A second galaxy missing dark matter in the NGC1052 group (1901.05973v3)

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.

• The paper reports the discovery of NGC1052-DF4, a galaxy similar to NGC1052-DF2, confirming its dark matter deficiency through stellar and globular cluster kinematics.
• Velocity dispersion measurements of NGC1052-DF4 and its globular clusters indicate its mass is dominated by stars with little to no dark matter present.
• The presence of two dark matter-deficient galaxies in the same group challenges standard galaxy formation models and indicates a potential new class requiring alternative explanations.

A Second Galaxy Lacking Dark Matter in the NGC 1052 Group

The paper authored by van Dokkum et al. reports on the discovery of a second ultra-diffuse galaxy, NGC1052-DF4 (DF4), which exhibits a significant lack of dark matter. This discovery follows the earlier identification of NGC1052-DF2 (DF2), another galaxy within the same group also noted for its apparent deficiency of dark matter. DF4 mirrors DF2 in several critical aspects, such as its size, morphology, surface brightness, and globular cluster population, suggesting that DF2 is not an anomalous case but represents a category of such galaxies. The paper makes use of data from the Low Resolution Imaging Spectrograph on the Keck I telescope and Hubble Space Telescope (HST) imaging to assert these findings.

Key Findings

• Galaxy Characteristics: DF4 shares similar characteristics with DF2, including size, surface brightness, and globular cluster makeup, with a structural fit to a low Sérsic index model. The intrinsic velocity dispersion of DF4 is reported as 4.2 km/s, aligning closely with expectations based solely on stellar mass, implying minimal dark matter presence.
• Globular Cluster Kinematics: Radial velocities of seven globular clusters associated with DF4 were measured, demonstrating a narrow velocity dispersion of merely 5.8 km/s. This further supports the notion that DF4, like DF2, lacks a significant dark matter halo.
• Implications on Mass and Dark Matter: Using modeling approaches such as the Tracer Mass Estimator, it was inferred that DF4's mass primarily resides in its stellar components, providing no substantial evidence for dark matter.

Implications

This paper's findings challenge traditional galaxy formation models, which postulate a gravitationally dominant dark matter halo as essential for galaxy formation. The presence of both DF2 and DF4, sharing similar unusual properties, necessitates reconsideration of galaxy evolution theories, particularly the role and distribution of dark matter in such systems. Traditional explanations involving rare orbital configurations or viewing angles are insufficient to account for multiple occurrences in close proximity. Instead, the possibility of alternate formation mechanisms, potentially involving tidal processes, must be explored, though these hypotheses remain speculative without further evidence.

Future Directions

The identification of DF4 alongside DF2 emphasizes the potential existence of a broader class of ultra-diffuse, dark matter-deficient galaxies. This compels further investigative efforts to elucidate their formation processes and external influences. Expanding surveys with telescopes capable of deep-sky imaging, like the Dragonfly Telephoto Array, are essential to uncover more instances within this galaxy category and to better understand their distribution and prevalence within various cosmic environments. Additionally, exploring the stellar dynamics and examining environments for potential tidal interactions or other unique formation pathways could provide further clarity.

In conclusion, these observations pose significant questions on fundamental ideas regarding galaxy formation and stress the necessity of re-evaluating current theories or developing novel ones that accommodate the existence of such galaxies without substantial dark matter components. The pursuit of this line of inquiry could significantly enhance our understanding of both galaxy morphology and dark matter dynamics.