Star formation in the Milky Way came in bursts

The formation of suns in the center of the Milky Way was not continuous

Two bursts of activity led to the formation of stars in the center of the Milky Way. New observations show that stellar births peaked about eight billion years ago; a second activity occurred about a billion years ago. Astronomers had previously assumed that the stars in the galaxy's comparatively small central disk had formed continuously. This new scenario affects theories about the origin and properties of the bar-shaped feature of the Milky Way's disk.

Image of the Milky Way’s central region with an angular resolution of 0.2 arcseconds, taken with the HAWK-I instrument on ESO’s Very Large Telescope for the stellar survey in the nuclear disk undertaken by Francisco Nogueras-Lara et al.

Astronomers have uncovered the previously unknown history of the stars near the center of our galaxy. Their work shows that the stars in question formed in two waves: more than 90 % of the stars formed at least eight billion years ago.  A second batch, responsible for around 5 % of the stars, formed in a short amount of time around one billion years ago. Between the two bursts of activity, there was a long quiescent period with hardly any new stars forming at all.

The stars in question belong to a dense, disk-shaped region with a diameter of about 1000 light-years (roughly 1 % of the diameter of the Milky Way’s majestic disk of stars), known as the nuclear disk. This disk surrounds the Milky Way’s innermost nuclear cluster of stars and its central supermassive black hole.

With their result of two intense episodes of star formation, the researchers have rewritten part of our home galaxy’s history: Previously, it had been assumed that the stars in the central region of the Milky Way had formed gradually over the past billions of years. The new timeline has consequences for a number of other astronomical phenomena.

Notably, it constrains the growth history of our galaxy’s central black hole. Gas flowing into the central regions of our galaxy drive both star formation and the increase in central black hole mass. The newly reconstructed star formation history indicates that our central black hole is likely to have reached most of its present mass earlier than eight billion years ago.

The brief, but intense burst of star formation activity one billion years ago is likely to be one of the most energetic events in the history of our galaxy. Hundreds of thousands of newly formed massive stars would have exploded as supernovae within millions of years.

The results also force astronomers to rethink another fundamental feature of our galaxy. The Milky Way is a barred spiral galaxy, with an elongated region estimated between 3,000 and 15,000 light-years long linking the inner ends of its two major spiral arms. Such bar structures are thought to be very efficient at funneling gas into a galaxy’s central region, which would lead to the formation of new stars.

The billions of years without star formation in the nuclear galactic disk forces astronomers to rethink this scenario. During those quiet years, gas was evidently not funneled into the Galactic center in sufficient amounts. Francisco Nogueras Lara (then Instituto de Astrofísica de Andalucía, now a post-doctoral researcher at MPIA), lead author of the article, says: “Either the galactic bar has come into existence only recently, or such bars are not as efficient in funneling gas as is commonly assumed. In the latter case, some event – like a close encounter with a dwarf galaxy – must have triggered the gas flow towards the Galactic center about one billion years ago.”

The reconstruction of the history of the nuclear Galactic disk makes use of some of the fundamental insights of astronomers into star formation. Stars only live for a certain span of time, which depends on their mass and chemical composition.

Whenever many stars have been born at the same time, which is a common occurrence, astronomers can look at the ensemble, plot the star’s brightness against the reddishness of their color (“colour-magnitude diagram”) and deduce how long ago the ensemble was formed. One among several age indicators is the “red clump” of stars that have already begun to fuse helium in their core regions. From the average brightness of stars in that clump, one can deduce the age of that group of stars.

But there is a catch: All those techniques require astronomers to study separate stars. For the Milky Way’s central regions, that is a highly challenging task. As seen from Earth, the Galactic center is hidden behind gigantic clouds of dust, requiring infrared observations to “look through” the clouds.

But then, such observations are bound to see too many stars in the Milky Way’s center! The Galactic center is very dense, with between a thousand and a hundred thousand stars in a cube with a side-length of one light-year. When astronomers observe very dense star fields of this kind, those stellar disks will overlap in the telescope image. Separating such fields into separate stars is difficult – but necessary if you want to reconstruct the formation history of the Galactic center.

Given those challenges, when Rainer Schödel (Instituto de Astrofísica de Andalucía, PI of the GALACTICNUCLEUS survey), MPIA’s Nadine Neumayer and their colleagues began planning to tackle the history of our Milky Way’s central region in late 2014, they knew they would have to find the right instrument for the job. As Neumayer explains: “We needed a near-infrared instrument with a large field of view, able to observe the Milky Way’s central region which is in the Southern Sky. ESO’s HAWK-I instrument was ideal for our survey.” HAWK-I is an infrared camera at the 8 meter Very Large Telescope at the Paranal Observatory of the European Southern Observatory (ESO) in Chile.

For their GALACTICNUCLEUS survey, the astronomers observed the Galactic center region with HAWK-I for 16 nights managing to obtain accurate photometry of more than three million stars. Using a special technique known as holographic imaging, the astronomers were able to distinguish between stars as little as 0.2 arc seconds apart. With this accuracy, you could distinguish two one-cent coins viewed from a distance of more than 8 kilometers. Two clearly visible “red clumps” in the resulting colour-magnitude diagram allowed for the reconstruction of the formation history of the Galactic nuclear disk.

As a next step, the astronomers are now studying the influence of dust on their observations (extinction and reddening). Taking into accounts the effect of dust should allow for even more precise reconstructions of the history of our galaxy’s central regions in the future.

 HOR / MP

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