A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44 (1606.06291v2)

Abstract: Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these Ultra Diffuse Galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here we present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of 47 km/s, which implies a dynamical mass of M_dyn=0.7x10^10 M_sun within its deprojected half-light radius of r_1/2=4.6 kpc. The mass-to-light ratio is M/L=48 M_sun/L_sun, and the dark matter fraction is 98 percent within the half-light radius. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in 0.4" seeing we infer that Dragonfly 44 has 94 globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r=4.6 kpc to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of ~10^12 M_sun, similar to the mass of the Milky Way. The existence of nearly-dark objects with this mass is unexpected, as galaxy formation is thought to be maximally-efficient in this regime.

• The paper reveals a stellar velocity dispersion of 47 km/s and a dynamical mass of 0.7×10^10 M⊙ within a 4.6 kpc half-light radius, implying a dark matter fraction of 98%.
• The paper identifies nearly 100 globular clusters, an anomalously high number for a low-luminosity galaxy, which supports the presence of an extensive dark matter halo.
• The paper estimates a total halo mass of approximately 10^12 M⊙ for Dragonfly 44, suggesting it is a 'failed' massive galaxy with characteristics similar to the Milky Way.

Overview of the Ultra Diffuse Galaxy Dragonfly 44

This paper examines the intriguing properties of the ultra diffuse galaxy (UDG) known as Dragonfly 44, identified within the Coma cluster. UDGs are notable for their large size and low surface brightness, posing challenges to conventional galaxy formation theories. The custom 33.5-hour spectral integration with DEIMOS on the Keck II telescope provides detailed measurements of the stellar kinematics of Dragonfly 44.

Key Findings

1. Stellar Velocity Dispersion and Mass Estimation: The velocity dispersion of the stars in Dragonfly 44 is measured at $\sigma = 47^{+8}_{-6}$ km/s, leading to a dynamic mass estimation of $$M_{\rm dyn}(<r_{1/2}) = 0.7^{+0.3}_{-0.2} \times 10^{10}$$ M$_{\odot}$ within its half-light radius of 4.6 kpc. The mass-to-light ratio, $M/L_I(<r_{1/2})$, is similarly noteworthy at $48^{+21}_{-14}$ M$_{\odot}$/L$_{\odot}$, conveying a high dark matter fraction of 98%.
2. Globular Cluster Population: High-resolution Gemini imaging identifies approximately 94 globular clusters associated with Dragonfly 44, a number typical for galaxies with significantly higher mass. This unexpectedly large number of globular clusters further supports the hypothesis that UDGs like Dragonfly 44 contain significant dark matter halos.
3. Dark Halo Mass: By comparing Dragonfly 44’s mass profile with NFW halo models, the estimated total halo mass is approximately $10^{12}$ M$_{\odot}$. This mass aligns closely with that of the Milky Way, which is surprising for a galaxy with such low luminosity and suggests that Dragonfly 44 may be a "failed" massive galaxy.

Implications and Theoretical Speculations

• Galaxy Evolution: The features of UDGs, particularly their high mass-to-light ratios and exceptional globular cluster populations, imply that various UDGs could be "failed" galaxies of much higher luminosity, potentially signaling that significant feedback mechanisms (such as supernovae, AGN activity, or tidal and gas interaction) halted their full development.
• Dark Matter and Galaxy Formation Models: These findings challenge current galaxy formation paradigms by demonstrating that such massive and dark galaxies can exist. They may necessitate revisions in models related to the efficiency of star formation in different halo mass regimes, highlighting the need for further investigation into how processes governing galaxy evolution can diverge in high-density environments like galaxy clusters.

Future Directions

Ongoing and future investigations, both observational and theoretical, should aim to delineate the properties and formation pathways of these UDGs more clearly:

• Expanded Surveys and Observations: Broader spectroscopic and imaging studies are needed to verify if phenomena observed in Dragonfly 44 hold for a more extensive sample of UDGs.
• Gravitational Lensing Studies: Observations leveraging weak gravitational lensing could provide independent assessments of mass distribution in UDGs.
• Numerical Simulations: Simulations that incorporate detailed feedback processes and environmental effects on galaxy evolution might be adjusted to reflect the unusual characteristics discovered in objects like Dragonfly 44.

This compelling glimpse into Dragonfly 44 not only enriches the understanding of one unique galaxy but also sheds light on broader dynamics in galaxy formation, subsequently driving further inquiry into the universe's more obscure constituents.