Discovery and Spectroscopic Confirmation of Aquarius III: A Low-Mass Milky Way Satellite Galaxy

Core Concepts

Aquarius III is a newly discovered low-luminosity, metal-poor, and kinematically cold stellar system located in the outer halo of the Milky Way, likely on first infall.

Abstract

The authors present the discovery and detailed characterization of Aquarius III, a new ultra-faint Milky Way satellite galaxy identified in the DECam Local Volume Exploration (DELVE) survey.

Through deeper follow-up imaging with DECam, the authors find that Aquarius III is a low-luminosity (MV = -2.5+0.3-0.5 mag), extended (r1/2 = 41+9-8 pc) stellar system located at a heliocentric distance of 85 ± 4 kpc. From Keck/DEIMOS spectroscopy, the authors identify 11 member stars and measure a mean heliocentric radial velocity of -13.1+1.0-0.9 km/s for the system. They place an upper limit of σv < 3.5 km/s (σv < 1.6 km/s) on its velocity dispersion at the 95% (68%) credible level.

Based on Calcium-Triplet-based metallicities of the six brightest red giant members, the authors find that Aquarius III is very metal-poor ([Fe/H] = -2.61 ± 0.21) with a statistically-significant metallicity spread (σ[Fe/H] = 0.46+0.26-0.14 dex). They interpret this metallicity spread as strong evidence that the system is a dwarf galaxy rather than a star cluster.

Combining the velocity measurement with Gaia proper motions, the authors find that Aquarius III is currently situated near its orbital pericenter in the outer halo (rperi = 78±7 kpc) and is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, making Aquarius III a potentially useful laboratory for probing galaxy formation physics in low-mass halos if further velocity measurements confirm its velocity dispersion is truly below σv ≲2 km/s.

Stats

Aquarius III has a heliocentric distance of 85 ± 4 kpc.
The system has a mean heliocentric radial velocity of -13.1+1.0-0.9 km/s.
Aquarius III has an upper limit on its velocity dispersion of σv < 3.5 km/s (σv < 1.6 km/s) at the 95% (68%) credible level.
The system is very metal-poor with [Fe/H] = -2.61 ± 0.21 and a metallicity spread of σ[Fe/H] = 0.46+0.26-0.14 dex.
Aquarius III has an orbital pericenter of rperi = 78±7 kpc.

Quotes

"Aquarius III is a low-luminosity (MV = -2.5+0.3-0.5, LV = 850+380-260 L⊙), extended (r1/2 = 41+9-8 pc) stellar system located at a heliocentric distance of 85 ± 4 kpc."
"From Keck/DEIMOS spectroscopy, the authors identify 11 member stars and measure a mean heliocentric radial velocity of -13.1+1.0-0.9 km/s for the system and place an upper limit of σv < 3.5 km/s (σv < 1.6 km/s) on its velocity dispersion at the 95% (68%) credible level."
"Based on Calcium-Triplet-based metallicities of the six brightest red giant members, the authors find that Aquarius III is very metal-poor ([Fe/H] = -2.61 ± 0.21) with a statistically-significant metallicity spread (σ[Fe/H] = 0.46+0.26-0.14 dex)."

Key Insights Distilled From

Discovery and Spectroscopic Confirmation of Aquarius III: A Low-Mass Milky Way Satellite Galaxy

by W. C... at arxiv.org 10-03-2024

https://arxiv.org/pdf/2410.00981.pdf

Deeper Inquiries

How does the orbital history and low velocity dispersion of Aquarius III compare to other ultra-faint dwarf galaxies, and what implications might this have for our understanding of galaxy formation in low-mass halos?

Aquarius III exhibits a unique orbital history characterized by its current position near its orbital pericenter at approximately 78 kpc from the Milky Way, suggesting it may be on its first infall into the Galactic halo. This is in contrast to many other ultra-faint dwarf galaxies, which often show signs of significant tidal disruption due to closer proximity to the Galactic disk and higher velocity dispersions. For instance, galaxies like Ursa Major II and Coma Berenices have higher velocity dispersions and are located at different orbital phases, indicating a more complex interaction with the Milky Way's gravitational field.
The low velocity dispersion of Aquarius III, measured at less than 3.5 km/s, further distinguishes it from other ultra-faint dwarf galaxies, which typically exhibit higher dispersions. This low dispersion suggests that Aquarius III has retained a more coherent stellar structure, likely due to its relatively recent accretion into the Milky Way's gravitational influence. The implications of these findings are significant for our understanding of galaxy formation in low-mass halos. They suggest that such systems may not have undergone extensive tidal stripping, allowing them to preserve their original stellar populations and chemical compositions. This preservation can provide valuable insights into the early formation processes of dwarf galaxies and the role of dark matter in shaping their evolution.

What additional observations, such as detailed chemical abundances or proper motion measurements, could help further constrain the nature and evolutionary history of Aquarius III?

To deepen our understanding of Aquarius III's nature and evolutionary history, several additional observations would be beneficial. First, detailed chemical abundance studies of its stellar population, particularly focusing on elements beyond iron (such as alpha elements and neutron-capture elements), could provide insights into the star formation history and the nucleosynthesis processes that occurred within the galaxy. Such measurements would help determine whether Aquarius III experienced a prolonged period of star formation or if it formed its stars in a more rapid, burst-like manner.
Second, precise proper motion measurements from future Gaia data releases could help elucidate the dynamical state of Aquarius III. By comparing its proper motion with that of other Milky Way satellites, we could better understand its orbital dynamics and interactions with the Milky Way's gravitational field. This information would be crucial for modeling its past trajectories and assessing the likelihood of future interactions or disruptions.
Lastly, obtaining a larger sample of member stars through further spectroscopic observations would enhance the statistical significance of the findings related to its velocity dispersion and metallicity distribution. This would allow for a more robust comparison with other ultra-faint dwarf galaxies, ultimately contributing to a clearer picture of the formation and evolution of such systems in the context of the broader galaxy formation framework.

Given the growing census of ultra-faint dwarf galaxies around the Milky Way, how might the properties of this population as a whole inform our broader models of structure formation and the nature of dark matter?

The growing census of ultra-faint dwarf galaxies around the Milky Way provides a critical dataset for testing and refining our models of structure formation and the nature of dark matter. These galaxies serve as observational probes of the small-scale structure of the universe, allowing us to investigate the predictions of the Lambda Cold Dark Matter (ΛCDM) paradigm.
The properties of ultra-faint dwarf galaxies, such as their luminosities, velocity dispersions, and metallicity distributions, can inform us about the mass function of dark matter halos and the efficiency of star formation in low-mass environments. For instance, the observed abundance of ultra-faint dwarfs compared to the predictions from simulations suggests that there may be a significant population of dark matter subhalos that do not host luminous galaxies, challenging our understanding of galaxy formation efficiency.
Moreover, the diversity in the properties of these dwarf galaxies, including their orbital histories and chemical compositions, can shed light on the processes of tidal stripping and merging in the context of hierarchical structure formation. By analyzing the relationships between these properties, we can better constrain the parameters of dark matter models, such as its nature (e.g., whether it is cold, warm, or self-interacting) and its role in galaxy formation.
In summary, the ultra-faint dwarf galaxy population not only enriches our understanding of the Milky Way's satellite system but also serves as a vital tool for probing the fundamental aspects of cosmology and the nature of dark matter, ultimately enhancing our models of structure formation in the universe.