Sunday, 12 October 2014

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.

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