Keck/ESO/HEIC: Rare Fossil Relic of Early Milky Way Discovered

[bystander]

Rare Fossil Relic of Early Milky Way Discovered
W.M. Keck Observatory | 2016 Sep 07

[img3="The Unusual Cluster Terzan 5 - Credit: NASA/ESA/Hubble/F. Ferraro"]https://cdn.spacetelescope.org/archives ... c1617a.jpg[/img3][hr][/hr]

A fossilized remnant of the early Milky Way harboring stars of hugely different ages has been discovered by an international team of astronomers. This stellar system located in the Galactic Bulge has the appearance of a globular cluster, but it is like no other cluster known. It contains stars remarkably similar to the most ancient stars in the Milky Way but also a significant population of young stars, thus bridging the gap in understanding between our galaxy's past and its present. The research presents a possible route for astronomers to unravel the mysteries of galaxy formation, and offers an unrivaled view into the complicated history of the Milky Way. The findings are being published in The Astrophysical Journal today.

The system, called Terzan 5, has been classified as a globular cluster since its discovery 40 years ago. Now, an Italian-led team of astronomers has discovered that Terzan 5, which is 19 000 light-years from Earth, is like no other globular cluster known.

To make the discovery, the team scoured data from the Advanced Camera for Surveys and Wide Field Camera 3 on board Hubble; the second generation Near Infrared Camera (NIRC2) on the W. M. Keck Observatory located on Maunakea, Hawaii; and the Multi-conjugate Adaptive Optics Demonstrator (MAD) at ESO’s Very Large Telescope in Chile. The team found compelling evidence there are two distinct kinds of stars in Terzan 5, which not only vary in the elements they contain, but have an age-gap of roughly 7 billion years. ...

Astronomers Discover Rare Fossil Relic of Early Milky Way
ESO Science Release | 2016 Sep 07

Hubble Discovers Rare Fossil Relic of Early Milky Way
ESA Hubble Science Release | 2016 Sep 07

The Age of the Young Bulge-like Population in the Stellar System Terzan 5:
Linking the Galactic Bulge to the High-z Universe - F. R. Ferraro et al

• Astrophysical Journal 828(2):75 (10 Sep 2016) DOI: 10.3847/0004-637X/828/2/75
  arXiv.org > astro-ph > arXiv:1609.01515 > 06 Sep 2016

[Ann]

That's so weird!

W.M. Keck Observatory wrote:This stellar system located in the Galactic Bulge has the appearance of a globular cluster, but it is like no other cluster known. It contains stars remarkably similar to the most ancient stars in the Milky Way but also a significant population of young stars, thus bridging the gap in understanding between our galaxy's past and its present.

Click to view full size image

Globular cluster M13. Photo: Hubble.

The best way to see if a globular cluster is really old is to see if it contains blue horizontal stars. This particular evolutionary stage only occurs in extremely metal-poor stars. The blue horizontal stars are typically numerous in the globulars where they are found, but they are also definitely fainter than the brightest red stars in the globulars.

Take a look at globular cluster M13 at left. The blue stars are very obvious and quite numerous, but they are definitely fainter than the bright red giants and asymtotic giant branch stars.

Terzan 5 is totally different. Check out this 758 KB image of it, or even this 4.3 MB image of it. There are two really blue stars at left, whihc are obviously foreground objects, but the other blue stars, mostly in the right part of the cluster, might well belong to it. But these stars are not particularly numerous at all. On the other hand, they are bright, in most cases as bright as the red giants, or even brighter.

That's so weird!!!

Could the all the bright blue stars simply be foreground objects and blue-looking because they are much less reddened than Terzan 5? If not, Terzan 5 is a complete weirdo!!!

Ann

[rstevenson]

I'm seeing a lot of blue stars in Terzan 5, Ann. I think the large bright blue ones may be confusing the issue. Perhaps they are just foreground stars.

Here's the center of Terzan 5, which I cropped out to exclude any of those bright blue stars. I greatly enhanced its saturation to make the colour differences clearer. Lots of blue, I think.

Rob

[bystander]

[url=http://www.eso.org/public/images/eso1630a/][b][i]The Unusual Cluster Terzan 5 - Credit: ESO/F. Ferraro[/i][/b][/url]

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Peering through the thick dust clouds of the galactic bulge an international team of astronomers has revealed the unusual mix of stars in the stellar cluster known as Terzan 5. The new results indicate that Terzan 5 is in fact one of the bulge's primordial building blocks, most likely the relic of the very early days of the Milky Way.

This picture is from the Multi-Conjugate Adaptive Optics Demonstrator (MAD), a prototype adaptive optics system used to demonstrate the feasibility of different techniques in the framework of the E-ELT and the second generation VLT Instruments. The star colours are from the Hubble image of the same star field.

[Ann]

I'm seeing a lot of blue stars in Terzan 5, Ann. I think the large bright blue ones may be confusing the issue. Perhaps they are just foreground stars.

Here's the center of Terzan 5, which I cropped out to exclude any of those bright blue stars. I greatly enhanced its saturation to make the colour differences clearer. Lots of blue, I think.

Terzan 5-detail, sat enhanced.jpg
Rob

Thanks, Rob!

I still think there are very few obviously blue stars in Terzan 5. There is a relatively bright one at about 4 o'clock. There is also a trio of relatively bright blue stars at about 7 o'clock.

There are also huge numbers of very small blue stars, but they don't make sense. They are too small and too numerous. I don't think they are blue at all, but rather the color balance is off. That wouldn't be surprising, since Terzan 5 is highly reddened.

If you check out the picture posted by bystander after your post, you'll see that there are hardly any blue stars there at all. That makes better sense to me. Globulars that are unsuffciently metal-poor to support blue horizontal stars also lack them. Such globulars certainly exist, and the second-brightest globular of the Milky Way, 47 Tuc, is a prime example.

Nevertheless, judging by that picture posted by bystander, there really are a few blue stars in Terzan 5, too many to be foreground or background stars. But also too few to be a "normal" globular cluster population of blue horizontal stars.

So some of the mystery remains. But maybe the answer has indeed already been stated,

It contains stars remarkably similar to the most ancient stars in the Milky Way but also a significant population of young stars...

That's it, probably. Now somebody just has to figure out how such a thing could have happened.

Ann