[Paper Review] The Dark Energy Survey view of the Sagittarius stream: discovery of two faint stellar system candidates

E. Luque, A. Pieres+64 more|Americanae (AECID Library)|Aug 13, 2016

Stellar, planetary, and galactic studies2 references3 citations

TL;DR

This paper presents the discovery of two faint stellar system candidates, DES J0111−1341 and DES J0225+0304, using data from the first two years of the Dark Energy Survey. Located at ~25 kpc heliocentric distance and showing old, metal-poor populations, their proximity to the Sagittarius stream's orbital plane and structural parameters suggest possible association with the stream, marking the first potential ultrafaint stellar systems linked to it and the first confirmed case of an ultrafaint satellite of a satellite if spectroscopic follow-up confirms binding and kinematic coherence.

ABSTRACT

We report the discovery of two new candidate stellar systems in the constellation of Cetus using the data from the first two years of the Dark Energy Survey (DES). The objects, DES J0111-1341 and DES J0225+0304, are located at a heliocentric distance of ~ 25 kpc and appear to have old and metal-poor populations. Their distances to the Sagittarius orbital plane, ~ 1.73 kpc (DES J0111-1341) and ~ 0.50 kpc (DES J0225+0304), indicate that they are possibly associated with the Sagittarius dwarf stream. The half-light radius (r_h ~ 4.55 pc) and luminosity (M_V ~ +0.3) of DES J0111-1341 are consistent with it being an ultrafaint stellar cluster, while the half-light radius (r_h ~ 18.55 pc) and luminosity (M_V ~ -1.1) of DES J0225+0304 place it in an ambiguous region of size-luminosity space between stellar clusters and dwarf galaxies. Determinations of the characteristic parameters of the Sagittarius stream, metallicity spread (-2.18 < [Fe/H] < -0.95) and distance gradient (23 kpc < D_sun < 29 kpc), within the DES footprint in the Southern hemisphere, using the same DES data, also indicate a possible association between these systems. If these objects are confirmed through spectroscopic follow-up to be gravitationally bound systems and to share a Galactic trajectory with the Sagittarius stream, DES J0111-1341 and DES J0225+0304 would be the first ultrafaint stellar systems associated with the Sagittarius stream. Furthermore, DES J0225+0304 would also be the first confirmed case of an ultrafaint satellite of a satellite.

Motivation & Objective

• To identify new faint stellar systems in the Southern Galactic halo using deep photometric data from the Dark Energy Survey.
• To investigate the possible association of newly discovered systems with the Sagittarius dwarf tidal stream based on spatial, photometric, and kinematic criteria.
• To determine the structural and population parameters of candidate systems to classify them as ultrafaint clusters or dwarf galaxies.
• To assess the metallicity spread and distance gradient of the Sagittarius stream within the DES footprint to support dynamical associations.

Proposed method

• Utilized deep optical imaging from the Dark Energy Survey (DES) Y2Q1 data release covering ~1000 deg² in the Southern celestial hemisphere.
• Constructed density maps and Hess diagrams in both Sagittarius orbital and Galactic coordinates to identify stellar overdensities and substructures.
• Applied photometric and structural analysis to derive half-light radii, absolute magnitudes, and color-magnitude diagram fitting for candidate systems.
• Used metallicity and distance gradient measurements of the Sagittarius stream from decontaminated Hess diagrams to compare with candidate system properties.
• Defined On and Off regions around the stream to model background star density and enhance detection significance of overdensities.
• Employed photometric distance estimates and population synthesis modeling to infer age, metallicity, and distance for the candidate systems.

Experimental results

Research questions

• RQ1 Are there previously undetected faint stellar systems associated with the Sagittarius dwarf tidal stream in the DES footprint?
• RQ2 Do the structural and photometric properties of the candidate systems align with those of ultrafaint stellar clusters or dwarf galaxies?
• RQ3 Is there kinematic or spatial evidence linking the candidates to the Sagittarius stream, particularly given their proximity to the stream's orbital plane?
• RQ4 What is the metallicity spread and distance gradient of the Sagittarius stream in the DES observed region, and how does it compare with known substructures?
• RQ5 Could DES J0225+0304 represent the first confirmed ultrafaint satellite of a satellite, given its ambiguous size-luminosity classification?

Key findings

• DES J0111−1341 has a half-light radius of 4.55 pc and an absolute V-band magnitude of +0.3, consistent with an ultrafaint stellar cluster.
• DES J0225+0304 has a half-light radius of 18.55 pc and an absolute V-band magnitude of -1.1, placing it in the ambiguous size-luminosity regime between clusters and dwarf galaxies.
• Both candidates are located at a heliocentric distance of approximately 25 kpc, with DES J0111−1341 at 26.5 kpc and DES J0225+0304 at 23.8 kpc.
• The candidates lie at distances of 1.73 kpc and 0.50 kpc from the Sagittarius stream's orbital plane, respectively, suggesting possible tidal association.
• The Sagittarius stream in the DES footprint exhibits a metallicity spread from [Fe/H] ≈ -2.18 to -0.95 and a distance gradient from 23 to 29 kpc, indicating multiple stellar populations.
• If confirmed via spectroscopic follow-up, DES J0111−1341 would be the first ultrafaint stellar system associated with the Sagittarius stream, and DES J0225+0304 would be the first confirmed ultrafaint satellite of a satellite.