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.

A new catalogue of over 200 million stars in the Milky Way galaxy

Recently one of the surveys that I'm a member of made a huge data release including over 200 million stars from our own galaxy, the Milky Way. We've made the data available to the entire world, not just the astronomical community, but everyone in the world! Astronomers love to share data!

The Isaac Newton Telescope on La Palma (Credit: ING)

The survey in question is the INT Photometric H-Alpha Survey (IPHAS), which has, over the last 11 years been using the Isaac Newton Telescope on the island of La Palma to survey our own galaxy to an unprecedented level of detail.

IPHAS is a photometric survey, meaning the goal of the survey is to measure photometry for all the sources it observes (astronomers refer to most objects as 'sources' until they're sure exactly what they are: stars, planets or galaxies). Photometry is the easiest measurement astronomers can make, since all you have to do is measure how bright an object is using a certain filter. IPHAS uses two 'broad' filters and one 'narrow' Hα filter (more on the awesomeness of this filter and what you can do with it in a future post).

The Milky Way (Credit: Wikimedia Commons)

Our own galaxy is visible to us as a glowing band of light stretching across the sky, known as the Milky Way. It appears this way to us because we (the Earth, the Sun and the entire solar system) are within the galaxy, and so we see the galaxy all around us, but concentrated in a band of light that makes up the disk of our galaxy. Astronomers refer to this as the Galactic Plane.

Surveying our galaxy is not easy because the stars can often be very crowded together (hence why they often appear blurred as a bright band of light instead of individual stars), so to do the job properly requires a high-resolution telescope and instrument, but also one that could invest a lot of time and effort into a single project such as this.

In 2003 the Isaac Newton Telescope was just such a telescope and so the IPHAS collaboration was born, led by Janet Drew (currently at the University of Hertfordshire where I am also based). I joined the collaboration in 2004 when I started my PhD at University College London and have been involved in the survey ever since, participating in the telescope observations, analysing data, and writing papers. Its been great to be involved in such a large and exciting project such as this.

After 11 years of taking data, sorting and organising the data, and doing some exciting science as well, the catalog is finally finished. It contains information on 219 million detected objects, the vast majority of which are stars in our own galaxy. It's an example of what astronomers (and data scientists) refer to as 'big data': huge catalogs of data, observations and measurements that can be sorted, filtered and analysed en masse.

One example of this is the image shown below, which was put together by IPHAS astronomer Hywel Farnhill. This map shows part of the Galactic Plane that IPHAS has been surveying, mostly covering the constellation of Cygnus, but stretching to Sagitta on the right.

IPHAS stellar density map (Credit: Hywel Farnhill)

This map is a stellar density map, showing the density of stars across the Galactic Plane, i.e. the number of stars detected by the IPHAS survey in each area of the plane, and its really detailed! Click here to see a high-resolution version of the image, zoom into it, and have a look at the exquisite level of detail that can be seen. The brightest parts are where we detect the most stars and the darkest parts are where we detect the least stars.

Most of this structure is actually due to the obscuring effects of interstellar dust, and not actually the distribution of stars in our galaxy (though this does contribute). Interstellar dust, which is mostly very small silicate rocks (similar to those found on Earth), absorbs starlight, particularly at visible wavelengths, and leads to the fine structure that you can see in this image. Tracing the distribution of this dust is really important to astronomers because of the huge influence it can have when it absorbs the light from the stars and galaxies we are trying to study.

You can read more about the survey on the IPHAS website, learn about the data release itself in this published paper by IPHAS astronomer and catalog maestro Geert Barentsen, or read the press release from the Royal Astronomical Society. If you'd like to access the data (and please do) you can get the entire set of data, for free, from the Vizier web archive.