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!

How our galaxy absorbs other galaxies

I want to take a break from talking about star clusters today to discuss something on a much larger scale: our galaxy! This is motivated by a visit yesterday from Dr Vasily Belokurov from Cambridge University who gave an excellent seminar on the size and structure of our galaxy from studies of how our galaxy grows.

Large galaxies like our Milky Way galaxy can grow by absorbing smaller dwarf galaxies. These dwarf galaxies are common in the Universe and when they get close to a massive galaxy like ours they are drawn towards it by gravity and begin to orbit the larger galaxy (they're often called satellite galaxies at this point).

When these small galaxies get really close to the large galaxy they begin to be disrupted by the gravitational force from the larger galaxy and can actually be torn to shreds, scattering the stars in the dwarf galaxy out into long tidal streams, as shown in the image below.

Tidal streams caused by orbiting satellite galaxies (Credit: David Law)

This process can take millions of years while the dwarf galaxy orbits and falls into the larger galaxy. This creates patterns of huge tidal streams emanating from these satellite dwarf galaxies and which encircle our own galaxy.

A famous example of this is the Sagittarius dwarf galaxy, which is about 82,000 light years from us and in the process of being stripped apart as it orbits the Milky Way. We can see this as a huge stream of stars that circles the sky known as the Sagittarius tidal stream. The image below shows this stream (and other streams) using data from the Sloan Digital Sky Survey - which we talked about in a previous post. The Milky Way will one day consume this galaxy entirely, absorbing all of its stars into the halo of our galaxy.

The Sagittarius Tidal Stream as seen in SDSS data (Credit: Vasily Belokurov)

Dr Belokurov studies tidal stream such as this to infer the large-scale structure of our galaxy. Because we are inside our galaxy it can be hard to determine it's full size and spatial extent, so this can be difficult work. This approach is kind of like inferring the structure of a city by tracing the motions of cars entering the city - even if you don't know where the buildings and places of interest are, you would be able to estimate where they are and how the city is structured by the motions of cars into and around the city. This is exactly what Dr Belokurov does, only by using the positions and motions of tidal streams he can infer the structure of the galaxy!

Talking about star clusters in Copenhagen

The absence of posts this week was because I've been taking a break from doing research at my desk and instead I've been at a conference to discuss research with other scientists studying star clusters. The conference was "The Early Life of Stellar Clusters: Formation and Dynamics" and was held in Copenhagen, Denmark, a really beautiful city.

Copenhagen, very pretty (albeit cold)

Conferences are important in science because they allow scientists distributed all over the world to come together and discuss the current state of research with other scientists. This can be particularly important if you work in a small field (such as astrophysics) where the world's experts are spread all around the world. There can be lots of debates and arguments, discussions with collaborators, as well as opportunities to start new collaborations with people you meet.

This meeting attracted about 50 people who all study different aspects of star clusters and who have come together to discuss them. Most of the attendees have given a presentation to the group, including myself (I gave my talk on the first day and was able to relax after that). I talked about my recent work on an OB association called Cygnus OB2 (more on that in a future post) and I think it was well received.

It was a great conference and I'm very grateful to the organisers for putting it together (and for giving me the opportunity to present my work!). The meeting has given me lots of ideas for future projects, has opened the door to future collaborations, and helped develop some existing projects. I'm looking forward to the next conference already!