A second galaxy missing dark matter in the NGC1052 group

Abstract

The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that it has little or no dark matter. Here we report the discovery of a second galaxy in this class, residing in the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of its size, surface brightness, and morphology; has a similar distance of $D=19.9\pm 2.8$ Mpc; and also has a population of luminous globular clusters extending out to 7 kpc from the center of the galaxy. Accurate radial velocities of the diffuse galaxy light and seven of the globular clusters were obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. The velocity of the diffuse light is identical to the median velocity of the clusters, $v_{\rm sys}=\langle v_{\rm gc} \rangle=1445$ km/s, and close to the central velocity of the NGC1052 group. The rms spread of the observed velocities is very small at $\sigma_{\rm obs}=5.8$ km/s. Taking observational uncertainties into account we determine an intrinsic velocity dispersion of $\sigma_{\rm intr}=4.2^{+4.4}_{-2.2}$ km/s, consistent with the expected value from the stars alone ($\sigma_{\rm stars}\approx 7$ km/s) and lower than expected from a standard NFW halo ($\sigma_{\rm halo}\sim 30$ km/s). We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects exists. The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood.