WHAT I REALLY WANT TO KNOW IS…
Did these stars once live in a globular?
Sky surveys are one of the most exciting fields of astronomy today. Specialised digital cameras and spectrographs fitted to wide-field telescopes are allowing us to learn about large numbers of stars, or galaxies, at a time.
I am a member of the team that is conducting one such census, the Apache Point Observatory Galactic Evolution Experiment (APOGEE). It is one of three being carried out as part of the Sloan Digital Sky Survey IV, using a 2.5m telescope in New Mexico.
APOGEE studies hundreds of thousands of stars in the Milky Way to find out what they are made of. It can observe 300 stars at once! What sets this experiment apart is that its spectrograph studies them in infrared light, and so is able to see deeper into the vast amount of dust in the disc and central bulge of our Galaxy. Dust extinguishes optical light very efficiently, but not so much infrared light.
The first survey by this instrument, APOGEE 1, collected data on 150,000 stars, determining their velocities and chemical makeup accurately. And in doing so, we made an unexpected discovery.
Nitrogen abundance
For the first time, it was possible to collect this data for over 5,000 stars within two kiloparsecs (about 6,500 light years) of the galactic centre. And we discovered that a sizeable number of these stars contained very high levels of nitrogen. Such high nitrogen abundance is known to be a characteristic of stars found in globular clusters, those ancient concentrated collections of stars found in the halo around the Milky Way.
The reason for the existence of these stars in the centre of our Galaxy has not been established yet. But the explanation that seems most likely is that these stars actually result from the destruction of an early population of globular clusters. What is particularly interesting is that, if you do the numbers based on our observations, we find that the Galaxy must have had something in the order of 10 times more globular clusters than the 150 or so that we see surviving today. Our discovery was quite serendipitous. We weren’t expecting it, though my previous research in the field of extragalactic astronomy, where I studied the cores of other galaxies, had indicated that it would be a possibility. In that early work, 10 years ago, I identified an abundance pattern in the cores of elliptical galaxies that was nitrogen enriched, and I suggested it was similar to what you see in globular clusters. And elliptical galaxies are due to mergers of galaxies, just as our Galaxy’s halo was formed by absorbing surrounding dwarf galaxies. The stars we identified have a velocity and spatial distribution that is indistinguishable from populations of other stars in the inner halo. That means they are not the result of a recent accretion process, but have been there for a long time. I suspect they originally existed in globular cluster systems belonging to dwarf galaxies that came together in the formation of the early halo. These clusters would have been drawn by gravity into the centre of our Galaxy and got disrupted by its tidal forces. We will follow up our findings by doing computer modelling to work out how this all happened. We will also carry out further observations using a clone of the APOGEE spectrograph fitted to the du Pont 2.5m telescope and also the European Southern Observatory’s Very Large Telescope, both in Chile. It will be a lot easier to study the inner Galaxy from there as it lies in the southern part of the sky. Studying our Galaxy helps us to learn more about galaxy formation in general because it is the only galaxy we can see in so much detail. It is also a way in which we can get a handle on our own origin.