The mystery of our ageing galaxy

In an attempt to resolve a stubborn stellar measurement problem – a Japanese team of astronomers have uncovered a tantalising mystery at the heart of our galaxy…just where are all the young stars? 

With a relatively small telescope in your backyard, you can find galaxies floating in the dark universe – although it isn’t easy to discern their shapes. With the larger telescopes being used by astronomers, it becomes clear the universe is full of galaxies with various shapes, spiral galaxies, elliptical galaxies, and so on. Understanding their formation and evolution is one of the key questions in modern astronomy. Since the Big Bang – approximately 13.8 billion years ago – galaxies formed and continued their evolution by forming stars and changing shapes. The first clue to understanding the evolution of a galaxy is its shape. For example, elliptical galaxies are mainly made of old stars, indicating the star formation in those galaxies happened in the early phase of the universe, while galaxies with irregular shapes tend to have ongoing star formation so that they are evolving rapidly.

The Milky Way galaxy is a spiral galaxy and, among hundreds of billions of stars, hosts the Sun. Each spiral galaxy has a disc of stars and interstellar gas and dust in which star formation continues, particularly within the spiral arms, through almost the entire history of the universe. You can observe the disc of the Milky Way galaxy with the naked eye if you go to the countryside where the night sky gets sufficiently dark. The Milky Way, the whity pale stream of light, is spread across the sky. Unlike other galaxies, we cannot see the shape of the Milky Way galaxy from outside, as such it hard to find out the detailed structure of the galaxy.

In the Milky Way galaxy, thousands of Cepheids are thought to exist and, once they are found and studied, will reveal the structure of the galaxy.

Astronomers try to determine distances of stars spread within the Milky Way galaxy in order to overcome this difficulty; placing each star in a three-dimensional map should illustrate the stellar distribution and eventually the shape of the galaxy. However, it is usually difficult to determine distances to stars. What we can usually measure is apparent brightness of a star, which depends on both the absolute brightness and the distance. Absolute brightness of stars range from over a million to less than 1% of the solar brightness, and – except in some special circumstances – it is usually hard to tell the absolute brightness of each star.

One group of stars where it is possible to measure distance is the Cepheids. These stars pulsate, expand and shrink, in a cyclic manner. Each Cepheid has a particular period, typically between 1 and 30 days, which depends on the absolute brightness. This is known as the period-luminosity relation. Because pulsation periods can be measured independently of distances, we can estimate absolute brightness of Cepheids, which can be used to determine their distances in combination with apparent brightness. Cepheids have indeed played important roles in measuring astronomical distances and the size of the universe, starting from the early twentieth century when Edwin Hubble in particular measured distances to Andromeda and other galaxies. In the Milky Way galaxy, thousands of Cepheids are thought to exist and, once they are found and studied, will reveal the structure of the galaxy. However, a vast amount of interstellar dust in the disc of the galaxy prevents observations of stars, and as such only about five hundred Cepheids have been identified within the Milky Way.

The interstellar dust causes extinction of stellar lights, especially within the disc section which is made up of dense dust clouds, meaning light can only travel a relatively short distance. The effect of the dust is larger in the optical wavelengths – the wavelengths mainly used for observing stars like Cepheids – while infrared light can travel significantly larger distances. This is why we started infrared observations searching for Cepheids in the Milky Way more than ten years ago.

Our telescope – the Infrared Survey Facility (IRSF) – is located in the Sutherland station of the South African Astronomical Observatory. This 1.4-meter telescope has an infrared camera, named SIRIUS, which can take three images in three different wave-bands, between 1.1 – 2.2 microns, simultaneously. In 2011, we discovered three Cepheids toward the center of the Milky Way. They belong to the rotating disc, the relatively small area with a diameter about 800 light years which surrounds the super-massive black hole sitting at the heart of the galaxy. This small disc is known to host stellar groups with a wide range of age – among this mix are the three Cepheids are relatively young, thought to be approximately 25 million years old among.

In the observation program which followed, we searched for Cepheids in a wider area covering 2.3 square degrees, about 10 times the area of the full moon, around the galaxy center. Based on the observation carried out from 2007 to 2012, our new study reports 29 classical Cepheids of which three were found in 2011. These new Cepheids are faint compared to other Cepheids known in the Milky Way, and thus are expected to be situated at large distances in the direction of the galaxy center. Caution is demanded, however, when we determine distances to stars behind the veil of interstellar dust even if they are Cepheids which exhibit the period-luminosity relation. In addition to the dilution effect of light due to the distance, extinction by interstellar dust decreases the stellar light reaching us and this should be taken into account when we compare the absolute and apparent brightness. To solve this problem, we utilised the relationship between extinction and wavelength. The stronger the extinction gets, the redder a star would appear because the extinction decreases with increasing wavelength. The Cepheids we discovered are rather red, which indicates that they are significantly affected by the interstellar dust extinction even in the infrared.

The lack of Cepheids indicates that the density of such young stars is much lower compared to the surrounding regions.

The interstellar extinction is notoriously difficult to handle in astronomical studies. For our purpose we need to convert the redness of a Cepheid into the amount of extinction – but the accurate form of the relationship between the two has proved to be elusive in spite of serious efforts by astronomers for decades. In the end, what enabled us to put a robust constraint on the interstellar extinction was the Cepheids located around the galaxy center. In addition to the three we reported in 2011, another very similar Cepheid was found in our new survey. Since they are affected by sizeable amounts of extinction, different assumptions on the wavelength dependency of extinction would lead to different estimates of distances. On the other hand, these four Cepheids are confirmed to be rotating within the small disc around the galaxy center based on spectroscopic measurements of velocity. There are converging measures of the distance of the galaxy center, around 26,000 light years, some of which are independent of the interstellar extinction. This allows us to pick up the wavelength dependency which gives distance estimates of the Cepheids consistent with the distance to the center. Once the extinction form is fixed toward this area of the sky, we can estimate distances to other Cepheids after the corrections of extinction.

The distribution of the Cepheids we found has a striking feature (Figure 1, Figure 2). Besides the four Cepheids concentrated to 800 light years around the galaxy center, there are no other Cepheids within roughly 8000 light years of the center. Our calculation shows that, in the center and outer part of the Milky Way disc, we should have detected more than 10 Cepheids. The lack of them suggests that there is a giant stellar void in the inner part of the Milky Way. It should be noted that this region of the void is not a blank gap. In the same region, there are clearly a large number of stars but they are considered to be very old, ~10 billion years or more. In contrast, Cepheids are representing groups of relatively young stars, 10 – 300 million years. The lack of Cepheids indicates that the density of such young stars is much lower compared to the surrounding regions. Correspondingly, star formation has been inactive in the recent times in this gap region; otherwise there would be groups of young stars and some of them should have been found as Cepheids. Our result gives the first evidence of the void of young stars based on distribution of individual stars around this region. Future larger infrared surveys are expected to achieve a more complete mapping of Cepheids, which would help us to draw the density variation more clearly.

[caption id="attachment_56823" align="alignnone" width="450"] Figure 1: Distribution of Cepheids we discovered, indicated by yellow points, and those previously known based on optical observations, indicated by white small dots.[/caption]

It is important to explain how such a gap of young stars and star formation appears in the inner part of the Milky Way. The key is to understand how and where gas gathers to form stars within the galaxy. Only when the gas density gets high enough, do stars start to form. For example, a small region around the galaxy center is the bottom of the gravitational potential and gas may well fall to generate stars, although there remains a question about where the gas actually came from to produce the young stars like the four Cepheids. On the other hand, spiral arms in the Milky Way disc also gather gas being used for star formation. The range of 800 – 8000 light years from the galaxy center is dominated by the spheroidal concentration of old stars. This spheroid is known to be elongated, and thus the structure is also called the bar; the resulting skewed gravitational potential affects the motion of gas. A model of the Milky Way should be built on such a bar-like gravitational potential to help explain how star formation is suppressed in the inner part. While astronomers today are approaching the edge of the universe, there remain mysteries around the center of our own galaxy at 26000 light years away behind the thick veil of interstellar dust.

[caption id="attachment_56824" align="alignnone" width="430"] Figure 2: An artist's impression of the implied distribution of young stars, represented here by Cepheids shown as blue stars plotted on the background of a drawing of the Milky Way. With the exception of a small clump in the Galactic centre, the central 8000 light years appear to have very few Cepheids, and hence very few young stars.[/caption]

Author: Professor Noriyuki Matsunaga is based at the Department of Astronomy, University of Tokyo. He studies the structure and evolution of the Milky Way galaxy as well as stellar evolution.