The core of our galaxy is represented by what we've never seen before

The core of our galaxy is represented by what we've never seen before

Using infrared data from HAWK-I, researchers first learned about the central core of the Milky Way, which helps them reconstruct the history of star formation in our galaxy.

The overall rate of star formation in our galaxy is low, and astronomers have long known it, but in the central core of the Milky Way, which corresponds to an area of about 1,300 light years around a supermassive black hole, the Arrow A* has been 10 times the average rate of star formation in the last 100 million years, and the core of our galaxy is as productive as the star flashes or as the hyperproductive galaxy of 10 billion years ago.

Using detailed infrared observations, scientists were able for the first time to look at many young stars in the galaxy core, showing that star formation began near the centre of the Milky Way and then developed outwards.

Galactic core stars and observational issues

When you look from the Earth, the stars that belong to the galaxy core are hidden behind a lot of dust, but the problem is easily solved by infrared, millimetre waves or radio observations. On these waves, light passes through the dust, allowing us to see the galaxy center.

The first problem is that the galaxy center is so full of stars that it is difficult to distinguish them. The exception is a few very bright giants that differ in their mass and can therefore be easily separated from their neighbors. Prior to the new analysis described here, astronomers found only about 10 per cent of the expected total star mass in the galaxy center, in two massive star formations and several isolated young stars. Where are all the other stars and what are their properties?

New HAWK-I Census of Infrared Stars

Francisco Nogeras-Lara, an independent Humboldt researcher in the Lisa Maitner Nadine Neumeyer group at the Max Planck Astronomy Institute, and their colleague Rainer Shedel at the Astrophysics Institute of Andalusia in Granada, Spain, found themselves in a unique position to find young stars missing from the Galactic Center.

Shedel is the main researcher of the central region of the Milky Way, covering a total area of 64,000 square light years around the galaxy centre, and Nogeras Lara managed the search.

To detect individual stars in a crowded region such as the galaxy core, a high resolution is required. VLT consists of telescopes with 8-metre mirrors. Using a method known as a holographic survey, which combines several images with short exposures to mitigate the erosion effects of the Earth ' s atmosphere, the study was able to draw up the most detailed map of the target region with a resolution of 0.2 angular seconds. When only a few stars were on the map earlier, the GALACTICNUCEUS provided individual data for 3 million stars.

There are different stages of star formation in the galaxy core.

In studying the images of GALACTICNUCLEUS, researchers immediately noticed that the area of the galaxy core known as the B1 Arrower was different, containing far more young stars that ionized the surrounding gas than in other regions. Thanks to new observations by Nogeras-Lara and his colleagues, they were able for the first time to study in detail the stars of the B1 Arrower.

In particular, since all the B1 stars are approximately the same distance from the Earth, and the distance between the Earth and the Galactic Center is known, astronomers have been able to restore the luminance of each star. This is the internal luminous intensity that corresponds to the number of light emitted by the star per unit of time. This has enabled them to study the statistical distribution of the star luminance for these stars, i.e. to calculate how many stars there were in each range of luminous intensity. By analysing these data, Nogeras-Lara, Shedel and their colleagues found that there are several different phases of star formation in the B1 Arrow:

  • The older population, formed about 7 billion years ago, can indicate that star formation in the central core began in the interior region and then spread to the outer regions. For other galaxies, this mechanism for the formation of the so-called nuclear disc, a small disk of stars surrounding the galaxy centre, has already been observed. There is a large population of much younger stars, which are only 10 million years old or even less, with a total mass of more than 400,000 solar masses. This is almost ten times the total mass of two previously known massive star formations in the central region.

Reconstruction of Star History in the Milky Way

Many of the stars discovered by researchers at the B1 Arrow are isolated and not part of a massive cluster. This suggests that they have emerged from one or more star associations, less limited by mutual gravity, which then quickly split around a galaxy centre for several million years, leaving many separate stars behind. Although this result is primarily directed at the B1 Arrower, it can also explain more generally why young stars in the center of the galaxy can only be detected by high-resolution research, such as real work: they have emerged in free associations, which then dispersed into separate stars.

However, in order to reconstruct the history of star education and the overall evolution of the galaxy core, astronomers seek to place their findings on a more solid basis than the infrared observations of HAWK-I. Nogeras-Lara and his colleagues plan to continue their observations using the high-precision spectrograph of KMOS installed on VLT. Spectrum observations will allow astronomers to identify directly some of the very young stars by their spectrum appearance. This would be an important cross-check of the results published now.