Eight New Milky Way Companions Discovered in First-Year Dark Energy Survey Data (1503.02584v2)

Abstract: We report the discovery of eight new Milky Way companions in ~1,800 deg^2 of optical imaging data collected during the first year of the Dark Energy Survey (DES). Each system is identified as a statistically significant over-density of individual stars consistent with the expected isochrone and luminosity function of an old and metal-poor stellar population. The objects span a wide range of absolute magnitudes (M_V from -2.2 mag to -7.4 mag), physical sizes (10 pc to 170 pc), and heliocentric distances (30 kpc to 330 kpc). Based on the low surface brightnesses, large physical sizes, and/or large Galactocentric distances of these objects, several are likely to be new ultra-faint satellite galaxies of the Milky Way and/or Magellanic Clouds. We introduce a likelihood-based algorithm to search for and characterize stellar over-densities, as well as identify stars with high satellite membership probabilities. We also present completeness estimates for detecting ultra-faint galaxies of varying luminosities, sizes, and heliocentric distances in the first-year DES data.

• The paper identifies eight candidate MW companions with absolute magnitudes from -2.2 to -7.4, detected as significant stellar over-densities in DES optical imaging.
• The paper employs a dual-method approach that integrates likelihood algorithms with spatial binning to robustly detect ultra-faint galaxies.
• The paper’s discoveries near the Magellanic Clouds expand the known census of MW satellites and offer new insights into galaxy formation and interaction dynamics.

Overview of "Eight New Milky Way Companions Discovered in First-Year Dark Energy Survey Data"

This paper presents the discovery of eight new stellar systems that are potential companions to the Milky Way, identified through optical imaging from the first year of the Dark Energy Survey (DES). These newly found satellite candidates were detected as statistically significant over-densities of stars consistent with characteristics typical of old, metal-poor stellar populations. The discoveries contribute to the census of Milky Way companion systems, addressing complexities in the galaxy formation narrative, particularly in the ultra-faint regime.

Key Findings

• Discovery and Detection: The eight new candidates range in absolute magnitude from $M_V = -2.2$ to $-7.4$, indicating a range of possible galaxy types, with heliocentric distances from 30kpc to 330kpc and physical sizes between 10pc and 170pc. Most notably, these characteristics suggest that several of these objects are likely to be ultra-faint galaxies.
• Methodology: A dual-method approach was employed including a likelihood-based algorithm and a spatial binning search. The likelihood method enhanced sensitivity by characterizing both the spatial and photometric distributions of stars, incorporating varying survey depths and uncertainties. The results emphasized the importance of multiple methodologies in the robust discovery of ultra-faint galaxies.
• Significance and Localization: The detected objects are strategically located near the Magellanic Clouds, which may imply gravitational associations. The candidates represent a potentially significant increase in the known population of Milky Way satellite galaxies, aiding the understanding of the Milky Way's galactic environment.

Implications

1. Ultrafaint Galaxy Population: These discoveries add significant volume to the known population of ultra-faint galaxies, which are critical for probing the lower limits of galaxy formation and the effects of dark matter halos at smaller scales.
2. Future Surveys and Cataloguing: The results reinforce the efficacy of deep, wide-field surveys in uncovering new galactic structures not previously identified by surveys such as the Sloan Digital Sky Survey (SDSS). The methodology and findings from this work will likely influence strategies for future astronomical surveys.
3. The Role of the Magellanic Satellites: Given the proximity of several candidates to the Magellanic Clouds, there is potential for these objects to reveal more about the interaction dynamics between the Magellanic system and the Milky Way. This could refine models of galaxy interaction and accretion.

Future Directions

• Spectroscopic Confirmation: Future spectroscopic studies will be essential to ascertain the nature of these candidate systems, distinguishing between dwarf galaxies and globular clusters by measuring velocity dispersions and metallicity spreads.
• Exploration of Sky Coverage and Depth: The impending additional data from continuing DES operations will enable further discoveries that can delineate the characteristics and distribution of Milky Way satellites.
• Theoretical Implications: These findings will help refine theoretical models of galaxy formation and evolution by providing empirical data at lower luminosity thresholds and higher distances than previously available.

Overall, the research delineated in this paper highlights the advances in astronomical search methodologies, the complexities of Milky Way satellite dynamics, and the ever-evolving picture of the Local Group, enriching the scientific dialogue surrounding the formation of these faint galactic systems within the paradigm of hierarchical structure formation.