Yasone-3: Compact Milky Way Satellite

• Yasone-3 is a compact Milky Way satellite defined by an old (≈12 Gyr) and metal-poor ([Fe/H] ≈ -2.0) stellar population.
• The detection pipeline employed a KDTree search, color-magnitude filtering via MIST isochrones, and Gaia DR3 astrometry for kinematic confirmation.
• Photometry and deep imaging reveal a half-light radius of ~2.09 pc, absolute magnitude of +2.52, and stellar mass of ~14.4 M☉, marking it as the faintest in its sample.

Searching arXiv for the specified paper and closely related context. {"^^^^^^^^2query2 Yasone catalogue: three new Milky Way satellites and ^^^^2ti:\2^^^^7 further hypercompact candidates\"","max_results":5,"sort_by":"submittedDate","sort_order":"descending"} {"^^^^^^^^2query2 catalogue hypercompact candidates Milky Way satellites","max_results":^^^^2ti:\2query2^^^^,"sort_by":"relevance","sort_order":"descending"} Yasone-3 is a compact stellar overdensity identified in the Pan-STARRS^^^^2ti:\2^^^^ DR2 catalogue and reported as one of three new low-latitude Milky Way satellites in "The Yasone catalogue: three new Milky Way satellites and ^^^^2ti:\2^^^^7 further hypercompact candidates" (&&&^^^^2query2^^^^&&&). It is characterized by structural and photometric properties consistent with an old, metal-poor population, with a best-fit age of PRESERVED_PLACEHOLDER_^^^^2query2^^^^, metallicity PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^, heliocentric distance $15^{+3}_{-5}\,\mathrm{kpc}$, physical half-light radius $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$, absolute magnitude $M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$, and stellar mass $14.4^{+11.2}_{-5.2}\,M_\odot$ (&&&^^^^2query2^^^^&&&). Within the Yasone sample, it is the faintest and lowest-mass of the three confirmed systems.

^^^^2ti:\2^^^^. Discovery and detection pipeline

Yasone-3 was first flagged as a compact stellar overdensity in the PS^^^^2ti:\2^^^^ DR2 catalogue by means of a two-dimensional KDTree search (&&&^^^^2query2^^^^&&&). Every PS^^^^2ti:\2^^^^ star with $g < 21$ and morphological point-source flags was compared to its neighbours within a $15''$ radius, described as the projected size of a $\sim 1\,\mathrm{pc}$ cluster at $\simeq 50\,\mathrm{kpc}$. Significance was gauged by drawing 2^^^^2query2query2^^^^ mock circles of the same radius randomly in a PRESERVED_PLACEHOLDER_^^^^2ti:\2query2^^^^ background annulus around each candidate; an overdensity was retained only if it exceeded the local background mean by PRESERVED_PLACEHOLDER_^^^^2ti:\2ti:\2^^^^.

The search then imposed colour-magnitude-distance filtering. Each PRESERVED_PLACEHOLDER_^^^^2ti:\22^^^^ overdensity was required to have at least 5^^^^2query2^^^^% of its stars within PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^3 of a synthetic MESA-Isochrones (MIST) track in both PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^4 and PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^5, with age, PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^6, distance, and PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^7 treated as free parameters. This procedure tied the overdensity detection directly to a stellar-population model rather than to surface density alone.

Gaia DR3 cross-match provided kinematic confirmation. Bright-end members had to form a PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^8 clump in proper-motion space; for Yasone-3, two Gaia stars were found within a PRESERVED_PLACEHOLDER_^^^^2ti:\2^^^^9 radius and were distinct from the field. Finally, deep $15^{+3}_{-5}\,\mathrm{kpc}$^^^^2query2^^^^ imaging with the GTC/OSIRIS instrument extended the colour-magnitude diagram to the main sequence turn-off, enabling a robust structural measurement.

2. Structural definition and size scale

The structural characterization of Yasone-3 was derived from the GTC $15^{+3}_{-5}\,\mathrm{kpc}$^^^^2ti:\2^^^^-band image, in which the half-light radius $15^{+3}_{-5}\,\mathrm{kpc}$2 is defined as the radius enclosing half of the 5^^^^2ti:\2^^^^ putative member stars (&&&^^^^2query2^^^^&&&). The measured angular half-light radius is

$15^{+3}_{-5}\,\mathrm{kpc}$3

Using the heliocentric distance $15^{+3}_{-5}\,\mathrm{kpc}$4, the physical half-light radius is given by

$15^{+3}_{-5}\,\mathrm{kpc}$5

This places Yasone-3 in a regime of extreme compactness. The source paper explicitly notes that the Yasone systems have compactness $15^{+3}_{-5}\,\mathrm{kpc}$6 and extremely low stellar masses $15^{+3}_{-5}\,\mathrm{kpc}$7. For Yasone-3 specifically, the measured size is intermediate between Yasone-^^^^2ti:\2^^^^ and Yasone-2.

A plausible implication is that the observational challenge is not merely one of low luminosity but of separating a physically small stellar system from a crowded Galactic foreground. The use of deep imaging to reach the main sequence turn-off is therefore central to establishing the object’s internal coherence.

3. Photometric characterization and stellar population inference

The apparent magnitudes in the GTC/PS^^^^2ti:\2^^^^ $15^{+3}_{-5}\,\mathrm{kpc}$8 band of the brightest members yield

$15^{+3}_{-5}\,\mathrm{kpc}$9

for Yasone-3 (&&&^^^^2query2^^^^&&&). Transforming to $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$^^^^2query2^^^^ with the Tonry et al. (2^^^^2query2ti:\22^^^^) relation,

$2.09^{+0.70}_{-0.42}\,\mathrm{pc}$^^^^2ti:\2^^^^

The absolute magnitude then follows from the adopted distance:

$2.09^{+0.70}_{-0.42}\,\mathrm{pc}$2

Isochrone fitting was performed by maximizing the number of stars within $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$3 of the theoretical MIST locus in colour-magnitude space. The source gives an equivalent likelihood form for $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$4 stars,

$2.09^{+0.70}_{-0.42}\,\mathrm{pc}$5

where $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$6 is the orthogonal displacement of star $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$7 from the isochrone track in colour-magnitude space and $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$8 is its photometric error.

For Yasone-3, the best-fit parameters are an age of $2.09^{+0.70}_{-0.42}\,\mathrm{pc}$9 and $M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$^^^^2query2^^^^. No spectroscopy is yet available; metallicity is inferred photometrically by the isochrone that matches the red-giant and main-sequence loci. The distance uncertainty is set by the span of isochrone distance moduli that still trace the CMD within $M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$^^^^2ti:\2^^^^.

4. Luminosity and stellar-mass estimate

The mass estimate for Yasone-3 is derived from its total $M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$2-band luminosity and an adopted colour-based mass-to-light ratio (&&&^^^^2query2^^^^&&&). Using

$M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$3

with

$M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$4

the result is

$M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$5

Adopting the colour-based mass-to-light ratio in $M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$6 from Into & Portinari (2^^^^2query2ti:\2^^^^3),

$M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$7

which implies

$M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$8

The stellar mass is then

$M_V = +2.52^{+0.48}_{-0.62}\,\mathrm{mag}$9

The source identifies Yasone-3 as the least massive of the three confirmed Yasone systems. This mass scale is exceptionally small for an object considered in the context of Milky Way satellites, and it is one of the key reasons the paper treats classification as unresolved pending dynamical data.

5. Placement within the Yasone sample

Within the trio Yasone-^^^^2ti:\2^^^^, Yasone-2, and Yasone-3, the paper reports the following values for Yasone-3: distance $14.4^{+11.2}_{-5.2}\,M_\odot$^^^^2query2^^^^, half-light radius $14.4^{+11.2}_{-5.2}\,M_\odot$^^^^2ti:\2^^^^, absolute magnitude $14.4^{+11.2}_{-5.2}\,M_\odot$2, stellar mass $14.4^{+11.2}_{-5.2}\,M_\odot$3, and metallicity $14.4^{+11.2}_{-5.2}\,M_\odot$4 (&&&^^^^2query2^^^^&&&). By comparison with its siblings, Yasone-3 is the faintest, in the sense of having the highest $14.4^{+11.2}_{-5.2}\,M_\odot$5, and the lowest-mass, while its size is intermediate between the more compact Yasone-^^^^2ti:\2^^^^ and the larger Yasone-2.

All three systems host old ($14.4^{+11.2}_{-5.2}\,M_\odot$6), metal-poor populations at heliocentric distances of $14.4^{+11.2}_{-5.2}\,M_\odot$7--$14.4^{+11.2}_{-5.2}\,M_\odot$8 and reside at Galactic latitudes $14.4^{+11.2}_{-5.2}\,M_\odot$9, where foreground contamination is modest. The shared parameter space suggests that Yasone-3 is not an isolated anomaly within the catalogue, but part of a small population selected by a common overdensity-plus-isochrone-plus-astrometry workflow.

The source places the three objects at the boundary between ultra-compact globular clusters and the faintest ultra-faint dwarf galaxies, citing Ursa Major III as an example of the latter comparison class. This suggests that Yasone-3 is significant less for any single extreme observable than for its location in a sparsely populated region of size-luminosity-mass space.

6. Classification and open problems

The classification of Yasone-3 remains open (&&&^^^^2query2^^^^&&&). The paper states that, without spectroscopic velocity dispersions or a dynamical mass measurement, it is not yet possible to determine whether the object is better interpreted as an ultra-compact globular cluster, one of the faintest ultra-faint dwarf galaxies, or a related hypercompact system.

The source also discusses the hypercompact-cluster scenario, in which an intermediate-mass black hole of mass $g < 21$^^^^2query2^^^^ could retain a stellar cusp of a few $g < 21$^^^^2ti:\2^^^^ over Gyr timescales. For Yasone-3, no kinematic cusp is yet detected. Future high-resolution spectroscopy and deep imaging to measure the core radius are identified as the relevant follow-up observations, particularly to test for an inflated core with $g < 21$2, described as a putative IMBH signature.

A common misconception would be to regard the current photometric and astrometric evidence as sufficient for a definitive dynamical classification. The source does not make that claim. Instead, it presents Yasone-3 as a compact, old, metal-poor stellar system with well-constrained photometric and structural parameters, but with its physical nature still contingent on spectroscopic follow-up.