the globular clusters ( \langle v _ { gc } \rangle = 1803 \pm 2 km s ^ { -1 } ) .
There is no evidence for rotation within the KCWI field of view .
We find a stellar velocity dispersion of \sigma _ { stars } = 8.5 ^ { +2.3 } _ { -3.1 } km s ^ { -1 } , consistent with the dispersion that was derived from the globular clusters .
The implied dynamical mass within the half-light radius r _ { 1 / 2 } = 2.7 kpc is M _ { dyn } = ( 1.3 \pm 0.8 ) \times 10 ^ { 8 } M _ { \odot } , similar to the stellar mass within that radius ( M _ { stars } = ( 1.0 \pm 0.2 ) \times 10 ^ { 8 } \mathrm { M } _ { \odot } ) .
With this confirmation of the low velocity dispersion of NGC1052-DF2 , the most urgent question is whether this “ missing dark matter problem ” is unique to this galaxy or applies more widely .