A Pride of Satellites in the Constellation Leo? Discovery of the Leo VI Milky Way Satellite Galaxy with DELVE Early Data Release 3 (2408.00865v2)

Abstract: We report the discovery and spectroscopic confirmation of an ultra-faint Milky Way (MW) satellite in the constellation of Leo. This system was discovered as a spatial overdensity of resolved stars observed with Dark Energy Camera (DECam) data from an early version of the third data release of the DECam Local Volume Exploration survey (DELVE EDR3). The low luminosity ($M_V = -3.56_{-0.37}^{+0.47}$ ; $L_V = 2300_{-700}^{+1200} L_\odot$), large size ($R_{1/2} = 90_{-30}^{+30}$ pc), and large heliocentric distance ($D = 111_{-6}^{+9}$ kpc) are all consistent with the population of ultra-faint dwarf galaxies (UFDs). Using Keck/DEIMOS observations of the system, we were able to spectroscopically confirm nine member stars, while measuring a tentative mass-to-light ratio of $700_{-500}^{+1400} M_\odot/L_\odot$ and a non-zero metallicity dispersion of $\sigma_{[\rm Fe/H]}=0.19_{-0.11}^{+0.14}$, further confirming Leo VI's identity as an UFD. While the system has a highly elliptical shape, $\epsilon = 0.54_{-0.29}^{+0.19}$, we do not find any conclusive evidence that it is tidally disrupting. Moreover, despite the apparent on-sky proximity of Leo VI to members of the proposed Crater-Leo infall group, its smaller heliocentric distance and inconsistent position in energy-angular momentum space make it unlikely that Leo VI is part of the proposed infall group.

• The paper presents the discovery and characterization of Leo VI, a new ultra-faint dwarf galaxy satellite of the Milky Way, identified using DELVE survey data.
• Leo VI exhibits properties typical of ultra-faint dwarfs, including low luminosity (MV = -3.56) and a high mass-to-light ratio (∼1000 M⊙/L⊙) indicating significant dark matter.
• Analysis suggests Leo VI is dynamically stable against tidal forces and is a valuable candidate for probing dark matter properties through astrophysical J-factor calculations.

The Discovery and Characterization of Leo VI: An Ultra-Faint Dwarf Galaxy in the Milky Way

The discovery and analysis detailed in this comprehensive paper presents Leo VI as a newly identified ultra-faint dwarf galaxy (UFD) satellite of the Milky Way. Utilizing the DECam Local Volume Exploration (DELVE) survey's Early Data Release 3 (EDR3), an effort was made to identify and characterize dark matter-dominated systems around our galaxy.

Identification and Initial Observations

Leo VI was identified through a systematic search of resolved stars in the DELVE EDR3 catalog using a matched-filter algorithm, designed to detect spatial overdensities indicative of potential dwarf galaxies. The preliminary search strategy was grounded in a combination of photometric signals and spatial distribution, utilizing specific parameters from isochrone models to isolate stellar populations typical of UFDs.

Structural and Photometric Properties

The characteristics of Leo VI place it firmly within the category of an ultra-faint dwarf galaxy. The system exhibits extremely low luminosity with an absolute magnitude of $M_V = -3.56_{-0.37}^{+0.47}$ and spans an azimuthally-averaged physical half-light radius of $r_{1/2} = 90_{-30}^{+30}$ pc. The high ellipticity ($\epsilon = 0.54_{-0.29}^{+0.19}$) aligns with other known UFDs, though such elliptic shapes could also imply potential tidal disruptions, which the paper addresses through dynamic considerations.

Spectroscopic Confirmation and Dynamics

Spectroscopic follow-up through Keck/DEIMOS confirmed a membership of 11 stars, validating Leo VI's status as a coherent stellar system. The velocity dispersion is measured at $\sigma_v = 3.60_{-1.39}^{+1.99}$ km/s, leading to a significant mass-to-light ratio indicative of dark matter presence ($1000_{-700}^{+1900} M_\odot/L_\odot$). Interestingly, Leo VI’s metallicity dispersion of $\sigma_{[\rm Fe/H]}=0.33_{-0.14}^{+0.19}$ reflects a non-zero metallicity variation typical of systems that have undergone multiple star formation episodes.

Implications of Tidal Influence and Galactic Associations

The paper hypothesizes on potential tidal effects driven by the high ellipticity and systemic motion, considering the galaxy's orbit and confluence with the Milky Way. However, the analysis indicates that Leo VI's average density significantly surpasses the local tidal forces exerted by the Milky Way, implying a stable dynamical state.

Furthermore, the proximity of Leo VI to other known satellite systems initially suggested potential association within a broader infall group. By comparing orbital parameters and heliocentric distances, the researchers evaluate but ultimately dismiss the likelihood of such an association, particularly due to discordant energy-angular momentum distributions.

Dark Matter Probing Potential and Future Directions

The astrophysical J-factor calculations reinforce Leo VI's efficacy as a candidate for probing dark matter properties, despite not having the largest such factor among known UFDs. As a part of broader analyses combining multiple such systems, Leo VI strengthens constraints on dark matter models via annihilation signal studies.

This paper represents a significant contribution to the understanding of satellite galaxies, particularly with Leo VI further supplementing the catalog of known UFDs orbiting the Milky Way. Future observational advancements, especially with next-generation telescopes like the Vera C. Rubin Observatory, are expected to further refine our understanding of such distant and dark matter-rich systems. The anticipated proliferation in UFD discoveries will continue to refine theoretical models on galaxy formation and the nature of dark matter.