Still Missing Dark Matter: KCWI High-Resolution Stellar Kinematics of NGC1052-DF2 (1901.03711v2)

Abstract: The velocity dispersion of the ultra diffuse galaxy NGC1052-DF2 was found to be $\sigma_{\rm gc}=7.8^{+5.2}_{-2.2} \ \mathrm{kms^{-1}}$, 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 $\sigma_{\rm instr}\approx 12 \ \mathrm{kms^{-1}}$. The systemic velocity of the galaxy is $v_{\rm sys}=1805\pm 1.1 \ \mathrm{kms^{-1}}$, in very good agreement with the average velocity of the globular clusters ($\langle v_{\rm gc}\rangle = 1803\pm 2 \ \mathrm{kms^{-1}}$). There is no evidence for rotation within the KCWI field of view. We find a stellar velocity dispersion of $\sigma_{\rm stars}=8.4 \pm 2.1 \ \mathrm{kms^{-1}}$, consistent with the dispersion that was derived from the globular clusters. The implied dynamical mass within the half-light radius $r_{1/2}=2.7 \ \mathrm{kpc}$ is $M_{\rm dyn}= (1.3 \pm 0.8) \times 10^8 $ M${\odot}$, similar to the stellar mass within that radius ($M{\rm stars}=(1.0 \pm 0.2) \times 10^8 \ \mathrm{M}_{\odot}$). 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.

• The paper analyzes high-resolution stellar kinematics of the ultra diffuse galaxy NGC1052-DF2 using the KCWI instrument to verify its apparent lack of dark matter.
• Measurements find a low stellar velocity dispersion of 8.5 km/s, confirming prior globular cluster-based estimates and strongly suggesting negligible dark matter within the galaxy's half-light radius.
• These results challenge standard galaxy formation models by identifying a galaxy with unusually low dark matter content, indicating potential diversity in dark matter distribution among ultra diffuse galaxies.

An Analysis of the Stellar Kinematics in the Ultra Diffuse Galaxy NGC1052-DF2: Implications for Dark Matter Content

The paper "Still Missing Dark Matter: KCWI High-Resolution Stellar Kinematics of NGC1052-DF2" by Danieli et al. explores the intriguing case of the ultra diffuse galaxy (UDG) NGC1052-DF2, noted for its apparent lack of dark matter. Through high-resolution spectroscopy conducted with the Keck Cosmic Web Imager (KCWI), the authors provide a detailed analysis of the stellar kinematics, aiming to validate previous low-velocity dispersion measurements based on the radial velocities of globular clusters.

Methodological Advances and Findings

Danieli et al. employ the KCWI to obtain an integrated spectrum of the diffuse stellar light from NGC1052-DF2, improving upon earlier measurements that were constrained by the number of globular clusters available for analysis. With a high instrumental resolution of approximately 12 km/s, the paper aims to directly measure the galaxy's stellar velocity dispersion. The systemic velocity is determined to be $v_{\rm sys}=1805 \pm 1.1$ km/s, which coincides closely with previous measurements from associated globular clusters.

The stellar velocity dispersion of NGC1052-DF2 is measured at $\sigma_{\rm stars}=8.5^{+2.3}_{-3.1}$ km/s, a value predominantly consistent with prior estimates based on globular clusters. This result underscores a significant deviation from what would be anticipated based on the stellar mass to halo mass relation, strongly suggesting a negligible or absent dark matter component within the confines of the half-light radius.

Implications and Theoretical Considerations

The findings presented by Danieli et al. pose significant questions for current galaxy formation theories, particularly regarding the distribution and role of dark matter in UDGs. The results imply that dark matter in NGC1052-DF2 does not dominate the kinematics at the scales probed, which contrasts with the expectations set by conventional models. Consequently, this challenges the presumed uniformity of the stellar mass - halo mass connection at low mass scales, possibly indicating greater scatter or even stochasticity in dark matter content among UDGs.

A crucial implication of this research is the potential identification of a subset of galaxies for which the dark matter content is unusually low, as hypothesized by the paper. This outcome could either represent an extreme instance of baryon-dominated systems or suggest alternative dark matter distribution scenarios or dynamics, such as tidal interactions affecting dark matter halos in specific environments.

Future Directions

The paper encourages subsequent observational campaigns to ascertain whether the "missing dark matter problem" typified by NGC1052-DF2 is an anomaly or reflective of a broader pattern among UDGs. Comprehensive mapping of stellar and globular cluster kinematics in more such galaxies would be instrumental in understanding the generality of these findings. Additionally, theoretical work on galaxy formation mechanisms in low-density regions or models of dark matter interactions might be essential to reconcile these observations with the broader cosmological framework.

In summary, the rigorous analysis of NGC1052-DF2's stellar kinematics elucidates a rare example of a galaxy with minimal dark matter influence in its dynamics, thereby stimulating further inquiry into the diversity of galaxy-dark matter interrelations and encouraging deeper theoretical refinements in dark matter-centric galaxy formation models.