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!

The largest astronomical catalogs ever made! (Part II of II)

In a previous post I introduced the 'Top 10' largest astronomical catalogs that are currently and publicly available for astronomers (and anyone) to download and use. For a quick reminder, here's the list:

1. The US Naval Observatory (USNO-B1) all-sky catalog, 1046 million entries (2003)
2. The Guide Star Catalog (GSC v2.3) all-sky catalog, 946 million entries (2006)
3. The Sloan Digital Sky Survey (SDSS DR9), 933 million entries (2012)
4. The Wide-field Infrared Survey Explorer (WISE) all-sky catalog, 748 million entries (2013)
5. The United Kingdom Infrared Deep Sky Survey (UKIDSS), 727 million entries (2012)
6. The 2 Micron All Sky Survey (2MASS), 471 million entries (2003)
7. The Deep Near Infrared Survey (DENIS) of the southern sky, 355 million entries (2005)
8. The INT Photometric H-Alpha Survey (IPHAS), 219 million entries (2014)
9. The USNO CCD Astrograph Catalog v4 (UCAC4), 114 million entries (2012)
10. The Galactic Legacy Infrared Mid-Plane Survey (GLIMPSE), 104 million entries (2008)

In that post I discussed five of these surveys (including the IPHAS survey that I work on - read more about that here) that all include astronomical objects observed in the visible part of the electromagnetic spectrum (that is the part that we can see with our own eyes). The other five of these surveys all include measurements made outside of this part of the spectrum, and its those I'd like to talk about today.

The electromagnetic spectrum, with wavelength increasing to the right, and frequency (or energy) increasing to the left. (Credit: NASA)

These five surveys are WISE, UKIDSS, 2MASS, DENIS, and GLIMPSE, and they're all infrared photometric surveys, which means they measure how bright astronomical sources are in the infrared. The infrared part of the electromagnetic spectrum includes radiation with a longer wavelength than that of the light we can see, though not as long as microwave radiation or radio waves. You may be aware that infrared radiation is what we commonly know of as heat or warmth. Any object with a temperature around body temperature or up to a few hundred Celsius will radiate energy in the infrared.

Astronomers often divide up the infrared part of the electromagnetic spectrum into the near-, mid- and far-infrared regions, ordered by how far they are from the visible part of the spectrum. The definition of these three regions isn't important, but originates in the different technologies required to detect radiation in each region (such as different CCD detectors).

The near-infrared is the most easily accessible part of the infrared spectrum, and three of these surveys cover this region. Perhaps the most commonly-used of these is 2MASS, an all-sky near-infrared survey, which operated from 1997 to 2001 at two observatories in Arizona and Chile. The 2MASS all-sky view of the Milky Way is a stunning panorama of our galaxy, aided by the ability of near-IR radiation to penetrate the dust that would otherwise absorb optically visible radiation.

The 2MASS all-sky view of the Milky Way (Credit: IPAC)

The DENIS survey was also a deep near-IR survey that finished in 2001, but which only covered the southern sky. The UKIDSS survey(s) are a series of very deep near-IR surveys (deeper than both 2MASS and DENIS) on the United Kingdom Infra-Red Telescope (UKIRT) in Hawaii that targeted certain areas of the sky for different science goals. Some of these areas include regions of our galaxy where stars are forming and other areas look away from our galaxy out into the darkest regions of space where other galaxies are easily studied.

The remaining two surveys, WISE and GLIMPSE, are mid-infrared surveys, operating at slightly longer wavelengths than the near-IR surveys.

The centre of our Milky Way galaxy seen in the mid-infrared from GLIMPSE (Credit: Spitzer GLIMPSE team)

GLIMPSE was a survey of the Milky Way galaxy with NASA's Spitzer Space Telescope, and WISE is also a NASA space telescope that performed an all-sky infrared survey. Because the Earth's atmosphere absorbs most mid-infrared radiation, mid-infrared telescopes have to operate from above the atmosphere.

An artist's impression of NASA's
Spitzer Space Telescope in orbit
(Credit: IPAC)

A further complication for mid-infrared telescopes is that the telescopes themselves have to be cooled to very low temperatures to prevent them from radiating infrared photons themselves! This is because, even in space objects will radiate infrared radiation depending on their temperature. These photons would contaminate the measurements made, and so the entire telescope has to be cooled to incredibly low temperatures, much lower than anything in our solar system, to prevent them from radiating significantly large amounts of infrared radiation. Both space telescopes used cryostats filled with liquid hydrogen to achieve this, allowing them to be cooled to less than -263 C (under 10 K).

That brings our list of the largest astronomical catalogs to a close, for the time being at least. Astronomy is moving ever so rapidly towards bigger and bigger catalogs and it is almost certain that these catalogs will be eclipsed in the near future. The most likely candidates to take the throne of largest astronomical catalog will probably come from either ESA's Gaia satellite or the Large Synoptic Survey Telescope (LSST). More on these in a future post!

The largest astronomical catalogs ever made! (Part I)

In a previous post I discussed a huge data release from one of the surveys I've worked on over the last 10 years, which included a catalog of over 200 million sources. This is a BIG catalog of data, but how does it compare to other large astronomical surveys? I wanted to find out!

To do this I searched the two main astronomical data archives, the Vizier and IRSA online databases. My criteria for including a catalog was that it had to be a single catalog with the same information and measurements made in the same way for all the entries in the catalog (usually referred to as 'sources', because they are the source of the light we are measuring). It could be a catalog of any type of object (stars, planets, galaxies) and it could be from data obtained at any wavelength (optical, infrared, ultraviolet) by any telescope in the world.

So here's the 'Top 10' astronomical catalogs currently (and publicly) available:

1. The US Naval Observatory (USNO-B1) all-sky catalog, 1046 million entries (2003)
2. The Guide Star Catalog (GSC v2.3) all-sky catalog, 946 million entries (2006)
3. The Sloan Digital Sky Survey (SDSS DR9), 933 million entries (2012)
4. The Wide-field Infrared Survey Explorer (WISE) all-sky catalog, 748 million entries (2013)
5. The United Kingdom Infrared Deep Sky Survey (UKIDSS), 727 million entries (2012)
6. The 2 Micron All Sky Survey (2MASS), 471 million entries (2003)
7. The Deep Near Infrared Survey (DENIS) of the southern sky, 355 million entries (2005)
8. The INT Photometric H-Alpha Survey (IPHAS), 219 million entries (2014)
9. The USNO CCD Astrograph Catalog v4 (UCAC4), 114 million entries (2012)
10. The Galactic Legacy Infrared Mid-Plane Survey (GLIMPSE), 104 million entries (2008)

It's great to see that our survey, IPHAS, has made it into the Top 10! Most of these names probably won't mean much to non-astronomers, but to astronomers these are the big catalogs that many of us use regularly. They represent hundreds of nights of telescope time and thousands of hours of valuable work.

They are primarily (all but one) photometric surveys, which as I mentioned in a previous post is pretty much the easiest measurement an astronomer can make, it's simply a measure of how bright an object is. You don't have to measure the shape of the object, or its exact position, just how bright it is - simply take a picture and 'count' up the amount of light!

Photographic (negative) plate showing the galaxy M33, taken
by Edwin Hubble in 1926 (Credit: University of Arizona)

Interestingly the first two of these catalogs, USNO-B1 and GSC, are compiled not from CCD observations (as are the other photometric catalogs on this list), but from photometry extracted from photographic plates. During the 20th century the night sky was routinely photographed from various observatories around the world and many of the photographic plates have been carefully stored since then in telescope and university archives.

Putting this data together into a single uniform catalog (or two catalogs with heavy overlaps) represents an important and valuable achievement, and this data has been useful for many studies.

These two photographic surveys used photographic plates sensitive to either red or blue light, so both are known as 'optical' surveys (meaning they are sensitive to light in the optical part of the electromagnetic spectrum). The same is also true for three other surveys on this list: SDSS (3rd), IPHAS (8th), and UCAC4 (9th), though these surveys all use CCD observations.

CCDs are superior to photographic plates for many reasons, so why are these modern CCD catalogs smaller than the catalogs compiled from photographic observations? Well this is mostly because the first two of these CCD surveys only covered a small area on the sky: SDSS is focussed on the Galactic halo (where it is easier to observe other galaxies, which was the goal of their survey) and IPHAS covers only the Northern Galactic Plane (the part of our own galaxy visible from the northern hemisphere). Compared to the entire all-sky area of 41,253 square degrees, these two surveys only cover 14,555 and 1800 square degrees, respectively.

The Galactic Plane of our Milky Way galaxy (Credit: Imgur)

The final optical survey, UCAC4, is principally a proper-motion survey and not a photometric survey. The objective here is not to measure how bright the sources are, but how fast the object is moving across the sky (known as its 'proper motion').

The simplest way to do this is to take two images of the same area of the sky separated by a few years. You then measure the positions of all the objects in each image and calculate how far they've moved between the images. In truth it is much more complex than this, because you're measuring objects moving across a curved surface (the sky is a curved surface) and most of our cameras and detectors are not as perfect as we'd like them to be (i.e. a straight line across the sky might not be perfectly straight on the image you record because the optics of the camera might distort it).

Because of these difficulties it isn't always possible to measure a proper motion for every source, hence the reason this catalog is smaller than the other optical all-sky catalogs. Despite these difficulties though this is a very large proper motion catalog and it will be many years before it is eclipsed in terms of size!

Next time we'll look at the other five catalogs on this list, which have all been produced from observations outside of the visible part of the electromagnetic spectrum.