Berkeley undergraduate proposes three isolated, quenched dwarf galaxies as backsplash candidates

A dwarf galaxy that is both isolated and quenched is extremely rare, according to the phys.org account ^[2]. Dwarfs out in low-density space usually keep the cold gas they need to make stars. The ones that have stopped are usually found in crowded regions or beside a large host, where hydrodynamic and gravitational forces siphon the gas away ^[6]. "In isolation, these galaxies are expected to be actively forming stars, as there are no larger galaxies to strip their star-forming gas, but the galaxies I discovered break this rule," Shapiro said ^[4].

Backsplash explains the pairing by putting the stripping in the past. "My paper argues that they may be among the first resolved 'backsplash' galaxy candidates, or galaxies that once passed close to a large neighbor, whose pressure removed their gas, before being flung outward," he said ^[5].

I like the design. Shapiro started with simulations, working out where backsplash dwarfs should sit around a Milky Way-like galaxy and where existing telescope archives could detect them ^[10]. Only then did he search, so the three galaxies also test a prediction about where such objects should be found. The account does not say how many dwarfs the search examined, or how many isolated ones in the same volume are still forming stars.

Isolation is a measurement in its own right. Much of the research went into estimating distances to establish it ^[12]. If a dwarf turns out to sit closer to a large galaxy than estimated, the ordinary explanation for its quenching comes back ^[6]. The account already allows that reading for one of the three ^[8], which leaves two without a named satellite host ^[14]. Other causes of their quenched state also remain to be ruled out, by Shapiro's own account ^[19].

Phys.org's write-up goes further than the candidates do. It says the findings show the standard model "likely holds true even in milder cosmic neighborhoods" ^[15]. It also says the paper lends credence to the idea that external forces must be what strips isolated, quenched dwarfs ^[17]. I think three objects whose distances are still being refined do not yet support those statements.

A population count is the stronger test. The paper proposes new methods for predicting how often backsplash galaxies occur ^[11], and Shapiro expects the Vera C. Rubin Observatory to reveal many more like his three ^[18]. A predicted frequency set against a counted one would check the abundance the standard model predicts ^[9].

Shapiro worked from archival telescope data ^[10] and from the published literature. "I was fortunate that many of the leading journals in the field are free to access, which helped me gain an understanding of the necessary techniques and background," he said ^[13].