Three quiet dwarf galaxies near M101 offer a test of a long-predicted journey
Julian Shapiro has identified three faint galaxies that may have lost their star-forming gas near a larger neighbour before moving outward. Their distances and histories remain uncertain.
Julian Shapiro, now an undergraduate at the University of California, Berkeley, has reported three faint dwarf-galaxy candidates near the spiral galaxy M101 in a study highlighted on October 7. The galaxies appear to have stopped forming stars despite their apparent separation from a large host. They may offer a way to test whether some small galaxies lose their star-forming gas during a close encounter and then move back outward, but their distances and past paths have yet to be confirmed.
The study, accepted for publication in The Astrophysical Journal, names the objects Shapiro DG-I, DG-II and DG-III. Its central proposal is that at least some could be ‘backsplash’ galaxies: objects that passed through the surroundings of a larger galaxy, were stripped of gas and subsequently travelled beyond its main halo. Simulations predict such objects, Shapiro writes, but definitive nearby examples have proved difficult to identify.
Why three galaxies near M101 stand out
Small galaxies found away from larger neighbours are generally expected to retain gas and keep producing stars. Galaxies that have stopped doing so are more commonly associated with denser environments, where interactions can remove that gas. The combination of apparent isolation and quiet stellar populations is therefore the puzzle posed by these three candidates, according to the UC Berkeley account of the work.
The objects are faint and only partly resolved in the available images. Shapiro reports absolute visual magnitudes of roughly −7.3 to −8.1 and half-light radii of about 110 to 140 parsecs. The paper identifies DG-I and DG-II in archival Hyper Suprime-Cam images and DG-III in the Hyper Suprime-Cam Legacy Archive. Simulations helped guide the search toward places where a backsplash population might be detectable.
Shapiro DG-I is also catalogued as MAGE1412+5650; the study says it was concurrently found by another survey. That detail matters when counting discoveries: the paper reports three candidate galaxies, while acknowledging the separate identification of one of them. The objects’ value lies in the unusual combination of their apparent locations and low star formation, rather than a claim that all three have already been proved to share the same history.
What the observations can establish so far
The paper uses provisional distance estimates from archival images, alongside modelling and the absence of detected ultraviolet emission in GALEX data, to assess the candidates. Those measurements are consistent with quiet stellar populations and a possible backsplash explanation. They do not trace an orbit through time or show directly that M101 stripped gas from any one of the galaxies. Shapiro describes DG-I and DG-III as the stronger isolated backsplash candidates.
DG-II has a specific competing explanation. It lies close enough to the spiral galaxy NGC 5585 in projection that it could be a faint satellite of that galaxy, rather than an object thrown outward after an encounter with M101. The uncertainty is consequential: knowing where a faint object appears on the sky does not, by itself, settle its three-dimensional distance from either larger galaxy.
In the UC Berkeley-supplied report, Shapiro said an encounter with a large neighbour could have stripped the galaxies of star-forming gas before they moved outward. He identified M101 as the principal proposed host, while recognising NGC 5585 as a possible explanation for DG-II. His wording presents a mechanism to investigate, not an observed account of the galaxies’ past movements.
Distance measurements are the next test
The study says the existing distance estimates are limited by the depth of survey images. Observations with the Hubble or James Webb Space Telescope could provide stronger distances and help distinguish a backsplash origin from other explanations, including stripping associated with the cosmic web. DG-III especially needs a better distance constraint. Until those measurements are available, the designation ‘candidate’ remains central to the finding.
Shapiro also examines how satellite numbers around nearby galaxies relate to the masses of their surrounding haloes. He reports a possible correlation after accounting for indicators of environmental density and says a larger sample is needed. M101’s sparse satellite population fits the relation examined in the paper and is consistent with expectations from the standard Lambda-CDM cosmological model. That population-level comparison is separate from establishing the orbital history of DG-I, DG-II or DG-III.
The paper points to the Vera C. Rubin Observatory as a future source of more faint candidates around galaxies like M101. A larger set could make the predicted backsplash population easier to compare with observations. For these three objects, the immediate question is narrower: whether better distances place them where the proposed journey requires, or favour another account of why they appear to have stopped making stars.
Sources and context
- Undergraduate student may have solved puzzle regarding isolated galaxies that have stopped producing starsPhys.org / Science X; story provided by University of California, Berkeley
- Discovery of Isolated, Quenched Candidate Backsplash Dwarf Galaxies near M101arXiv; Julian Shapiro, original study accepted to The Astrophysical Journal
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