Tuesday, July 20, 2021

Flows in the sky and rivers of stars

Those that approach the Milky Way look like skeins of wool.  They're really long streams of stars, like time boxes, that hold clues about our galaxy's history and what's inside and outside, like the infamous dark matter.
 Star trails are nothing more than associations of stars grouped into elongated filaments that wrap around a host galaxy.  These filaments are produced when a globular storage or satellite dwarf galaxy, called flux progenitors, are stressed and cracked by the tidal forces of the host galaxy.  Stars are carried in streams, like sheep by a celestial shepherd, in tidal streams that later tend to orbit around the host galaxy.

 In addition to our galaxy, there are stellar fluxes in other galaxies such as NGC 5907, although the Milky Way walks over 60 cataloged fluxes.  Just think, of these stellar bands, only a very few are linked to a parent known as a surviving globular cluster, while the others are still unknown.

 In a recent study by Ana Bonaca (Center for Astrophysics | Harvard & Smithsonian), a team of scientists took advantage of the Gaia space observatory to investigate the origins of 23 cold stellar fluxes in the Milky Way's halone, whose results showed that only one of the rivers of stars plausibly came from a globular cluster born in the Milky Way.  Instead, the vast majority came from dwarf galaxies and globular clusters that found our galaxy in the river of time.  What was found is that the 23 streams have similar properties, and likely come from the same parents.

Understanding the origin of these stellar fluxes will allow us to better trace their paths, even taking into account the gravitational interactions they were subjected to, not only to understand the evolution of galaxies, but also to map the general distribution of dark matter in our galaxy to the various Stairs.

 Figure 1: NGC 5907, and the faint streams of stars revolving around it.  Credits: Jay GaBany
 Figure 2: Orbital energy compared to angular momentum of stars in 23 of the Milky Way's stellar fluxes (color data), compared to field stars (black data).  Bonaca et al.
 Figure 3: Sky map showing the 6 globular clusters (crosses) that the authors associate with 8 stellar flows (circles).  Adapted by Bonaca et al.

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