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The Hercules Satellite A Galactic Transitional Fossil

by Jon Voisey on September 12, 2010

Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated

structures consisting of stars, which are known as tidal streams, as shown in this artist

impression. Credit: Jon Lomberg

On Friday, I wrote about thepopulation of the thick diskand how surveys are revealin

that this portion of ourgalaxy is largely made ofstars stolen from cannibalized dwargalaxies. This fits in well with many other pieces of evidence to build up the general

picture ofgalactic formation that suggests galaxies form through the combination of

many small additions as opposed to a single, gigantic collapse. While many streams o

what is, presumably, tidally shredded galaxies span the outskirts ofthe Milky Way, an

other objects exist that are still fully formed galaxies, few objects have yet been

identified as a satellite that is undergoing the process of tidal disruption.

A new study, to be published in the October issue of the Astrophysical Journal suggest

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that the Hercules satellite galaxy may be one of the first of this intermediary forms

discovered.

In the past decade, numerous minor stellar systems have been discovered in the halo o

ourMilky Way galaxy. The properties of these systems have suggested to astronomer

that they are faint galaxies in their own right. Although many have elongated and

elliptical shapes (averaging an ellipticity of 0.47; 0.15 higher than that of brighter

dwarf galaxies that orbit further out), simulations have suggested that even these

stretched dwarfs are still able to remain largely cohesive. In general, the galaxy will

remain intact until it is stretched to an ellipticity of 0.7. At this point, a minor galaxy

will lose ~90% of its member stars and dissolve into a stellar stream.

In 2008, Munoz et al. reported the first Milky Way satellite that was clearly over this

limit. The Ursa MajorI satellite was shown to have an ellipticity of 0.8. Munoz

suggested that this, as well as the Hercules and Ursa Major II dwarfs were undergoin

tidal break up.

The new paper, by Nicolas Martin and Shoko Jin, further analyzes this proposition fothe Hercules satellite by going further and examining the orbital characteristics to

ensure that their passage would continue to distort the galaxy sufficiently. The system

already contains an ellipticity of 0.68, which puts it just under the theoretical limit.

The team looked to see just how closely the satellite would pass to our own galactic

center. The closer it passed, the more disruption it would feel. By projecting the orbit

they estimated the galaxy would come within ~6 kiloparsecs of the galactic center

which is about 40% of the radius of the galaxy overall. While this may not seem

especially close Martin and Jin report that they cannot conclude that it will beinsufficient. They state that disruption would be dependent on the properties of the

stellar system at that time of its journey in the Milky Way potential and, as such, out o

reach to the current observer.

However, there were some telling signs that the dwarf may already be shedding stars

Along the major axis of the galaxy, deep imaging has revealed a smallernumber of

stars that does not appear to be bound to the galaxy itself. Photometry of these stars ha

shown that their distribution on a color-magnitude diagram is strikingly similar to tha

of the Hercules galaxy itself.At this point, we cannot fully determine if the Hercules galaxy is doomed to become

another stellar stream around the Milky Way, but if it is not truly in the process of

breaking up, it seems to be on the very edge.

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