Surveys, surveys, surveys

I have been in Naples, Italy, this week for a conference about the many wonderful astronomical surveys that have been produced recently. The last decade has been a rich time for survey science, thanks in part to the pioneering surveys at the turn of the century (such as SDSS and 2MASS), but also to the work of new dedicated survey telescopes such as VST and VISTA. These large surveys have provided astronomers with huge samples of stars and galaxies with which to investigate many important aspects of how stars and galaxies form and evolve.

The European Southern Observatory's (ESO) facilities at Cerro Paranal,
including the VLT, VST and VISTA telescopes (Credit: Wikipedia)

It is with one of these telescopes, the VLT Survey Telescope (VST), that I have been using data from over the last few years for the VPHAS+ survey, the VLT Photometric H-Alpha Survey. This is the follow-up survey to IPHAS, the INT Photometric H-Alpha Survey, which I discussed in a previous post.

The VPHAS+ survey was actually the subject of the talk I gave at the conference today. I presented both an outline of the survey and some of the most exciting science results that have come out of the survey in recent years. It was quite an honour to present so much exciting science from across our survey collaboration, and I'm grateful to everyone in the survey who contributed to the work and helped me prepare the presentation. If you want to learn more you can watch my presentation on youTube here (my talk starts at 56:00).

This conference has been a great opportunity to learn both about surveys going on at the moment and some of the surveys planned for the future. As well as the traditional photometric surveys, there are now surveys designed to study how astronomical objects change over time by making repeat observations of the same areas of the sky. Some surveys are already doing excellent work in this area, such as the VVV survey (Vista Variables in the Via Lactae), but there are also some very exciting projects planned for the future, such as the Large Synoptic Survey Telescope (LSST).

Finally, there have been looks to the future of astronomical surveys and particularly the future spectroscopic surveys that will, instead of just measuring the brightness of sources in a few bands, actually take detailed spectra that can be used to determine the physical properties of the sources. The European Southern Observatory, ESO, has plans to convert one of its survey telescopes, VISTA, from being an imaging telescope to being a spectroscopic telescope. This is something that I really look forward to seeing and hopefully using. These are really exciting times for survey astronomy!

Searching for stars that run away

So it's observing proposal season and Wednesday was the ESO deadline, which means panicked proposal writing and last-minute scribbling all round.

ESO is the European Southern Observatory, Europe's premier observatory in the southern hemisphere, which operates some of the largest and most advanced telescopes in the world. This includes the four behemoths that make up the VLT, the Very Large Telescope (except there's four of them), each housing a 8.2m mirror and some of the best astronomical instruments in the world.

ESO's Very Large Telescope(s) in Chile (Credit: Wikimedia Commons)

One of the proposals we submitted is part of work by my colleague Mike Mohr-Smith to improve the census of massive stars in our galaxy and identify where they formed. Massive stars are very rare and live very short lives (on astronomical timescales at least), but affect the evolution of other stars and the galaxy as a whole in very important ways. How massive stars form is a major unanswered question in astronomy, and since they live such short lives finding these stars and tracing them back to their birth-sites is an important avenue of research.

So we've recently identified a number of hitherto-undiscovered massive stars in the vicinity of one of the young massive star clusters in our galaxy, Westerlund 2. The most massive stars we know of are nearly always found deep within star clusters, and some people have suggested that they can only form in such environments, so finding massive stars near a massive star cluster, but not within it, is very interesting.

The massive star cluster Westerlund 2 (Credit: Robert Gendler)

So the question arises, did these stars form outside of the cluster (which would make them very special) or did they form in the cluster but have since been ejected? The first step in answering this question is to measure the speed these stars are moving relative to the cluster. If the stars have been ejected (known as runaway stars) they should be moving very fast away from the cluster, but if they formed in isolation their velocities will be much lower.

By taking high resolution spectroscopy of these stars we can measure their speeds by observing the shifts in the positions of known spectral lines due to the Doppler effect. This is the same effect that causes the pitch of a siren to change as a vehicle moves towards you and then away from you, but instead of affecting sound waves it is shifting light waves.

Representation of the Doppler-shift effect on spectral lines
(Credit: University of Virginia)

This image shows a spectrum of light, which is light split into its constituent parts using a prism. What was originally white light has now been split into all the colours of the rainbow (it's actually the same effect that causes a rainbow!).

On top of the rainbow you can see dark lines, which are known as spectral lines. These are caused by atoms of different elements absorbing light at certain wavelengths (in certain parts of the spectrum). When an object is moving away from us, it's light is redshifted, meaning spectral lines shift towards the red part of the spectrum, while when an object is moving towards us it's light is blueshifted, meaning spectral lines shift towards the blue part of the spectrum. The faster an object is moving relative to us, the greater the shift in the position of the spectral lines. So by observing the spectrum of light from a star, measuring the positions of it's spectral lines and comparing them to the positions we know they should be at we can determine how fast the star is moving towards or away from us.

So our objective is to use one of the instruments on the VLT to acquire high-resolution spectroscopy, measure the positions of the spectral lines and therefore the speed the stars are moving relative to the stars in the cluster. With this information we can answer the question of whether these very massive stars formed inside the cluster or outside of the cluster.

That's our plan at least, and that's what we've written to the friendly people at ESO asking them if we can use their telescope to do this project. I'll let you know what they say!

Star formation across Carina and Sagittarius

A stunning image from the European Southern Observatory (ESO) released this week showing a wide-field view of star formation across the Carina-Sagittarius spiral arm.

The image is from the ESO's La Silla Observatory in Chile, which offers unparalleled views of the southern hemisphere's skies thanks to its high elevation in the Andes mountains and low levels of cloud coverage.

Star formation across Carina and Sagittarius (Credit: ESO)

The image shows two prominent star-forming regions: NGC 3603 (left) and NGC 3576 (right), both in the Carina-Sagittarius spiral arm of our galaxy. This spiral arm is the nearest major spiral arm to us as we look towards the centre of our galaxy. The spiral arm spans about a third of the night sky as we see it wrapped around the Galactic Centre, and contains many of the famous star forming regions and nebulae in the night sky.

Interestingly, this area of the spiral arm is the part where the arm curves away from us, causing long stretches of the arm to be superimposed along our sight-line. This means that NGC 3603 is about twice as far away from us as NGC 3576, despite the two regions appearing close to each other on the night sky and appearing as one extended region.