Dwarf Galaxy or Big Globular Cluster?

February 18, 2019 - Comment

AAS Nova brings us the story of a newly found cluster of stars. However the jury’s out as as to whether this group is a typical historic stellar cluster or one thing extra. The lately found object FSR 1758 is the intense area pictured within the middle of this picture. Is that this object a


AAS Nova brings us the story of a newly found cluster of stars. However the jury’s out as as to whether this group is a typical historic stellar cluster or one thing extra.

Globular cluster or dwarf galaxy?

The lately found object FSR 1758 is the intense area pictured within the middle of this picture. Is that this object a globular cluster or a dwarf galaxy?
Barbá et al. 2019

You would possibly assume that we’d already found all the big clusters of stars orbiting our galaxy. Surprisingly, there are nonetheless detections to be made — such because the lately found cluster FSR 1758. However is this massive group of stars an unlimited globular cluster? Or a newly detected dwarf galaxy?

A New Cluster

Zooming In

A large-field view (2.5° x 2.5°; prime) and a zoomed-in view (18’ x 18’; backside) centered on FSR 1758 from the DECaPS survey.
Barbá et al. 2019

FSR 1758 was first found final yr, hiding within the extraordinarily dense bulge on the middle of our galaxy. Objects within the galactic bulge are very tough to detect: because of the excessive density of surrounding stars and mud, not a lot of the sunshine of bulge objects makes it to us.

Primarily based on the restricted observations we initially had of FSR 1758 — and since clusters residing within the galactic bulge are sometimes anticipated to be of low mass — it was assumed that this object was a globular cluster: a spherical assortment of stars that each one fashioned across the similar time from the identical cloud and are sure by their mutual gravity.

However may the issue peering into the galactic bulge imply that we’re lacking essential particulars? One other idea posits that the a part of FSR 1758 we’re seeing is as a substitute the nucleus of a faint dwarf galaxy orbiting the Milky Manner; we merely can’t see the fainter outer reaches of the galaxy.

So which is it: globular cluster or dwarf galaxy? A crew of scientists led by Rodolfo Barbá (College of La Serena, Chile) have gathered observations to seek out out.

Revealing Particulars

Barbá and collaborators use three completely different units of observations to discover FSR 1758: optical information from Gaia’s DR2 and the DECam Airplane Survey, and near-infrared information from the VISTA Variables within the Through Lactea Prolonged Survey. From these information, the authors decide the cluster’s place and distance, in addition to its measurement, metallicity, absolute magnitude, and correct movement.

Proper motions of stars

This plot reveals the spatial distribution of stars which have frequent correct movement, suggesting that they belong to FSR 2758. Past the seen cluster of stars on the middle of the group, FSR 1758 seems to have a potential bigger, prolonged construction, suggesting it could be the nucleus of a dwarf galaxy.
Barbá et al. 2019

FSR 1758 has numerous intriguing properties. If it’s a globular cluster, it’s one of many largest round our galaxy — and the half that we see might be simply the metaphorical tip of the iceberg, as a lot of its inhabitants is probably going hidden by contamination and reddening on account of its location within the galactic bulge. Moreover, FSR 1758’s properties don’t match recognized relationships for globular clusters, such because the correlation between measurement and metallicity.

Lastly, the authors discover further asymmetrically distributed stars additional out within the area with motions and colours indicating that in addition they belong to FSR 1758. These recommend that the cluster could also be extra prolonged than initially thought and may need tidal tails. These indicators help an image by which FSR 1758 is the nucleus of a dwarf galaxy — which the authors tentatively title the Scorpius dwarf galaxy.

Although we nonetheless don’t have a definitive reply about FSR 1758’s nature, we are able to hope that future spectral information for its stars will settle the talk. And within the meantime, it’s good to be reminded that our galaxy continues to be hiding some shocking discoveries.

Quotation
“A Sequoia within the Backyard: FSR 1758—Dwarf Galaxy or Big Globular Cluster?,” Rodolfo H. Barbá et al. 2019 Astrophysical Journal Letters 870 L24. doi:10.3847/2041-8213/aaf811

This publish initially appeared on AAS Nova, which options analysis highlights from the journals of the American Astronomical Society.



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