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Deciding Dwarf Galaxy or Globular Cluster

When I was young and learning about galaxies of course I was impressed how well we understood these distant objects having such diversity.
Over the years I realized astronomers are still learning and that initial perception of clear understanding was wrong.

Earlier this year a new object was found and there is a debate whether it is a globular cluster or a dwarf galaxy; in either case it is hiding behind the obstruction of the Milky Way central bulge.

The debate reveals our assumptions for these objects.

Astronomers have access to new data available with a variety of recent instruments as part of sky surveys gathering details for many objects with each scan. That recent data enabled this discovery.

In this story FSR 1758 is either a dwarf galaxy or a globular cluster. The researchers have named it Scorpius dwarf galaxy.

On a first impression those two objects should be easily distinguished. They are not because FSR 1758 is so obscured it is difficult to determine the correct locations of its stars in 3-D; this is required to analyze stars in the object rather than just in the line of sight or just seem to be around it.


Spatial distribution of stars that have common proper motion, suggesting that they belong to FSR 2758. Beyond the visible custer of stars at the center of the group, FSR 1758 appears to have a possible larger extended structure, suggesting it may be the nucleus of a dwarf galaxy.
FSR 1758 has a number of intriguing properties. If it’s a globular cluster, it’s one of the largest around our galaxy — and the part that we see is probably just the metaphorical tip of the iceberg, as much of its population is likely hidden by contamination and reddening due to its location in the galactic bulge. Furthermore, FSR 1758’s properties don’t fit known relationships for globular clusters, such as the correlation between size and metallicity.

Lastly, the authors find additional asymmetrically distributed stars further out in the field with motions and colors indicating that they also belong to FSR 1758. These suggest that the cluster may be more extended than originally thought and might have tidal tails. These signs support a picture in which FSR 1758 is the nucleus of a dwarf galaxy — which the authors tentatively name the Scorpius dwarf galaxy.
Though we still don’t have a definitive answer about FSR 1758’s nature, we can hope that future spectral data for its stars will settle the debate.

my comment:

Metallicity is critical to every analysis; it is the basis for our understanding stars. The story explicitly mentions this problem. "FSR 1758’s properties don’t fit known relationships for globular clusters, such as the correlation between size and metallicity."

From this statement, one could wonder whether the conclusion this object is a dwarf galaxy was based on the finding it is not compatible with our 'understanding' of a globular cluster(GC).

The story mentions FSR 1758 could be one of the largest GC near the Milky Way.

for reference from Wikipedia ===

Omega Centauri (or NGC 5139) is a globular cluster in the constellation of Centaurus located at a distance of 15,800 light-years, it is the largest globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars and a total mass equivalent to 4 million solar masses,making it the most massive globular cluster of the Milky Way.
Omega Centauri is so distinctive from the other galactic globular clusters that it is thought to have an alternative origin as the core remnant of a disrupted dwarf galaxy.

The stars in the core of Omega Centauri are so crowded that they are estimated to average only 0.1 light years away from each other. The internal dynamics have been analyzed using measurements of the radial velocities of 469 stars.The members of this cluster are orbiting the center of mass with a peak velocity dispersion of 7.9 km/s. The mass distribution inferred from the kinematics is slightly more extended than, though not strongly inconsistent with, the luminosity distribution.

my comment:
One must note Omega Centauri has uncertainty about its origin as a GC. It is also distinctive among the GC implying a lack of consistent  observations among them. One could wonder which is the prototype for other GC to be compared.

The density of stars in the core is both surprising and impressive.

continuing Omega Centauri excerpt with part 2 ===

Using instruments at the Gemini Observatory to measure the speed of stars swirling in the cluster's core, E. Noyola and colleagues found that stars closer to the core are moving faster than stars farther away. This measurement was interpreted to mean that unseen matter at the core is interacting gravitationally with nearby stars. By comparing these results with standard models, the astronomers concluded that the most likely cause was the gravitational pull of a dense, massive object such as a black hole. They calculated the object's mass at 40,000 solar masses.
== end 2

My comment:

One could expect this discovery will be compared to Omega Centauri. That GC could be unique or its conclusions wrong.

The reference to 'standard models' is problematic.

Black holes should never be mentioned with GC stars.

I posted on October 4 the topic "Globular Cluster Stars"
That post is relevant here.
That post was about a story claiming a black hole caused a star to zoom in NGC 3201 which is a GC.
GC stars move radially and so they move only by electromagnrtic forces not by gravity so a black hole is a silly explanation of any behavior in a GC.

One can only hope the status of FSR1758 will be done objectively, not distorted by assumptions which could be incorrect.

They claim future data will resolve the debate but historically many assumptions are rarely questioned.

An objective analysis is probably unlikely so the Milky Way probably gained a new dwarf galaxy.


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